Patent Publication Number: US-7707874-B2

Title: Misfire determination device and method for internal combustion engine, and vehicle including misfire determination device

Description:
INCORPORATION BY REFERENCE 
   The disclosures of Japanese Patent Application No. 2007-319329 filed on Dec. 11, 2007 and Japanese Patent Application No. 2008-191398 filed on Jul. 24, 2008 including the specifications, drawings and abstracts are incorporated herein by reference in their entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an internal-combustion-engine misfire determination device and method, and to a vehicle including the device, and more specifically, to an internal-combustion-engine misfire determination device and method that determine the occurrence of a misfire in a multi-cylinder internal combustion engine, of which an output shaft is connected, through a torsion element, to a downstream shaft downstream of the torsion element, and to a vehicle including such a misfire determination device. 
   2. Description of the Related Art 
   As this kind of misfire determination device, a device has been proposed that, in a vehicle in which vibration control is performed using an electric motor so as to cancel variation in torque of a crankshaft of an engine, calculates the amount of correction of torque by which the torque output from the electric motor for vibration control using the electric motor is corrected, and detects misfires in the engine based on the amount of correction of torque output from the electric motor (see Japanese Patent Application Publication No. 2001-65402 (JP-A-2001-65402), for example). 
   In a system installed in a vehicle in which the crankshaft of the engine is connected to the downstream side through a torsion element, such as a damper, variation in torque of the crankshaft due to explosive combustion in the engine induces resonance of the torsion element and the downstream components including the torsion element, and the resonance causes variation in rotation of the crankshaft. As a result, even when it is tried to detect the occurrence of a misfire in one of the cylinders of the engine based on the variation in rotation of the crankshaft, the occurrence of a misfire cannot be accurately detected. When vibration control against variation in torque of the crankshaft of the engine is performed using an electric motor, resonance of the torsion element and the downstream components including the torsion element may be induced, and in this case, the accuracy in detecting the occurrence of a misfire in one of the cylinders of the engine is further reduced. 
   SUMMARY OF THE INVENTION 
   The invention provides an internal-combustion-engine misfire determination device and method that determine the occurrence of a misfire in a multi-cylinder internal combustion engine, of which an output shaft is connected, through a torsion element, to a downstream shaft downstream of the torsion element, and provides a vehicle including the device. 
   A first aspect of the invention relates to an internal-combustion-engine misfire determination device for determining an occurrence of a misfire in a multi-cylinder internal combustion engine, of which an output shaft is connected, through a torsion element, to a downstream shaft downstream of the torsion element. The internal-combustion-engine misfire determination device includes: an output-shaft rotational speed detection portion for detecting an output shaft rotational speed that is the rotational speed of the output shaft; a downstream shaft rotational speed detection portion for detecting a downstream shaft rotational speed that is the rotational speed of the downstream shaft; a resonance influence component calculation portion that is configured to acquire the detected output shaft rotational speed and the detected downstream shaft rotational speed, and that calculates a resonance influence component that is the component caused by an influence of resonance due to torsion of the torsion element on the output shaft rotational speed, based on the detected output shaft rotational speed and the detected downstream shaft rotational speed; and a misfire determination portion for determining the occurrence of the misfire in the internal combustion engine based on a rotational speed for determination that is obtained by subtracting the calculated resonance influence component from the detected output shaft rotational speed. 
   With the above configuration, it is possible to accurately determine the occurrence of a misfire in the internal combustion engine based on the output shaft rotational speed and the downstream shaft rotational speed even when the resonance due to torsion of the torsion element occurs. 
   In the internal-combustion-engine misfire determination device according to this aspect, the resonance influence component calculation portion may be configured to perform at least one of acquisition of the detected output shaft rotational speed and acquisition of the detected downstream shaft rotational speed via communication, extract a frequency component caused by the resonance from the acquired output shaft rotational speed, and a frequency component caused by the resonance from the acquired downstream shaft rotational speed, estimate a delay time due to the communication based on a phase difference between the extracted frequency components and the acquired output shaft rotational speed, perform the acquisitions of the output shaft rotational speed and the downstream shaft rotational speed between which a time interval corresponding to the estimated delay time due to the communication is interposed, and calculate the resonance influence component caused by the influence of the resonance on the output shaft rotational speed, based on the output shaft rotational speed and the downstream shaft rotational speed of which the acquisitions are performed with the time interval interposed therebetween. With the above configuration, when at least one of the output shaft rotational speed and the downstream shaft rotational speed is acquired via communication, it is possible to reduce the influence of the delay due to the communication on the accuracy in determining the occurrence of a misfire in the internal combustion engine based on the output shaft rotational speed and the downstream shaft rotational speed. 
   In the internal-combustion-engine misfire determination device according to this aspect, the resonance influence component calculation portion may estimate the delay time due to the communication in such a manner that the delay time is estimated to be longer as timing of ignition by an ignition device capable of performing ignitions independently in each of cylinders of the internal combustion engine is retarded. With the above configuration, it is possible to more properly estimate the delay time due to the communication. 
   In the internal-combustion-engine misfire determination device according to this aspect, the resonance influence component calculation portion may extract, as the frequency component caused by the resonance, a frequency component whose frequency is once per two rotations of the output shaft of the internal combustion engine. 
   In the internal-combustion-engine misfire determination device according to this aspect, the resonance influence component calculation portion may extract the frequency component, caused by the resonance, by applying a filtering process that does not attenuate the frequency component caused by the resonance but attenuates bands other than the resonance frequency. With the above configuration, it is possible to more accurately estimate the delay time due to communication because it is possible to detect the phase difference with the use of the resonance frequency components of the output shaft rotational speed and the downstream shaft rotational speed only. 
   In the internal-combustion-engine misfire determination device according to this aspect, the filtering process may be a process using a band-pass filter. 
   In the internal-combustion-engine misfire determination device according to this aspect, the resonance influence component calculation portion may estimate the delay time due to the communication based on an after-smoothing phase difference obtained by smoothing the phase difference between both the extracted frequency components. With the above configuration, the misfire determination device works even when the internal combustion engine is in a transitional state. 
   In the internal-combustion-engine misfire determination device, the downstream shaft rotational speed detection portion may output the calculated downstream shaft rotational speed to the resonance influence component calculation portion via communication, and the resonance influence component calculation portion may directly acquire the output shaft rotational speed detected by the output shaft rotational speed detection portion without the communication, and acquire the downstream shaft rotational speed detected by the downstream shaft rotational speed detection portion via the communication. 
   In the internal-combustion-engine misfire determination device according to this aspect, the resonance influence component calculation portion may estimate the delay time due to the communication based on combustion conditions in the internal combustion engine. 
   In the internal-combustion-engine misfire determination device according to this aspect, the resonance influence component calculation portion may calculate a torsion angle of the torsion element based on the acquired output shaft rotational speed and the acquired downstream shaft rotational speed, and calculate the resonance influence component based on the calculated torsion angle, a spring constant of the torsion element, and a moment of inertia on the internal combustion engine side of the torsion element. The resonance influence component calculation portion may calculate the torsion angle by integrating a value obtained by subtracting the acquired downstream shaft rotational speed from the acquired output shaft rotational speed, and calculate the resonance influence component by integrating a product of the torsion angle and a constant ratio between the spring constant and the moment of inertia. 
   A second aspect of the invention relates to a vehicle including a multi-cylinder internal combustion engine, of which an output shaft is connected, through a torsion element, to a downstream shaft downstream of the torsion element. The vehicle further includes an output-shaft rotational speed detection means for detecting an output shaft rotational speed that is the rotational speed of the output shaft; a downstream shaft rotational speed detection means for detecting a downstream shaft rotational speed that is the rotational speed of the downstream shaft; a resonance influence component calculation means that is configured to perform acquisition of the detected output shaft rotational speed and acquisition of the detected downstream shaft rotational speed, and calculates the resonance influence component caused by the influence of the resonance on the output shaft rotational speed, based on the acquired output shaft rotational speed and the acquired downstream shaft rotational speed; and a misfire determination means for determining the occurrence of the misfire in the internal combustion engine based on a rotational speed for determination that is obtained by subtracting the calculated resonance influence component from the detected output shaft rotational speed. 
   The above configuration also brings about the effects brought about by the above internal-combustion-engine misfire determination device, that is, it is possible to accurately determine the occurrence of a misfire in the internal combustion engine even when resonance due to torsion of the torsion element occurs, and it is also possible to reduce the influence of the delay due to communication on the accuracy in determining the occurrence of a misfire in the internal combustion engine based on the output shaft rotational speed and the downstream shaft rotational speed when at least one of the output shaft rotational speed and the downstream side shaft rotational speed is acquired via the communication. 
   The vehicle according to this aspect may further include an electric motor that outputs mechanical power to the downstream-side shaft downstream of the torsion element, and the downstream shaft rotational speed detection portion may detect an electric motor rotational speed that is the rotational speed of the electric motor, and convert the detected electric motor rotational speed to obtain the downstream shaft rotational speed. With the above configuration, it is possible to use the high precision sensor for detecting the rotational speed of the electric motor as the downstream shaft rotational speed detection portion. In addition, it is possible to accurately determine the occurrence of a misfire in the internal combustion engine even when vibration control for controlling the vibration caused by the variation in torque on the axle side is performed using the electric motor. 
   The vehicle according to this aspect may further include an electric power/mechanical power input/output device that is connected to the downstream-side shaft and an axle and that receives and outputs mechanical power from and to the downstream-side shaft and the axle side, which involves input and output of electric power and mechanical power to and from the electric power/mechanical power input/output device. The electric motor may be connected to the axle side so as to be able to output mechanical power to the axle side. The downstream-side shaft rotational speed detection portion may detect a drive state in which the electric power/mechanical power input/output device is driven, and perform a calculation based on the detected electric motor rotational speed and the detected drive state to detect the downstream-side shaft rotational speed. With the above configuration, it is possible to accurately determine the occurrence of a misfire in the internal combustion engine even when vibration control for controlling the vibration caused by the variation in torque on the axle side is performed using the electric power/mechanical power input/output device. 
   A third aspect of the invention relates to an internal-combustion-engine misfire determination method of determining an occurrence of a misfire in a multi-cylinder internal combustion engine, of which an output shaft is connected, through a torsion element, to a downstream shaft downstream of the torsion element. The internal-combustion-engine misfire determination method includes: detecting an output shaft rotational speed that is the rotational speed of the output shaft; detecting a downstream shaft rotational speed that is the rotational speed of the downstream shaft; calculating a resonance influence component that is the component caused by an influence of resonance due to torsion of the torsion element on the output shaft rotational speed, based on the detected output shaft rotational speed and the detected downstream shaft rotational speed; and determining the occurrence of the misfire in the internal combustion engine based on a rotational speed for determination that is obtained by subtracting the calculated resonance influence component from the detected output shaft rotational speed. 
   The internal-combustion-engine misfire determination method according to this aspect may include acquiring the detected output shaft rotational speed and the detected downstream shaft rotational speed via communication; extracting a frequency component caused by the resonance from the acquired output shaft rotational speed; extracting a frequency component caused by the resonance from the acquired downstream shaft rotational speed; estimating a delay time due to the communication based on a phase difference between the extracted frequency components and the acquired output shaft rotational speed; performing acquisition of the output shaft rotational speed and acquisition of the downstream shaft rotational speed between which a time interval corresponding to the estimated delay time due to the communication is interposed; and calculating the resonance influence component that is the component caused by the influence of the resonance on the output shaft rotational speed, based on the output shaft rotational speed and the downstream shaft rotational speed of which the acquisitions are performed with the time interval interposed therebetween. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
       FIG. 1  is a configuration diagram showing an outline of a configuration of a hybrid car  20  in which an internal-combustion-engine misfire determination device according to an embodiment of the invention is installed; 
       FIG. 2  is a configuration diagram showing an outline of a configuration of an engine  22  according to the embodiment of the invention; 
       FIG. 3  is a flow chart showing an example of a misfire determination process performed by an engine ECU  24  according to the embodiment of the invention; 
       FIG. 4  is a flow chart showing an example of a rotational-speed-for-determination calculation process performed by the engine ECU  24  according to the embodiment of the invention; 
       FIG. 5  is a flow chart showing an example of a downstream-of-damper rotational speed calculation process performed by an electronic control unit  70  for a hybrid system according to the embodiment of the invention; 
       FIG. 6  is a flow chart showing an example of a communication delay time estimation process performed by the engine ECU  24  according to the embodiment of the invention; 
       FIG. 7  is an explanatory diagram showing an example of characteristics of a band-pass filter according to the embodiment of the invention; 
       FIG. 8  is an explanatory diagram showing an example of a relation between rotational speed Ne of the engine  22  and after-filtering rotational speed FNe according to the embodiment of the invention; 
       FIG. 9  is an explanatory diagram showing a relation between phases of the after-filtering rotational speed FNe and after-filtering rotational speed FNd according to the embodiment of the invention; 
       FIG. 10  is an explanatory diagram showing an example of a base phase difference setting map according to the embodiment of the invention; 
       FIG. 11  is an explanatory diagram showing an example of a correction value setting map according to the embodiment of the invention; 
       FIG. 12  is a flow chart showing an example of a rotational-speed-for-determination calculation process according to the embodiment of the invention; 
       FIG. 13  is a configuration diagram showing an outline of a configuration of a hybrid car  120  according to a modification of the embodiment of the invention; and 
       FIG. 14  is a configuration diagram showing an outline of a configuration of a hybrid car  220  according to a modification of the embodiment of the invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Modes for carrying out the invention will be described below using embodiments. 
     FIG. 1  is a configuration diagram showing an outline of a configuration of a hybrid car  20  in which an internal-combustion-engine misfire determination device is installed, according to an embodiment of the invention. As shown in  FIG. 1 , the hybrid car  20  of this embodiment includes: an engine  22 , a three-axis power distribution/integration mechanism  30  that is connected to a crankshaft  26 , which serves as an output shaft of the engine  22 , through a damper  28 , which serves as a torsion element; a motor MG 1  capable of generating electricity that is connected to the power distribution/integration mechanism  30 ; a speed reduction gear  35  fixed to a ring gear shaft  32   a  that is connected to the power distribution/integration mechanism  30 ; a motor MG 2  connected to the speed reduction gear  35 ; and an electronic control unit  70  for a hybrid system (hereinafter referred to as the hybrid ECU  70 ), which controls the whole vehicle. An electronic control unit  24  for an engine, which mainly controls the engine  22 , a crank position sensor  140 , which detects the rotational position of the crankshaft  26  of the engine  22 , described later, and rotational position detection sensors  43 ,  44 , which detect the rotational positions of the motors MG 1 , MG 2 , function as the misfire determination device for an internal combustion engine of this embodiment. 
   The engine  22  is an eight-cylinder internal combustion engine capable of outputting mechanical power using hydrocarbon fuel, such as gasoline or light oil, for example. As shown in  FIG. 2 , in the engine  22 , air cleaned by an air cleaner  122  is taken in through a throttle valve  124 , the intake air and gasoline are mixed by injecting gasoline from a fuel injection valve  126  provided for each cylinder, the mixture is taken into a combustion chamber through an intake valve  128  and explosively combusted by the electric spark of an ignition plug  130 , and the reciprocation motion of a piston  132  that is pushed down by the energy of the combustion is converted into the rotational motion of the crankshaft  26 . The exhaust gas from the engine  22  is discharged into the outside through a purification device (three-way catalyst)  134  that removes harmful components, such as carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx). 
   The engine  22  is controlled by the electronic control unit  24  for an engine (hereinafter, referred to as the engine ECU  24 ). The engine ECU  24  is a microprocessor including a CPU  24   a  as a main component, and includes, in addition to the CPU  24   a , a ROM  24   b  for storing processing programs, a RAM  24   c  for temporarily storing data, and input and output ports and a communication port (not shown). Supplied to the engine ECU  24  through the input port are signals from various sensors for detecting status values of the engine  22 , that is, a signal indicating the crank position (crank angle CA) from the crank position sensor  140  for detecting the rotational position (crank angle CA) of the crankshaft  26 , a signal indicating the coolant temperature from a coolant temperature sensor  142  for detecting the temperature of coolant of the engine  22 , a signal indicating the cam position from a cam position sensor  144  for detecting the rotational position of a cam shaft for opening/closing the intake valve  128  and exhaust valve  130  for taking and discharging gas into and from the combustion chamber, a signal indicating the throttle position from a throttle valve position sensor  146  for detecting the position of the throttle valve  124 , a signal indicating the intake air amount Q from an air flow meter  148  that is attached in an intake pipe, a signal indicating the temperature of the intake air from a temperature sensor  149  that is also attached in the intake pipe, a signal indicating the air/fuel ratio AF from an air/fuel ratio sensor  135   a , and a signal indicating oxygen concentration from an oxygen sensor  135   b . In addition, output from the engine ECU  24  through the output port are various control signals for driving the engine  22 , that is, for example, a drive signal to be sent to the fuel injection valve  126 , a drive signal to be sent to a throttle motor  136  for adjusting the position of the throttle valve  124 , a control signal to be sent to an ignition coil  138  that is integrated with an igniter, and a control signal to be sent to a variable valve timing mechanism  150  capable of varying the open/close timing of the intake valve  128 . The engine ECU  24  communicates with the hybrid ECU  70 , and controls the operation of the engine  22  using control signals from the hybrid ECU  70 , and at the same time, outputs data concerning the operational status of the engine  22  as needed. The above-described crank position sensor  140  is an electromagnetic pickup sensor having a timing rotor that is fixed so as to rotate in synchronization with the crankshaft  26  and in which teeth are formed at ten-degree intervals and a void corresponding to two teeth is created for detecting the reference position. The crank position sensor  140  generates a shaped wave every time the crankshaft  26  rotates  10  degrees. The engine ECU  24  calculates, as a rotational speed Ne of the engine  22 , the rotational speed during each 30-degree rotation of the crankshaft  26  based on the shaped wave received from the crank position sensor  140 . 
   The power distribution/integration mechanism  30  includes a sun gear  31  that is an external gear, a ring gear  32  that is an internal gear arranged concentrically with the sun gear  31 , a plurality of pinion gears  33  that mesh with the sun gear  31  and the ring gear  32 , and a carrier  34  that rotatably and revolvably supports the plurality of pinion gears  33 . The power distribution/integration mechanism  30  is thus constructed in the form of a planetary gear mechanism that effects differential operation in which the sun gear  31 , the ring gear  32  and the carrier  34  are used as rotary elements. In the power distribution/integration mechanism  30 , the crankshaft  26  of the engine  22  is connected, through the damper  28 , to a carrier shaft  34   a  that is connected to the carrier  34 , the motor MG 1  is connected to the sun gear  31 , and the speed reduction gear  35  is connected to the ring gear  32  through the ring gear shaft  32   a . The power distribution/integration mechanism  30  distributes the mechanical power that is input from the engine  22  through the carrier  34  to the sun gear  31  side and the ring gear  32  side according to the gear ratio when the motor MG 1  functions as an electric generator, and on the other hand, the power distribution/integration mechanism  30  integrates the mechanical power that is input from the engine  22  through the carrier  34  and the mechanical power that is input from the motor MG 1  through the sun gear  31  and outputs the integrated mechanical power to the ring gear  32  side when the motor MG 1  functions as an electric motor. The mechanical power output to the ring gear  32  is ultimately output from the ring gear shaft  32   a  to driving wheels  63   a ,  63   b  of the vehicle through a gear mechanism  60  and a differential gear  62 . 
   The motors MG 1  and MG 2  are known synchronous generator/motors that operate as electric generators and electric motors, and exchange electric power with a battery  50  through inverters  41 ,  42 . Electric power lines  54  that connect the battery  50  and the inverters  41 ,  42  are a positive bus and a negative bus that are shared by the inverters  41 ,  42 , so that another motor can use the electric power generated by one of the motor MG 1  and the motor MG 2 . Thus, the battery  50  is charged by the electric power generated by the motor MG 1  or the motor MG 2  or discharged due to a shortage of electric power. When the input and output of electric power between the motors MG 1 , MG 2  are balanced, the battery  50  is neither charged nor discharged. Driving of the motor MG 1  and driving of the motor MG 2  are both controlled by an electronic control unit  40  for motors (hereinafter, referred to as the motor ECU  40 ). Supplied to the motor ECU  40  are signals required to control driving of the motors MG 1 , MG 2 , that is, for example, signals from the rotational position detection sensors  43 ,  44  for detecting the rotational positions of the rotors of the motors MG 1 , MG 2 , and signals indicating the phase currents applied to the motors MG 1 , MG 2  that are detected by current sensors (not shown). Switching control signals are output from the motor ECU  40  to the inverters  41 ,  42 . The motor ECU  40  communicates with the hybrid ECU  70 , and controls driving of the motors MG 1 , MG 2  using control signals from the hybrid ECU  70 , and at the same time, outputs data concerning the operational status of the motors MG 1 , MG 2  to the hybrid ECU  70  as needed. The rotational position detection sensors  43 ,  44  each include a resolver. The motor ECU  40  calculates the rotational speeds Nm 1 , Nm 2  of the motors MG 1 , MG 2  every predetermined time period (every 50 μs or every 100 μs, for example) based on the signals from the rotational position detection sensors  43 ,  44 . 
   The battery  50  is controlled by the electronic control unit  52  for a battery (hereinafter, referred to as the battery ECU  52 ). Supplied to the battery ECU  52  are signals required to control the battery  50 , that is, a signal indicating the voltage across terminals of the battery  50  that is output from a voltage sensor (not shown) placed between the terminals of the battery  50 , a signal indicating the charging/discharging electric current that is output from a current sensor (not shown) attached to one of the electric power lines  54  connected to the output terminals of the battery  50 , and a signal indicating the battery temperature Tb that is output from a temperature sensor  51  attached to the battery  50 . The battery ECU  52  outputs data concerning conditions of the battery  50  to the hybrid ECU  70  via communication as needed. The battery ECU  52  also calculates the state of charge (SOC) based on the integral value of the charging/discharging electric current that is detected by the current sensor in order to control the battery  50 . 
   The engine ECU  70  is a microprocessor including a CPU  72  as a main component, and includes, in addition to the CPU  72 , a ROM  74  for storing processing programs, a RAM  76  for temporarily storing data, and input and output ports and a communication port (not shown). Supplied to the hybrid ECU  70  through the input port are an ignition signal from an ignition switch  80 , a signal indicating the shift position SP from a shift position sensor  82  that detects the position of a shift lever  81 , a signal indicating the accelerator pedal operation amount Acc from an acceleration pedal position sensor  84  that detects the amount of depression of an accelerator pedal  83 , a signal indicating the brake pedal position BP from a brake pedal position sensor  86  that detects the amount of depression of a brake pedal  85 , and a signal indicating the vehicle speed V from a vehicle speed sensor  88 . The hybrid ECU  70  is connected to the engine ECU  24 , the motor ECU  40 , and the battery ECU  52  via the communication port, and exchanges various control signals and data with the engine ECU  24 , the motor ECU  40 , and the battery ECU  52 . In this embodiment, the communication between the ECUs is performed using a communication protocol called Controller Area Network (CAN). According to CAN, data is transmitted/received in units called frames, and an ID number determining the priority is assigned to each frame, and the ECUs determine whether to receive the data based on the ID. 
   In the hybrid car  20  of the embodiment constructed as described above, the required torque that should be output to the ring gear shaft  32   a  is calculated based on the accelerator pedal operation amount Acc corresponding to the amount of depression of the accelerator pedal  83  by a driver and on the vehicle speed V, and operation of the engine  22 , operation of the motor MG 1 , and operation of the motor MG 2  are controlled such that the required mechanical power corresponding to the required torque is output to the ring gear shaft  32   a . Modes for controlling operation of the engine  22 , the motor MG 1 , and the motor MG 2  include: a torque conversion operation mode in which operation of the engine  22  is controlled such that the mechanical power that fulfills the required mechanical power is output from the engine  22 , operation of the motor MG 1  and operation of the motor MG 2  are controlled such that all the mechanical power output from the engine  22  is subjected to the torque conversion performed by the power distribution/integration mechanism  30 , the motor MG 1 , and the motor MG 2  and is output to the ring gear shaft  32   a ; a charge/discharge operation mode in which operation of the engine  22  is controlled so as to output a mechanical power corresponding to the sum of the required mechanical power and the electric power needed for charging/discharging of the battery  50 , and operation of the motor MG 1  and operation of the motor MG 2  are controlled such that all of or part of the mechanical power output from the engine  22  is subjected to the torque conversion performed by the power distribution/integration mechanism  30 , the motor MG 1  and the motor MG 2 , and the required mechanical power is output to the ring gear shaft  32   a , which involves charging/discharging of the battery  50 ; and a motor operation mode in which operation control is performed such that operation of the engine  22  is stopped and the mechanical power corresponding to the required mechanical power is output from the motor MG 2  to the ring gear shaft  32   a.    
   Next, an operation performed to determine whether there is a misfire in one of cylinders of the engine  22  mounted on the hybrid car  20  of the embodiment constructed as described above will be described.  FIG. 3  is a flow chart showing an example of a misfire determination process performed by the engine ECU  24 . This routine is repeatedly performed every predetermined time period. 
   When the misfire determination process is performed, the CPU  24   a  of the engine ECU  24  acquires a rotational speed Nj(CA) for determination (step S 100 ), and performs a process of calculating a 30-degree rotation time T 30 (CA) that is required for the crankshaft  26  to rotate 30 degrees based on the reciprocal of the acquired rotational speed Nj(CA) for determination (step S 110 ). The rotational speed Nj(CA) for determination is a rotational speed obtained by subtracting a component Nde caused by the influence of resonance (resonance influence component) due to torsion of the damper  28  from the rotational speed Ne of the engine  22 . The rotational speed Nj(CA) for determination is calculated in a process of calculating the rotational speed for determination. For convenience of explanation, the process of calculating the rotational speed Nj(CA) for determination will be described later. 
   Next, the difference (T 30 (ATDC 30 )−T 30 (ATDC 90 )) between the 30-degree rotation time T 30 (ATDC 30 ) at the point 30 degrees after the top dead center of a compression stroke of the cylinder that is the subject of the misfire determination (ATDC 30 ) and T 30 (ATDC 90 ) at the point 90 degrees after the same top dead center (ATDC 90 ) is calculated as a time difference TD 30  (step S 120 ), and the difference (difference between two time differences TD 30 s, the latter of which is calculated 360 degrees after the point at which the former is calculated)(TD 30 −TD 30 (360 degrees ago)) between the calculated time difference TD 30  and the value calculated as the time difference TD 30  360 degrees before the point at which the current time difference TD 30  is calculated, is calculated as a determining value J 30  (step S 130 ). The calculated determining value J 30  is compared with a threshold value Jref (step S 140 ). When the determining value J 30  is greater than the threshold value Jref, it is determined that there is a misfire in the subject cylinder (step S 150 ), and the misfire determination process is exited. When the determining value J 30  is equal to or less than the threshold value Jref, it is determined that there is no misfire in the subject cylinder, and the misfire determination process is exited. Considering the angles relative to the compression top dead center, and the acceleration of the piston  132  due to combustion (explosion) in the engine  22 , it should be understood that the time difference TD 30  has a negative value when the combustion is normal in the cylinder, and has a positive value when there is a misfire in the cylinder. Thus, when the combustion (explosion) in the subject cylinder is normal, the determining value J 30  becomes a value close to zero, and on the other hand, when there is a misfire in the subject cylinder, the determining value J 30  becomes a positive value greater than the absolute value of the time difference TD 30  of the cylinder in which the combustion is normal. Accordingly, when a value close to the absolute value of the time difference TD 30  of the cylinder in which the combustion is normal is set as the threshold value Jref, it is possible to accurately determine the occurrence of a misfire in the subject cylinder. 
   Next, the process of calculating the rotational speed Nj(CA) for determination will be described. In the process of calculating the rotational speed Nj(CA) for determination, as shown in the rotational-speed-for-determination calculation process in  FIG. 4 , the CPU  24   a  of the engine ECU  24  acquires a communication delay time Td (step S 200 ), and acquires, every 30 degrees of rotation of the crankshaft, the crank angle CA, the rotational speed Ne(CA) of the engine  22 , and the rotational speed on the power distribution/integration mechanism  30  side of the damper  28 , that is, a rotational speed Nd(CA) downstream of the damper, which is the rotational speed of the carrier shaft  34   a , with the acquired communication delay time Td taken into account (step S 210 ). The communication delay time Td is the delay time due to communication performed until the downstream-of-damper rotational speed Nd is acquired in step S 210 , and is estimated in a process of estimating the delay time due to communication, based on the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd. Of the rotational speeds Ne of the engine  22  each calculated by the engine ECU  24  every time the crankshaft  26  rotates 30 degrees based on the shaped waves sent from the crank position sensor  140 , the rotational speed at the crank angle CA is acquired as the rotational speed Ne(CA) of the engine  22 . Of the rotational speeds Nd calculated by the hybrid ECU  70  in the downstream-of-damper rotational speed calculation process, the rotational speed Nd at the crank angle CA that is received the communication delay time Td in advance of when the rotational speed Ne(CA) of the engine  22  is received is acquired as the downstream-of-damper rotational speed Nd(CA) via communication. It is preferable that, as the rotational speed Ne of the engine  22 , one that is received when the rotational speed Ne of the engine  22  that is used in the calculation of the communication delay time Td is received be acquired.  FIG. 5  is a flow chart showing an example of the downstream-of-damper rotational speed calculation process.  FIG. 6  is a flow chart showing an example of the communication-delay-time estimation process. The downstream-of-damper rotational speed calculation process shown in  FIG. 5  and the communication-delay-time estimation process shown in  FIG. 6  will be described below in this order. 
   In the process of calculating the downstream-of-damper rotational speed Nd, as shown in the downstream-of-damper rotational speed calculation process shown in  FIG. 5 , the CPU  72  of the hybrid ECU  70  acquires the rotational speeds Nm 1 , Nm 2  of the motors MG 1 , MG 2  (step S 300 ), and calculates the downstream-of-damper rotational speed Nd according to the following equation (1) using the acquired rotational speeds Nm 1 , Nm 2  of the motors MG 1 , MG 2 , the gear ratio p of the power distribution/integration mechanism  30  (the number of teeth of the sun gear/the number of teeth of the ring gear), and the gear ratio Gr of the speed reduction gear  35  (step S 310 ). Then, the calculated downstream-of-damper rotational speed Nd is transmitted to the engine ECU  24  (step S 320 ), and this process is exited. The rotational speeds Nm 1 , Nm 2  that are calculated based on the signals from the rotational position detection sensors  43 ,  44  are acquired via communication. Thus, because the downstream-of-damper rotational speed Nd(CA) that is calculated by the hybrid ECU  70  according to the following equation (1) using the rotational speeds Nm 1 , Nm 2  of the motors MG 1 , MG 2  supplied from the motor ECU  40  via communication is acquired by the engine ECU  24  via communication, communication delay occurs, and the acquired downstream-of-damper rotational speed Nd is different from the actual, or current, downstream-of-damper rotational speed Nd.
 
 Nd=[Nm 2·Gr+ρ· Nm 1]/(1+ρ)   (1)
 
   In the communication-delay-time estimation process, as shown by the communication-delay-time estimation process shown in  FIG. 6 , the CPU  24   a  of the engine ECU  24  acquires the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd (step S 400 ). The acquisitions of the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd have been described above. Subsequently, a rotational speed FNe after filtering is calculated by passing the acquired rotational speed Ne of the engine  22  through a band-pass filter, and a rotational speed FNd after filtering is calculated by passing the acquired downstream-of-damper rotational speed Nd through the same band-pass filter (step S 410 ). The band-pass filter extracts, from the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd, the resonance frequency components caused by torsion of the damper  28 .  FIG. 7  shows an example of the band-pass filter. Assuming that the resonance due to torsion of the damper  28  occurs in the cycle in which misfires occur, that is, the cycle in which the crankshaft  26  rotates twice (half of the rotational cycle), when the rotational speed Ne of the engine  22  is 1000 rpm, a filter that does not damp 8 Hz, which is the resonance frequency, and significantly damps (to one tenth or below, for example) the other bands may be used as the band-pass filter. In this way, it is possible to make the signals indicating the after-filtering rotational speeds FNe, FNd have smooth sinusoidal waveforms with low noise.  FIG. 8  shows an example of the rotational speed Ne of the engine  22  and the after-filtering rotational speed FNe. 
   After the after-filtering rotational speeds FNe, FNd are calculated in this way, the peak point of the after-filtering rotational speed FNe and the peak point of the after-filtering rotational speed FNd are compared to calculate a phase difference Δθ (step S 420 ), and the calculated phase difference Δθ is smoothed to calculate a phase difference Δθsmo after smoothing (step S 430 ). The communication delay time Td is estimated based on the after-smoothing phase difference Δθsmo, the rotational speed Ne of the engine  22 , and the load (the intake air amount Q sent from the air flow meter  148 ) (step S 440 ), and the process is exited. An example of the relations between the phases of the after-filtering rotational speeds FNe, FNd is shown in  FIG. 9 . Because the crankshaft  26  is connected, through the damper  28 , to the carrier shaft  34   a  downstream of the damper  28 , a phase delay of the downstream-of-damper rotational speed Nd relative to the rotational speed Ne of the engine  22  occurs due to physical delay characteristics. Meanwhile, the engine ECU  24  directly calculates the rotational speed Ne of the engine  22  based on the signal from the crank position sensor  140 , and acquires the downstream-of-damper rotational speed Nd that is calculated by the hybrid ECU  70  based on the rotational speeds Nm 1 , Nm 2  of the motors MG 1 , MG 2  supplied from the motor ECU  40  via communication. Thus, the downstream-of-damper rotational speed Nd has a phase delay relative to the rotational speed Ne of the engine  22  caused by the delay due to communication performed until the downstream-of-damper rotational speed Nd is received. The physical delay characteristics of the damper  28  depend only on the rotational speed Ne of the engine  22  (also depend on the load in some cases), and it is therefore possible to determine the characteristics based on the rotational speed Ne of the engine  22  and the load. Thus, by detecting the phase difference between the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd, it is possible to estimate the communication delay time Td based on the phase difference, the rotational speed Ne of the engine  22 , and the load. In this embodiment, because the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd contain much noise, it is not easy to directly detect the phase difference between the rotational speeds Ne, Nd. Thus, the communication delay time Td is estimated by detecting the phase difference Δθ between the rotational speeds FNe, FNd after converting the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd into the after-filtering rotational speeds FNe, FNd having a smooth sinusoidal waveforms with low noise with the use of the band-pass filter that extracts the resonance frequency component caused by torsion of the damper  28 . With regard to the estimation of the communication delay time Td, in this embodiment, the relations between the rotational speed Ne of the engine  22 , the load (intake air amount), and the base phase difference Δθb are empirically determined, the determined relations are stored in the form of maps in the ROM  24   b  in advance, and, when the rotational speed Ne of the engine  22  and the load are given, the corresponding base phase difference Δθb is derived, and the estimation of the communication delay time Td is performed based on the difference between the derived base phase difference Δθb and the calculated after-smoothing phase difference Δθsmo.  FIG. 10  shows an example of the map. The base phase difference Δθb is the empirically-determined phase difference between the after-filtering rotational speeds FNe, FNd that are obtained when the average communication delay time is taken into consideration that is the ordinary delay time due to communication performed until the downstream-of-damper rotational speed Nd is received by the engine ECU  24  while the engine  22  is in a steady operation state. Thus, the difference between the base phase difference Δθb and the after-smoothing phase difference Δθsmo can be considered as the deviation from the average communication delay time, and it is therefore possible to estimate the communication delay time Td based on the sum of the time corresponding to the deviation and the average communication delay time. The reason why the after-smoothing phase difference Δθsmo is used to estimate the communication delay time Td is to obtain a favorable follow-up characteristics with respect to the change in the rotational speed Ne of the engine  22  and the change in the load because the base phase difference Δθb is determined on the assumption that the engine is in a steady operation state. 
   As described above, in the communication-delay-time estimation process shown in  FIG. 6 , the communication delay time Td is estimated using the phase difference between the after-filtering rotational speeds FNe, FNd that are obtained by extracting the resonance frequency components caused by torsion of the damper  28  from the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd. Because resonance occurs in the cycle in which the engine  22  misfires, the communication-delay-time estimation process may be performed only when the determining value J 30  is close to the threshold value Jref, which means that there is a possibility that a misfire in the engine  22  is occurring. In this case, the above-described average communication delay time may be set as the communication delay time Td. 
   When the process returns to step S 210  of the rotational-speed-for-determination calculation process shown in  FIG. 4 , and the crank angle CA, the rotational speed Ne(CA) of the engine  22 , and the downstream-of-damper rotational speed Nd(CA) that is received the estimated communication delay time Td in advance of when the corresponding rotational speed Ne(CA) is received are acquired, the torsion angle θd(CA) of the damper  28  is calculated according to the following equation (2) using the rotational speed Ne(CA) of the engine  22  and the downstream-of-damper rotational speed Nd(CA) (step S 220 ). A noise-containing resonance influence component Nden(CA) containing low-frequency noise is calculated as the influence of resonance of the damper  28  on the rotational speed of the engine  22 , according to the following equation (3) using a constant ratio (K/J) that is the ratio between the spring constant K of the damper  28  and a moment of inertia J on the engine  22  side of the damper  28  and the calculated torsion angle θd(CA) (step S 230 ).
 
 θd (CA)=∫{ Ne (CA)− Nd (CA)} dt    (2)
 
 Nden (CA)=( K/J )·∫θ d (CA) dt    (3)
 
   In order to eliminate low-frequency noise in the noise-containing resonance influence component Nden(CA), the noise-containing resonance influence component Nden(CA) is passed through a high-pass filter to calculate a resonance influence component Nde(CA) (step S 240 ), and the rotational speed Nj(CA) for determination is calculated by subtracting the calculated resonance influence component Nde(CA) from the rotational speed Ne(CA) of the engine  22  (step S 250 ). With regard to the high-pass filter, it suffices that the cut-off frequency is set so that the resonance frequency of the damper  28  is not damped, while the band of frequencies lower than the resonance frequency is damped. When such a high-pass filter is used, it is possible to eliminate the low-frequency components accumulated due to the integrations according to the above-described equations (2) and (3). 
   The rotational speed Nj(CA) for determination that is calculated in the rotational-speed-for-determination calculation process is obtained by subtracting the resonance influence component Nde(CA), which is the component caused by the influence of resonance due to torsion of the damper  28  from the rotational speed detected by the crank position sensor  140  and calculated, that is, the rotational speed Ne of the engine  22  that is the rotational speed subjected to the influence of the resonance due to torsion of the damper  28 . Thus, the rotational speed Nj(CA) for determination reflects only the rotational fluctuation caused by the explosion (combustion) and the misfire in each cylinder of the engine  22 . Thus, when the misfire determination in the engine  22  is performed using the rotational speed Nj(CA) for determination, it is possible to accurately determine the occurrence of a misfire in the engine  22  even when the resonance caused by torsion of the damper  28  is occurring. 
   According to the misfire determination device for an internal combustion engine that is mounted on the hybrid car  20  of the above-described embodiment, the after-filtering rotational speeds FNe, FNd are calculated by passing the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd on the downstream side of the damper  28  through a band-pass filter that extracts, from the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd, the resonance frequency components caused by torsion of the damper  28 , and the delay time Td due to communication performed until the downstream-of-damper rotational speed Nd is received by the engine ECU  24  is estimated based on the phase difference Δθ between the calculated after-filtering rotational speeds FNe, FNd and on the rotational speed Ne of the engine  22 . Then, the rotational speed Ne(CA) of the engine  22  is calculated, and the downstream-of-damper rotational speed Nd(CA) that is received the estimated communication delay time Td in advance of when the corresponding rotational speed Ne(CA) of the engine  22  is received, is acquired. Then, the resonance influence component Nde(CA) is calculated with the use of the rotational speed Ne(CA) of the engine  22  and the downstream-of-damper rotational speed Nd(CA), the rotational speed Nj(CA) for determination is calculated by subtracting the resonance influence component Nde(CA) from the rotational speed Ne(CA) of the engine  22 , and the occurrence of a misfire in the engine  22  is determined based on the rotational speed Nj(CA) for determination. The resonance influence component Nde(CA) can be calculated based on the rotational speed Ne(CA) of the engine  22  and the downstream-of-damper rotational speed Nd(CA), in which the influence of delay due to communication performed until the downstream-of-damper rotational speed Nd is received by the engine ECU  24  is eliminated. Thus, it is possible to accurately determine the occurrence of a misfire in the engine  22  even when there is a resonance due to torsion of the damper  28  by determining the occurrence of a misfire in the engine  22  based on the rotational speed Nj(CA) for determination obtained by subtracting the resonance influence component Nde(CA) from the rotational speed Ne(CA) of the engine  22 . In addition, because the after-smoothing phase difference Δθsmo obtained by smoothing the phase difference Δθ between the after-filtering rotational speeds FNe, FNd is used, it is possible to accurately estimate the communication delay time Td not only when the engine  22  is in a steady state but also when the engine  22  is in a transitional state. 
   In the misfire determination process performed in the misfire determination device for the internal combustion engine mounted on the hybrid car  20  of the above-described embodiment, although it is not assumed that vibration control for controlling the vibration based on the variation in torque of the ring gear shaft  32   a  connected to the axel shaft side is performed using the motors MG 1 , MG 2 , it is possible to determine the occurrence of a misfire in the engine  22  with the use of the above-described misfire determination process even when the vibration control is performed with the use of the motors MG 1 , MG 2 . 
   Although the misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment is configured such that the engine ECU  24 , the motor ECU  40 , and the hybrid ECU  70  can communicate with each other with the use of CAN, the communication is not limited to that using CAN. Other communication systems, such as the communication using Direct Memory Access (DMA), may be used. 
   In the misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment, the engine ECU  24  directly calculates the rotational speed Ne of the engine  22  based on the signal from the crank position sensor  140 , acquires, via communication, the downstream-of-damper rotational speed Nd that is calculated by the hybrid ECU  70  based on the rotational speeds Nm 1 , Nm 2  of the motors MG 1 , MG 2  supplied from the motor ECU  40  via communication, and calculates the rotational speed Nj(CA) for determination with the use of the engine ECU  24  based on such a rotational speed Ne and a downstream-of-damper rotational speed Nd. However, the rotational speed Ne of the engine  22  may be calculated by another ECU and acquired via communication, and the downstream-of-damper rotational speed Nd that is directly calculated may be used. Alternatively, the rotational speed Ne of the engine  22  may be calculated by another ECU and acquired via communication, and the downstream-of-damper rotational speed Nd that is calculated by another ECU than the ECU that calculates the rotational speed Ne may be used. 
   In the misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment, the base phase difference Δθb used in estimating the communication delay time Td is determined on the assumption that the base phase difference Δθb is the phase difference between the after-filtering rotational speeds FNe, FNd in which the average delay time due to communication performed until the downstream-of-damper rotational speed Nd is received by the engine ECU  24  while the engine  22  is in a steady operation state is taken into consideration. However, the phase difference determined may be a phase difference between the after-filtering rotational speeds FNe, FNd obtained when the delay due to communication performed until the downstream-of-damper rotational speed Nd is received by the engine ECU  24  while the engine  22  is in a steady operation is not taken into consideration. In this case, the phase difference between the after-filtering rotational speeds FNe, FNd directly corresponds to the communication delay time Td. 
   Although, in the misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment, the communication delay time Td is estimated based on the phase difference between the after-filtering rotational speeds FNe, FNd, rotational speed Ne of the engine  22 , and the load (intake air amount), the communication delay time Td may be estimated based on the phase difference between the after-filtering rotational speeds FNe, FNd and the rotational speed Ne of the engine  22  without taking account of the load of the engine  22 . 
   Although, in the misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment, the communication delay time Td is estimated using the after-smoothing phase difference Δθsmo that is obtained by smoothing the phase difference Δθ between the after-filtering rotational speeds FNe, FNd, the communication delay time Td may be estimated using the phase difference Δθ that is not smoothed. 
   Although, in the hybrid car  20  of the embodiment, the communication delay time Td is estimated based on the after-smoothing phase difference Δθsmo, the rotational speed Ne of the engine  22 , and the load (the intake air amount Q sent from the air flow meter  148 ), the communication delay time Td may be estimated with the combustion conditions (the timing Tf of ignition by the ignition plug  130 , for example) additionally taken into consideration. In this case, for example, the base phase difference Δθb may be derived based on the rotational speed Ne of the engine  22  and the load (intake air amount Q) as in the case of the embodiment, a correction value Δθα may be set based on the ignition timing Tf, and the communication delay time Td may be estimated based on the sum of the average communication delay time and the time corresponding to the difference between the sum of the after-smoothing phase difference Δθsmo and the derived base phase difference Δθb and the correction value Δθα (Δθb+Δθα) (deviation from the above-described average communication delay time). The correction value Δθα is set by empirically determining the relation between the ignition timing Tf and the correction value Δθα in advance, storing the relation as a correction value setting map, and, when ignition timing Tf is given, deriving the corresponding correction value Δθα from the stored map.  FIG. 11  shows an example of the correction value setting map. In  FIG. 11 , “Tf 1 ” is the ignition timing for maximizing the output torque from the engine  22 . In the example shown in  FIG. 11 , the correction value Δθα is set such that the correction value Δθα linearly increases as the ignition timing Tf is retarded relative to the timing Tf 1 . It is conceivable that the communication delay time Td depends not only on the rotational speed Ne of the engine  22  and the load, but also on the combustion conditions in the engine  22 . Nevertheless, the communication delay time Td can be further accurately estimated by estimating the communication delay time Td with the ignition timing Tf taken into consideration in this way. As a result, even when ignition is performed by the ignition plug  130  at a maximally retarded timing or a timing slightly earlier than the maximally retarded timing in order to warm up the three-way catalyst of the purification device  134 , the communication delay time Td can be further accurately estimated. Although, in this modification, as shown in  FIG. 11 , the correction value Δθα is set such that the correction value Δθα linearly increases as the ignition timing Tf is retarded, the correction value Δθα may be set such that the correction value Δθα increases in a curve or stepwise at 1 or greater intervals as the ignition timing Tf is retarded. In this modification, the communication delay time Td is estimated based on the sum of the average communication delay time and the time corresponding to the difference between the after-smoothing phase difference Δθsmo and the sum of the base phase difference Δθb and the correction value Δθα (Δθb+Δθα) (deviation from the above-described average communication delay time). However, a configuration may be adopted in which the communication delay time Td is estimated based on the after-smoothing phase difference Δθsmo, the rotational speed Ne of the engine  22 , and the load (the intake air amount Q) as in the case of the process of step S 440  of the embodiment, the correction value ΔTd depending on the ignition timing Tf may be set, and the communication delay time Td may be again estimated by adding the correction value ΔTd to the estimated communication delay time Td. In this case, the correction value ΔTd may be set such that the correction value ΔTd increases as the ignition timing is retarded. 
   In the misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment, the torsion angle θd(CA) of the damper  28  is calculated based on the rotational speed Ne(CA) of the engine  22  and the downstream-of-damper rotational speed Nd(CA) on the downstream side of the damper  28 , the noise-containing resonance influence component Nden(CA) is calculated based on the spring constant K, the constant ratio (K/J), and the torsion angle θd(CA), the noise-containing resonance influence component Nde(CA) is passed through a high-pass filter to calculate the resonance influence component Nde(CA), the rotational speed Nj(CA) for determination is calculated by subtracting the resonance influence component Nde(CA) from the rotational speed Ne(CA) of the engine  22 , and the occurrence of a misfire in the engine  22  is determined based on the rotational speed Nj(CA) for determination. However, any calculation method may be used as long as the resonance influence component Nde(CA) is calculated using the rotational speed Ne(CA) of the engine  22  and the downstream-of-damper rotational speed Nd(CA) on the downstream side of the damper  28 . The resonance influence component Nde(CA) does not have to be calculated by passing the noise-containing resonance influence component Nden(CA) through a high-pass filter. 
   Although, in the misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment, the downstream-of-damper rotational speed Nd is calculated based on the rotational speeds Nm 1 , Nm 2  of the motors MG 1 , MG 2 , a rotational speed sensor may be provided for the carrier shaft  34   a  to directly detect the rotational speed of the carrier shaft  34   a , and the detected rotational speed may be used as the downstream-of-damper rotational speed Nd. 
   In the misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment, in the process of calculating the rotational speed Nj(CA) for determination, the noise-containing resonance influence component Nden(CA) is calculated according to the above equation (3) using the torsion angle θd(CA) of the damper  28  that is calculated with the use of the rotational speed Ne(CA) of the engine  22  and the downstream-of-damper rotational speed Nd(CA), and using the constant ratio (K/J) that is the ratio between the spring constant K of the damper  28  and the moment of inertia J on the engine  22  side of the damper  28 . However, the component obtained by reflecting, in the spring force term of the damper  28  calculated according to the equation (3), a gain g and a phase β, which are influences of the damping force term of the damper  28  on the spring force term thereof may be calculated as the noise-containing resonance influence component Nden(CA). A flow chart of the rotational-speed-for-determination calculation process in this case is shown in  FIG. 12 . In this rotational-speed-for-determination calculation process, after the torsion angle θd(CA) of the damper  28  is calculated, the spring force term Nk is calculated that is calculated on the assumption that the left hand side of the equation (3) is the spring force term Nk of the damper  28  (step S 232 ), the gain g and the phase β that are influences of the damping force term of the damper  28  on the spring force term Nk according to the following equations (4) and (5) based on the rotational speed Ne(CA) of the engine  22  (step S 234 ), and the noise-containing resonance influence component Nden(CA) is calculated with the calculated gain g and phase β reflected in the spring force term Nk (step S 236 ). In order to eliminate low-frequency noise in the noise-containing resonance influence component Nden(CA), the noise-containing resonance influence component Nden(CA) is passed through a high-pass filter to calculate a resonance influence component Nde(CA) (step S 240 ), and the rotational speed Nj(CA) for determination is calculated by subtracting the calculated resonance influence component Nde(CA) from the rotational speed Ne(CA) of the engine  22  (step S 250 ). Next, the gain g and the phase β will be described that are influences of the damping force term of the damper  28  on the spring force term Nk. 
   
     
       
         
           
             
               
                 g 
                 = 
                 
                   
                     1 
                     + 
                     
                       
                         
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                             2 
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                             ⁢ 
                             
                                 
                             
                             ⁢ 
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                         2 
                       
                       · 
                       
                         
                           ( 
                           
                             Cdamp 
                             Kdamp 
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
           
             
               
                 β 
                 = 
                 
                   
                     tan 
                     
                       - 
                       1 
                     
                   
                   ⁢ 
                   
                     
                       
                         ( 
                         
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                           ⁢ 
                           
                               
                           
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                         ) 
                       
                       · 
                       Cdamp 
                     
                     Kdamp 
                   
                 
               
             
             
               
                 ( 
                 5 
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   Assume that the rotational angular velocity of the damper  28  that is a component that exerts an influence on the crankshaft  26  is ωe-damp, the angular velocity of the crankshaft  26  is ωe, the angular velocity of the shaft on the downstream side of the damper  28  is ωinp, the rotational angle of the crankshaft  26  is θe, the rotational angle of the shaft downstream of the damper  28  is θinp, the spring constant of the damper  28  is Kdamp, the constant of the damping force term of the damper  28  is Cdamp, and the moment of inertia of on the engine  22  side of the damper  28  is Ie. Then, the component ωe-damp that is an influence of the damper  28  on the crankshaft  26  can be expressed by the equation (6), which can be transformed into the equation (7). The first term on the right hand side of the equation (6)) is the spring force term, and the second term on the right hand side thereof is the damping force term. 
   
     
       
         
           
             
               
                 
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                 ( 
                 6 
                 ) 
               
             
           
           
             
               
                 
                   ω 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   e 
                   ⁢ 
                   
                     - 
                   
                   ⁢ 
                   damp 
                 
                 = 
                 
                   
                     
                       Kdamp 
                       Ie 
                     
                     ⁢ 
                     
                       ∫ 
                       
                         ∫ 
                         
                           
                             ( 
                             
                               
                                 ω 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 inp 
                               
                               - 
                               
                                 ω 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 e 
                               
                             
                             ) 
                           
                           ⁢ 
                           
                             ⅆ 
                             
                               t 
                               2 
                             
                           
                         
                       
                     
                   
                   + 
                   
                     
                       Cdamp 
                       Ie 
                     
                     ⁢ 
                     
                       ∫ 
                       
                         
                           ( 
                           
                             
                               ω 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               inp 
                             
                             - 
                             
                               ω 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               e 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           ⅆ 
                           t 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
         
       
     
   
   When it is assumed that the where, frequency of misfires when there is a misfire in one of the cylinders of the engine  22  is f, the amplitude of the torsional angular velocity of the damper  28  is α, and the torsional angular velocity of the damper  28  is expressed by the equation (8), the equation (7) can be transformed into the equation (9). By comparing the first term, which is the spring force term, on the right hand side of the second line and the third line of the equation (9), the above equations (4) and (5) can be obtained. 
   
     
       
         
           
             
               
                 
                   ( 
                   
                     
                       ω 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       inp 
                     
                     - 
                     
                       ω 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       e 
                     
                   
                   ) 
                 
                 = 
                 
                   α 
                   · 
                   
                     sin 
                     ⁡ 
                     
                       ( 
                       
                         2 
                         ⁢ 
                         π 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         f 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 ( 
                 8 
                 ) 
               
             
           
           
             
               
                 
                   
                     
                       
                         
                           ω 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           e 
                           ⁢ 
                           
                             - 
                           
                           ⁢ 
                           damp 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               Kdamp 
                               Ie 
                             
                             ⁢ 
                             
                               ∫ 
                               
                                 ∫ 
                                 
                                   
                                     ( 
                                     
                                       α 
                                       · 
                                       
                                         sin 
                                         ⁡ 
                                         
                                           ( 
                                           
                                             2 
                                             ⁢ 
                                             π 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             f 
                                           
                                           ) 
                                         
                                       
                                     
                                     ) 
                                   
                                   ⁢ 
                                   
                                     ⅆ 
                                     
                                       t 
                                       2 
                                     
                                   
                                 
                               
                             
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             Cdamp 
                             Ie 
                           
                           ⁢ 
                           
                             ∫ 
                             
                               
                                 ( 
                                 
                                   α 
                                   · 
                                   
                                     sin 
                                     ⁡ 
                                     
                                       ( 
                                       
                                         2 
                                         ⁢ 
                                         π 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         f 
                                       
                                       ) 
                                     
                                   
                                 
                                 ) 
                               
                               ⁢ 
                               
                                 ⅆ 
                                 t 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               Kdamp 
                               Ie 
                             
                             · 
                             
                               
                                 
                                   - 
                                   α 
                                 
                                 · 
                                 
                                   sin 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       2 
                                       ⁢ 
                                       π 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       f 
                                     
                                     ) 
                                   
                                 
                               
                               
                                 
                                   ( 
                                   
                                     2 
                                     ⁢ 
                                     π 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     f 
                                   
                                   ) 
                                 
                                 2 
                               
                             
                           
                           + 
                           
                             
                               Cdamp 
                               Ie 
                             
                             · 
                             
                               
                                 
                                   - 
                                   α 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   cos 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       2 
                                       ⁢ 
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                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       f 
                                     
                                     ) 
                                   
                                 
                               
                               
                                 ( 
                                 
                                   2 
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                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   f 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               - 
                               α 
                             
                             
                               Ie 
                               · 
                               
                                 
                                   ( 
                                   
                                     2 
                                     ⁢ 
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     f 
                                   
                                   ) 
                                 
                                 2 
                               
                             
                           
                           ⁢ 
                           
                             
                               
                                 
                                   Kdamp 
                                   2 
                                 
                                 + 
                                 
                                   
                                     
                                       ( 
                                       
                                         2 
                                         ⁢ 
                                         π 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         f 
                                       
                                       ) 
                                     
                                     2 
                                   
                                   · 
                                   
                                     Cdamp 
                                     2 
                                   
                                 
                               
                             
                             · 
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           sin 
                           ⁡ 
                           
                             ( 
                             
                               
                                 2 
                                 ⁢ 
                                 π 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 f 
                               
                               + 
                               β 
                             
                             ) 
                           
                         
                       
                     
                   
                 
                 ⁢ 
                 
                   
 
                 
                 ⁢ 
                 
                   where 
                   , 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       tan 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       β 
                     
                     = 
                     
                       
                         
                           ( 
                           
                             2 
                             ⁢ 
                             π 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             f 
                           
                           ) 
                         
                         · 
                         Cdamp 
                       
                       Kdamp 
                     
                   
                 
               
             
             
               
                 ( 
                 9 
                 ) 
               
             
           
         
       
     
   
   When it is assumed that misfires consecutively occur in one of the cylinders of the engine  22 , a misfire occurs per two rotations of the crankshaft  26 , and the frequency f of misfires can be calculated as f=Ne/120 using the rotational speed Ne of the engine  22 . Thus, the gain g and the phase β that are influences of the damping force term of the damper  28  on the spring force term Nk can be calculated by substituting, into the equations (4) and (5), the values of the frequency f of misfires calculated using the rotational speed Ne of the engine  22 , the spring constant Kdamp obtained by multiplying the constant ratio (K/J) by the moment of inertia J empirically obtained in advance, and the constant Cdamp empirically obtained in advance. Needless to say, the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd can be replaced by the angular velocity ωe of the crankshaft  26  and the angular velocity ωinp of the shaft downstream of the damper  28  by multiplying the rotational speed Ne(CA) of the engine  22  and the downstream-of-damper rotational speed Nd by a conversion constant, such as 2π/60. 
   By calculating the gain g and the phase β that are influences of the damping force term of the damper  28  on the spring force term Nk, and calculating the noise-containing resonance influence component Nden(CA) with the calculated gain g and phase β reflected in the spring force term Nk, it is possible to more properly calculate the noise-containing resonance influence component Nden(CA), and it is possible to more properly calculate the rotational speed Nj(CA) for determination. As a result, it is possible to more accurately determine the occurrence of a misfire in the engine  22 . 
   The misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment acquires the crank angle CA, the rotational speed Ne(CA) of the engine  22 , and the rotational speeds Nm 1 (CA), Nm 2 (CA) of the motors MG 1 , MG 2  that are sampled every 30 degrees of rotation of the crankshaft, calculates the downstream-of-damper rotational speed Nd(CA) and the resonance influence component Nde(CA), and calculates the rotational speed Nj(CA) for determination. However, the crank angle at which the rotational speed Nj(CA) for determination is calculated is not limited, and therefore, the resonance influence component Nden(CA) and the rotational speed Nj(CA) may be calculated every 10 degrees or 5 degrees of rotation of the crankshaft. 
   In the misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment, the occurrence of a misfire in the engine  22  is determined by determining the 30-degree rotation time T 30 (CA) from the rotational speed Nj(CA) for determination, calculating the time difference TD 30 , which is the difference between the 30-degree rotation time T 30 (ATDC 30 ) at the point 30 degrees after the top dead center of a compression stroke of the cylinder that is the subject of the misfire determination (ATDC 30 ) and T 30 (ATDC 90 ) at the point 90 degrees after the same top dead center (ATDC 90 ), and calculating the determining value J 30 , which is the difference in the time differences TD 30 , the latter of which is calculated 360 degrees after the point at which the former is calculated. However, other calculation methods may be used to determine the occurrence of a misfire in the engine  22  as long as the occurrence of a misfire in the engine  22  is determined using the rotational speed Nj(CA) for determination. 
   Although, in the misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment, the occurrence of a misfire in one of the cylinders of the 8-cylinder engine  22  is determined, the number of cylinders is not limited as long as the device determines the occurrence of a misfire in one of the cylinders of a multi-cylinder engine, that is, for example, the occurrence of a misfire in one of the cylinders of a 6-cylinder engine is determined, or the occurrence of a misfire in one of the cylinders of a 4-cylinder engine is determined. 
   Although, in the misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment, the occurrence of a misfire in the engine  22  in a system in which the motor MG 2  is connected to the ring gear shaft  32   a  through the speed reduction gear  35  is determined, the occurrence of a misfire in the engine  22  in a system in which the motor MG 2  is connected to the ring gear shaft  32   a  through a transmission instead of the speed reduction gear  35  may be determined. Alternatively, the occurrence of a misfire in the engine  22  in a system in which the motor MG 2  is directly connected to the ring gear shaft  32   a  without the speed reduction gear  35  or the transmission interposed therebetween may be determined. 
   The misfire determination device for an internal combustion engine mounted on the hybrid car  20  of the embodiment determines the occurrence of a misfire in the engine  22  of the vehicle provided with the power distribution/integration mechanism  30  and the motor MG 2 , the power distribution/integration mechanism  30  connected to the crankshaft  26  of the engine  22  through the damper  28 , which serves as a torsion element, and connected to the ring gear shaft  32   a  and the rotary shaft of the motor MG 1 , the motor MG 2  connected to the ring gear shaft  32   a  through the speed reduction gear  35 . However, the invention is applicable when the crankshaft of the engine is connected to the downstream side through the damper, which serves as a torsion element. Thus, the occurrence of a misfire in the engine  22  in a system in which the mechanical power from the motor MG 2  is transmitted to the axle (the axle connected to wheels  64   a ,  64   b  in  FIG. 13 ) different from the axle (the axle connected to the wheels  63   a ,  63   b ) to which the ring gear shaft  32   a  is connected, as illustrated by a hybrid car  120  of a modification shown in  FIG. 13 , may be determined. Alternatively, as illustrated by a hybrid car  220  of a modification shown in  FIG. 14 , the engine  22 , in which the occurrence of a misfire is determined, may be provided with a double-rotor generator  230  that has an inner rotor  232  connected to the crankshaft  26  of the engine  22  through the damper  28  and an outer rotor  234  connected to the axle side on which the mechanical power is output to the driving wheels  63   a ,  63   b , and that transmits part of the mechanical power from the engine  22  to the axle side and converts the remaining mechanical power into electric power. In this case, the motor MG 2  may be connected to the axle side through the speed reduction gear  35  or the transmission, or may be connected to the axle side without the speed reduction gear  35  or the transmission interposed therebetween. 
   Relations between the main components of the embodiments and the main elements of the inventions described in the “SUMMARY OF THE INVENTION” section will now be described. In the embodiment, the crank position sensor  140  that detects the rotational position of the crankshaft  26  and the engine ECU  24  that calculates, as the rotational speed Ne of the engine  22 , the rotational speed during each 30-degree rotation of the crankshaft  26  based on the shaped waves received from the crank position sensor  140  are an example of the “output-shaft rotational-speed detection portion”. The rotational position detection sensors  43 ,  44  that detect the rotational positions of the rotors of the motors MG 1 , MG 2 , the motor ECU  40  that calculates the rotational speeds Nm 1 , Nm 2  of the motors MG 1 , MG 2  based on the signals from the rotational speed detection sensors  43 ,  44 , and the hybrid ECU  70  that calculates the downstream-of-damper rotational speed Nd, which is the rotational speed of the carrier shaft  34   a  (an example of the downstream shaft) downstream of the damper  28  based on the rotational speeds Nm 1 , Nm 2  of the motors MG 1 , MG 2  are an example of the “downstream shaft rotational-speed detection portion”. An example of the “resonance-influence component calculation portion” is the engine ECU  24  that performs the steps of S 200  to S 240  shown in  FIG. 4  in which: the engine ECU  24  calculates the after-filtering rotational speeds FNe, FNd that are obtained by extracting the resonance frequency components caused by torsion of the damper  28  from the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd with the use of the band-pass filter; the engine ECU  24  performs the communication-delay-time estimation process shown in  FIG. 6  in which the delay time Td due to communication performed until the downstream-of-damper rotational speed Nd is received by the engine ECU  24  via communication is estimated based on the phase difference between the calculated after-filtering rotational speeds FNe, FNd and on the rotational speed Ne of the engine  22 ; the engine ECU  24  acquires the rotational speed Ne of the engine  22  and the downstream-of-damper rotational speed Nd that is received the estimated communication delay time Td in advance of when the rotational speed Ne of the engine  22  is received; the engine ECU  24  calculates the torsion angle θb of the damper  28  according to the equation (2) using the acquired rotational speed Ne of the engine  22  and downstream-of-damper rotational speed Nd; the engine ECU  24  calculates the noise-containing resonance influence component Nden(CA) containing low-frequency noise as the influence of resonance of the damper  28  on the rotational speed of the engine  22  using a constant ratio (K/J) that is the ratio between the spring constant K of the damper  28  and a moment of inertia J on the engine  22  side of the damper  28  and the torsion angle θd; and the engine ECU  24  calculates the resonance influence component Nde(CA) by eliminating low-frequency noise with the use of a high-pass filter. An example of the “misfire determination portion” is the engine ECU  24  that performs the process of S 250  shown in  FIG. 4 , in which the rotational speed Nj(CA) for determination is calculated by subtracting the resonance influence component Nde(CA) from the rotational speed Ne(CA) of the engine  22 , and that also performs the misfire determination process shown in  FIG. 3  in which the occurrence of a misfire in the engine  22  is determined using the rotational speed Nj(CA) for determination. The motor MG 2  that outputs power to the carrier shaft  34   a  side downstream of the damper  28 , that is, the downstream ring gear shaft  32   a , through the speed reduction gear  35  is an example of the “electric motor”. The power distribution/integration mechanism  30  connected to the carrier shaft  34   a  downstream of the damper  28  and to the axle-side ring gear shaft  32   a  and the motor MG 1  connected to the sun gear  31  of the power distribution/integration mechanism  30  are an example of the “electric power/mechanical power input/output device”. The relations between the main components of the embodiments and the main elements of the inventions described in the “SUMMARY OF THE INVENTION” section do not limit the elements of the inventions described in the “SUMMARY OF THE INVENTION” section because the embodiments are an example for specifically describing a mode for carrying out the inventions described in the “SUMMARY OF THE INVENTION” section. 
   Although the embodiment has been described as the misfire determination device for an internal combustion engine mounted on the hybrid car  20 , the invention may be applied to the misfire determination device for an internal combustion engine mounted on a car that includes neither a vehicle-driving electric motor nor an electric generator. The invention may be applied to the misfire determination device for an internal combustion engine mounted on a vehicle other than automobiles, or a mobile object, such as a boat and ship, or an aircraft, and may also be applied to the misfire determination device for an internal combustion engine installed in a fixed facility. 
   “When at least one of the output shaft rotational speed and the downstream shaft rotational speed is acquired via communication” in the invention includes, in addition to “when the output shaft rotational speed is directly acquired without communication, and the downstream-of-damper rotational speed is acquired via communication”, “when the output shaft rotational speed is acquired via communication, and the downstream-of-damper rotational speed is directly acquired without communication”, and “when the output shaft rotational speed is acquired via communication, and the downstream-of-damper rotational speed is also acquired via communication”. In the latter case, the communication delay time corresponds to the difference in communication delay times. The combustion conditions in the internal combustion engine in the invention include the timing of ignition by the ignition device capable of performing ignitions independently in each of cylinders of the internal combustion engine. 
   While modes for carrying out the invention have been described using the embodiment, the invention is not limited to such an embodiment at all, and the invention can be implemented in various forms without departing from the spirit and scope of the invention.