Patent Publication Number: US-7213543-B2

Title: Technique of detecting failure of compression ratio varying mechanism and controlling internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technique of varying a compression ratio in an internal combustion engine. More specifically the invention pertains to a technique of detecting a failure or trouble arising in a compression ratio varying mechanism and controlling an internal combustion engine. 
     2. Description of the Related Art 
     The internal combustion engine is advantageously small in size but is capable of outputting relatively large power. Because of these advantages, the internal combustion engine is widely used as the power source of diverse transportation facilities, such as automobiles, ships and boats, and aircraft, and as the power source of various stationary machines and equipment. The internal combustion engine makes a compressed air-fuel mixture subjected to combustion in a combustion chamber, converts the combustion pressure generated through the combustion into mechanical power, and takes out the mechanical power. 
     A technique of varying the compression ratio of the air-fuel mixture according to the driving conditions of the internal combustion engine has been proposed to improve a conversion efficiency into mechanical power (that is, a thermal efficiency) and increase an maximum output. Setting a low compression ratio ensures a sufficiently high maximum output under the conditions of high loading. Setting a high compression ratio enhances the thermal efficiency under the conditions of medium or low loading. The optimum ignition timing depends upon the compression ratio. The lower compression ratio advances the optimum ignition timing. The general control procedure thus changes the ignition timing with a variation in compression ratio. 
     One proposed technique fixes the ignition timing to the setting suitable for the high compression ratio, when any failure arises in the course of varying the compression ratio in the internal combustion engine (see Japanese Patent Laid-Open Gazette No. 1-35047). The technique of this cited reference fixes the ignition timing to the setting for the high compression ratio, when the compression ratio is locked in at the high compression ratio. The technique prevents the ignition timing from advancing to the setting suitable for the low compression ratio, in the case of a lock-in of the compression ratio, thus lowering the potential for abnormal combustion called knocking. 
     The above prior art technique lowers the potential for knocking but may not allow the internal combustion engine to be driven stably, when some trouble or failure arises in the course of varying the compression ratio. Even when the compression ratio is locked in at the low compression ratio, this proposed technique fixes the ignition timing to the setting suitable for the high compression ratio. Such fixation is disadvantageous to stable driving of the internal combustion engine. The low compression ratio is undesirable for quick and stable combustion of the air-fuel mixture in a combustion chamber. For stable combustion of the air-fuel mixture, the ignition timing is thus to be changed to the adequate setting with a decrease in compression ratio. Diverse controls are carried out in the internal combustion engine with the aim of improving the thermal efficiency or of reducing the emission. Some of such controls have adverse effects on stable combustion. Execution of such controls in the state of a lock-in of the low compression ratio may lower the stability of combustion and prevent the internal combustion engine from being driven stably. 
     SUMMARY OF THE INVENTION 
     The object of the invention is thus to provide a technique that enables an internal combustion engine equipped with a compression ratio varying mechanism to be driven stably, even in the case of the occurrence of any failure or trouble in the compression ratio varying mechanism. 
     In order to attain at least part of the above and the other related objects, the present invention is directed to an internal combustion engine that compresses an air-fuel mixture of a fuel and the air and makes the compressed air-fuel mixture subjected to combustion in a combustion chamber to generate power. The internal combustion engine includes: a compression ratio varying mechanism that varies a compression ratio as an indicator representing a degree of compression of the air-fuel mixture; a compression ratio control module that controls actuation of the compression ratio varying mechanism, so as to regulate the compression ratio according to a driving condition of the internal combustion engine; a failure detection module that detects occurrence of a failure in the compression ratio varying mechanism; and a specific control restriction module that, in response to detection of the occurrence of a failure, restricts execution of a specific control that has adverse effects on stable combustion of the air-fuel mixture. 
     Another application of the invention is a corresponding control method of the internal combustion engine. The invention is accordingly directed to a control method of an internal combustion engine that compresses an air-fuel mixture of a fuel and the air and makes the compressed air-fuel mixture subjected to combustion in a combustion chamber to generate power. The control method includes the steps of: controlling actuation of a compression ratio varying mechanism, which varies a compression ratio as an indicator representing a degree of compression of the air-fuel mixture, according to a driving condition of the internal combustion engine, so as to regulate the compression ratio of the internal combustion engine; detecting occurrence of a failure in the compression ratio varying mechanism; and in response to detection of the occurrence of a failure, restricting execution of a specific control that has adverse effects on stable combustion of the air-fuel mixture. 
     The internal combustion engine of the invention and the corresponding control method of the internal combustion engine restrict execution of the specific control having adverse effects on stable combustion of the air-fuel mixture, when any failure or trouble arises in the compression ratio varying mechanism. This arrangement ensures stable combustion of the air-fuel mixture and thereby stable driving of the internal combustion engine, even in the case of the occurrence of a failure in the compression ratio varying mechanism. 
     In one embodiment of the internal combustion engine, the compression ratio varying mechanism changes over the compression ratio between at least two different levels, that is, a first compression ratio of a lowest level and a second compression ratio of a highest level. In this embodiment, detection of a non-variable state of the compression ratio to at least the second compression ratio in the compression ratio varying mechanism indicates the occurrence of a failure. 
     The higher compression ratio enhances combustion of the air-fuel mixture. Under the condition of a high compression ratio, a control operation having adverse effects on stable combustion is often carried out by taking into account this tendency. While a failure in the compression ratio varying mechanism does not allow the compression ratio to be set to a high level, execution of this control operation heightens the potential for poor combustion. The arrangement of determining the occurrence of a failure based on detection of a non-variable state of the compression ratio to the second compression ratio effectively prevents poor combustion and advantageously ensures stable driving of the internal combustion engine. 
     The internal combustion engine may have a function of detecting a lock-in compression ratio, at which the compression ratio varying mechanism is locked in. In this structure, the occurrence of a failure may be determined, when the lock-in compression ratio is different from the second compression ratio. 
     When the lock-in compression ratio is equal to the second compression ratio, the specific control having adverse effects on stable combustion of the air-fuel mixture does not substantially interfere with the stable combustion. In such cases, the internal combustion engine is driven stably even under the specific control to enhance a thermal efficiency or to reduce emission. 
     In the internal combustion engine having a variable air-fuel ratio between a stoichiometric air-fuel ratio and a lean air-fuel ratio, a control operation of setting the lean air-fuel ratio may be restricted, in response to detection of the occurrence of a failure in the compression ratio varying mechanism. Restriction of the control operation of setting the lean air-fuel ratio may narrow a driving condition for setting the lean air-fuel ratio, may reduce a lean degree of the air-fuel ratio, or may be a combination thereof. The control operation of setting the lean air-fuel ratio may otherwise be prohibited. 
     As is known in the art, the lean air-fuel ratio of the air-fuel mixture tends to lower the stability of combustion. When any failure is detected in the compression ratio varying mechanism, restriction of the control operation of setting the lean air-fuel ratio desirably prevents unstable combustion of the air-fuel mixture. 
     In one embodiment, the internal combustion engine carries out an ignition delay control to retard the ignition timing, when the internal combustion engine is in a cold state. This ignition delay control may be restricted, in response to detection of the occurrence of a failure in the compression ratio varying mechanism. Restriction of the ignition delay control may narrow a driving condition for delaying the ignition timing, may reduce a degree of ignition delay, or may be a combination thereof. The ignition delay control may otherwise be prohibited. 
     When the internal combustion engine is in the cold state, the ignition delay control may be carried out to retard the ignition timing from the optimum ignition timing that ensures the most favorable combustion state. The delay of the ignition timing tends to lower the stability of combustion. When any failure is detected in the compression ratio varying mechanism, restriction of the ignition delay control to retard the ignition timing desirably prevents unstable combustion of the air-fuel mixture. 
     In another example, the internal combustion engine comprises an EGR mechanism to recirculate part of the combustion exhaust, which is produced by combustion of the air-fuel mixture, into the combustion chamber and an EGR control module that controls the amount of the recirculated combustion exhaust by operating said EGR mechanism according to the driving condition of said internal combustion engine. This recirculation by said EGR mechanism is restricted, in response to detection of the occurrence of a failure in the compression ratio varying mechanism. Restriction of the EGR control may narrow a driving condition for carrying out the EGR control, may reduce a flow of combustion exhaust (EGR gas) to be recirculated into the combustion chamber, or may be a combination thereof. The EGR control may otherwise be prohibited. One method of recirculating the combustion exhaust into the combustion chamber may lead a partial flow of the combustion exhaust, which is discharged from the combustion chamber, from an exhaust conduit back to an intake conduit. Another method may cause part of the combustion exhaust to be ejected from the combustion chamber to the intake conduit and to be taken in again with the flow of the air. 
     Execution of the EGR control to recirculate the combustion exhaust tends to lower the stability of combustion of the air-fuel mixture. When any failure is detected in the compression ratio varying mechanism, restriction of the EGR control desirably prevents unstable combustion of the air-fuel mixture. 
     In one preferable application, the internal combustion engine detects a lock-in of the compression ratio varying mechanism and restricts a control specification of the control operation having adverse effects on the stable combustion to an allowable range corresponding to each lock-in compression ratio. 
     In the case of detection of the occurrence of a failure in the compression ratio varying mechanism, execution of the control operation having adverse effects on the stable combustion in the allowable range desirably enables the internal combustion engine to be driven without lowering the stability of combustion. 
     In one preferable embodiment, the internal combustion engine has an intake conduit that leads a supply of intake air to the combustion chamber, a first fuel injection valve that injects the fuel in the intake conduit, and a second fuel injection valve that injects the fuel into the combustion chamber. At least one of the first fuel injection valve and the second fuel injection valve is actuated to inject the fuel according to the driving condition of the internal combustion engine. In this embodiment, actuation of the first fuel injection valve to inject the fuel is restricted, in response to detection of the occurrence of a failure. 
     While the compression ratio varying mechanism has a failure, injection of the fuel in the intake conduit may cause a phenomenon called backfire, which astonishes the driver. The backfire will be discussed in detail later. The arrangement of restricting the fuel injection from the first fuel injection valve and allowing only the second fuel injection valve to inject the fuel effectively prevents the occurrence of backfire. 
     These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates the structure of an engine in one embodiment of the invention; 
         FIG. 2  is a flowchart showing a control flow in a cold state in an engine control routine of a first embodiment; 
         FIG. 3  is a flowchart showing a control flow in a warm-up state in the engine control routine of the first embodiment; 
         FIG. 4  conceptually shows a map of adequate settings of the compression ratio against the driving conditions; 
         FIG. 5  conceptually shows maps of adequate settings of the ignition timing against the driving conditions with respect to various settings of the compression ratio; 
         FIG. 6  conceptually shows a map of a variation in warm-up ignition delay against the temperature of cooling water; 
         FIG. 7  conceptually shows a map of adequate settings of the EGR valve opening against the driving conditions; 
         FIG. 8  conceptually shows maps of adequate settings of the air-fuel ratio against the driving conditions with respect to various settings of the compression ratio; 
         FIG. 9  conceptually shows a map of adequate settings of the fuel injection mode against the driving conditions; and 
         FIG. 10  is a flowchart showing an engine control routine executed in a second embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Some modes of carrying out the invention are discussed below as preferred embodiments in the following sequence: 
     A. System Structure 
     B. First Embodiment 
     B-1. Control in Cold State 
     B-2. Control in Warm-up State 
     C. Second Embodiment 
     A. System Structure 
       FIG. 1  schematically illustrates the structure of an engine  10  including a compression ratio varying mechanism in one embodiment of the invention. As illustrated, the engine  10  mainly includes a cylinder head  20 , a cylinder block assembly  30 , a main moving assembly  40 , intake conduits  50 , exhaust conduits  58 , EGR conduits  70 , and an engine control unit (hereafter referred to as ECU)  60 . 
     The cylinder block assembly  30  has an upper block  31  with the cylinder head  20  mounted thereon and a lower block  32  to receive the main moving assembly  40  therein. An actuator  33  is interposed between the upper block  31  and the lower block  32 . The actuator  33  is driven to vertically move the upper block  31  relative to the lower block  32 . Tubular cylinders  34  are formed in the upper block  31 . 
     The main moving assembly  40  has pistons  41  received inside the cylinders  34 , a crankshaft  43  rotating inside the lower block  32 , and connecting rods  42  connecting the pistons  41  with the crankshaft  43 . The pistons  41 , the connecting rods  42 , and the crankshaft  43  constitute a crank mechanism. Rotation of the crankshaft  43  slides up and down each piston  41  in the corresponding cylinder  34 , while the vertical sliding motion of the piston  41  rotates the crankshaft  43  in the lower block  32 . 
     Attachment of the cylinder head  20  to the cylinder block assembly  30  gives spaces defined by a lower face of the cylinder head  20  (a face coming into contact with the upper block  31 ), the cylinders  34 , and the pistons  41 . These spaces function as combustion chambers. The upward movement of the upper block  31  by actuation of the actuator  33  moves the cylinder head  20  up to increase the inner volume of each combustion chamber, thus lowering the compression ratio. The downward movement of the cylinder head  20  with the upper block  31 , on the other hand, reduces the inner volume of each combustion chamber to heighten the compression ratio. 
     The compression ratio may be measured by a compression ratio sensor  63 , which is located in the lower block  32 . In the structure of this embodiment, a stroke sensor is applied for the compression ratio sensor  63  and measures a relative position of the upper block  31  to the lower block  32  to specify the compression ratio. The stroke sensor is not restrictive at all, but any other suitable method is applicable to detect the compression ratio. For example, a pressure sensor located in the cylinder head  20  may be used to measure the pressure in the combustion chamber and specify the compression ratio based on the observed pressure. 
     The cylinder head  20  has intake ports  23  to take the air into the corresponding combustion chambers and exhaust ports  24  to discharge the exhaust gas from the corresponding combustion chambers. An intake valve  21  is set at an opening of each intake port  23  to the combustion chamber, and an exhaust valve  22  is set at an opening of each exhaust port  24  to the combustion chamber. The intake valves  21  and the exhaust valves  22  are driven by a cam mechanism with the vertical motions of the pistons  41 . The on-off control of the intake valves  21  and the exhaust valves  22  at respective adequate timings in synchronism with the motions of the pistons  41  takes the air into the combustion chambers and discharges the exhaust gas from the combustion chambers. The cylinder head  20  has an ignition plug  27 , which ignites the air-fuel mixture with a spark in the combustion chambers. 
     Each intake conduit  50  is connected with the intake port  23  of the cylinder head  20  to lead the air to the cylinder head  20 . An air cleaner  51  is set on an upstream end of the intake conduits  50 . The engine  10  of the embodiment is a 4-cylinder engine and has four combustion chambers. The intake conduits  50  of the four combustion chambers join together at a serve tank  54 . The supply of the air goes through the air cleaner  51  for removal of dust and foreign substances, is distributed by the serge tank  54  into the intake conduits  50  of the respective combustion chambers, and is flown into the respective combustion chambers via the intake ports  23 . A throttle valve  52  is located in each intake conduit  50  upstream the serve tank  54 . The opening of the throttle valve  52  is regulated by an electric actuator  53  to control the quantity of the air flown into the combustion chamber. Each combustion chamber has two fuel injection valves  26  and  55 . The fuel injection valve  26  located in the cylinder head  20  gives a direct spray of the fuel into the combustion chamber, whereas the fuel injection valve  55  located in the intake conduit  50  gives a spray of the fuel in the intake conduit  50  toward the intake port  23 . The supply of the fuel injected from the fuel injection valve  26  or from the fuel injection valve  55  is vaporized to form a mixture of the fuel and the air (air-fuel mixture) in each combustion chamber. 
     Each exhaust conduit  58  is connected with the exhaust port  24  of each combustion chamber to lead and release the exhaust gas discharged from the combustion chamber to the outside. The EGR conduit  70  connects the exhaust conduit  58  with the intake conduit  50 . Part of the exhaust gas flowing through the exhaust conduit  58  is recirculated to the intake conduit  50  via the EGR conduit  70  and is fed with the intake air into the combustion chamber. An EGR valve  72  is located in the middle of the EGR conduit  70 . The opening of the EGR valve  72  is regulated to control the flow of the exhaust gas recirculation (EGR gas). 
     The ECU  60  is constructed by a microcomputer including a central processing unit (CPU), a ROM, a RAM, and an input/output circuit, which are mutually connected via a bus. The ECU  60  receives required information from a crank angle sensor  61  attached to the crankshaft  43  and an accelerator opening sensor  62  built in an accelerator pedal, and controls actuation of the fuel injection valves  26  and  55  and the ignition plug  27  at adequate timings to make the air-fuel mixture subjected to combustion in the combustion chambers and generate the power. The ECU  60  also controls actuation of the electric actuator  53  to regulate the flow of intake air and actuation of the actuator  33  to vary the compression ratio. The CU  60  detects a warm-up state of the engine  10 , based on an output of a water temperature sensor  64  located in the upper block  31 . 
     In the engine  10  having the configuration discussed above, some trouble or failure may arise in the actuator  33  or in another component of the compression ratio varying mechanism to lock in the current compression ratio. One example of such trouble is locking of the actuator  33  to lock in the current low setting of the compression ratio or to prevent the compression ratio from being set to a higher level while allowing the compression ratio to be varied only in a low range. As described previously, the low compression ratio is disadvantageous to stable combustion. The occurrence of such failure worsens the combustion conditions and may lead to an unstable drive of the engine  10 . The engine  10  of the embodiment has a control strategy as discussed below to ensure a stable drive of the engine  10 , even when some trouble or failure arises in the compression ratio varying mechanism. 
     B. First Embodiment 
     An engine control routine for controlling the operations of the engine  10  in a first embodiment is discussed below with reference to the flowcharts of  FIGS. 2 and 3 . 
     When the engine control routine starts, the ECU  60  first receives inputs of driving conditions of the engine  10  (step S 100 ). The driving conditions input here are a revolution speed of the engine  10  or an engine speed Ne and an accelerator opening qac. The engine speed Ne is computed from the output of the crank angle sensor  61 , and the accelerator opening qac is measured by the accelerator opening sensor  62 . 
     The ECU  60  subsequently determines whether the engine  10  is in a cold state (step S 102 ). The ECU  60  specifies the temperature of cooling water in the upper block  31 , based on the output of the water temperature sensor  64 . It is determined that the engine  10  is in the cold state (step S 102 : yes), when the specified temperature of cooling water is not higher than a preset level. It is determined that the engine  10  is not in the cold state (step S 102 : no), on the other hand, when the specified temperature of cooling water is higher than the preset level. The control of the engine  10  in the cold state is different from the control of the engine  10  in the non-cold state (that is, in a warm-up state). The description first regards the control flow in the cold state and then the control flow in the warm-up state. 
     B-1. Control in Cold State 
     When it is determined that the engine  10  is in the cold state (step S 102 : yes), the ECU  60  sets a compression ratio of the engine  10 , based on the input driving conditions (step S 104 ). The adequate settings of the compression ratio against the driving conditions, the engine speed Ne and the accelerator opening qac, as parameters are stored in the form of a map in the ROM of the ECU  60 .  FIG. 4  conceptually shows a map of adequate settings of the compression ratio against the driving conditions, which is stored in the ROM. A similar map of adequate settings of the compression ratio in the warm-up state, as well as the map in the cold state shown in  FIG. 4 , is stored in the ROM of the ECU  60 . The engine in the cold state has the higher potential for unstable combustion of the air-fuel mixture. In order to ensure stable combustion, the settings of the compression ratio in the map in the cold state are higher than those in the map in the warm-up state. The ECU  60  refers to this map, reads the setting of the compression ratio corresponding to the input driving conditions, and actuates the actuator  33  to set the compression ratio in the engine  10  at step S 104 . 
     After setting the compression ratio, the ECU  60  sets an air-fuel ratio (step S 106 ). The air-fuel ratio is an indicator representing a concentration of the fuel in the air-fuel mixture and is calculated by dividing the weight of the air included in the air-fuel mixture by the weight of the fuel. The procedure of this embodiment sets a stoichiometric air-fuel ratio, regardless of the driving conditions, when the engine  10  is in the cold state. At the stoichiometric air-fuel ratio, the air and the fuel are mixed to attain just sufficient combustion. When the engine  10  is in the cold state, the stoichiometric air-fuel ratio is set to ensure stable combustion of the air-fuel mixture. The air-fuel ratio having a lower concentration of the fuel than that of the stoichiometric air-fuel ratio is called the lean air-fuel ratio. The air-fuel ratio having a higher concentration of the fuel than that of the stoichiometric air-fuel ratio is called the rich air-fuel ratio. 
     After setting the air-fuel ratio, the ECU  60  sets a fuel injection mode (step S 108 ). As shown in  FIG. 1 , the engine  10  of the embodiment has the two fuel injection valves  26  and  55 . Actuation of the fuel injection valve  26  set in the cylinder head  20  gives a direct spray of the fuel into the combustion chamber. The fuel is accordingly localized in the combustion chamber to form a section of the higher fuel concentration (that is, the lower air-fuel ratio) and a section of the lower fuel concentration (that is, the higher air-fuel ratio). The air-fuel mixture having an adequate distribution of the air-fuel ratio in the combustion chamber desirably saves the total quantity of the fuel and enhances the thermal efficiency of the engine  10 . The mode of directly injecting the fuel into the combustion chamber is called the in-cylinder injection mode. 
     Actuation of the fuel injection valve  55  set in the intake conduit  50 , on the other hand, gives an injection of the fuel in the intake conduit  50 . The injected fuel is vaporized, is mixed with the air, and is taken into the combustion chamber. In this case, the fuel and the air are well blended to form the homogeneous air-fuel mixture in the combustion chamber. The homogeneous air-fuel mixture set to have the stoichiometric air-fuel ratio ensures the most stable combustion. The homogeneous air-fuel mixture set to have the lower air-fuel ratio than the stoichiometric air-fuel ratio (that is, the higher fuel concentration), on the other hand, enables output of the maximum power. The mode of injecting the fuel in the intake conduit  50  is called the port injection mode. 
     The in-cylinder injection may be adopted in combination with the port injection. Such combination makes part of the fuel injected in the intake conduit  50  and the residual fuel directly injected into the combustion chamber. The air-fuel mixture having the high fuel concentration is localized in a partial area of the combustion chamber, while the homogeneous air-fuel mixture having the lower fuel concentration is present in the other area of the combustion chamber. The injection amounts and the injection timings of these two fuel injection valves  26  and  55  are regulated according to the driving conditions to form the air-fuel mixture having an appropriate distribution of the air-fuel ratio in the combustion chamber. This exploits the high performance of the engine  10 . 
     Since the engine  10  is in the cold state, the ECU  60  selects the port injection mode for the fuel injection to ensure stable combustion of the air-fuel mixture at step S 108 . The stoichiometric air-fuel ratio has been set at step S 106 . The homogeneous air-fuel mixture having the stoichiometric air-fuel ratio is accordingly formed in the combustion chamber to ensure   stable combustion of the air-fuel mixture, while the engine  10  is in the cold state. 
     The ECU  60  subsequently sets an ignition timing (step S 110 ) The ignition timing is set by referring to a map, as in the case of the compression ratio. Adequate settings of the ignition timing against the driving conditions, the engine speed Ne and the accelerator opening qac, as parameters with respect to various settings of the compression ratio are stored in the form of maps in the ROM of the ECU  60 .  FIG. 5  conceptually shows maps of adequate settings of the ignition timing against the driving conditions with respect to various settings of the compression ratio. The ECU  60  refers to a map for the current compression ratio set at step S 104  and sets the ignition timing corresponding to the input driving conditions at step S 110 . 
     After setting the ignition timing, the ECU  60  determines whether the compression ratio varying mechanism has any failure or trouble (step S 112 ). As described above, the upper block  31  of the engine  10  is vertically moved relative to the lower block  32  to vary the compression ratio in the engine  10 . Namely the relative position of the upper block  31  to the lower block  32  immediately specifies the compression ratio actually set in the engine  10 . The actual compression ratio of the engine  10  is accordingly specified, based on the relative position of the upper block  31  detected by the compression ratio sensor  63  located in the lower block  32 . When the observedl compression ratio is identical with the compression ratio set at step S 104  in the engine control routine, the ECU  60  determines that the compression ratio varying mechanism functions normally. When the specified compression ratio is different from the setting of the compression ratio, on the other hand, the ECU  60  determines that the compression ratio varying mechanism has some failure or trouble. 
     When it is determined that there is no failure or trouble in the compression ratio varying mechanism (step S 112 : no), the ECU  60  sets a warm-up ignition delay (step S 114 ). The warm-up ignition delay is an operation carried out when the engine  10  is driven in the cold state, and retards the ignition timing from the standard timing to quickly warm the engine  10  up. The delay of the ignition timing from the standard timing lowers the thermal efficiency of the engine, that is, the conversion rate of thermal energy generated by combustion into mechanical power, while increasing the energy released as heat with the exhaust gas. This quickly warms up the engine or the emission control catalyst. An extreme delay of the ignition timing, however, causes unstable combustion of the air-fuel mixture. The adequate ignition timing is thus to be set with a variation in temperature of the engine. In the structure of this embodiment, a variation in adequate ignition delay against the temperature of cooling water is experimentally determined in advance and is stored in the form of a map as shown in  FIG. 6  in the ROM of the ECU  60 . The ECU  60  receives the observed temperature of cooling water in the engine  10 , which is measured by the water temperature sensor  64  located in the upper block  31 , and refers to the map of  FIG. 6  to set the adequate delay of the ignition timing at step S 114  in the flowchart of  FIG. 2 . 
     After setting the warm-up ignition delay, the ECU  60  sets the opening of the EGR valve  72  (step S 116 ). The EGR (exhaust gas recirculation) operation recirculates part of the exhaust gas to the combustion chamber for combustion with the air-fuel mixture. The EGR operation lowers the combustion temperature of the air-fuel mixture and thereby lowers the concentration of nitrogen oxides NOx included in the exhaust gas. The structure of this embodiment recirculates part of the exhaust gas from the exhaust conduit  58  to the intake conduit  50  via the EGR conduit  70  as the EGR operation as shown in  FIG. 1 . The flow of the exhaust gas recirculation (the flow of the EGR gas) is controlled by regulating the opening of the EGR valve  72  located in the EGR conduit  70 . The optimum flow of the EGR gas, that is, the adequate EGR valve opening, depends upon the driving conditions of the engine. In the structure of this embodiment, adequate settings of the EGR valve opening against the driving conditions, that is, the engine speed Ne and the accelerator opening qac, as parameters in the cold state of the engine are determined in advance and are stored in the form of a map as shown in  FIG. 7  in the ROM of the ECU  60 . The ECU  60  refers to this map, reads the adequate setting of the EGR valve opening corresponding to the input driving conditions, and actually sets the opening of the EGR valve  72  at step S 116 . 
     On completion of the settings of the compression ratio, the air-fuel ratio, the fuel injection mode, the ignition timing, the warm-up ignition delay, and the EGR valve opening according to the driving conditions, the ECU  60  calculates the amount of fuel injection based on these settings and injects the fuel at an adequate timing (step S 120 ), and ignites the air-fuel mixture with a spark from the ignition plug  27  in the combustion chamber at the adequate timing, which is determined by taking into account the warm-up ignition delay (step S 122 ). This causes combustion of the air-fuel mixture in the combustion chamber and generates power. 
     When it is determined that the compression ratio varying mechanism has some failure or trouble (step S 112 : yes), the engine control routine skips the processing of step S 114  to set the warm-up ignition delay and the processing of step S 116  to set the EGR valve opening but prohibits the EGR operation (step S 118 ), because of the reasons discussed below. 
     When the engine  10  is driven in the cold state, the warm-up ignition delay accelerates the warm-up of the engine  10 . The warm-up ignition delay retards the ignition timing from the adequate ignition timing and thus adversely affects the stable combustion of the air-fuel mixture. The warm-up ignition delay is set in the range of ensuring stable combustion. But when any failure or trouble arises in the compression ratio varying mechanism, the warm-up ignition delay may lead to unstable combustion. For example, when the current low setting of the compression ratio is locked in, the combination of the lock-in of the low compression ratio with the adverse effects of the warm-up ignition delay may cause unstable combustion. The same problem is found when the compression ratio is not settable to a higher level but is variable only in a low range, for example, when the compression ratio is not settable to the high setting  15  but is selectable only between the lower settings  10  and  13 . 
     Like the warm-up ignition delay, the EGR operation adversely affects the stable combustion. The exhaust gas remaining after combustion of the air-fuel mixture is basically an incombustible inert gas. The EGR operation leads the flow of this inert gas with the air-fuel mixture to the combustion chamber and accordingly has the adverse effects on the stable combustion. When any failure or trouble arises in the compression ratio varying mechanism, the EGR operation may lead to unstable combustion, because of the same reason discussed above with regard to the warm-up ignition delay. 
     Because of the reasons discussed above, the engine control routine of this embodiment skips the settings of the warm-up ignition delay and the EGR valve opening and prohibits the EGR operation at step S 118 , in the case of the occurrence of any failure or trouble in the compression ratio varying mechanism. The EGR operation is prohibited by setting the EGR valve  72  at its full closed position. In this case, the subsequent fuel injection control (step S 120 ) and ignition timing control (step S 122 ) are carried out without the warm-up ignition delay and the EGR operation. This ensures stable combustion of the air-fuel mixture in the combustion chamber, regardless of the setting of the compression ratio in the engine  10 . 
     The ECU  60  determines whether the driver gives an engine stop instruction to stop the engine  10  (step S 124 ). When it is determined that the driver gives an engine stop instruction (step S 124 : yes), the engine control routine shown in  FIG. 2  is terminated. When it is determined that the driver gives no engine stop instruction (step S 124 : no), on the other hand, the engine control routine returns to step S 100  and repeats the above series of processing. In the course of the processing, the temperature of cooling water in the engine  10  gradually rises and the engine  10  is set in the warm-up state. The engine control routine then proceeds to the control in the warm-up state as discussed below. 
     B-2. Control in Warm-up State 
     The following description regards the control executed when the engine  10  is in the warm-up state, that is, in the case of a negative answer at step S 102 .  FIG. 3  is a flowchart showing a control flow executed when the engine  10  is in the warm-up state. When the control flow of  FIG. 3  starts, the ECU  60  first sets the compression ratio (step S 140 ). This is almost equivalent to the processing of step S 104  in the flowchart of  FIG. 2  to set the compression ratio in the cold state. The ECU  60  refers to the map of the adequate settings of the compression ratio in the warm-up state, which is stored in the ROM of the ECU  60 , reads the setting of the compression ratio corresponding to the input driving conditions, and actuates the actuator  33  to set the compression ratio in the engine  10  at step S 140 . As mentioned previously, the settings of the compression ratio in the map to be referred to in the warm-up state are lower than those in the map of  FIG. 4  to be referred to in the cold state. 
     The ECU  60  subsequently determines whether any failure or trouble arises in the compression ratio varying mechanism (step S 142 ). As described above, the procedure of detecting the occurrence of a failure or trouble compares the actual compression ratio measured by the compression ratio sensor  63  with the compression ratio set corresponding to the driving conditions at step S 140 . When the observed compression ratio is identical with the setting of the compression ratio, it is determined that the compression ratio varying mechanism has no failure or trouble (step S 142 : no). When the observed compression ratio is different from the setting of the compression ratio, on the other hand, it is determined that the compression ratio varying mechanism has some failure or trouble (step S 142 : yes). 
     In the case of a negative answer at step S 142 , that is, when it is determined that the compression ratio varying mechanism has no failure or trouble, the ECU  60  sets the air-fuel ratio (step S 144 ). When the engine  10  is in the cold state, the stoichiometric air-fuel ratio is set at step S 106  in the flowchart of  FIG. 2 . When the engine  10  is in the warm-up state, on the other hand, an adequate air-fuel ratio is set according to the driving conditions. Adequate settings of the air-fuel ratio against the driving conditions, the engine speed Ne and the accelerator opening qac, as parameters with respect to various settings of the compression ratio are determined in advance and are stored in the form of maps in the ROM of the ECU  60 .  FIG. 8  conceptually shows maps of adequate settings of the air-fuel ratio against the driving conditions with respect to various settings of the compression ratio, which are stored in the ROM. The ECU  60  refers to a map for the current setting of the compression ratio and sets the adequate air-fuel ratio corresponding to the input driving conditions at step S 144 . 
     After setting the air-fuel ratio, the ECU  60  sets the fuel injection mode (step S 146 ). As described previously with reference to  FIG. 1 , the engine  10  of the embodiment has the two fuel injection valves  26  and  55  to directly inject the supply of fuel into the combustion chamber (the in-cylinder injection mode) and to inject the supply of fuel in the intake conduit  50  (the port injection mode), prior to the flow-in of the air and the injected fuel to the combustion chamber. In the in-cylinder injection mode, the fuel injection timing is adequately set to form a high fuel concentration area and a low fuel concentration area in the combustion chamber. For example, the fuel injection control is executed to form a high fuel concentration area in the vicinity of the ignition plug  27  and a low fuel concentration area in the residual part of the combustion chamber. Such control assures stable combustion of the air-fuel mixture having the lean air-fuel ratio. In the port injection mode, the fuel is injected in the intake conduit  50  and is flown together with the air into the combustion chamber. This well mixes the fuel with the air and forms the homogeneous air-fuel mixture in the combustion chamber. The port injection mode is thus advantageous to quick combustion of the air-fuel mixture to generate a large power. The engine  10  of the embodiment takes advantage of the two fuel injection valves  26  and  55  and selectively sets the in-cylinder injection mode or the port injection mode to the fuel injection mode. The ECU  60  sets the adequate fuel injection mode according to the input driving conditions at step S 146 . 
       FIG. 9  conceptually shows a map of the fuel injection mode against the driving conditions, which is stored in the ROM of the ECU  60 . As illustrated in this map, the port injection mode is selected as the fuel injection mode under the driving conditions of the high engine speed Ne and/or the large accelerator opening qac that require the large power output. The in-cylinder injection mode is selected as the fuel injection mode in the other driving conditions. The ECU  60  refers to this map and selects the adequate fuel injection mode according to the driving conditions at step S 146 . 
     After setting the air-fuel ratio and the fuel injection mode, the ECU  60  sets the ignition timing and the EGR valve opening (steps S 148  and S 150 ). The processing of these steps is substantially identical with the settings in the cold state discussed above (see steps S 110  and S 116  in the flowchart of  FIG. 2 ). Maps of adequate settings of the ignition timing in the warm-up state similar to those of  FIG. 5  and a map of adequate settings of the EGR valve opening in the warm-up state similar to that of  FIG. 7  are stored in the ROM of the ECU  60 . The ECU  60  refers to the maps of the adequate settings of the ignition timing in the warm-up state to set the ignition timing at step S 148 , and refers to the map of the adequate settings of the EGR valve opening in the warm-up state to set the EGR valve opening at step S 150 . When it is determined that the engine  10  is in the warm-up state and that the compression ratio varying mechanism has no failure or trouble, the ECU  60  carries out the fuel injection control (step S 120  in the flowchart of  FIG. 2 ) and the ignition timing control (step S 122 ) with the settings of the air-fuel ratio, the fuel injection mode, and the ignition timing to adequately drive the engine  10 . 
     When it is determined that the compression ratio varying mechanism has any trouble or failure (step S 142 : yes), on the other hand, the ECU  60  sets the stoichiometric air-fuel ratio (step S 152 ). Setting the stoichiometric air-fuel ratio ensures stable combustion of the air-fuel mixture, regardless of the setting of the compression ratio, as described previously. The ECU  60  subsequently sets the in-cylinder injection mode to the fuel injection mode (step S 154 ), in order to prevent the occurrence of backfire. In general, the air-fuel mixture quickly combusts to heighten the inner pressure of the combustion chamber, press the piston  41  down, and generate power. The lock-in of the compression ratio at the low level, however, may lead to the slow and poor combustion of the air-fuel mixture even during the downward movement of the piston  41 . A further extension of the slow combustion to the subsequent intake cycle may cause a backflow of the hot exhaust from the combustion chamber into the intake conduit  50  via the intake valve  21  and combust the fuel present in the intake conduit  50 . This phenomenon is called backfire. 
     The backfire is typically observed when the engine is driven at the rich air-fuel ratio. The backfire is the phenomenon occurring when the combustion of the air-fuel mixture continues even during the downward movement of the piston  41 . In the case of the lean air-fuel ratio of the air-fuel mixture, the combustion often dies out during the downward movement of the piston  41 . This does not cause the backfire. The control flow of this embodiment sets the stoichiometric air-fuel ratio at step S 152  to ensure the stable combustion of the air-fuel mixture. This is the backfire-susceptible condition. The backfire makes a dreadful noise to astonish the driver and may even damage the serve tank  54 . The processing of step S 154  sets the in-cylinder injection mode to the fuel injection mode by taking into account the potential for the backfire. In the in-cylinder injection mode, the fuel is not present in the intake conduit  50 . This perfectly precludes the possibility of the backfire. 
     The ECU  60  then refers to the maps of the adequate settings of the ignition timing stored in the ROM of the ECU  60  and sets the ignition timing (step S 156 ). Here the control flow may use the maps of the adequate settings of the ignition timing in the cold state shown in  FIG. 5  to set the ignition timing. As described above, the control flow in the cold state carries out the warm-up ignition delay to retard the ignition timing from the adequate setting of the ignition timing. The control flow in the warm-up state, however, does not carry out the warm-up ignition delay and sets the practically adequate ignition timing based on the maps of  FIG. 5 . Another applicable procedure may store maps of adequate settings of the ignition timing for the stoichiometric air-fuel ratio in the warm-up state in the ROM of the ECU  60  and refer to these maps to set the ignition timing. 
     After setting the ignition timing, the ECU  60  prohibits the EGR operation (step S 158 ). The EGR operation adversely affects the stable combustion of the air-fuel mixture, as described previously. The processing of step S 158  thus sets the EGR valve  72  at its full closed position and prohibits the EGR operation to ensure stable combustion of the air-fuel mixture. 
     The ECU  60  carries out the fuel injection control (step S 120 ) and the ignition timing control (step S 122 ) with the settings of the air-fuel ratio, the fuel injection mode, and the ignition timing. When the compression ratio varying mechanism has any failure or trouble, the control flow prohibits the EGR operation and forms the air-fuel mixture having the stoichiometric air-fuel ratio, which is stably combusted in the combustion chamber. Direct injection of the fuel into the combustion chamber eliminates the possibility of the backfire. 
     The ECU  60  then determines whether the driver gives an engine stop instruction to stop the engine  10  (step S 124 ). When it is determined that the driver gives no engine stop instruction (step S 124 : no), the engine control routine returns to step S 100  and repeats the above series of processing. When it is determined that the driver gives an engine stop instruction (step S 124 : yes), on the other hand, the engine control routine shown in  FIG. 2  is terminated. 
     As described above, when the compression ratio varying mechanism has some failure or trouble, the control procedure of the first embodiment avoids the operations having adverse effects on stable combustion of the air-fuel mixture, that is, setting of the lean air-fuel ratio, delay of the ignition timing, and the EGR operation. One possible modification may avoid such operations having adverse effects on stable combustion of the air-fuel mixture, only when the compression ratio is locked in at the low level or when the compression ratio is not settable to a higher level. This modified procedure does not avoid these operations, when there is no possibility of poor combustion, as in the case of a lock-in of the compression ratio at the high level. 
     C. Second Embodiment 
     The procedure of the first embodiment skips the operations having adverse effects on the stable combustion of the air-fuel mixture, when any failure or trouble arises in the compression ratio varying mechanism. Another applicable procedure may restrict these operations to an allowable range corresponding to the observed compression ratio, in the case of the occurrence of a failure or trouble. This is described below as a second embodiment. 
       FIG. 10  is a flowchart showing an engine control routine executed in the second embodiment. 
     When the engine control routine of the second embodiment starts, the ECU  60  first receives inputs of driving conditions of the engine  10  (step S 200 ). The driving conditions input here are the engine speed Ne and the accelerator opening qac. 
     The ECU  60  sets the compression ratio in the engine  10  (step S 202 ). The compression ratio is set by referring to the map of the adequate settings of the compression ratio against the driving conditions (see  FIG. 4 ), which is stored in the ROM of the ECU  60 , like the first embodiment. The ECU  60  subsequently specifies the actual compression ratio set in the engine  10 , based on the output of the compression sensor  63  (step S 204 ), and compares the observed compression ratio with the setting of the compression ratio to detect the occurrence of any failure or trouble (step S 206 ). When the observed compression ratio is identical with the setting of the compression ratio, it is determined that the compression ratio varying mechanism has no failure or trouble (step S 206 : no). The ECU  60  then sets the air-fuel ratio and the ignition timing according to the driving conditions (step S 208 ). Like the first embodiment, the maps of the adequate settings of the air-fuel ratio against the driving conditions as parameters (see  FIG. 8 ) and the maps of the adequate settings of the ignition timing against the driving conditions (see  FIG. 5 ) are prepared in advance and are stored in the ROM of the ECU  60 . The processing of step S 208  refers to these maps and sets the adequate air-fuel ratio and ignition timing according to the input driving conditions and the compression ratio. 
     The ECU  60  calculates the amount of fuel injection based on the setting of the air-fuel ratio and actuates the fuel injection valve  26  at the adequate timing (step S 212 ). This forms the air-fuel mixture having the preset air-fuel ratio in the combustion chamber. The ECU  60  then actuates the ignition plug  27  at the ignition timing set at step S 208  to ignite the air-fuel mixture (step S 214 ). This causes quick combustion of the air-fuel mixture in the combustion chamber and generates power. 
     When the observed compression ratio is different from the setting of the compression ratio, on the other hand, it is determined that the compression ratio varying mechanism has some failure or trouble (step S 206 : yes). In this case, the ECU  60  sets the air-fuel ratio and the ignition timing according to the observed compression ratio (step S 210 ). The maps of the adequate settings of the air-fuel ratio with respect to various settings of the compression ratio as shown in  FIG. 8  are prepared in advance and are stored in the ROM of the ECU  60 . The maps of the adequate settings of the ignition timing with respect to various settings of the compression ratio as shown in  FIG. 5  are also stored in the ROM of the ECU  60 . The processing of step S 210  refers to the maps for the observed compression ratio and sets the air-fuel ratio and the ignition timing. For example, when the observed compression ratio is e=10 and the compression ratio set at step S 202  is e=15, the ECU  60  refers to the maps for the compression ratio e=10 to set the air-fuel ratio and the ignition timing. When the observed compression ratio is e=11, the ECU  60  interpolates the maps for the compression ratio e=10 and those for the compression ratio e=13 and computes the air-fuel ratio and the ignition timing corresponding to the compression ratio e=11. 
     After the setting of the air-fuel ratio and the ignition timing according to the observed compression ratio, the ECU  60  carries out the fuel injection control (step S 212 ) and the ignition timing control (step S 214 ). When the compression ratio varying mechanism has any failure or trouble, the control flow of the second embodiment specifies the compression ratio actually set in the engine  10  and sets the air-fuel ratio and the ignition timing in an allowable range corresponding to the observed compression ratio. This arrangement ensures stable operations of the engine  10  without worsening the combustion state of the air-fuel mixture, even when some failure or trouble arises in the compression ratio varying mechanism. 
     The control routine of the second embodiment does not include the EGR control and the warm-up ignition delay control. Such omission is only for the purpose of clarification of explanation. The control routine of the second embodiment may thus include the EGR control and the warm-up ignition delay control according to the requirements, like the first embodiment. 
     The embodiments discussed above are to be considered in all aspects as illustrative and not restrictive. There may be many modifications, changes, and alterations without departing from the scope or spirit of the main characteristics of the present invention. All changes within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 
     The scope and spirit of the present invention are indicated by the appended claims, rather than by the foregoing description.