Patent Publication Number: US-2016230693-A1

Title: Vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This nonprovisional application claims the benefit of Japanese Patent Application No. 2015-024288 filed on Feb. 10, 2015 with the Japan Patent Office, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to a vehicle including a fuel pump, an injection valve injecting fuel supplied from the fuel pump to an engine, and a fuel pressure sensor detecting a supply pressure of the fuel generated by the fuel pump. 
     2. Description of the Background Art 
     Japanese Patent Laying-Open No. 2013-68127 discloses that a fault diagnosis of a fuel pressure sensor is conducted for a vehicle which includes a fuel pump, an injection valve injecting fuel supplied from the fuel pump to an engine, and the fuel pressure sensor detecting a supply pressure of the fuel generated by the fuel pump. For the fault diagnosis of the fuel pressure sensor, the fuel pressure is increased to a diagnosis fuel pressure higher than a fuel pressure for normal use. Based on whether or not an output of the fuel pressure sensor at this time is a value representing the diagnosis fuel pressure, it is determined whether or not the fuel pressure sensor has a fault. 
     SUMMARY 
     However, regarding the vehicle disclosed in Japanese Patent Laying-Open No. 2013-68127, if a fault diagnosis of the fuel pressure sensor is conducted under the condition for example that the engine load ratio (the ratio of the intake air quantity to the quantity of drawable air) is low, the fuel pressure which is increased to the diagnosis fuel pressure in spite of the low load ratio (small intake air quantity) could cause excessive fuel (rich air-fuel ratio), and accordingly cause deterioration of the emission performance. 
     Some embodiments described herein conduct a fault diagnosis of a fuel pressure sensor without deteriorating the emission performance. 
     A vehicle according to the present disclosure includes: an engine; a fuel pump; an injection valve configured to inject fuel supplied from the fuel pump to the engine; a fuel pressure sensor configured to detect a supply pressure of the fuel generated by the fuel pump; and a control apparatus configured to perform a fuel pressure increasing process of increasing the supply pressure of the fuel generated by the fuel pump to a second fuel pressure higher than a first fuel pressure, and conduct a fault diagnosis of the fuel pressure sensor based on a value detected by the fuel pressure sensor during the fuel pressure increasing process. In a case where a fuel injection quantity of the injection valve becomes excessive with respect to a load ratio of the engine during the fuel pressure increasing process, the control apparatus performs a load ratio increasing process of changing an operating point of the engine so that the load ratio of the engine increases. 
     With such a configuration, in the case where the fuel pressure increasing process performed when a fault diagnosis of the fuel pressure sensor is conducted causes the fuel injection quantity to become excessive with respect to the engine load ratio, the load ratio increasing process is performed to increase the engine load ratio. Accordingly, the intake air quantity is increased and thus excessive fuel (rich air-fuel ratio) is prevented. As a result, a fault diagnosis of the fuel sensor can be conducted without deteriorating the emission performance. 
     In some embodiments, the load ratio increasing process is a process of increasing the load ratio of the engine by decreasing a rotational speed of the engine while keeping an output of the engine as it is. 
     With such a configuration, the load ratio increasing process can be performed to increase the engine load ratio while keeping the engine output as it is. 
     In some embodiments, the fuel injection quantity of the injection valve is proportional to a product of the supply pressure of the fuel generated by the fuel pump and an injection time of the injection valve. A control range of the injection time of the injection valve is limited to a minimum injection time or more. In the case where a target fuel injection quantity determined by an intake air quantity and a target air-fuel ratio is less than a minimum injection quantity determined by the second fuel pressure and the minimum injection time, the control apparatus determines that the fuel injection quantity of the injection valve is excessive with respect to the load ratio of the engine and performs the load ratio increasing process. 
     With such a configuration, even when the injection time of the injection valve is limited to a minimum injection time or more, a fault diagnosis of the fuel pressure sensor can be conducted without deteriorating the emission performance. 
     In some embodiments, the control apparatus continues the load ratio increasing process until the target fuel injection quantity becomes larger than the minimum injection quantity. 
     With such a configuration, the load ratio increasing process can be performed to ensure that the target fuel injection quantity is made larger than the minimum injection quantity. 
     In some embodiments, the first fuel pressure is a control fuel pressure used when a fault diagnosis of the fuel pressure sensor is not conducted. The second fuel pressure is a diagnosis fuel pressure used when a fault diagnosis of the fuel pressure sensor is conducted. 
     With such a configuration, even when the fuel pressure is increased to the diagnosis fuel pressure which is not used under normal control, deterioration of the emission performance can be prevented. 
     In some embodiments, the injection valve is a port injection valve injecting fuel to an intake passage of the engine. 
     With such a configuration, a fault diagnosis of the fuel pressure sensor detecting the fuel pressure of the port injection valve can be conducted without deteriorating the emission performance. 
     The foregoing and other objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a vehicle. 
         FIG. 2  is a diagram showing a configuration of an engine and a fuel supply apparatus. 
         FIG. 3  is a diagram showing injection characteristics of a port injection valve. 
         FIG. 4  is a diagram showing a correlation between fuel pressure P and output voltage V of a low-pressure fuel pressure sensor in the case where the low-pressure fuel pressure sensor is in a normal condition. 
         FIG. 5  is a diagram showing a relation between fuel pressure P, injection time T, and fuel injection quantity Q. 
         FIG. 6  is a diagram for illustrating details of an engine load ratio increasing process. 
         FIG. 7  is a flowchart showing a process procedure in the case where an engine ECU conducts a fault diagnosis of a low-pressure fuel pressure sensor. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will hereinafter be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference characters, and a description thereof will not be repeated. 
     [Basic Configuration of Vehicle] 
       FIG. 1  is a block diagram showing a configuration of a vehicle  1 . Referring to  FIG. 1 , vehicle  1  includes an engine  10 , a fuel supply apparatus  15 , motor generators  20 ,  30 , a power split device  40 , a reduction mechanism  58 , drive wheels  62 , a power control unit (PCU)  60 , a battery  70 , and a control apparatus  100 . 
     This vehicle  1  is a series-parallel-type hybrid vehicle and configured to travel with at least one of engine  10  and motor generator  30  acting as a driving source. 
     Engine  10 , motor generator  20 , and motor generator  30  are coupled to each other via power split device  40 . To a rotational shaft  16  of motor generator  30  which is coupled to power split device  40 , reduction mechanism  58  is connected. Rotational shaft  16  is coupled via reduction mechanism  58  to drive wheels  62  and also coupled via power split device  40  to a crankshaft of engine  10 . 
     Power split device  40  is a planetary gear train including a sun gear, a pinion gear, a carrier, and a ring gear. The pinion gear meshes with the sun gear and the ring gear. The carrier supports the pinion gear so that the pinion gear can rotate about its axis, and is coupled to engine  10 . The sun gear is coupled to motor generator  20 . The ring gear is coupled via rotational shaft  16  to motor generator  30  and drive wheels  62 . 
     Power split device  40  has a characteristic that the rotational speed of the sun gear, the rotational speed of the carrier, and the rotational speed of the ring gear have a relation therebetween represented by a straight line connecting these rotational speeds together in a nomographic chart (a relation that in the case where two of respective values of the rotational speeds are determined, the remaining value is accordingly determined). Therefore, an appropriate adjustment of the rotational speed of motor generator  20  which is coupled to the sun gear allows power split device  40  to function as an electrical continuously-variable transmission capable of continuously varying the ratio between the rotational speed (namely vehicle speed) of drive wheels  62  coupled to the ring gear and the rotational speed of engine  10  coupled to the carrier. 
     While the present embodiment will be described regarding the case where the present disclosure is applied to a hybrid vehicle including power split device  40  (electrical continuously-variable transmission), the vehicle to which the present disclosure is applicable may be any vehicle having a structure capable of adjusting the engine rotational speed regardless of the vehicle speed. For example, the present disclosure is also applicable to a vehicle including a mechanical continuously-variable transmission between the engine and the drive wheels, for example. 
     Motor generators  20  and  30  are both a well-known synchronous generator motor which can operate as either a generator or a motor. Motor generators  20  and  30  are connected to PCU  60 , and PCU  60  is connected to battery  70 . 
     Control apparatus  100  includes a power management electronic control unit (hereinafter PM-ECU)  140 , an engine electronic control unit (hereinafter engine ECU)  141 , a motor electronic control unit (hereinafter motor ECU)  142 , and a battery electronic control unit (hereinafter battery ECU)  143 . 
     PM-ECU  140  is connected via a communication port (not shown) to engine ECU  141 , motor ECU  142 , and battery ECU  143 . PM-ECU  140  communicates a variety of control signals and data with engine ECU  141 , motor ECU  142 , and battery ECU  143 . 
     Motor ECU  142  is connected to PCU  60  and controls driving of motor generators  20  and  30 . Battery ECU  143  calculates the remaining capacity (hereinafter SOC (State Of Charge)) of battery  70 , based on the integral value of the charging/discharging current of battery  70 . 
     Engine ECU  141  is connected to engine  10  and fuel supply apparatus  15 . Engine ECU  141  receives signals from a variety of sensors detecting the operating condition of engine  10  (such as accelerator pedal position sensor, throttle opening position sensor, engine rotational speed sensor, engine water temperature sensor, and air-fuel ratio sensor), and performs operational control such as fuel injection control, ignition control, and intake air quantity control. 
     For example, engine ECU  141  controls the throttle opening position (intake air quantity) based on the vehicle speed and the accelerator pedal position, for example. Engine ECU  141  also performs feedback control for the fuel injection quantity so that the air-fuel ratio detected by an air-fuel ratio sensor (not shown) provided on an exhaust passage becomes a target air-fuel ratio (stoichiometric air-fuel ratio, for example). For example, in the case where the intake air quantity increases to cause the air-fuel ratio to have a value of a lean air-fuel ratio, with respect to the target air-fuel ratio, engine ECU  141  increases the fuel injection quantity so that the air-fuel ratio becomes closer to the target air-fuel ratio. 
     [Configuration Involved in Fuel Supply] 
       FIG. 2  is a diagram showing a configuration of engine  10  and fuel supply apparatus  15  involved in fuel supply. In the present embodiment, a vehicle to which the present disclosure is applied is a hybrid vehicle in which a dual-injection-type internal combustion engine using in-cylinder injection and port injection in combination, such as for example a four-series-cylinder gasoline engine, is adopted as an internal combustion engine. 
     Referring to  FIG. 2 , engine  10  includes an intake manifold  36 , a throttle valve  37 , an intake port  21 , and four cylinders  11  provided in a cylinder block. 
     In an intake stroke of each cylinder  11 , intake air AIR is drawn from an intake pipe into each cylinder  11  through intake manifold  36  and intake port  21 . 
     The quantity of air drawn into each cylinder  11  (intake air quantity) is adjusted based on the opening position (throttle opening position θ) of throttle valve  37 . Throttle opening position θ is controlled based on a control signal from engine ECU  141 . In the following description, the ratio of the intake air quantity to the quantity of air which can be drawn into each cylinder  11  (quantity of drawable air) is referred to as “engine load ratio.” 
     Fuel supply apparatus  15  includes a low-pressure fuel supply mechanism  50  and a high-pressure fuel supply mechanism  80 . Low-pressure fuel supply mechanism  50  includes a fuel delivery unit  51 , a low-pressure fuel pipe  52 , a low-pressure delivery pipe  53 , a low-pressure fuel pressure sensor  53   a , and a port injection valve  54 . 
     High-pressure fuel supply mechanism  80  includes a high-pressure pump  81 , a check valve  82   a , a high-pressure fuel pipe  82 , a high-pressure delivery pipe  83 , a high-pressure fuel pressure sensor  83   a , and an in-cylinder injection valve  84 . 
     In-cylinder injection valve  84  is an injector for in-cylinder injection having an injection hole portion  84   a  exposed in a combustion chamber of each cylinder  11 . When in-cylinder injection valve  84  is opened, compressed fuel in high-pressure delivery pipe  83  is injected into combustion chamber from injection hole portion  84   a  of in-cylinder injection valve  84 . 
     Engine ECU  141  is configured to include a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), an input interface circuit, and an output interface circuit, for example. Engine ECU  141  receives an engine start/stop command from the PM-ECU in  FIG. 1 , and accordingly controls engine  10  and fuel supply apparatus  15 . 
     Engine ECU  141  calculates a necessary fuel injection quantity for each combustion, based on the accelerator pedal position, the intake air quantity (throttle opening position θ), the engine rotational speed, and the air-fuel ratio, for example. Based on the calculated fuel injection quantity, engine ECU  141  outputs, at an appropriate time, an injection command signal for example to port injection valve  54  and in-cylinder injection valve  84 . 
     While the present embodiment is described regarding the case where low-pressure fuel supply mechanism  50  and high-pressure fuel supply mechanism  80  are provided, the present disclosure is also applicable to a configuration where high-pressure fuel supply mechanism  80  is not provided but low-pressure fuel supply mechanism  50  is provided. 
     In the following, low-pressure fuel supply mechanism  50  will be described in more detail. Fuel delivery unit  51  includes a fuel tank  511 , a feed pump  512 , a suction filter  513 , a fuel filter  514 , and a relief valve  515 . 
     Fuel tank  511  stores fuel, such as gasoline for example, to be consumed by engine  10 . Suction filter  513  prevents foreign matter from being sucked in. Fuel filter  514  removes foreign matter in discharge fuel. 
     Relief valve  515  is opened when the pressure of fuel discharged from feed pump  512  reaches an upper limit pressure, and is kept closed while the fuel pressure is less than the upper limit pressure. 
     Low-pressure fuel pipe  52  connects fuel delivery unit  51  to low-pressure delivery pipe  53 . It should be noted that low-pressure fuel pipe  52  is not limited to the fuel pipe, and may be one member through which a fuel passage is formed, or a plurality of members between which a fuel passage is formed. 
     Low-pressure delivery pipe  53  is connected, at one end along the direction in which cylinders  11  are arranged in series, to low-pressure fuel pipe  52 . To low-pressure delivery pipe  53 , port injection valve  54  is coupled. To low-pressure delivery pipe  53 , a low-pressure fuel pressure sensor  53   a  detecting the inside fuel pressure is attached. 
     Port injection valve  54  is an injector for port injection having an injection hole portion  54   a  exposed in intake port  21  which is associated with each cylinder  11 . Port injection valve  54  is a needle valve which is opened upon being energized through a control signal from engine ECU  141 . When port injection valve  54  is opened, the fuel compressed by feed pump  512  and present in low-pressure delivery pipe  53  is injected from injection hole portion  54   a  into intake port  21 . 
     Based on a command signal transmitted from engine ECU  141 , feed pump  512  is driven and stopped. 
     Feed pump  512  can draw fuel from inside fuel tank  511 , compress the drawn fuel, and then discharge the fuel. Further, under control by engine ECU  141 , feed pump  512  can change the discharge quantity per unit time [m 3 /sec] and the discharge pressure [kPa: kilopascal]. 
     The fact that feed pump  512  is controlled in this way may provide a number of benefits. First of all, in order to prevent the inside fuel from vaporizing when the engine temperature becomes a high temperature, it is necessary to apply a pressure to low-pressure delivery pipe  53  to the extent that does not cause vaporization of the fuel. However, if the applied pressure is too high, a high load is exerted on the pump and thus the energy loss is large. Because the pressure applied for preventing vaporization of the fuel varies depending on the temperature, a minimal pressure can be applied to low-pressure delivery pipe  53  to thereby reduce the energy loss. Further, feed pump  512  can be appropriately controlled so that a quantity of fuel corresponding to the quantity of fuel consumed by the engine is discharged, to thereby save the uselessly applied energy. This is therefore advantageous in term of improvement of fuel economy, as compared with the configuration where the pressure is once applied more than necessary and then a pressure regulator is used to make the pressure constant. 
     [Injection Characteristics of Port Injection Valve] 
       FIG. 3  is a diagram showing injection characteristics of port injection valve  54 . In  FIG. 3 , the horizontal axis represents injection time T (the time for which the valve is opened) of port injection valve  54 , and the vertical axis represents fuel injection quantity Q of port injection valve  54 . In the following, the pressure of fuel in low-pressure delivery pipe  53  that has been compressed by feed pump  512  is referred to as “fuel pressure P.” 
     Fuel injection quantity Q of port injection valve  54  is basically proportional to the product of fuel pressure P and injection time T. Therefore, with respect to constant fuel pressure P, fuel injection quantity Q linearly increases with injection time T. This is a characteristic (linear controllability for fuel injection quantity Q) of port injection valve  54 . 
     However, port injection valve  54  is a needle valve which is opened upon being energized, and the above-described linear controllability for fuel injection quantity Q is lost when the injection time T (energization time) is considerably short, specifically in the region less than a predetermined value T 0 . In view of this, in the present embodiment, the control range of injection time T is limited to the range from a minimum injection time Tmin which is slightly longer than predetermined value T 0 . Accordingly, the linear controllability for fuel injection quantity Q is ensured, and injection time T (energization time) can be controlled to thereby precisely control fuel injection quantity Q. 
     Due to the influence of the fact that the control range of injection time T is limited to at least minimum injection time Tmin, fuel injection quantity Q is also limited to at least minimum injection quantity Qmin which is determined based on fuel pressure P and minimum injection time Tmin. 
     In the present embodiment, fuel pressure P may be switched between a normal control pressure P 1  which is used during normal control (a fault diagnosis of low-pressure fuel pressure sensor  53   a  is not conducted), and a diagnosis pressure P 2  (&gt;P 1 ) which is used during a fault diagnosis of low-pressure fuel pressure sensor  53   a , as will be described later herein. With respect to injection time T kept constant at minimum injection time Tmin, the higher the fuel pressure P, the larger the fuel injection quantity Q. Therefore, as shown in  FIG. 3 , relative to a minimum fuel injection quantity (hereinafter “minimum injection quantity Qmin 1 ”) when fuel pressure P is normal control pressure P 1 , a minimum fuel injection quantity (hereinafter “minimum injection quantity Qmin 2 ”) when fuel pressure P is diagnosis pressure P 2  is larger. 
     [Fault Diagnosis of Low-Pressure Fuel Pressure Sensor  53   a]   
     In order to variably control the supply pressure of fuel (fuel pressure P) generated by feed pump  512 , it is necessary to ensure the reliability of the value detected by low-pressure fuel pressure sensor  53   a  mounted on low-pressure delivery pipe  53  storing fuel for port injection. 
     For this sake, engine ECU  141  in the present embodiment regularly conducts a fault diagnosis of low-pressure fuel pressure sensor  53   a . For the fault diagnosis of low-pressure fuel pressure sensor  53   a , engine ECU  141  increases fuel pressure P to diagnosis pressure P 2  corresponding to the valve-opening pressure of relief valve  515 . Based on whether or not low-pressure fuel pressure sensor  53   a  detects, at this time, a value representing the valve-opening pressure, engine ECU  141  determines whether or not low-pressure fuel pressure sensor  53   a  has a fault. 
       FIG. 4  is a diagram showing a correlation between fuel pressure P [unit: kPa] and output voltage V [unit: V (volt)] of low-pressure fuel pressure sensor  53   a  in the case where low-pressure fuel pressure sensor  53   a  is in a normal condition. As shown in  FIG. 4 , in the case where low-pressure fuel pressure sensor  53   a  is in a normal condition, the higher the fuel pressure P, the higher the output voltage V of low-pressure fuel pressure sensor  53   a.    
     During normal control (in the case where a fault diagnosis of low-pressure fuel pressure sensor  53   a  is not conducted), engine ECU  141  controls feed pump  512  so that fuel pressure P is normal control pressure P 1  (400 kPa for example). At this time, if low-pressure fuel pressure sensor  53   a  is in a normal condition, output voltage V of low-pressure fuel pressure sensor  53   a  is a value representing voltage V 1  corresponding to normal control pressure P 1 . 
     In contrast, in the case where a fault diagnosis of low-pressure fuel pressure sensor  53   a  is to be conducted, engine ECU  141  performs control for increasing fuel pressure P to diagnosis pressure P 2  (650 kPa for example) which is higher than normal control pressure P 1 , by increasing the output of feed pump  512  (this control is hereinafter also referred to as “fuel pressure increasing process”). Specifically, engine ECU  141  performs feed-forward control on feed pump  512  so that fuel pressure P becomes diagnosis pressure P 2 . Diagnosis pressure P 2  is a fuel pressure corresponding to the valve opening pressure of relief valve  515 . 
     If low-pressure fuel pressure sensor  53   a  is in a normal condition during the fuel pressure increasing process, output voltage V of low-pressure fuel pressure sensor  53   a  is a value representing voltage V 2  corresponding to diagnosis pressure P 2 . Thus, based on whether or not the value detected by low-pressure fuel pressure sensor  53   a  during the fuel pressure increasing process is a value around voltage V 2  which corresponds to diagnosis pressure P 2 , engine ECU  141  determines whether or not low-pressure fuel pressure sensor  53   a  has a fault. 
     [Engine Load Ratio Increasing Process during Fault Diagnosis] 
     As described above, in the case where a fault diagnosis of low-pressure fuel pressure sensor  53   a  is to be performed, engine ECU  141  performs the fuel pressure increasing process of increasing fuel pressure P to diagnosis pressure P 2 . However, the increase of fuel pressure P to diagnosis pressure P 2  could cause fuel injection quantity Q to become larger than required, and accordingly deteriorate the emission performance 
       FIG. 5  is a diagram showing a relation between fuel pressure P, injection time T, and fuel injection quantity Q. With the horizontal axis representing fuel pressure P and the vertical axis representing injection time T, fuel injection quantity Q is proportional to the product of fuel pressure P and injection time T, and therefore, a curve for constant fuel injection quantity Q is represented by an inverse proportion curve as shown in  FIG. 5 . 
     In the case where the engine load ratio is low, the intake air quantity is accordingly small. Then, air-fuel ratio control (a process of performing feedback control for fuel injection quantity Q for making the air-fuel ratio equal to a target air-fuel ratio) is done to control fuel injection quantity Q so that fuel injection quantity Q is also small. In the example shown in  FIG. 5 , a case is shown where the engine load ratio is low and fuel injection quantity Q has a predetermined value Q 0  which is relatively low. In order to cause fuel injection quantity Q to be predetermined value Q 0  while fuel pressure P is normal control pressure P 1 , it is necessary that injection time T be time T 1  (the point of intersection of the inverse proportion curve representing Q=Q 0  and P=P 1 ). Although this time T 1  is a relatively short time, time T 1  is slightly higher than minimum injection time Tmin. Thus, engine ECU  141  can set injection time T to time T 1 . 
     When fuel pressure P has been increased from normal control pressure P 1  to diagnosis pressure P 2 , it is necessary, for making fuel injection quantity Q equal to predetermined value Q 0 , to set injection time T to time T 2  (point of intersection of the inverse proportion curve representing Q=Q 0  and P=P 2 ) which is still shorter than time T 1 . However, this time T 2  is shorter than minimum injection time Tmin, and thus injection time T must be set to minimum injection time Tmin which is longer than time T 2 . Consequently, fuel injection quantity Q becomes minimum injection quantity Qmin 2  larger than predetermined value Q 0  and therefore the fuel injection quantity Q becomes excessive with respect to the engine load ratio. Thus, the air-fuel ratio becomes a rich ratio and the emission performance is deteriorated. 
     In view of the above, in the case where the fuel injection quantity Q becomes excessive with respect to the engine load ratio during the fuel pressure increasing process, engine ECU  141  performs control so that the engine load ratio is increased (hereinafter referred to as “engine load ratio increasing process.”). 
       FIG. 6  is a diagram for illustrating details of the engine load ratio increasing process. As shown in  FIG. 6 , with the horizontal axis representing the engine rotational speed and the vertical axis representing the engine load ratio, the engine output is proportional to the product of the engine rotational speed and the engine load ratio, and therefore, a curve for a constant engine output (constant-engine-power curve) is represented by an inverse proportion curve as shown in  FIG. 6 . 
     On such a constant-engine-power curve, engine ECU  141  changes the engine operating point so that the engine rotational speed is decreased and thereby the engine load ratio is increased. Thus, the above-described “engine load ratio increasing process” is a process of changing the engine operating point so that the engine rotational speed is decreased while keeping the engine output as it is, and thereby the engine load ratio is increased, during a fault diagnosis of low-pressure fuel pressure sensor  53   a . The engine load ratio increasing process which increases the engine load ratio causes the intake air quantity to increase and thus changes the air-fuel ratio to a leaner air-fuel ratio, and therefore, the air-fuel ratio is prevented from becoming a rich air-fuel ratio. Consequently, a fault diagnosis of low-pressure fuel pressure sensor  53   a  can be conducted without causing deterioration of the emission performance. 
     It should be noted that the present embodiment can decrease the engine rotational speed during the engine load ratio increasing process, by adjusting the rotational speed of motor generator  20 . 
       FIG. 7  is a flowchart showing a process procedure in the case where engine ECU  141  conducts a fault diagnosis of low-pressure fuel pressure sensor  53   a . This flowchart is repeatedly followed at predetermined intervals while engine ECU  141  is operating. 
     In step (step is hereinafter abbreviated as “S”)  10 , engine ECU  141  determines whether or not a fault diagnosis of low-pressure fuel pressure sensor  53   a  has already been conducted during the current trip. It should be noted that “trip” is a measure representing a single travel, and is typically a period from the time a user starts a vehicle system to the time the user thereafter stops the vehicle system. 
     In the case where a fault diagnosis of low-pressure fuel pressure sensor  53   a  has already been conducted during the current trip (YES in S 10 ), engine ECU  141  directly ends the process without performing the subsequent operations in S 11  to S 15 . In this way, the fault diagnosis of low-pressure fuel pressure sensor  53   a  is prevented from being conducted multiple times during one trip. Namely, the frequency at which the fault diagnosis of low-pressure fuel pressure sensor  53   a  is conducted is once per trip. 
     In the case where a fault diagnosis of low-pressure fuel pressure sensor  53   a  has not yet been conducted during the current trip (NO in S 10 ), engine ECU  141  determines in S 11  whether or not a diagnosis condition is met. This determination is made for the purpose of determining whether or not the current situation is appropriate for the fault diagnosis of low-pressure fuel pressure sensor  53   a . For example, in the case of a highland for example where the atmospheric pressure is lower than a predetermined value, engine ECU  141  determines that the diagnosis condition is not met because it is expected that an accurate result cannot be derived from the diagnosis. In the case where the diagnosis condition is not met (NO in S 11 ), engine ECU  141  directly ends the process without performing the subsequent operations in S 12  to S 15 . 
     In the case where the diagnosis condition is met (YES in S 11 ), engine ECU  141  performs in S 12  the above-described fuel-pressure increasing process. Specifically, engine ECU  141  performs feed-forward control on feed pump  512  so that fuel pressure P is increased from normal control pressure P 1  to diagnosis pressure P 2 . 
     In S 13 , it is determined whether or not a target fuel injection quantity Qtag is larger than a threshold value Qsh. This determination is made for the purpose of determining whether or not the fuel pressure increasing process in S 12  causes the fuel to become excessive due to the increase of fuel pressure P to diagnosis pressure P 2 . In the present embodiment, target fuel injection quantity Qtag is a fuel injection quantity necessary for achieving the target air-fuel ratio with the current throttle opening (intake air quantity) as it is. Namely, target fuel injection quantity Qtag is a value determined by using the intake air quantity and the target air-fuel ratio. Threshold value Qsh is a fuel injection quantity (namely minimum injection quantity Qmin) which is obtained when fuel pressure P is diagnosis pressure P 2  and injection time T is minimum injection time Tmin (see  FIG. 3 ). 
     In the case where target fuel injection quantity Qtag is less than threshold value Qsh, it is necessary, in order to achieve the target air-fuel ratio, that injection time T be less than minimum injection time Tmin. Actually, however, injection time T can only be decreased to minimum injection time Tmin, and therefore, actual fuel injection quantity Q is larger than target fuel injection quantity Qtag. 
     In view of the above, in the case where target fuel injection quantity Qtag is smaller than threshold value Qsh (NO in S 13 ), engine ECU  141  performs in S 14  the above-described engine load ratio increasing process. Specifically, engine ECU  141  changes the engine operating point so that the engine rotational speed is decreased by a predetermined value while keeping the engine output as it is (on the constant-engine-power curve), and thereby the engine load ratio is increased by a predetermined value (see  FIG. 6 ). The engine load ratio increasing process increases the engine load ratio and accordingly increases the intake air quantity, and therefore, target fuel injection quantity Qtag is also increased. 
     The increase of the engine load ratio (increase of target fuel injection quantity Qtag) by the engine load ratio increasing process is continued until target fuel injection quantity Qtag becomes larger than threshold value Qsh (namely until injection time T becomes longer than minimum injection time Tmin) Accordingly, the condition where the fuel is excessive is reliably prevented. 
     In the case where the engine load ratio increasing process causes target fuel injection quantity Qtag to become larger than threshold value Qsh (YES in S 13 ), engine ECU  141  performs a fault diagnosis process for low-pressure fuel pressure sensor  53   a . In this fault diagnosis process, engine ECU  141  determines, in the case where the output of low-pressure fuel pressure sensor  53   a  is a value representing voltage V 2  which corresponds to diagnosis pressure P 2 , low-pressure fuel pressure sensor  53   a  is in a normal condition. Otherwise, engine ECU  141  determines that low-pressure fuel pressure sensor  53   a  has a fault. 
     As seen from the foregoing, in the case where engine ECU  141  in the present embodiment conducts a fault diagnosis of low-pressure fuel pressure sensor  53   a , engine ECU  141  increases fuel pressure P to diagnosis pressure P 2 . However, in the case where the increase of fuel pressure P to diagnosis pressure P 2  causes fuel to become excessive (rich air-fuel ratio), engine ECU  141  changes the engine operating point so that the engine rotational speed is decreased while keeping the engine output as it is, and thereby the engine load ratio is increased. Accordingly, with the engine output kept as it is, the intake air quantity can be increased to suppress the state where the fuel is excessive. As a result, the fault diagnosis of the fuel pressure sensor can be conducted without deteriorating the emission performance. 
     Regarding the above-described embodiment, the above description is given of the case where the engine load ratio increasing process in S 14  is performed after the fuel-pressure increasing process in S 12  is performed. However, the engine load ratio increasing process may be performed before the fuel pressure increasing process is performed. Namely, before the fuel pressure increasing process is performed, it may be predicted whether or not the fuel pressure increasing process will cause target fuel injection quantity Qtag to become smaller than threshold value Qsh. Then, in the case where it is predicted that the fuel pressure increasing process will cause target fuel injection quantity Qtag to become smaller than threshold value Qsh, the engine load ratio increasing process may be performed in advance and thereafter the fuel pressure increasing process may be performed. 
     Although particular embodiments have been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the claimed subject matter.