Patent Publication Number: US-10767592-B2

Title: Failure diagnosis device for in-cylinder pressure sensor

Description:
TECHNICAL FIELD 
     The technology disclosed herein relates to a failure diagnosis device for an in-cylinder pressure sensor. 
     BACKGROUND OF THE DISCLOSURE 
     JP2010-144670A discloses an abnormality detecting device for an in-cylinder pressure sensor which detects the pressure inside a combustion chamber of an engine. The in-cylinder pressure sensor of this device includes a deforming part which deforms by the in-cylinder pressure, and a strain gage adhered to the deforming part. When the material composition of the deforming part is changed due to the influences of heat, etc. and the modulus of elasticity increases, the deforming part becomes difficult to be elastically deformed, and the gain of the output value of the in-cylinder pressure sensor decreases. Therefore, this device detects the gain of the output value of the in-cylinder pressure sensor, and when the gain is low, it concludes that the in-cylinder pressure sensor is abnormal. 
     In order to prevent a false diagnosis, when the gain of the output value of the in-cylinder pressure sensor is low and the output waveform is deviated in time, this device concludes that the engine is abnormal, and on the other hand, when the gain of the output value of the in-cylinder pressure sensor is low and the output waveform is not deviated in time, it concludes that the in-cylinder pressure sensor is not abnormal. 
     The present inventors have determined the gain of the output value of the in-cylinder pressure sensor, as a result of diligent examinations. Specifically, although the detailed mechanism is unknown, it was newly determined that the symmetry of the signal value of the in-cylinder pressure sensor collapses, when the in-cylinder pressure sensor has failed by being damaged under the influence of heat etc. In this case, a difference between the signal value of the in-cylinder pressure sensor at the timing where the crank angle is advanced by a given amount from a compression top dead center, and the signal value of the in-cylinder pressure sensor at the timing where the crank angle is retarded by the same amount from the compression top dead center increases. 
     Although the technology disclosed in JP2010-144670A takes the gain of the output value of the in-cylinder pressure sensor into consideration, since it does not take the symmetry of the output value at all into consideration, the present inventors have noticed that there is room for improvement for the accuracy of the abnormality diagnosis. 
     SUMMARY OF THE DISCLOSURE 
     The technology disclosed herein increases an accuracy of a failure diagnosis of an in-cylinder pressure sensor. 
     According to one aspect of the present disclosure, a failure diagnosis device for an in-cylinder pressure sensor is provided. The device includes an in-cylinder pressure sensor disposed so as to face to the inside of a combustion chamber of an engine mounted to an automobile, and configured to output a signal corresponding to a pressure inside the combustion chamber, and an engine controller comprised of circuitry configured to execute a diagnosis module into which the signal of the in-cylinder pressure sensor is inputted and diagnose a failure of the in-cylinder pressure sensor based on the signal of the in-cylinder pressure sensor. 
     The diagnosis module includes a reading module configured to read the signal of the in-cylinder pressure sensor at a predefined first timing that is a timing retarded by a specific crank angle from a compression top dead center, and the signal of the in-cylinder pressure sensor at a predefined second timing that is a timing advanced by the specific crank angle from the compression top dead center, and a failure determining module configured to determine that the in-cylinder pressure sensor has failed when the failure determining module determines that a difference between a signal value of the in-cylinder pressure sensor at the first timing and a signal value of the in-cylinder pressure sensor at the second timing exceeds a predefined threshold. 
     According to this configuration, the diagnosis module compares the difference between the signal value of the in-cylinder pressure sensor at the first timing and the signal value of the in-cylinder pressure sensor at the second timing with the predefined threshold. The failure determining module determines that the in-cylinder pressure sensor has failed when the difference exceeds the threshold. Thus, the failure diagnosis of the in-cylinder pressure sensor can accurately be performed. 
     The diagnosis module may make the threshold smaller as a speed of the engine increases. 
     The difference between the signal value of the in-cylinder pressure sensor at the first timing and the signal value of the in-cylinder pressure sensor at the second timing becomes larger by being influenced by cooling loss. The influence of the cooling loss may become a factor of error when diagnosing the failure of the in-cylinder pressure sensor, although it is unrelated to the failure of the in-cylinder pressure sensor. In order to improve the accuracy of the failure diagnosis of the in-cylinder pressure sensor, it is effective to set the threshold larger than the difference influenced by the cooling loss. 
     On the other hand, when the engine speed is high, since the cooling loss per unit time decreases, the difference described above becomes smaller. Thus, in this case, the threshold is set to be smaller than that when the engine speed is low, it is effective to improve the accuracy of the failure diagnosis of the in-cylinder pressure sensor. 
     The diagnosis module may make the threshold larger as an amount of air filled up in the combustion chamber increases. 
     When the air amount filled up in the combustion chamber is large, a leak of air from fitting parts of piston rings during the compression stroke increases, and therefore, the pressure inside the combustion chamber after the compression top dead center falls. As a result, the difference between the signal value of the in-cylinder pressure sensor at the first timing and the signal value of the in-cylinder pressure sensor at the second timing may become large. This influence may become a factor of the error when diagnosing the failure of the in-cylinder pressure sensor, although it is unrelated to the failure of the in-cylinder pressure sensor. In order to improve the accuracy of the failure diagnosis of the in-cylinder pressure sensor, it is effective to set the threshold larger than the difference influenced by the leak loss. 
     According to this configuration, when the air amount filled into the combustion chamber is large, the diagnosis module sets the threshold larger than when the air amount is small. Thus, it becomes advantageous for performing the failure diagnosis of the in-cylinder pressure sensor more accurately. 
     The failure determining module may repeatedly perform a comparison of the difference with the threshold. The failure determining module may determine that the in-cylinder pressure sensor has failed when a determination that the difference exceeds the threshold is made continuously a plurality of times. 
     For example, as a result of the mating of the parts which constitute the in-cylinder pressure sensor being changed during a compression stroke, the pressure inside the combustion chamber may fall after a compression top dead center. Although such a pressure fall is only a temporary phenomenon, there is a possibility of causing a false diagnosis of the in-cylinder pressure sensor. 
     On the other hand, since the pressure inside the combustion chamber falls continuously when the in-cylinder pressure sensor actually fails, it becomes advantageous for performing the failure diagnosis of the in-cylinder pressure sensor more accurately if the determination by the failure determining module is repeatedly performed as described above. 
     The diagnosis module may include an informing device configured to inform of the failure when the failure determining module determines the failure of the in-cylinder pressure sensor. 
     According to this configuration, the user is informed of the failure of the in-cylinder pressure sensor, and as a result, the broken in-cylinder pressure sensor is replaced, for example. 
     The failure diagnosis device may further include an engine control module into which a signal of one or more sensors at least including the in-cylinder pressure sensor is inputted and configured to operate the engine based on the signal of the one or more sensors. The engine control module may stop a supply of fuel to the engine, when a fuel cut condition is satisfied, while the automobile travels. The diagnosis module may perform the failure diagnosis of the in-cylinder pressure sensor, while the engine control module stops the supply of fuel to the engine. 
     While the engine control module stops the fuel supply to the engine, the combustion is not performed in the combustion chamber. That is, the signal value of the in-cylinder pressure sensor increases and decreases only depending on the volume change of the combustion chamber. The failure determining module can perform the failure diagnosis of the in-cylinder pressure sensor more accurately based on the difference between the signal value of the in-cylinder pressure sensor at the first timing and the signal value of the in-cylinder pressure sensor at the second timing. 
     Moreover, the failure diagnosis device may further include an ignition plug, disposed so as to face to the inside of the combustion chamber, and configured to ignite a mixture gas inside the combustion chamber in response to an ignition signal of the engine control module. A portion of the mixture gas inside the combustion chamber may start combustion accompanied by flame propagation, by forcible ignition of the ignition plug, and remaining unburnt mixture gas may then combust by self-ignition. The engine control module may output the ignition signal to the ignition plug before a target timing so that the unburnt mixture gas self-ignites at the target timing. The engine control module may estimate a timing at which the unburnt mixture gas self-ignites based on the signal of the in-cylinder pressure sensor. 
     The present applicant proposes SPCCI (SPark Controlled Compression Ignition) combustion which is a combination of SI (Spark Ignition) combustion and CI (Compression Ignition) combustion. SI combustion is a combustion accompanied by the flame propagation by forcibly igniting the mixture gas inside the combustion chamber, and CI combustion is a combustion which starts by the mixture gas inside the combustion chamber being compressed and ignited. SPCCI combustion is a form of combustion in which, by forcibly igniting the mixture gas inside the combustion chamber, the combustion by flame propagation starts, and then, due to the heat generated by the SI combustion and the pressure build-up inside the combustion chamber by flame propagation, unburnt mixture gas inside the combustion chamber carries out CI combustion. 
     CI combustion is performed when the in-cylinder temperature reaches an ignition temperature defined by composition of the mixture gas. If the in-cylinder temperature reaches the ignition temperature near the compression top dead center and then CI combustion is performed, fuel efficiency of SPCCI combustion is maximized. 
     Meanwhile, in SPCCI combustion, if CI combustion takes place near the compression top dead center, the in-cylinder pressure may rise excessively, and thereby combustion noise may become excessive. In such a case, since CI combustion will take place when a piston descends considerably during expansion stroke if the ignition timing is retarded, combustion noise can be reduced. However, fuel efficiency of the engine decreases. 
     In order to achieve both the reduction of combustion noise and the improvement of fuel efficiency in the engine which performs SPCCI combustion, SPCCI combustion must be controlled so that a combustion waveform which changes according to the advancing of a crank angle becomes a suitable combustion waveform. 
     In order to control SPCCI combustion, for example, a CI combustion start timing θci can be used as a parameter indicative of a characteristic of SPCCI combustion. The CI combustion start timing θci is a timing at which unburnt mixture gas self-ignites. If an actual θci is advanced exceeding a target θci, since CI combustion occurs at a timing near the compression top dead center, combustion noise increases. Then, in order to reduce combustion noise, the engine control module must recognize the actual θci. 
     If the actual θci can be estimated, the engine control module can bring the actual θci close to the target θci by adjusting the ignition timing according to a deviation of the actual θci from the target θci. For example, while the actual θci is advanced exceeding the target θci, the engine control module can retard the ignition timing, and, as a result of the actual θci being retarded, combustion noise can be reduced. 
     The present applicant has also proposed a technique for accurately estimating θci based on the signal of the in-cylinder pressure sensor. 
     In the engine which performs SPCCI combustion, the diagnosing the failure of the in-cylinder pressure sensor accurately makes it possible to reduce combustion noise and improve fuel efficiency. 
     The engine controller may be connected to a warning lamp provided to the automobile, and may cause the warning lamp to light up when the in-cylinder pressure sensor is determined to have failed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating a configuration of an engine. 
         FIG. 2  is a view illustrating a configuration of a combustion chamber, where an upper drawing is a plan view of a combustion chamber, and a lower drawing is a cross-sectional view taken along a line II-II. 
         FIG. 3  is a block diagram illustrating a configuration of an engine control device. 
         FIG. 4  is a cross-sectional view illustrating a configuration of an in-cylinder pressure sensor. 
         FIG. 5  is a graph illustrating a waveform of SPCCI combustion. 
         FIG. 6  is a block diagram illustrating a functional configuration of an engine controller. 
         FIG. 7  is a graph illustrating a change in a close timing of an intake valve with respect to an engine load. 
         FIG. 8  is a block diagram illustrating a functional configuration according to a failure diagnosis device for the in-cylinder pressure sensor. 
         FIG. 9  is a graph illustrating a waveform of a signal which is outputted from the in-cylinder pressure sensor when the in-cylinder pressure sensor is normal, and a waveform of the signal outputted when the sensor has failed. 
         FIG. 10  is a graph illustrating a pressure difference when the in-cylinder pressure sensor is normal, and a threshold according to the failure diagnosis of the in-cylinder pressure sensor. 
         FIG. 11  is a graph illustrating the pressure difference according to an engine load. 
         FIG. 12  is a graph illustrating the pressure difference according to an EGR gas amount. 
         FIGS. 13A and 13B  illustrate a flowchart of procedure of the failure diagnosis of the in-cylinder pressure sensor. 
         FIG. 14  illustrates a relation between an engine speed and a delay, where the upper graph illustrates a relation between the engine speed and a delay cycle, and the lower graph illustrates a relation between the engine speed and a delay time. 
         FIG. 15  is a time chart illustrating a change in each parameter related to the failure diagnosis of the in-cylinder pressure sensor. 
         FIG. 16  is a time chart illustrating a change in each parameter during the failure diagnosis of the in-cylinder pressure sensor. 
         FIG. 17  is a flowchart illustrating a part of procedure of the failure diagnosis of the in-cylinder pressure sensor, which is different from  FIG. 13B . 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Hereinafter, one embodiment of a failure diagnosis device for an in-cylinder pressure sensor will be described in detail with reference to the accompanying drawings. The following description is one example of the failure diagnosis device for the in-cylinder pressure sensor. 
       FIG. 1  is a view illustrating a configuration of an engine of a compression ignition type provided with the failure diagnosis device for the in-cylinder pressure sensor.  FIG. 2  is a view illustrating a configuration of a combustion chamber of the engine. Note that in  FIG. 1 , the intake side is on the left side, and the exhaust side is on the right side. In  FIG. 2 , the intake side is on the right side, and the exhaust side is on the left side.  FIG. 3  is a block diagram illustrating a configuration of an engine control device. 
     An engine  1  is a four-stroke reciprocating engine which operates by a combustion chamber  17  repeating an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke. The engine  1  is mounted on an automobile with four wheels. The automobile travels by the operation of the engine  1 . Fuel of the engine  1  is gasoline in this example. The fuel may be any liquid fuel which at least contains gasoline. For example, the fuel may be gasoline which contains bioethanol, etc. 
     (Configuration of Engine) 
     The engine  1  is provided with a cylinder block  12 , and a cylinder head  13  placed thereon. A plurality of cylinders  11  are formed inside the cylinder block  12 . In  FIGS. 1 and 2 , although only one cylinder  11  is illustrated, the engine  1  is a multi-cylinder engine in this example. 
     In each cylinder  11 , a piston  3  is inserted so as to be slidable. The piston  3  is coupled to a crankshaft  15  through a connecting rod  14 . The piston  3 , the cylinder  11  and the cylinder head  13  define the combustion chamber  17 . Note that the term “combustion chamber” as used herein may be used in a broader sense. That is, the “combustion chamber” may mean a space formed by the piston  3 , the cylinder  11 , and the cylinder head  13 , regardless of the position of the piston  3 . Moreover, in the following description, the term “in-cylinder” as used herein may also be used as a synonymous of the combustion chamber. 
     As illustrated in the lower drawing of  FIG. 2 , a lower surface of the cylinder head  13 , i.e., a ceiling surface of the combustion chamber  17  is comprised of an inclined surface  1311  and an inclined surface  1312 . The inclined surface  1311  is a rising gradient which rises from the intake side toward an axis X 2  of an injector  6  (described later). The inclined surface  1312  is a rising gradient which rises from the exhaust side toward the axis X 2  of the injector  6 . The ceiling surface of the combustion chamber  17  is of a so-called “pent roof” shape. 
     An upper surface of the piston  3  bulges toward the ceiling surface of the combustion chamber  17 . A cavity  31  is formed in the upper surface of the piston  3 . The cavity  31  is dented from the upper surface of the piston  3 . The cavity  31  has a shallow dish shape in this example. The center of the cavity  31  is offset on the exhaust side from a center axis X 1  of the cylinder  11 . 
     A geometric compression ratio of the engine  1  is set 10 or more and 30 or less. The engine  1  performs SPCCI (SPark Controlled Compression Ignition) combustion which is a combination of SI (spark ignition) combustion and CI (compressed ignition) combustion, as will be described later. SPCCI combustion controls CI combustion using a heat generation and a pressure buildup by SI combustion. The engine  1  is a compression ignition engine. However, this engine  1  does not have to make the temperature of the combustion chamber  17  high when the piston  3  reaches a compression top dead center. Thus, the engine  1  can set the geometric compression ratio comparatively low. If the geometric compression ratio can be made lower, it becomes advantageous to a reduction of cooling loss, and a reduction of mechanical loss. In a regular fuel specification (i.e., a low octane fuel of which the octane number is about 91), the geometric compression ratio of the engine  1  may be 14 to 17, and in a high-octane specification (i.e., a high octane fuel of which the octane number is about 96), the geometric compression ratio may also be 15 to 18. 
     An intake port  18  is formed in the cylinder head  13  for each cylinder  11 . Although illustration is omitted, the intake port  18  includes two intake ports, a first intake port and a second intake port. The intake port  18  communicates with the combustion chamber  17 . The intake port  18  is a so-called tumble port. That is, the intake port  18  has such a shape that a tumble flow is formed inside the combustion chamber  17 . 
     An intake valve  21  is disposed in each intake port  18 . The intake valve  21  opens and closes a channel between the combustion chamber  17  and each intake port  18 . The intake valve  21  is opened and closed by a valve operating mechanism at a given timing. The valve operating mechanism may be a variable valve operating mechanism in which the valve timing and/or the valve lift is variable. In this example, as illustrated in  FIG. 3 , the variable valve operating mechanism has an intake electric S-VT (Sequential-Valve Timing)  23 . The intake electric S-VT  23  continuously changes a phase of an intake cam shaft within a given range. The open timing and the close timing of the intake valve  21  change continuously. Note that the intake valve operating mechanism may have a hydraulic S-VT, instead of the electric S-VT. 
     An exhaust port  19  is formed also in the cylinder head  13  for each cylinder  11 . The exhaust port  19  also has two exhaust ports of a first exhaust port and a second exhaust port. The exhaust port  19  communicates with the combustion chamber  17 . 
     An exhaust valve  22  is disposed in each exhaust port  19 . The exhaust valve  22  opens and closes a channel between the combustion chamber  17  and each exhaust port  19 . The exhaust valve  22  is opened and closed by the valve operating mechanism at a given timing. This valve operating mechanism may be a variable valve operating mechanism in which the valve timing and/or the valve lift are variable. In this example, as illustrated in  FIG. 3 , the variable valve operating mechanism has an exhaust electric S-VT  24 . The exhaust electric S-VT  24  continuously changes a phase of an exhaust cam shaft within a given range. The open timing and the close timing of the exhaust valve  22  change continuously. Note that the exhaust valve operating mechanism may have a hydraulic S-VT, instead of the electric S-VT. 
     The intake electric S-VT  23  and the exhaust electric S-VT  24  adjust a length of an overlap period where both the intake valve  21  and the exhaust valve  22  open. As the length of the overlap period becomes longer, the residual gas inside the combustion chamber  17  can be purged. Moreover, by adjusting the length of the overlap period, internal EGR (Exhaust Gas Recirculation) gas can be introduced into the combustion chamber  17 . The intake electric S-VT  23  and the exhaust electric S-VT  24  constitute an internal EGR system. Note that the internal EGR system is not necessarily comprised of the S-VTs. 
     The injector  6  is attached to the cylinder head  13  for each cylinder  11 . The injector  6  directly injects the fuel into the combustion chamber  17 . The injector  6  is one example of a fuel injection part. The injector  6  is disposed at a location where the inclined surface  1311  and the inclined surface  1312  intersect with each other. As illustrated in  FIG. 2 , the axis X 2  of the injector  6  is located on the exhaust side of the center axis X 1  of the cylinder  11 . The axis X 2  of the injector  6  is parallel to the center axis X 1 . The axis X 2  of the injector  6  coexists with the center of the cavity  31 . The injector  6  opposes to the cavity  31 . Note that the axis X 2  of the injector  6  may coexist with the center axis X 1  of the cylinder  11 . In such a configuration, the axis X 2  of the injector  6  may coexist with the center of the cavity  31 . 
     Although detailed illustration of the injector  6  is omitted, it is comprised of a multi nozzle hole type fuel injection valve having a plurality of nozzle holes. As illustrated by a two-dot chain line in  FIG. 2 , the injector  6  injects the fuel so that fuel spray may spread radially from the center of the combustion chamber  17 . In this example, the injector  6  has ten nozzle holes, and the nozzle holes are arranged so as to be equally spaced in the circumferential direction. 
     The injector  6  is connected to a fuel supply system  61 . The fuel supply system  61  includes a fuel tank  63 , and a fuel supply path  62  which connects the fuel tank  63  to the injector  6 . The fuel tank  63  stores the fuel. A fuel feed pump  65  and a common rail  64  are provided in the fuel supply path  62 . The fuel feed pump  65  pumps the fuel to the common rail  64 . In this example, the fuel feed pump  65  is a plunger-type pump driven by the crankshaft  15 . The common rail  64  stores at a high fuel pressure the fuel which is pumped from the fuel feed pump  65 . When the injector  6  opens, the fuel stored inside the common rail  64  is injected into the combustion chamber  17  from the nozzle holes of the injector  6 . The fuel supply system  61  is capable of supplying the fuel at the high pressure of 30 MPa or higher to the injector  6 . The pressure of the fuel supplied to the injector  6  may be changed according to the operating state of the engine  1 . Note that the configuration of the fuel supply system  61  is not limited to the configuration described above. 
     An ignition plug  25  is attached to the cylinder head  13  for each cylinder  11 . The ignition plug  25  forcibly ignites mixture gas inside the combustion chamber  17 . The ignition plug  25  is one example of an ignition part. In this example, the ignition plug  25  is disposed on the intake side of the center axis X 1  of the cylinder  11 . The ignition plug  25  is located between the two intake ports  18 . The ignition plug  25  is attached to the cylinder head  13  so as to be inclined from above to below in a direction toward the center of the combustion chamber  17 . As illustrated in  FIG. 2 , an electrode of the ignition plug  25  projects into the combustion chamber  17  and is located near the ceiling surface of the combustion chamber  17 . Note that the ignition plug  25  may be disposed on the exhaust side of the center axis X 1  of the cylinder  11 . Moreover, the ignition plug  25  may be disposed on the center axis X 1  of the cylinder  11 . 
     An intake passage  40  is connected to one side surface of the engine  1 . The intake passage  40  communicates with the intake port  18  of each cylinder  11 . The gas to be entered into the combustion chamber  17  flows through the intake passage  40 . An air cleaner  41  is disposed at an upstream end of the intake passage  40 . The air cleaner  41  filters fresh air. A surge tank  42  is disposed near a downstream end of the intake passage  40 . A part of the intake passage  40  downstream of the surge tank  42  constitutes an independent passage which branches for each cylinder  11 . A downstream end of the independent passage is connected to the intake port  18  of each cylinder  11 . 
     A throttle valve  43  is disposed between the air cleaner  41  and the surge tank  42  in the intake passage  40 . The throttle valve  43  adjusts an amount of the fresh air introduced into the combustion chamber  17  by adjusting an opening of the valve thereof. 
     A supercharger  44  is disposed in the intake passage  40  downstream of the throttle valve  43 . The supercharger  44  boosts the gas entering into the combustion chamber  17 . In this example, the supercharger  44  is a mechanical supercharger driven by the engine  1 . The mechanical supercharger  44  may be roots type, Lysholm type, vane type, or centrifugal type. 
     An electromagnetic clutch  45  is provided between the supercharger  44  and the engine  1 . The electromagnetic clutch  45  transmits or intercepts a driving force from the engine  1  to the supercharger  44  between the supercharger  44  and the engine  1 . By an ECU  10  switching the interception (disengagement) and the connection (engagement) of the electromagnetic clutch  45 , ON/OFF of the supercharger  44  is switched, as will be described later. 
     An intercooler  46  is disposed in the intake passage  40  downstream of the supercharger  44 . The intercooler  46  cools gas compressed by the supercharger  44 . The intercooler  46  may be, for example, a water cooling type or an oil cooling type. 
     The intake passage  40  is connected to a bypass passage  47 . The bypass passage  47  connects a part of the intake passage  40  upstream of the supercharger  44  to a part downstream of the intercooler  46 . The bypass passage  47  bypasses the supercharger  44  and the intercooler  46 . An air by-pass valve  48  is disposed in the bypass passage  47 . The air by-pass valve  48  adjusts a flow rate of gas flowing through the bypass passage  47 . 
     The ECU  10  causes the air by-pass valve  48  to fully open, when the supercharger  44  is turned OFF (i.e., when the electromagnetic clutch  45  is disengaged or released). The gas flowing through the intake passage  40  bypasses the supercharger  44  and enters into the combustion chamber  17 . The engine  1  operates in a non-boosting state, i.e., a natural aspiring state. 
     When the supercharger  44  is turned ON, the engine  1  operates in a boosting state. The ECU  10  adjusts an opening of the air by-pass valve  48 , when the supercharger  44  is turned ON (i.e., when the electromagnetic clutch  45  is engaged or connected). A portion of gas which passed the supercharger  44  flows back upstream of the supercharger  44  through the bypass passage  47 . When the ECU  10  adjusts the opening of the air by-pass valve  48 , the pressure of the gas entering into the combustion chamber  17  varies. That is, a boost pressure varies. Note that a phrase “during the boost” as used herein may refer to time when the pressure inside the surge tank  42  exceeds the atmospheric pressure, and a phrase “during the non-boost” as used herein may refer to time when the pressure inside the surge tank  42  becomes lower than the atmospheric pressure. 
     In this example, a supercharging system  49  is comprised of the supercharger  44 , the bypass passage  47 , and the air by-pass valve  48 . 
     The engine  1  has a swirl generating part which generates a swirl flow inside the combustion chamber  17 . The swirl flow flows as illustrated by white arrows in  FIG. 2 . The swirl generating part has a swirl control valve  56  attached to the intake passage  40 . Although detailed illustration of the swirl control valve  56  is omitted, it is disposed in a secondary passage, among a primary passage connected with one of the two intake ports  18  and the secondary passage connected with the other intake port  18 . The swirl control valve  56  is an opening controllable valve in which a cross-section of the secondary passage can be choked. When the opening of the swirl control valve  56  is small, since a flow rate of intake air entering into the combustion chamber  17  from one intake port  18  is relatively large, and on the other hand, a flow rate of intake air entering into the combustion chamber  17  from the other intake port  18  is relatively small, the swirl flow inside the combustion chamber  17  becomes stronger. When the opening of the swirl control valve  56  is large, since the flow rates of intake air entering into the combustion chamber  17  from the two intake ports  18  become equal, the swirl flow inside the combustion chamber  17  becomes weaker. When the swirl control valve  56  is fully opened, the swirl flow does not occur. 
     An exhaust passage  50  is connected to the other side surface of the engine  1 . The exhaust passage  50  communicates with the exhaust port  19  of each cylinder  11 . The exhaust passage  50  is a passage through which the exhaust gas discharged from the combustion chamber  17  flows. Although the detailed illustration of an upstream part of the exhaust passage  50  is omitted, it constitutes an independent passage which branches for each cylinder  11 . An upstream end of the independent passage is connected to the exhaust port  19  of each cylinder  11 . 
     An exhaust gas purification system having a plurality of catalytic converters is disposed in the exhaust passage  50 . The upstream catalytic converter is disposed inside an engine bay, although the corresponding illustration is omitted. The upstream catalytic converter has a three-way catalyst  511  and a GPF (Gasoline Particulate Filter)  512 . The downstream catalytic converter is disposed outside the engine bay. The downstream catalytic converter has a three-way catalyst  513 . Note that the exhaust gas purification system is not limited to the configuration of the illustrated example. For example, the GPF may be omitted. Moreover, the catalytic converter is not limited to what has the three-way catalyst. Further, the arranged order of the three-way catalyst and GPF may be suitably changed. 
     An EGR passage  52  is connected between the intake passage  40  and the exhaust passage  50 . The EGR passage  52  is a passage for recirculating a part of exhaust gas to the intake passage  40 . The EGR passage  52  constitutes an external EGR system. An upstream end of the EGR passage  52  is connected to the exhaust passage  50  between the upstream catalytic converter and the downstream catalytic converter. A downstream end of the EGR passage  52  is connected to the part of the intake passage  40  upstream of the supercharger  44 . EGR gas flowing through the EGR passage  52  enters into the part of the intake passage  40  upstream of the supercharger  44 , without passing through the air by-pass valve  48  in the bypass passage  47 . 
     An EGR cooler  53  of a water cooling type is disposed in the EGR passage  52 . The EGR cooler  53  cools exhaust gas. An EGR valve  54  is also disposed in the EGR passage  52 . The EGR valve  54  adjusts a flow rate of exhaust gas flowing through the EGR passage  52 . By adjusting the opening of the EGR valve  54 , a recirculating amount of the cooled exhaust gas (i.e., external EGR gas) can be adjusted. 
     This engine  1  has the external EGR system and the internal EGR system, as an EGR system  55 . The external EGR system can supply exhaust gas to the combustion chamber  17  at a lower temperature than that of the internal EGR system. 
     The control device of the compression ignition engine  1  is provided with the ECU (Engine Control Unit)  10  for operating the engine  1 . The ECU  10  is an engine controller based on a well-known microcomputer, and as illustrated in  FIG. 3 , it includes a microcomputer  101  having a processor such as a central processing unit (CPU) which executes programs, a memory  102  which is comprised of, for example, RAM (Random Access Memory) and ROM (Read Only Memory) and stores the programs and data, and an interface (I/F) circuit  103  which outputs and inputs an electrical signal. 
     As illustrated in  FIGS. 1 and 3 , the ECU  10  is connected to various kinds of sensors SW 1 -SW 17 . Each of the sensors SW 1 -SW 17  outputs a signal to the ECU  10 . The sensors include the following sensors. 
     Airflow sensor SW 1 : Disposed in the intake passage  40  downstream of the air cleaner  41  and outputs a signal corresponding to the flow rate of fresh air flowing through the intake passage  40 ; 
     First intake air temperature sensor SW 2 : Disposed in the intake passage  40  downstream of the air cleaner  41  and outputs a signal corresponding to the temperature of fresh air flowing through the intake passage  40 ; 
     First pressure sensor SW 3 : Disposed in the intake passage  40  downstream of the connected position of the EGR passage  52  and upstream of the supercharger  44 , and outputs a signal corresponding to the pressure of gas entering into the supercharger  44 ; 
     Second intake air temperature sensor SW 4 : Disposed in the intake passage  40  downstream of the supercharger  44  and upstream of the connected position of the bypass passage  47 , and outputs a signal corresponding to the temperature of gas flowed out of the supercharger  44 ; 
     Intake pressure sensor SW 5 : Attached to the surge tank  42  and outputs a signal corresponding to the pressure of gas downstream of the supercharger  44 ; 
     In-cylinder pressure sensor SW 6 : Attached to the cylinder head  13  corresponding to each cylinder  11  and outputs a signal corresponding to the pressure inside each combustion chamber  17 ; 
     Exhaust temperature sensor SW 7 : Disposed in the exhaust passage  50  and outputs a signal corresponding to the temperature of exhaust gas discharged from the combustion chamber  17 ; 
     Linear O 2  sensor SW 8 : Disposed in the exhaust passage  50  upstream of the upstream catalytic converter and outputs a signal corresponding to oxygen concentration in exhaust gas; 
     Lambda O 2  sensor SW 9 : Disposed downstream of the three-way catalyst  511  of the upstream catalytic converter and outputs a signal corresponding to the oxygen concentration in exhaust gas; 
     Water temperature sensor SW 10 : Attached to the engine  1  and outputs a signal corresponding to the temperature of cooling water; 
     Crank angle sensor SW 11 : Attached to the engine  1  and outputs a signal corresponding to a rotation angle of the crankshaft  15 ; 
     Accelerator opening sensor SW 12 : Attached to an accelerator mechanism and outputs a signal corresponding to an accelerator opening proportional to an operating amount of the accelerator; 
     Intake cam angle sensor SW 13 : Attached to the engine  1  and outputs a signal corresponding to a rotation angle of the intake cam shaft; 
     Exhaust cam angle sensor SW 14 : Attached to the engine  1  and outputs a signal corresponding to a rotation angle of the exhaust cam shaft; 
     EGR differential pressure sensor SW 15 : Disposed in the EGR passage  52  and outputs a signal corresponding to a differential pressure between upstream and downstream of the EGR valve  54 ; 
     Fuel pressure sensor SW 16 : Attached to the common rail  64  of the fuel supply system  61  and outputs a signal corresponding to the pressure of fuel supplied to the injector  6 ; and 
     Third intake air temperature sensor SW 17 : Attached to the surge tank  42  and outputs a signal corresponding to the temperature of gas inside the surge tank  42 , i.e., the temperature of intake air entering into the combustion chamber  17 . 
     The ECU  10  determines the operating state of the engine  1  based on the signals from the sensors SW 1 -SW 17 , and calculates a control amount of each device according to predefined control logic. The control logic is stored in the memory  102 . The control logic includes calculating target amounts and/or the control amounts by using a map stored in the memory  102 . 
     The ECU  10  outputs electrical signals according to the calculated control amounts to the injectors  6 , the ignition plugs  25 , the intake electric S-VT  23 , the exhaust electric S-VT  24 , the fuel supply system  61 , the throttle valve  43 , the EGR valve  54 , the electromagnetic clutch  45  of the supercharger  44 , and the air by-pass valve  48  and the swirl control valve  56 . 
     For example, the ECU  10  sets a target torque of the engine  1  based on the signal from the accelerator opening sensor SW 12  and the map, and determines a target boost pressure. Then, the ECU  10  performs a feedback control for adjusting the opening of the air by-pass valve  48  based on the target boost pressure and the differential pressure before and after the supercharger  44  obtained from the signals of the first pressure sensor SW 3  and the intake pressure sensor SW 5 . By this feedback control, the boost pressure becomes the target boost pressure. 
     Moreover, the ECU  10  sets a target EGR ratio (i.e., a ratio of EGR gas to all the gas inside the combustion chamber  17 ) based on the operating state of the engine  1  and the map. Then, the ECU  10  determines a target EGR gas amount based on the target EGR ratio and an intake air amount based on the signal from the accelerator opening sensor SW 12 , and performs a feedback control for adjusting the opening of the EGR valve  54  based on the differential pressure before and after the EGR valve  54  obtained from the signal of the EGR differential pressure sensor SW 15 . By this feedback control, the external EGR gas amount entering into the combustion chamber  17  becomes the target EGR gas amount. 
     Further, the ECU  10  performs an air-fuel ratio feedback control, when a given control condition is satisfied. Specifically, based on the oxygen concentrations in exhaust gas obtained from the signals of the linear O 2  sensor SW 8  and the lambda O 2  sensor SW 9 , the ECU  10  adjusts a fuel injection amount of the injector  6  so that the air-fuel ratio of mixture gas becomes a desired value. 
     Note that the details of other controls of the engine  1  performed by the ECU  10  will be described later. 
     The ECU  10  is connected to an informing device  57 . The informing device  57  is comprised of, for example, a warning lamp provided to an instrument panel. As will be described later, when a failure diagnosis device  100  for the in-cylinder pressure sensor SW 6  diagnoses a failure of the in-cylinder pressure sensor SW 6 , the informing device  57  informs a user of the failure. 
     (Configuration of in-Cylinder Pressure Sensor) 
       FIG. 4  illustrates a configuration of the in-cylinder pressure sensor SW 6 . The in-cylinder pressure sensor SW 6  has a diaphragm  71  disposed so as to face to the inside of the combustion chamber  17 . The diaphragm  71  is made of material having flexibility. The diaphragm  71  is disposed at a tip end of the in-cylinder pressure sensor SW 6 . A circumferential edge of the diaphragm  71  is supported by a housing. The housing is comprised of an outer housing  72  and an inner housing  73 . When the pressure inside the combustion chamber  17  increases, the pressure pushes an external surface of the diaphragm  71  so that a central part of the diaphragm  71  which is not supported by the outer housing  72  and the inner housing  73  is bent. 
     The outer housing  72  is fixed to the cylinder head  13  of the engine  1 , although the corresponding illustration is omitted. The outer housing  72  is cylindrical in which a tip end thereof is opened. The diaphragm  71  is attached to a tip-end face of the outer housing  72 . The circumferential edge of the diaphragm  71  is fixed to the outer housing  72  by welding. 
     The inner housing  73  is fitted into the outer housing  72 . The inner housing  73  is located at a tip-end part of the outer housing  72 . The inner housing  73  is comprised of a combination of a plurality of parts. The inner housing  73  is also cylindrical. The circumferential edge of the diaphragm  71  is also fixed to the inner housing  73  by welding. 
     The inner housing  73  is biased by a biasing member  74  toward the tip end of the in-cylinder pressure sensor SW 6 . The biasing member  74  is disposed inside the outer housing  72 , on a base-end side of the in-cylinder pressure sensor SW 6  from the inner housing  73  (i.e., upward in  FIG. 4 ). 
     A piezo-electric element  75  is disposed inside the inner housing  73 . The piezo-electric element  75  is deformed by the diaphragm  71  being bent, and outputs a weak current corresponding to an amount of the deformation. 
     A pedestal  76  is attached to a tip-end part of the piezo-electric element  75 . The pedestal  76  has a protrusion  761  in the central part thereof, which protrudes toward the tip end of the in-cylinder pressure sensor SW 6 . The protrusion  761  is located inside a through-hole  731  provided to a tip-end part of the inner housing  73 . 
     A central protrusion  711  which protrudes toward the base end of the in-cylinder pressure sensor SW 6  is provided to a central part on inner surface of the diaphragm  71 , integrally with the diaphragm  71 . The central protrusion  711  of the diaphragm  71  contacts the protrusion  761  of the pedestal  76 . When the central part of the diaphragm  71  is bent, the pedestal  76  is pushed by the central protrusion  711  toward the base end of the in-cylinder pressure sensor SW 6 , thereby deforming the piezo-electric element  75 . 
     An electrode  77  is attached to a base-end part of the piezo-electric element  75 . The weak current of the piezo-electric element  75  is outputted through the electrode  77 . 
     A base-end part of the electrode  77  is supported by an electrode support part  78 . The electrode support part  78  is also comprised of a combination of a plurality of members. The electrode support part  78  is welded to the inner housing  73 . An electrically conductive part  79  is disposed inside the electrode support part  78 . The electrically conductive part  79  extends toward the base end of the in-cylinder pressure sensor SW 6 . A base end of the electrically conductive part  79  is connected to a charge amplifier  710  provided to the in-cylinder pressure sensor SW 6 . The charge amplifier  710  amplifies the weak current of the piezo-electric element  75  and outputs it to the ECU  10 . 
     A compression spring  791  is disposed between the electrode  77  and the electrically conductive part  79 . The compression spring  791  electrically connects between the electrode  77  and the electrically conductive parts  79 . 
     An annular insulating part  712  is provided between an integrated part comprised of the pedestal  76 , the piezo-electric element  75  and the electrode  77 , and the inner housing  73 . In  FIG. 4 , the insulating part  712  is a portion colored in black. 
     (Concept of SPCCI Combustion) 
     The engine  1  performs combustion by compression self-ignition in a given operating state, mainly for the purpose of an improvement of fuel efficiency and an improvement of exhaust emission performance. The combustion by self-ignition largely changes at the timing of self-ignition, when the temperature inside the combustion chamber  17  varies before the compression is started. Thus, the engine  1  performs SPCCI combustion which is a combination of SI combustion and CI combustion. 
     SPCCI combustion is a form of combustion in which the ignition plug  25  forcibly ignites a mixture gas inside the combustion chamber  17  to cause the mixture gas to carry out SI combustion by flame propagation, and when the temperature inside the combustion chamber  17  increases by heat generated by the SI combustion and the pressure inside the combustion chamber  17  increases by flame propagation, unburnt mixture gas carries out CI combustion by self-ignition. 
     By adjusting an amount of the heat generation of SI combustion, the variation in the temperature inside the combustion chamber  17  before the compression is started is absorbable. By the ECU  10  adjusting an ignition timing, mixture gas can be self-ignited at a target timing. 
     In SPCCI combustion, the heat generation during SI combustion is milder than the heat generation during CI combustion. As illustrated in  FIG. 5 , in a waveform of a heat generation rate (dQ/dθ) of SPCCI combustion, SI combustion is smaller in the rising slope than CI combustion. Moreover, a rate of pressure fluctuation inside the combustion chamber  17  (dp/dθ) of SI combustion also becomes milder than that of CI combustion. 
     When the unburnt mixture gas carries out self-ignition after the start of SI combustion, the waveform slope of the heat generation rate may change from small to large at the timing of self-ignition. The waveform of the heat generation rate may have a point of inflection X at the timing when CI combustion starts. 
     After the start of CI combustion, SI combustion and CI combustion are performed in parallel. Since the heat generation is larger during CI combustion than SI combustion, the heat generation rate becomes relatively high. However, since CI combustion is performed after the compression top dead center, it is avoided that the waveform slope of the heat generation rate becomes excessively large. The rate of pressure fluctuation (dp/dθ) also becomes comparatively mild during CI combustion. 
     The rate of pressure fluctuation (dp/dθ) can be used as an index indicative of combustion noise. As described above, since SPCCI combustion can reduce the rate of pressure fluctuation (dp/dθ), it becomes possible to avoid the combustion noise becoming excessively large. The combustion noise of the engine  1  is lowered within an allowable level. 
     SPCCI combustion is finished when CI combustion ends. CI combustion is shorter in the combustion period than SI combustion. The combustion end timing is earlier in SPCCI combustion than in SI combustion. 
     The waveform of the heat generation rate in SPCCI combustion is formed so that a first heat generation part Q SI  formed by SI combustion, and a second heat generation part Q CI  formed by CI combustion continue in this order. 
     Here, a SI ratio is defined as a parameter indicative of a characteristic of SPCCI combustion. The SI ratio is herein defined as an index related to a ratio of the heat amount generated by SI combustion to all the heat amount generated by SPCCI combustion. The SI ratio is a heat amount ratio generated by two types of combustion of different combustion forms. When the SI ratio is high, the ratio of SI combustion is high, and when the SI ratio is low, the ratio of CI combustion is high. If the ratio of SI combustion in SPCCI combustion is high, it becomes advantageous to the reduction of combustion noise. If the ratio of CI combustion in SPCCI combustion is high, it becomes advantageous to the improvement of fuel efficiency of the engine  1 . 
     The SI ratio may be defined as a ratio of the heat amount generated by SI combustion to the heat amount generated by CI combustion. That is, in SPCCI combustion where a crank angle at which CI combustion starts is set as a CI combustion start timing θci, SI ratio=Q SI /Q CI  from an SI combustion area Q SI  on an advance side of θci, and a CI combustion area Q CI  including θci and on a retard side of θci, in a waveform  801  illustrated in  FIG. 5 . 
     (Engine Control Logic) 
       FIG. 6  is a block diagram illustrating a functional configuration of the ECU  10  which performs the control logic of the engine  1 . The ECU  10  operates the engine  1  according to the control logic stored in the memory  102 . Specifically, the ECU  10  determines the operating state of the engine  1  based on the signals from the sensors SW 1 -SW 17 , and performs calculations for adjusting the properties inside the combustion chamber  17 , the injection amount, an injection timing, and the ignition timing so that the combustion inside the combustion chamber  17  becomes the combustion of the SI ratio according to the operating state. 
     The ECU  10  controls SPCCI combustion using two parameters of the SI ratio and θci. Specifically, the ECU  10  defines a target SI ratio and a target θci corresponding to the operating state of the engine  1 , and adjusts the temperature inside the combustion chamber  17  and the ignition timing so that an actual SI ratio becomes the target SI ratio and an actual θci becomes the target θci. The temperature inside the combustion chamber  17  is adjusted by adjusting the temperature and/or the amount of exhaust gas entering into the combustion chamber  17 . 
     The ECU  10  first reads the signals from the sensors SW 1 -SW 17  through the I/F circuit  103 . Next, a target SI ratio/target θci setting module  101   a  of the microcomputer  101  of the ECU  10  determines the operating state of the engine  1  based on the signals from the sensors SW 1 -SW 17 , and sets the target SI ratio (i.e., the target heat amount ratio) and the target CI combustion start timing θci. The target SI ratio is defined according to the operating state of the engine  1 . The target SI ratio is stored in a target SI ratio memory  1021  of the memory  102 . The target SI ratio/target θci setting module  101   a  sets the target SI ratio low when the load of the engine  1  is low, and on the other hand, it sets the target SI ratio high when the load of the engine  1  is high. When the load of the engine  1  is low, both the reduction of combustion noise and the improvement of fuel efficiency can be achieved by increasing the ratio of CI combustion in SPCCI combustion. On the other hand, when the load of the engine  1  is high, it becomes advantageous to the reduction of combustion noise by increasing the ratio of SI combustion in SPCCI combustion. 
     As described above, θci means the crank angle timing when CI combustion starts in SPCCI combustion (see  FIG. 5 ). The target θci is also defined according to the operating state of the engine  1 . The target θci is stored in a target θci memory  1022  of the memory  102 . Combustion noise can be reduced if θci is on the retard side. Fuel efficiency of the engine  1  can be improved if θci is on the advance side. The target θci is set as much as possible to the advance side within the range where combustion noise can be kept within the allowable level. 
     A target in-cylinder property setting module  101   b  sets target in-cylinder properties for realizing the set target SI ratio and target θci based on a model stored in the memory  102 . Specifically, the target in-cylinder property setting module  101   b  sets a target temperature, a target pressure, and other target properties inside the combustion chamber  17 . 
     An in-cylinder property controlling module  101   c  sets the opening of the EGR valve  54 , the opening of the throttle valve  43 , the opening of the air by-pass valve  48 , the opening of the swirl control valve  56 , a phase angle of the intake electric S-VT  23  (i.e., a valve timing of the intake valve  21 ), and a phase angle of the exhaust electric S-VT  24  (i.e., a valve timing of the exhaust valve  22 ), which are required for realizing the target in-cylinder properties. The in-cylinder property controlling module  101   c  sets the control amounts of these devices based on the map stored in the memory  102 . The in-cylinder property controlling module  101   c  outputs control signals to the EGR valve  54 , the throttle valve  43 , the air by-pass valve  48 , the swirl control valve (SCV)  56 , the intake electric S-VT  23 , and the exhaust electric S-VT  24  based on the set control amounts. By each device operating based on the signal of the ECU  10 , the property inside the combustion chamber  17  becomes the target property. 
     Further, the in-cylinder property controlling module  101   c  calculates a predicted value of the property, and an estimated value of the property inside the combustion chamber  17  based on the set control amount of each device. The property predicted value is a value obtained by predicting the property inside the combustion chamber  17  before the intake valve  21  is closed. The property predicted value is used for setting the injection amount of the fuel on the intake stroke, as will be described later. The property estimated value is a value obtained by estimating the property inside the combustion chamber  17  after the intake valve  21  is closed. The property estimated value is used for setting the injection amount of the fuel on the compression stroke, and the ignition timing, as will be described later. 
     A first injection amount setting module  101   d  sets the injection amount of the fuel on the intake stroke based on the property predicted value. When performing a divided injection on the intake stroke, an injection amount of each divided injection is set. Note that if the injection of the fuel is not performed on the intake stroke, the first injection amount setting module  101   d  sets the injection amount of the fuel to zero. A first injection controlling module  101   e  outputs the control signal to the injector  6  so that the injector  6  injects the fuel into the combustion chamber  17  at a given injection timing. The first injection controlling module  101   e  outputs an injection result of the fuel on the intake stroke. 
     A second injection amount setting module  101   f  sets the injection amount of the fuel on the compression stroke based on the property estimated value and the injection result of the fuel on the intake stroke. Note that when not injecting the fuel on the compression stroke, the second injection amount setting module  101   f  sets the injection amount of the fuel to zero. A second injection controlling module  101   g  outputs the control signal to the injector  6  so that the injector  6  injects the fuel into the combustion chamber  17  at an injection timing based on the preset map. The second injection controlling module  101   g  outputs an injection result of the fuel on the compression stroke. 
     An ignition timing setting module  101   h  sets an ignition timing based on the property estimated value and the injection result of the fuel on the compression stroke. An ignition controlling module  101   i  outputs the control signal to the ignition plug  25  so that the ignition plug  25  ignites the mixture gas inside the combustion chamber  17  at the set ignition timing. 
     Here, when the ignition timing setting module  101   h  predicts that the temperature inside the combustion chamber  17  becomes lower than the target temperature based on the property estimated value, it advances the injection timing on the compression stroke more than the injection timing based on the map so that the ignition timing can be advanced. Moreover, when the ignition timing setting module  101   h  predicts that the temperature inside the combustion chamber  17  becomes higher than the target temperature based on the property estimated value, it retards the injection timing on the compression stroke more than the injection timing based on the map so that the ignition timing can be retarded. 
     That is, if the temperature inside the combustion chamber  17  is low, after SI combustion begins by jump-spark ignition, the timing at which the unburnt mixture gas carries out self-ignition (CI combustion start timing θci) is delayed, and the SI ratio is deviated from the target SI ratio. In this case, an increase of the unburnt fuel and a degradation of the exhaust emission performance are caused. 
     Therefore, when it is predicted that the temperature inside the combustion chamber  17  becomes lower than the target temperature, the first injection controlling module  101   e  and/or the second injection controlling module  101   g  advance the injection timing, and the ignition timing setting module  101   h  advances the ignition timing. Since a sufficient heat generation becomes possible by SI combustion, by making the start of SI combustion earlier, it can prevent that the timing θci of self-ignition of unburnt mixture gas is delayed when the temperature inside the combustion chamber  17  is low. As a result, θci approaches the target θci, and the SI ratio approaches the target SI ratio. 
     Moreover, if the temperature inside the combustion chamber  17  is high, the unburnt mixture gas carries out self-ignition immediately after SI combustion begins by jump-spark ignition, and thereby the SI ratio is deviated from the target SI ratio. In this case, combustion noise increases. 
     Therefore, when it is predicted that the temperature inside the combustion chamber  17  becomes higher than the target temperature, the first injection controlling module  101   e  and/or the second injection controlling module  101   g  retard the injection timing, and the ignition timing setting module  101   h  retards the ignition timing. Since the start of SI combustion is delayed, it can prevent that the timing θci of self-ignition of unburnt mixture gas becomes early when the temperature inside the combustion chamber  17  is high. As a result, the θci approaches the target θci, and the SI ratio approaches the target SI ratio. 
     By the ignition plug  25  igniting the mixture gas, SI combustion or SPCCI combustion is carried out inside the combustion chamber  17 . The in-cylinder pressure sensor SW 6  measures a change in the pressure inside the combustion chamber  17 . 
     The measurement signal of the in-cylinder pressure sensor SW 6  is inputted into a θci deviation calculating module  101   k . The θci deviation calculating module  101   k  estimates the CI combustion start timing θci based on the measurement signal of the in-cylinder pressure sensor SW 6 , and calculates a deviation of the estimated CI combustion start timing θci from the target θci. The θci deviation calculating module  101   k  outputs the calculated θci deviation to the target in-cylinder property setting module  101   b . The target in-cylinder property setting module  101   b  corrects the model based on the θci deviation. The target in-cylinder property setting module  101   b  sets the target in-cylinder properties in the subsequent cycles by using the corrected model. 
     The control logic of this engine  1  adjusts the SI ratio and θci so as to correspond to the operating state of the engine  1  by a property setting device including the throttle valve  43 , the EGR valve  54 , the air by-pass valve  48 , the swirl control valve  56 , the intake electric S-VT  23 , and the exhaust electric S-VT  24 . 
     As one example of control of the property setting device,  FIG. 7  illustrates a change in the close timing IVC of the intake valve  21  with respect to the load of the engine  1 . In this figure, the close timing IVC of the intake valve  21  is more advanced as it goes upward. When the close timing IVC of the intake valve  21  is advanced, since the open timing of the intake valve  21  is also advanced, a positive overlap period where both the intake valve  21  and the exhaust valve  22  open becomes longer. Therefore, the amount of EGR gas entering into the combustion chamber  17  increases. 
     Here, when the engine  1  is in a specific operating state, the engine  1  is operated with an air-fuel ratio (A/F) of mixture gas at a stoichiometric air-fuel ratio or a substantially stoichiometric air-fuel ratio, and a gas-fuel ratio (G/F) leaner than the stoichiometric air-fuel ratio. Therefore, the engine  1  can secure the purification performance of exhaust gas by using the three-way catalyst and improves fuel efficiency. When the load of the engine  1  is low, the amount of fuel supply is small. When the load of the engine  1  is low, the ECU  10  sets the close timing IVC of the intake valve  21  at a timing on the retard side. The amount of air entering into the combustion chamber  17  is limited so as to correspond to the small amount of fuel supply. Moreover, since the amount of EGR gas entering into the combustion chamber  17  is also limited, combustion stability can be secured. 
     Since the amount of fuel supply increases when the load of the engine  1  becomes higher, combustion stability increases. The ECU  10  sets the close timing IVC of the intake valve  21  at a timing on the advance side. Therefore, the amount of air entering into the combustion chamber  17  increases, and the amount of EGR gas entering into the combustion chamber  17  increases. 
     When the load of the engine  1  becomes further higher, the temperature inside the combustion chamber  17  becomes further higher. Therefore, the amount of internal EGR gas is reduced and the amount of external EGR gas is increased so that the temperature inside the combustion chamber  17  does not become excessively high. In order to achieve this, the ECU  10  again sets the close timing IVC of the intake valve  21  at a timing on the retard side. 
     When the load of the engine  1  becomes further higher, the amount of fuel supply increases. In order to introduce the amount of air which makes A/F of mixture gas be the stoichiometric air-fuel ratio or the substantially stoichiometric air-fuel ratio into the combustion chamber  17  with respect to the large amount of fuel supply, the supercharger  44  boosts. When the supercharger  44  starts boosting, the ECU  10  again sets the close timing of the intake valve  21  at a timing on the advance side. Since the amount of air entering into the combustion chamber  17  increases and the positive overlap period where both the intake valve  21  and the exhaust valve  22  open is provided, the residual gas inside the combustion chamber  17  can be purged. 
     In this way, the control logic of the engine  1  performs a rough adjustment of the SI ratio by adjusting the properties inside the combustion chamber  17 . The control logic of the engine  1  is also configured to adjust the SI ratio and θci by adjusting the injection timing and the ignition timing of fuel. By the adjustment of the injection timing and the ignition timing, the difference between the cylinders can be corrected, and a fine adjustment of the self-ignition timing can be performed, for example. By the adjustment of the SI ratio at the two steps, the engine  1  can accurately achieve the desired SPCCI combustion corresponding to the operating state. 
     (Combustion Noise Control) 
     Since SPCCI combustion is the combustion form which is a combination of SI combustion and CI combustion, each of knock resulting from SI combustion and knock resulting from CI combustion may occur. Here, if the knock resulting from SI combustion is referred to as the SI knock, and the knock resulting from CI combustion is the CI knock, the SI knock is a phenomenon in which the unburnt gas outside an area where the mixture gas carried out SI combustion inside the combustion chamber  17  rapidly combusts by an abnormal partial self-ignition (a partial self-ignition clearly different from normal CI combustion), and the CI knock is a phenomenon in which the main components of the engine  1  (the cylinder block, the cylinder head, the piston, the crank journal parts, etc.) resonate due to the pressure fluctuation caused by CI combustion. The SI knock appears as loud noise at a frequency of about 6.3 kHz, which is caused by organ-pipe oscillation occurring inside the combustion chamber  17  by the partial self-ignition. On the other hand, the CI knock appears as loud noise at a frequency of about 1 to 4 kHz (more strictly, a plurality of frequencies included in this frequency range), which is caused by the resonance of the primary components of the engine  1 . Thus, the SI knock and the CI knock appear as noise at different frequencies resulting from the different factors. 
     The ECU  10  controls SPCCI combustion so that both the SI knock and the CI knock do not occur. Specifically, the ECU  10  calculates an SI knock index value relevant to the SI knock, and a CI knock index value relevant to the CI knock by carrying out a Fourier transform of the detection signal of the in-cylinder pressure sensor SW 6 . The SI knock index value is an in-cylinder pressure spectrum near 6.3 kHz which increases with the generation of the SI knock, and the CI knock index value is an in-cylinder pressure spectrum near 1 to 4 kHz which increases with the generation of the CI knock. 
     Then, the ECU  10  determines a θci limit according to the predefined map so that each of the SI knock index value and the CI knock index value does not exceed an allowable limit, and compares θci determined from the operating state of the engine  1  with the θci limit. If θci limit is the same as or on the advance side of θci, θci is set as the target θci, and on the other hand, if the θci limit is on the retard side of θci, the θci limit is set as the target θci. Both the SI knock and the CI knock are reduced by this control. 
     (Failure Diagnosis of in-Cylinder Pressure Sensor) 
     As described above, the engine  1  which performs SPCCI combustion performs the ignition control and the control for reducing combustion noise by using the detection signal of the in-cylinder pressure sensor SW 6 . In the engine  1 , the detection signal of the in-cylinder pressure sensor SW 6  is important. If a misdetection signal is outputted by a failure of the in-cylinder pressure sensor SW 6 , trouble may be caused in the operational control of the engine  1 . Therefore, the engine  1  is provided with the failure diagnosis device  100  for the in-cylinder pressure sensor SW 6 . 
       FIG. 8  illustrates a configuration of the failure diagnosis device  100  for the in-cylinder pressure sensor SW 6 . The failure diagnosis device  100  includes a diagnosis module  111  and an engine control module  112 . The diagnosis module  111  and the engine control module  112  are functional blocks constituted by the ECU  10 . The engine control module  112  controls the operation of the engine  1 . Here, the engine control module  112  performs a fuel cut control of the engine  1 . Specifically, the engine control module  112  stops the supply of the fuel to the engine  1  through the injector  6 , when a slowdown fuel cut condition is satisfied, during the travel of the automobile. The engine control module  112  determines that the slowdown fuel cut condition is satisfied based on the detection signal of the accelerator opening sensor SW 12 . 
     When the fuel supply stops, the engine  1  carries out the fuel-cut operation. The ignition plug  25  does not perform any ignition during the fuel-cut operation. The intake electric S-VT  23  changes the valve timing of the intake valve  21  to a preset target valve timing. The target valve timing is a valve timing suitable for a resume from the fuel-cut operation. The engine control module  112  changes the valve timing of the intake valve  21  to the target valve timing through the intake electric S-VT  23 , after stopping the fuel supply to the engine  1 . 
     The diagnosis module  111  diagnoses the failure of the in-cylinder pressure sensor SW 6  when a given condition is satisfied. 
     In detail, the diagnosis module  111  diagnoses the failure of the in-cylinder pressure sensor SW 6 , while the engine  1  continues a steady operation for a given period of time. While the engine  1  continues the steady operation, variations of the combustion pressure of the mixture gas, the temperature of the wall surface of the combustion chamber, the ratio of specific heat of the mixture gas, etc. become smaller, as compared with a period while the engine  1  performs a transitional operation, for example. If the failure diagnosis of the in-cylinder pressure sensor SW 6  is performed when the environment inside the cylinder is stable, the variation in the output of the in-cylinder pressure sensor SW 6  unrelated to the failure can be reduced. 
     Moreover, the diagnosis module  111  diagnoses the failure of the in-cylinder pressure sensor SW 6 , also while the engine  1  carries out the fuel cut operation, in addition to while the engine  1  continues the steady operation. Accordingly, the diagnosis module  111  can diagnose the failure of the in-cylinder pressure sensor SW 6  based on the pressure variation inside the combustion chamber  17  which is not influenced by the combustion of the mixture gas. Moreover, since the ignition plug  25  does not ignite the fuel while the engine  1  carries out the fuel cut operation, it is also advantageous that the detection signal of the in-cylinder pressure sensor SW 6  is not influenced by noise of the ignition plug  25 . 
     —Primary Configuration of Functional Blocks— 
     The diagnosis module  111  includes, as its primary components, an operating state determining module  1111  which determines the steady operation, the fuel cut operation, etc., an estimating module  1114  which estimates the in-cylinder pressure at a post-top timing (+α° CA) which is a timing retarded by a specific crank angle from a compression top dead center, a reading module  1113  which reads the detection signal of the in-cylinder pressure sensor SW 6  at the post-top timing, and a failure determining module  1112  which determines the failure of the in-cylinder pressure sensor SW 6  in response to signals outputted from the estimating module  1114  and the reading module  1113 . 
     The operating state determining module  1111  determines that the engine  1  continues the steady operation for the given period of time. Here, the operating state determining module  1111  determines that the engine  1  carries out the steady operation, when the operating state of the engine  1  is maintained constant or substantially constant. In detail, the operating state determining module  1111  determines that the engine  1  carries out the steady operation when at least one of an amount of air filled up in the combustion chamber  17 , an amount of EGR gas contained in the mixture gas inside the combustion chamber  17 , and an amount of fuel supplied to the combustion chamber  17 , fall within a given range. 
     In more detail, the operating state determining module  1111  determines the amount of air filled up in the combustion chamber  17 , the amount of EGR gas contained in the mixture gas inside the combustion chamber  17 , and the amount of fuel supplied to the combustion chamber  17  based on the detection signals of the sensors SW 1 -SW 17 . The operating state determining module  1111  determines that the engine  1  carries out the steady operation when the amount of change in each of the three amounts is determined to be below a given value. 
     Then, the operating state determining module  1111  determines that “the engine  1  continues the steady operation for the given period of time,” when the engine  1  continues the steady operation for a set period of time (in this example, several seconds). 
     Moreover, the operating state determining module  1111  determines that “the engine  1  starts the fuel cut operation,” when it is determined that the slowdown fuel cut condition of the engine controller  112  is satisfied. 
     The estimating module  1114  estimates the in-cylinder pressure at the post-top timing based on the operating state of the engine  1 , when the operating state determining module  1111  determines the continuation of the steady operation, or when the operating state determining module  1111  determines the fuel cut operation. Below, the value of the in-cylinder pressure estimated by the estimating module  1114  is referred to as the “post-top estimated value.” The post-top estimated value indicates an estimated value of the in-cylinder pressure expected to be reached if the in-cylinder pressure sensor SW 6  has not failed. 
     The post-top estimated value estimated by the estimating module  1114  is inputted into the failure determining module  1112 . The post-top timing is illustration of a “first timing.” 
     The reading module  1113  reads a value of the detection signal of the in-cylinder pressure sensor SW 6  at the post-top timing (i.e., a post-top signal value), and a value of the detection signal of the in-cylinder pressure sensor SW 6  at the pre-top timing (−α° CA) advanced by the same specific crank angle as the post-top timing from the compression top dead center (i.e., a pre-top signal value). The pre-top timing is illustration of a “second timing.” 
     Note that the specific crank angle is set so that the post-top timing becomes within the first half of an expansion stroke. Here, the “first half” may also be an early stage, for example, when the expansion stroke is divided into three stages of the early stage, a middle stage, and a later stage. The specific crank angle may be, for example, near 60° C.A. The influence of the cooling loss can be reduced by setting so that the post-top timing becomes the first half of the expansion stroke. Therefore, the accuracy of the failure diagnosis can be improved. 
     Moreover, according to the operating state of the engine  1 , the specific crank angle may be set on real time. In that case, the specific crank angle is set after the valve close of the intake valve  21  is completed, not during the transition of the valve close of the intake valve  21  by the intake electric S-VT  23 . Further, in each cycle, the specific crank angle may be set to be before the ignition timing. By setting in this way, the influence of the close operation of the intake valve  21  and the influence of the combustion of the mixture gas can be reduced. Therefore, the accuracy of the failure diagnosis can be improved. 
     That is, in order to improve the accuracy of the failure diagnosis of the in-cylinder pressure sensor SW 6 , it is desirable to set the specific crank angle so that the pre-top timing becomes before the ignition timing and the post-top timing becomes in the first half of the expansion stroke, and performs such setting after the close timing IVC of the intake valve  21 . 
     The pre-top signal value read by the reading module  1113  is inputted into the estimating module  1114 . The estimating module  1114  estimates the post-top estimated value based on the inputted pre-top signal value. On the other hand, the post-top signal value read by the reading module  1113  is inputted into the failure determining module  1112 . 
     The failure determining module  1112  determines the failure of the in-cylinder pressure sensor SW 6  based on the output fall of the detection signal of the in-cylinder pressure sensor SW 6 . As will be described later, the output fall of the detection signal of the in-cylinder pressure sensor SW 6  originates an abnormality of electrical insulation of the insulated part  712  of the in-cylinder pressure sensor SW 6 . The failure determining module  1112  determines that the in-cylinder pressure sensor SW 6  is failed based on a comparison of the post-top estimated value estimated by the estimating module  1114  with the post-top signal value read by the reading module  1113 . This comparison is indirectly performed through the threshold based on the post-top estimated value and the difference based on the post-top signal value. 
     In more detail, the failure determining module  1112  determines that the in-cylinder pressure sensor SW 6  is failed, when the difference between the post-top signal value and the pre-top signal value exceeds the threshold defined corresponding to the post-top estimated value. 
     The diagnosis module  111  is further provided with a threshold setting module  1115 . The threshold setting module  1115  sets the threshold based on the post-top estimated value. The failure determining module  1112  reads the threshold set by the threshold setting module  1115 . 
     If the failure determining module  1112  determines the failure of the in-cylinder pressure sensor SW 6 , it informs of the failure through the informing device  57 . Therefore, the user is informed that the in-cylinder pressure sensor SW 6  has failed. 
     —Functional Blocks Relevant to Limitation of Failure Determination— 
     Further, as functional blocks relevant to limitation of the failure determination, the diagnosis module  111  includes a limiting module  1117  which limits the determination of the failure by the failure determining module  1112 , and a delay determining module  1118  and a valve timing determining module  1119  which output signals to the limiting module  1117 . 
     While the engine  1  continues the steady operation, or while the engine  1  performs the fuel cut operation, the limiting module  1117  limits the failure diagnosis of the in-cylinder pressure sensor SW 6  until the valve timing of the intake valve  21  becomes the target valve timing. The in-cylinder pressure sensor SW 6  outputs the signal corresponding to the pressure variation resulting from a volume change of the combustion chamber  17 , etc. The failure determining module  1112  performs the failure determination based on the detection signal of the in-cylinder pressure sensor SW 6  corresponding to such a pressure variation. Since the timing at which the compression of the gas inside the combustion chamber  17  is started changes when the close timing of the intake valve  21  is changed, the pressure and the maximum pressure inside the combustion chamber  17  during a compression stroke also change. As a result, even if the in-cylinder pressure sensor SW 6  has not failed, the output of the in-cylinder pressure sensor SW 6  varies, and therefore, the accuracy of the failure diagnosis of the in-cylinder pressure sensor SW 6  is deteriorated. By limiting the failure diagnosis of the in-cylinder pressure sensor SW 6  until the close timing of the intake valve  21  becomes the target timing, the failure determining module  1112  can perform the failure determination of the in-cylinder pressure sensor SW 6  when the close timing of the intake valve  21  is a specific timing. Therefore, the accuracy of the failure diagnosis of the in-cylinder pressure sensor SW 6  can be improved. 
     A detection signal of the intake cam angle sensor SW 13  is inputted into the valve timing determining module  1119 . The valve timing determining module  1119  outputs a signal to the limiting module  1117 , when it determines that the valve timing of the intake valve  21  becomes the target valve timing based on the detection signal of the intake cam angle sensor SW 13 . 
     While performing the fuel cut operation, the limiting module  1117  limits that the failure determining module  1112  determines the failure of the in-cylinder pressure sensor SW 6  for a set period of time after the supply of the fuel to the engine  1  is stopped. Immediately after stopping the fuel supply to the engine  1 , the environment inside the combustion chamber  17  is not stable. For example, immediately after stopping the fuel supply to the engine  1 , a ratio of specific heat of the gas inside the combustion chamber  17  may not become constant because the EGR gas which remains inside the EGR passage  52  enters into the combustion chamber  17 . Moreover, a change in the temperature of the wall surface inside the combustion chamber  17  is sometimes large immediately after stopping the fuel supply to the engine  1 . As a result, even if the in-cylinder pressure sensor SW 6  does not fail, the output of the in-cylinder pressure sensor SW 6  varies, and the accuracy of the diagnosis of failure of the in-cylinder pressure sensor SW 6  is deteriorated. 
     Therefore, the limiting module  1117  limits that the failure determining module  1112  determines the failure of the in-cylinder pressure sensor SW 6  for the preset period after stopping the fuel supply to the engine  1 . Accordingly, the diagnosis module  111  can more accurately diagnose the failure of the in-cylinder pressure sensor SW 6  during the fuel-cut operation. 
     The diagnosis module  111  is provided with a delay determining module  1118 . The delay determining module  1118  counts the number of cycles of the engine  1 . The delay determining module  1118  is a timer for measuring that the preset period described above has lapsed. The delay determining module  1118  starts the count of the number of cycles when a signal indicating that the engine  1  performs the fuel-cut operation is received from the operating state determining module  1111 . If the delay determining module  1118  determines that a preset number of cycles has lapsed after stopping the supply of the fuel to the engine  1 , it outputs a signal to the limiting module  1117 . Note that the delay determining module  1118  may measure a period of time after stopping the fuel supply to the engine  1 , instead of counting the number of cycles. 
     (Specific Configuration Relevant to Failure Diagnosis) 
       FIG. 9  illustrates one example of the detection signal outputted when the in-cylinder pressure sensor SW 6  is normal, and the detection signal outputted when the in-cylinder pressure sensor SW 6  has failed. In  FIG. 9 , the horizontal axis is a crank angle, where 0 represents a compression top dead center. In  FIG. 9 , the vertical axis is the pressure inside the combustion chamber  17  (in-cylinder pressure), which is proportional to the detection signal of the in-cylinder pressure sensor SW 6 . 
     As illustrated in  FIG. 9 , if the in-cylinder pressure sensor SW 6  is normal, the in-cylinder pressure becomes the maximum near the compression top dead center, and becomes substantially symmetrical centering on the crank angle at which the pressure becomes the maximum. When the in-cylinder pressure sensor SW 6  has not failed (when the sensor is normal), the post-top signal value slightly falls below the pre-top signal value by the influence of the cooling loss, etc. 
       FIG. 10  illustrates the pressure difference when the sensor is normal. The pressure difference illustrated in  FIG. 10  indicates a difference obtained by subtracting the post-top signal value when the sensor is normal (post-top estimated value) from the pre-top signal value. 
     As illustrated in  FIG. 10 , since the cooling loss per unit time decreases as the engine speed increases, the in-cylinder pressure after the compression top dead center becomes higher, in general. Therefore, the pressure difference when the sensor is normal becomes smaller, in general. 
     Moreover, as illustrated in  FIG. 10 , when the amount of air filled up into the combustion chamber  17  (in-cylinder air amount) is large, a leak of air during the compression stroke increases, and therefore, the in-cylinder pressure after the compression top dead center falls in general. Therefore, the pressure difference when the sensor is normal becomes smaller, in general. 
     The tendency illustrated in  FIG. 10  is a tendency when the in-cylinder pressure sensor SW 6  operates normally. On the other hand, the present inventors obtained new knowledge about the tendency when the in-cylinder pressure sensor SW 6  has failed, as a result of diligent examinations. 
     Specifically, it is newly found out that the post-top signal value fluctuates greatly as compared with the pre-top signal value, when the in-cylinder pressure sensor SW 6  is failed under the influence of heat etc. Therefore, if the in-cylinder pressure sensor SW 6  is failed, the post-top signal value which is actually detected relatively deviates largely from the estimated post-top estimated value which is expected to be reached if the sensor is normal. According to the examination of the present inventors, it is found out that the post-top signal value falls when the insulation abnormality occurs in the insulated part  712 . In this case, as a result of the symmetry of the signal value of the in-cylinder pressure sensor SW 6  collapsing, the difference between the pre-top signal value and the post-top estimated value, and the post-top signal value becomes larger. 
     Therefore, as described above, the diagnosis module  111  determines that the in-cylinder pressure sensor SW 6  has failed based on the comparison of the post-top estimated value estimated beforehand with the post-top signal value read by the reading module  1113 . 
     While the engine  1  continues the steady operation, the variation in the output unrelated to the failure of the in-cylinder pressure sensor SW 6  can be reduced. Therefore, while the variation in the output unrelated to the failure can be reduced, it becomes possible to diagnose at the timing where the output fluctuates notably when the failure occurs. Therefore, the failure relevant to the insulated part  712  can be diagnosed more accurately. 
     The estimating module  1114  estimates the post-top estimated value as described above. The estimating module  1114  estimates the post-top estimated value based on the pre-top signal value. A map corresponding to the pressure difference illustrated in  FIG. 10  is stored in the memory  102  of the ECU  10 . The estimating module  1114  estimates the post-top estimated value by reading the pressure difference based on the operating state of the engine  1 , based on the map, and subtracting the pressure difference from the pre-top signal value. 
     The post-top estimated value is in agreement with the post-top signal value when the in-cylinder pressure sensor SW 6  operates normally, as clear from this estimating technique. Therefore, the difference between the post-top estimated value and the pre-top signal value indicates the tendency illustrated in  FIG. 10 . 
     That is, the estimating module  1114  estimates the post-top estimated value higher in general as the engine speed increases. This means that the influence of the cooling loss is taken into consideration. The influence of the cooling loss may become a factor of the error when estimating the in-cylinder pressure, although it is unrelated to the failure of the in-cylinder pressure sensor SW 6 . In order to perform the failure diagnosis more accurately, it is effective to take the influence of the cooling loss into consideration. 
     Moreover, the estimating module  1114  estimates the post-top estimated value lower in general as the amount of air filled up into the combustion chamber  17  increases. This means that the leak from the fitting parts of the piston rings is taken into consideration. This phenomenon can be a factor of the error when estimating the in-cylinder pressure, although it is unrelated to the failure of the in-cylinder pressure sensor SW 6 . In order to perform the failure diagnosis more accurately, it is effective to take the leak from the fitting parts into consideration. 
     Moreover, as a tendency peculiar to the time when the mixture gas combusts inside the combustion chamber  17  (while the engine  1  continues the steady operation), the estimating module  1114  estimates the pressure inside the combustion chamber  17  larger as the load of the engine  1  increases. 
     The combustion pressure becomes higher as the load of the engine  1  increases. The present inventors confirmed that, when the combustion pressure is high, the mating between the diaphragm  71  and the piezo-electric element  75  becomes stronger, and as a result, the signal value of the in-cylinder pressure sensor SW 6  appears to be smaller. This phenomenon reduces the post-top signal value greatly as compared with the pre-top signal value. Such a tendency can be a factor of the error when estimating the in-cylinder pressure, although it is unrelated to the failure of the in-cylinder pressure sensor SW 6 . In order to perform the failure diagnosis more accurately, it is effective to take the influence of the mating into consideration. 
     Specifically, the estimating module  1114  estimates the post-top estimated value lower as the load of the engine  1  increases. This becomes advantageous when diagnosing the failure of the in-cylinder pressure sensor SW 6  more accurately. 
     For example, when the air/fuel ratio is constant, the load of the engine  1  becomes higher as the amount of air filled up into the combustion chamber  17  increases. Therefore, the pressure difference used for the estimation of the post-top estimated value indicates the same tendency as or a similar tendency to the in-cylinder air amount, as illustrated in  FIG. 11 . 
     Moreover, for example, the ratio of specific heat of the mixture gas, and therefore, the combustion temperature become lower as the amount of EGR gas increases. In this case, it can be considered that the influence of the combustion to the in-cylinder pressure sensor SW 6  becomes relatively smaller and the pressure difference when the sensor is normal becomes smaller (see  FIG. 12 ). Such a tendency can be a factor of the error when estimating the in-cylinder pressure, although it is unrelated to the failure of the in-cylinder pressure sensor SW 6 . Therefore, in order to improve the accuracy of the failure diagnosis, it is effective to take the amount of EGR gas into consideration. 
     Specifically, the estimating module  1114  estimates the post-top estimated value higher as the amount of EGR gas contained in the mixture gas increases. This becomes advantageous when diagnosing the failure of the in-cylinder pressure sensor SW 6  more accurately. 
     The diagnosis module  111  sets the threshold corresponding to the post-top estimated value. Specifically, the threshold setting module  1115  sets the threshold so as to be larger than a difference obtained by subtracting the post-top estimated value from the pre-top signal value (i.e., the pressure difference when the sensor is normal). The threshold is set larger as the post-top estimated value becomes smaller. 
     When such a configuration is adopted, the diagnosis module  111  sets the threshold smaller, for example, as the engine speed increases. Similarly, the diagnosis module  111  sets the threshold larger, for example, as the amount of air filled up in the combustion chamber  17  increases. Accordingly, it becomes possible to set the threshold larger than the pressure difference resulting from the cooling loss, or set the threshold larger than the difference resulting from the leak loss. Therefore, the accuracy of the failure diagnosis can be improved. 
     The failure determining module  1112  indirectly compares the post-top estimated value with the post-top signal value. In detail, the failure determining module  1112  compares the threshold defined according to the post-top estimated value with the difference between the pre-top signal value and the post-top signal value, and when the compared difference exceeds the threshold, it determines that the in-cylinder pressure sensor SW 6  has failed. Therefore, the failure diagnosis of the in-cylinder pressure sensor SW 6  can be performed more accurately. 
     Note that the failure determining module  1112  may repeatedly perform the comparison of the difference between the pre-top signal value and the post-top signal value with the threshold, and when the determination that the difference compared in this way exceeds the threshold is made continuously a plurality of times, it may determine that the in-cylinder pressure sensor SW 6  has failed. 
     For example, as a result of the mating of the parts which constitute the in-cylinder pressure sensor SW 6  being changed during a compression stroke, the pressure inside the combustion chamber  17  may fall after a compression top dead center. Although such a pressure fall is only a temporary phenomenon, there is a possibility of causing a false diagnosis of the in-cylinder pressure sensor SW 6 . 
     On the other hand, since the pressure inside the combustion chamber  17  falls continuously when the in-cylinder pressure sensor SW 6  has actually failed, it becomes advantageous for performing the failure diagnosis of the in-cylinder pressure sensor SW 6  more accurately if the determination by the failure determining module  1112  is repeatedly performed as described above. 
     (Procedure of Failure Diagnosis of in-Cylinder Pressure Sensor) 
       FIGS. 13A and 13B  illustrate a flowchart of a procedure of the failure diagnosis of the in-cylinder pressure sensor SW 6 , which is executed by the failure diagnosis device  100 . At Step S 1  after a start of the procedure, the failure diagnosis device  100  reads the detection signals from the sensors SW 1 -SW 17 . 
     In the subsequent Step S 2 , the operating state determining module  1111  determines whether the engine  1  continues the steady operation for the given period of time based on the detection signals of the sensors SW 1 -SW 17 . Specifically, the operating state determining module  1111  determines that the engine  1  carries out the steady operation when an amount of change in each of the amount of air filled up in the combustion chamber  17 , the amount of EGR gas contained in the mixture gas inside the combustion chamber  17 , and the amount of fuel supplied to the combustion chamber  17  is determined to be below a given value. For example, the amount of fuel supplied to the combustion chamber  17  may be determined through the detection signal of the accelerator opening sensor SW 12 . 
     If it is determined that the steady operation continues at Step S 2 , the process shifts to Step S 12 . If it is determined that the steady operation does not continue, the process shifts to Step S 3 . In the former case, a determination according to the close timing of the intake valve  21  (Step S 12 ) is performed, and the process then shifts to processing which performs the failure diagnosis of the in-cylinder pressure sensor SW 6  (Steps S 13 -S 24 ). In the latter case, via a determination of whether to perform the fuel cut operation (Steps S 3 -S 9 ), processing according to the limitation of the failure diagnosis is performed (Steps S 10 -S 11 ), and the process then shifts to the processing to perform the failure diagnosis (Steps S 13 -S 24 ). 
     Specifically, at Step S 12 , the valve timing determining module  1119  of the diagnosis module  111  determines whether the close timing of the intake valve  21  becomes the target timing or substantially the target timing. While the determination at Step S 12  is NO, the process repeats Step S 12 . While the process repeats Step S 12 , the limiting module  1117  limits the execution of the failure diagnosis by the failure determining module  1112 . If the determination at Step S 12  becomes YES, the process shifts to Step S 13 . 
     On the other hand, at Step S 3 , the operating state determining module  1111  determines whether the slowdown fuel cut condition is satisfied. Specifically, the operating state determining module  1111  determines whether the accelerator opening becomes zero based on the detection signal of the accelerator opening sensor SW 12 . If the accelerator opening becomes zero and the slowdown fuel cut condition is satisfied, the process shifts to Step S 4 . If the slowdown fuel cut condition is not satisfied, the process returns. 
     At Step S 4 , the operating state determining module  1111  determines whether the engine water temperature exceeds a given value based on the detection signal of the water temperature sensor SW 10 . If the engine water temperature exceeds the given value, the fuel cut is executed. If the engine water temperature does not exceed the given value, the fuel cut is not executed. If the determination at Step S 4  is YES, the process shifts to Step S 5 . If the determination at Step S 4  is NO, the process returns. 
     At Step S 5 , the operating state determining module  1111  determines whether the opening of the EGR valve  54  becomes zero or substantially zero. The EGR valve  54  is closed during the fuel cut. If the determination at Step S 5  is YES, the process shifts to Step S 6 , and on the other hand, if the determination at Step S 5  is NO, the process returns. 
     At Step S 6 , the engine control module  112  stops the supply of the fuel to the engine  1  through the injector  6  (i.e., the fuel cut). At the subsequent Step S 7 , the engine control module  112  changes the valve timing of the intake valve  21  through the intake electric S-VT  23  to a target valve timing set during the fuel-cut operation. 
     At Step S 8 , the operating state determining module  1111  determines whether a stop condition of the slowdown fuel cut is satisfied. For example, the engine control module  112  stops the fuel cut when the engine speed falls excessively. Moreover, the fuel cut is stopped when the accelerator opening exceeds zero. If the determination at Step S 8  is YES, the process shifts to Step S 9  where the engine control module  112  stops the slowdown fuel cut. If the determination at Step S 8  is NO, the process shifts to Step S 10 . 
     At Step S 10 , the valve timing determining module  1119  of the diagnosis module  111  determines whether the close timing of the intake valve  21  becomes a target timing, or substantially the target timing. While the determination is NO at Step S 10 , the process repeats Step S 10 . The limiting module  1117  limits the execution of the failure diagnosis by the failure determining module  1112  during the repetition of Step  10 . If the determination at Step S 10  becomes YES, the process shifts to Step S 11 . 
     At Step S 11 , the delay determining module  1118  of the diagnosis module  111  determines whether a delay cycle has lapsed after the fuel cut is started. Here, an upper chart  141  of  FIG. 14  illustrates a relation between the engine speed and the delay cycle. The delay cycle is constant regardless of the engine speed. By passing a given number of cycles, a gas exchange can be performed for each combustion chamber  17  at least one time, and therefore, the environment in each combustion chamber  17  is stabilized. 
     Note that as described above, the delay determining module  1118  may measure time instead of counting of the number of cycles of the engine  1 . A lower chart  142  of  FIG. 14  illustrates a relation between the engine speed and a delay time. The delay time is shorter as the engine speed increases. That is because the time required for one cycle becomes shorter as the engine speed increases. 
     Returning to the flowchart of  FIG. 13A , if the determination at Step S 11  is NO, the process repeats Step S 11 . The limiting module  1117  limits the execution of the failure diagnosis by the failure determining module  1112 . If the determination at Step S 10  becomes YES, the process shifts to Step S 13 . 
     The limiting module  1117  limits the failure diagnosis of the in-cylinder pressure sensor SW 6  until two conditions are satisfied. The two conditions include the close timing of the intake valve  21  becoming the target timing, and the delay cycle (or the delay time) being lapsed after the fuel cut is started. Accordingly, since the failure determining module  1112  can perform the failure diagnosis of the in-cylinder pressure sensor SW 6  when the inside of the combustion chamber  17  is in the same state, it can improve the accuracy of the failure diagnosis. 
     The process at and after Step S 13  illustrated in the flowchart of  FIG. 13B  is common to the case where the engine  1  continues the steady operation and the case where the delay cycle has lapsed after the fuel cut is started. 
     Specifically, at Step S 13 , the diagnosis module  111  sets the specific crank angle based on the close timing of the intake valve  21  and the ignition timing of the ignition plug  25 . In this example, the diagnosis module  111  sets the specific crank angle so that the post-top timing becomes in the first half of the expansion stroke and the pre-top timing becomes before the ignition timing. 
     Then, at Step S 14 , the reading module  1113  of the diagnosis module  111  reads the detection signal of the in-cylinder pressure sensor SW 6 . In detail, the reading module  1113  sets the pre-top timing based on the specific crank angle set at Step S 13  and reads the pre-top signal value corresponding to the setting. 
     Then, at Step S 15 , the estimating module  1114  of the diagnosis module  111  estimates the post-top signal value (post-top estimated value) which is expected to be realized if the in-cylinder pressure sensor SW 6  has not failed, based on the pre-top signal value. Then, at Step S 16  which continues from Step S 15 , the threshold setting module  1115  of the diagnosis module  111  sets the threshold corresponding to the post-top estimated value. 
     Then, at step S 17 , the reading module  1113  of the diagnosis module  111  again reads the detection signal of the in-cylinder pressure sensor SW 6 . In detail, the reading module  1113  sets the post-top timing based on the specific crank angle set at Step S 13 , and reads the post-top signal value corresponding to the setting. 
     Then, at step S 18 , the failure determining module  1112  of the diagnosis module  111  compares the post-top estimated value with the post-top signal value. In detail, the failure determining module  1112  compares the threshold set based on the post-top estimated value with the difference obtained by subtracting the post-top signal value from the pre-top signal value. 
     At Step S 19 , the failure determining module  1112  determines whether the difference between the pre-top signal value and the post-top signal value exceeds the threshold based on the post-top estimated value. Since it is considered that the in-cylinder pressure sensor SW 6  has failed when the difference exceeds the threshold, the process shifts to Step S 20  where the failure determining module  1112  increments a first failure determining counter by 1. Since it is considered that the in-cylinder pressure sensor SW 6  does not fail when not exceeding the threshold, the process shifts to Step S 21  where the failure determining module  1112  makes the failure determining counter 0. 
     Then, at Step S 22 , the failure determining module  1112  determines whether the first failure determining counter exceeds a given value. The given value may be about 3 to 5, for example. If the determination of Step S 22  is NO, the process returns. If the determination of Step S 22  is YES, the process shifts to Step S 23 . That is, if the failure determining module  1112  determines the failure of the in-cylinder pressure sensor SW 6  continuously about several times, the failure determining module  1112  concludes at Step S 23  that the in-cylinder pressure sensor SW 6  has failed. A false diagnosis can be prevented by diagnosing the failure of the in-cylinder pressure sensor SW 6  based on the plurality of determinations. 
     At the subsequent Step S 24  after S 23 , the failure determining module  1112  informs of the failure through the informing device  57 . Therefore, the user is informed of the failure of the in-cylinder pressure sensor SW 6 . As a result, the broken in-cylinder pressure sensor SW 6  is replaced, for example. 
     (Time Chart) 
       FIGS. 15 and 16  are time charts illustrating a change in each parameter when the failure diagnosis device  100  of the in-cylinder pressure sensor SW 6  performs the failure diagnosis of the in-cylinder pressure sensor SW 6  according to the flowchart of  FIGS. 13A and 13B . In  FIGS. 15 and 16 , the horizontal axis indicates time. 
       FIG. 15  is a time chart when performing the failure diagnosis while the engine  1  continues the steady operation, and  FIG. 16  is a time chart when performing the failure diagnosis upon the fuel cut operation. 
     —Failure Diagnosis while Continuing Steady Operation— 
     First, while the automobile travels, the operator releases the accelerator pedal which he/she has stepped on to return the accelerator opening, the accelerator opening becomes gradually smaller, and the accelerator opening then becomes substantially constant at a time t 1  (refer to a waveform  151 ). Accordingly, the amount of fuel supplied to the combustion chamber  17  also becomes substantially constant. As a result, it is determined that the change in the amount of fuel becomes below a given amount. 
     Moreover, the valve opening of the EGR valve  54  becomes gradually smaller with the accelerator opening, and the valve opening of the EGR valve  54  becomes substantially constant at the time t 1  (refer to a waveform  152 ). Accordingly, the amount of EGR gas contained in the mixture gas formed inside the combustion chamber  17  also becomes substantially constant. As a result, it is determined that the amount of change in the EGR gas becomes below a given value. 
     Note that although not illustrated in  FIG. 15 , in the time chart illustrated here, the amount of air filled up into the combustion chamber  17  also becomes substantially constant at the time t 1 . As a result, it is determined that the change in the amount of air filled up into the combustion chamber  17  becomes below a given value at the time t 1 . 
     In response to these determinations, the ECU  10  determines that the engine  1  carries out the steady operation. A steady operation flag indicating that the engine  1  carries out the steady operation becomes 1 from 0, as illustrated in a waveform  153 , at the time t 1 . When the steady operation flag becomes 1, the operating state determining module  1111  counts a lapsed time after the steady operation flag becomes 1 from 0 (refer to a waveform  155 ). This count may be performed by directly measuring time, or indirectly measuring through the number of cycles. 
     At this time, the close timing of the intake valve  21  is changed to the target timing corresponding to the steady operation (refer to a waveform  154 ). According to the phase difference between the valve close timing before the change and the target timing, time required for the close timing of the intake valve  21  reaching the target timing varies. 
     As illustrated in a waveform  155 , the operating state determining module  1111  determines that the engine  1  continues the steady operation for a given set period of time, at a time t 2 . Then, the operating state determining module  1111  determines that the valve timing of the intake valve  21  reaches the target timing, at a time t 3 . Thus, when the conditions of that the steady operation continues and the valve timing of the intake valve  21  becomes the target valve timing are both satisfied, the execution flag of the failure diagnosis becomes 1 from 0, as illustrated in a waveform  156 , at the time t 3 . 
     When the execution flag of the failure diagnosis becomes 1, the failure determining module  1112  starts the failure determination of the in-cylinder pressure sensor SW 6 . Then, at a time t 4 , if the engine  1  shifts to the transitional operation from the steady operation when the operator steps on the accelerator pedal, the steady operation flag becomes 0 from 1. Simultaneously, the failure-diagnosis execution flag also becomes 0 from 1, in order to stop the failure diagnosis of the in-cylinder pressure sensor SW 6 . The count of the lapsed time is also reset. 
     —Failure Diagnosis while Carrying Out Fuel Cut Operation— 
     First, when the operator returns the accelerator pedal that was stepped on while the automobile travels, the accelerator opening becomes gradually smaller and, for example, the accelerator opening becomes zero at a time t 1  (refer to a waveform  161 ). The opening of the EGR valve  54  becomes gradually smaller with the accelerator opening, and the opening of the EGR valve  54  also becomes zero at the time t 1  (refer to a waveform  162 ). Note that although not illustrated in  FIG. 16 , the water temperature of the engine  1  exceeds the given value, and therefore, the slowdown fuel cut is possible. As illustrated by a waveform  163 , an F/C flag becomes 1 from 0 at the time t 1 . When the F/C flag becomes 1, the engine control module  112  stops the supply of fuel. Therefore, the engine  1  carries out the fuel-cut operation after the time t 1 . 
     The close timing of the intake valve  21  is changed to the preset target timing. 
     According to a phase difference between the valve close timing before the change and the target timing, the time required for the close timing of the intake valve  21  reaching the target timing varies. If the phase difference is large, as illustrated by a solid line in a waveform  164 , the time required for the close timing of the intake valve  21  reaching the target timing becomes longer. On the other hand, if the phase difference is small, as illustrated by a one-dot chain line in the waveform  164 , the time required for the close timing of the intake valve  21  reaching the target timing becomes shorter. 
     As illustrated in a waveform  165 , the delay determining module  1118  starts the count of cycle when the fuel cut begins. The delay cycle may be 7 to 9 cycles, for example. In the example of  FIG. 16 , the delay cycle has lapsed at a time t 2 . 
     Here, the delay cycle is set so as to be between the longest time (t 3 −t 1 ) and the shortest time (t 3 ′−t 1 ), which are required for the close timing of the intake valve  21  reaching the target timing. The failure diagnosis of the in-cylinder pressure sensor SW 6  is performed while the engine  1  carries out the fuel-cut operation. Once the engine  1  ends the fuel-cut operation, the failure diagnosis of the in-cylinder pressure sensor SW 6  cannot be performed. If the failure diagnosis of the in-cylinder pressure sensor SW 6  needs to be performed frequently, it is desirable to perform the failure diagnosis promptly after the fuel supply to the engine  1  is stopped. In order to raise the diagnostic frequency, it is more advantageous to lessen the delay cycle as much as possible. It becomes advantageous to raise the frequency of the failure diagnosis if the lapsed time of the delay cycle is shortened to be shorter than the longest change time required for the valve timing of the intake valve  21  becoming the target timing. 
     On the other hand, if the lapsed time of the delay cycle is made longer than the shortest change time required for the valve timing of the intake valve  21  becoming the target timing, it becomes advantageous to improve the accuracy of the failure diagnosis, because the state inside the combustion chamber  17  becomes stable. By adjusting the delay cycle (or the delay time), both the improvement of the accuracy of the failure diagnosis and the increase in the frequency of the failure diagnosis can be achieved. 
     In the example illustrated in  FIG. 16 , at a time t 3 , the conditions of that the delay cycle has lapsed and the valve timing of the intake valve  21  becomes the target valve timing are both satisfied. As illustrated in a waveform  166 , at a time t 3 , an execution flag of the failure diagnosis becomes 1 from 0, and the failure determining module  1112  executes the failure determination of the in-cylinder pressure sensor SW 6 . 
     Note that in the example illustrated in  FIG. 16 , when the time point at which the valve timing of the intake valve  21  becomes the target timing is earlier (time t 3 ′), the execution flag of the failure diagnosis become 1 from 0 at the time t 2 , as illustrated by an one-dot chain line in the waveform  166 . 
     Then, at a time t 4 , when the operator steps on the accelerator pedal and the accelerator opening becomes larger than 0, the F/C flag becomes 0 for stopping the fuel cut. At the same time, since the failure diagnosis of the in-cylinder pressure sensor SW 6  is also stopped, the failure diagnosis execution flag also becomes 0. 
     Other Embodiments 
     —Modification of Diagnostic Method— 
     In the above embodiment, the diagnosis module  111  is configured to determine the threshold based on the post-top estimated value, but it is not limited to this configuration. The threshold may directly be determined based on the pre-top signal value and the operating state of the engine  1 , without the intervening post-top estimated value. In this case, the threshold indicates the same tendency as the pressure difference illustrated in  FIGS. 10 to 12 . 
     —Modification of Flowchart— 
       FIG. 17  illustrates a modification of the flowchart according to the failure diagnosis of the in-cylinder pressure sensor. Steps S 20 -S 24  of  FIG. 13B  are replaced by Steps S 20 A-S 26  of  FIG. 17 . 
     First, at Step S 19 , if the failure determining module  1112  determines that the difference between the pre-top signal value and the post-top signal value exceeds the threshold based on the post-top estimated value, the process shifts to Step S 20 A, and on the other hand, if the difference is determined to be below the threshold, the process shifts to Step S 21 A. 
     When the process shifts to Step S 20 A, since it is thought that the in-cylinder pressure sensor SW 6  is failed, the failure determining module  1112  increments a failure determining counter by 1 and decrements a normality determining counter by 1. On the other hand, at Step S 21 A, since it is considered that the in-cylinder pressure sensor SW 6  has not failed, the failure determining module  1112  decrements the failure determining counter by 1 and increments the normality determining counter by 1. 
     At the subsequent Step S 22 , the failure determining module  1112  determines whether the failure determining counter exceeds the given value. If the determination at Step S 22  is YES, the process shifts to Step S 23 . That is, since the frequency of the determination as the in-cylinder pressure sensor SW 6  being failure is higher than the frequency of the determination as the in-cylinder pressure sensor SW 6  not being failed, the failure determining module  1112  concludes that the in-cylinder pressure sensor SW 6  has failed, and at the subsequent Step S 24 , the failure determining module  1112  informs of the failure through the informing device  57 . 
     On the other hand, if the determination at Step S 22  is NO, the process shifts to Step S 25 . At Step S 25 , the failure determining module  1112  determines whether the normality determining counter exceeds the given value. If the determination at Step S 25  is YES, the process shifts to Step S 26 . Since the frequency of the determination as the in-cylinder pressure sensor SW 6  has not failed is higher than the frequency of the determination as the in-cylinder pressure sensor SW 6  being failed, the failure determining module  1112  concludes that the in-cylinder pressure sensor SW 6  does not fail, and makes the failure determining counter 0. Moreover, the failure determining module  1112  also makes the normality determining counter 0. On the other hand, if the determination at Step S 25  is NO, the process returns. 
     Thus, the failure diagnosis device  100  can prevent the false diagnosis by diagnosing the failure of the in-cylinder pressure sensor SW 6  using the two kinds of counters of the normal determining counter and the failure determining counter. 
     Instead of the configuration, the operating state determining module  1111  may determine an amount of change per unit time of each of the amount of air filled up in the combustion chamber  17 , the amount of EGR gas contained in the mixture gas inside the combustion chamber  17 , and the amount of fuel supplied to the combustion chamber  17 . In this case, when all of these three amounts of change are determined to be below the respective given values, it can be determined that the engine  1  carries out the steady operation. 
     Note that the technology disclosed herein is not limited to be applied to the engine  1  of the above configuration. The engine  1  may adopt various configurations. 
     It should be understood that the embodiments herein are illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof, are therefore intended to be embraced by the claims. 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
         
           
               1  Engine 
               100  Failure Diagnosis Device 
               1111  Operating State Determining Module 
               1112  Failure Determining Module 
               1113  Reading Module 
               1114  Estimating Module 
               17  Combustion Chamber 
               25  Ignition Plug 
               71  Diaphragm 
               75  Piezo-electric Element 
             SW 1  Airflow Sensor 
             SW 2  First Intake Air Temperature Sensor 
             SW 3  First Pressure Sensor 
             SW 4  Second Intake Air Temperature Sensor 
             SW 5  Intake Pressure Sensor 
             SW 6  In-cylinder Pressure Sensor 
             SW 7  Exhaust Temperature Sensor 
             SW 8  Linear O 2  Sensor 
             SW 9  Lambda O 2  Sensor 
             SW 10  Water Temperature Sensor 
             SW 11  Crank Angle Sensor 
             SW 12  Accelerator Opening Sensor 
             SW 13  Intake Cam Angle Sensor 
             SW 14  Exhaust Cam Angle Sensor 
             SW 15  EGR Differential Pressure Sensor 
             SW 16  Fuel Pressure Sensor 
             SW 17  Third Intake Air Temperature Sensor