Misfire determination system and method for internal combustion engine, vehicle including misfire determination system for internal combustion engine, and system for and method of estimating rigidity of torsion element

An internal-combustion-engine misfire determination system includes: detection sections that detects the rotational speeds of the output shaft and the downstream shaft; a rigidity estimation section that performs a rigidity estimation process in which frequency components caused by a resonance due to torsion of the torsion element are extracted from the rotational speeds, and the rigidity of the torsion element is estimated based on a value obtained by comparing amplitudes of both of the extracted frequency components and on the detected output shaft rotational speed; a resonance influence component calculation section that calculates a resonance influence component caused by an influence of the resonance on the output shaft rotational speed; and a misfire determination section that determines 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 output shaft rotational speed.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No 2007-321002 filed on Dec. 12, 2007 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a misfire determination system for an internal combustion engine, a vehicle, a system for estimating the rigidity of a torsion element, a misfire determination method for an internal combustion engine, and a method of estimating the rigidity of a torsion element. More specifically, the invention relates to an internal-combustion-engine misfire determination system for determining the occurrence of a misfire in a multi-cylinder internal combustion engine of which the output shaft is connected, through a torsion element, to a downstream shaft downstream of the torsion element, a vehicle including such a misfire determination system, a system for estimating the rigidity of a torsion element that is interposed between the output shaft of a multi-cylinder internal combustion engine and a downstream shaft, a misfire determination method for determining the occurrence of a misfire in an internal combustion engine, and a method of estimating the rigidity of a torsion element.

2. Description of the Related Art

A misfire determination system for an internal combustion engine has been proposed that, in a vehicle in which vibration control is performed using an electric motor so as to cancel fluctuation in torque (rotational fluctuation) of a crankshaft of an engine, calculates the amount of adjustment of torque that is output from the electric motor for vibration control, and detects misfiring in the engine based on the amount of adjustment of torque made by the electric motor (see Japanese Patent Application Publication No. 2001-65402 (JP-A-2001-65402), for example).

In a misfire determination system installed in a vehicle in which a damper (torsion element) is interposed between the crankshaft of the engine and a downstream shaft, fluctuation 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. The resonance causes rotational fluctuation of the crankshaft, and 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 rotational fluctuation of the crankshaft, the occurrence of a misfire cannot be accurately detected. The rotational fluctuation of the crankshaft that is caused by resonance is affected by the rotational speed of the crankshaft and the rotational speed of the downstream shaft, and in addition, affected by the rigidity (spring constant) of the torsion element. The manufacturing error and the chronological change are large with respect to the spring constant of the torsion element, and it is therefore desirable to accurately estimate the spring constant when the rotational fluctuation of the crankshaft caused by resonance is analyzed.

SUMMARY OF THE INVENTION

Torsion-element rigidity estimation system and method according to the invention accurately estimate a rigidity of a torsion element that is interposed between an output shaft of a multi-cylinder internal combustion engine and a downstream shaft. In addition, internal-combustion-engine misfire determination system and method, and a vehicle according to the invention accurately determine the occurrence of a misfire in an internal combustion engine with the use of such a rigidity of the torsion element.

A first aspect of the invention relates to an internal-combustion-engine misfire determination system. The internal-combustion-engine misfire determination system is an internal-combustion-engine misfire determination system for determining 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. The internal-combustion-engine misfire determination system includes: an output-shaft rotational speed detection section that detects an output shaft rotational speed that is the rotational speed of the output shaft; a downstream shaft rotational speed detection section that detects a downstream shaft rotational speed that is the rotational speed of the downstream shaft; a rigidity estimation section that estimates a rigidity of the torsion element based on the output shaft rotational speed and the downstream shaft rotational speed; and a misfire determination section that determines the occurrence of the misfire in the internal combustion engine based on the detected output shaft rotational speed and the estimated rigidity of the torsion element.

The rigidity estimation section may perform a rigidity estimation process in which a frequency component caused by a resonance due to torsion of the torsion element is extracted from the detected output shaft rotational speed, a frequency component caused by the resonance is extracted from the detected downstream shaft rotational speed, and the rigidity of the torsion element is estimated based on a value obtained by comparing amplitudes of both of the extracted frequency components and on the detected output shaft rotational speed.

The internal-combustion-engine misfire determination system may further include a resonance influence component calculation section that calculates a resonance influence component caused by an influence of the resonance on the output shaft rotational speed based on the estimated rigidity of the torsion element, the detected output shaft rotational speed, and the detected downstream shaft rotational speed, and the misfire determination section determines 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.

According to the above aspect, a frequency component caused by a resonance due to torsion of the torsion element is extracted from an output shaft rotational speed that is the rotational speed of the output shaft, a frequency component caused by the resonance is extracted from a downstream shaft rotational speed that is the rotational speed of the downstream shaft, the rigidity of the torsion element is estimated based on a value obtained by comparing amplitudes of both of the extracted frequency components and on the output shaft rotational speed, a resonance influence component caused by an influence of a resonance on the output shaft rotational speed is calculated based on the estimated rigidity of the torsion element, the output shaft rotational speed and the downstream shaft rotational speed, and the occurrence of the misfire in the internal combustion engine is determined based on a rotational speed for determination that is obtained by subtracting the calculated resonance influence component from the output shaft rotational speed. The rigidity of the torsion element is estimated by extracting the frequency component, caused by the resonance, of the output shaft rotational speed and the frequency component, caused by the resonance, of the downstream shaft rotational speed and comparing the amplitudes of both of the extracted frequency components, so that it is possible to accurately estimate the rigidity of the torsion element even when a manufacturing error and/or a chronological change occurs in the torsion element. In addition, because a resonance influence component caused by the influence of a resonance due to torsion of the torsion element on the output shaft rotational speed is calculated based on the thus estimated rigidity of the torsion element, the output shaft rotational speed and the downstream shaft rotational speed, and the occurrence of the misfire in the internal combustion engine is determined based on a rotational speed for determination that is obtained by subtracting the calculated resonance influence component from the output shaft rotational speed, it is possible to accurately determine the occurrence of a misfire in the internal combustion engine even when the resonance due to torsion of the torsion element occurs.

A second aspect of the invention relates to a vehicle, the 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; and the internal-combustion-engine misfire determination system according to the first aspect.

The above aspect also brings about the effects brought about by the above internal-combustion-engine misfire determination system according to the first aspect, that is, for example, it is possible to accurately estimate the rigidity of the torsion element even when a manufacturing error and/or a chronological change occurs in the torsion element, and 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.

A third aspect of the invention relates to a torsion-element rigidity estimation system. The torsion-element rigidity estimation system estimates a rigidity of a torsion element that is interposed between an output shaft of a multi-cylinder internal combustion engine and a downstream shaft. The torsion-element rigidity estimation system includes: an output-shaft rotational speed detection section that detects an output shaft rotational speed that is the rotational speed of the output shaft; a downstream shaft rotational speed detection section that detects a downstream shaft rotational speed that is the rotational speed of the downstream shaft; and a rigidity estimation section that extracts a resonance frequency component caused by torsion of the torsion element from the detected output shaft rotational speed, extracts the resonance frequency component from the detected downstream shaft rotational speed, and estimates the rigidity of the torsion element based on a value obtained by comparing amplitudes of both of the extracted frequency components and on the detected output shaft rotational speed.

According to the above aspect, a frequency component caused by a resonance due to torsion of the torsion element is extracted from an output shaft rotational speed that is the rotational speed of the output shaft, a frequency component caused by the resonance is extracted from a downstream shaft rotational speed that is the rotational speed of the downstream shaft, and the rigidity of the torsion element is estimated based on a value obtained by comparing amplitudes of both of the extracted frequency components and on the output shaft rotational speed. Thus, it is possible to more properly estimate the rigidity of the torsion element even when a manufacturing error and/or a chronological change occurs in the torsion element.

A fourth aspect of the invention relates to an internal-combustion-engine misfire determination method. The internal-combustion-engine misfire determination method is an internal-combustion-engine misfire determination method for determining 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. The internal-combustion-engine misfire determination method extracts a frequency component caused by a resonance due to torsion of the torsion element from an output shaft rotational speed that is the rotational speed of the output shaft, extracts a frequency component caused by the resonance from a downstream shaft rotational speed that is the rotational speed of the downstream shaft, estimates the rigidity of the torsion element based on a value obtained by comparing amplitudes of both of the extracted frequency components and on the output shaft rotational speed, calculates a resonance influence component caused by an influence of a resonance on the output shaft rotational speed, based on the estimated rigidity of the torsion element, the output shaft rotational speed and the downstream shaft rotational speed, and determines 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 output shaft rotational speed.

According to the above aspect, a frequency component caused by a resonance due to torsion of the torsion element is extracted from an output shaft rotational speed that is the rotational speed of the output shaft, a frequency component caused by the resonance is extracted from a downstream shaft rotational speed that is the rotational speed of the downstream shaft, the rigidity of the torsion element is estimated based on a value obtained by comparing amplitudes of both of the extracted frequency components and on the output shaft rotational speed, a resonance influence component caused by an influence of a resonance on the output shaft rotational speed is calculated based on the estimated rigidity of the torsion element, the output shaft rotational speed and the downstream shaft rotational speed, and the occurrence of the misfire in the internal combustion engine is determined based on a rotational speed for determination that is obtained by subtracting the calculated resonance influence component from the output shaft rotational speed. The rigidity of the torsion element is estimated by extracting the frequency component, caused by the resonance, of the output shaft rotational speed and the frequency component, caused by the resonance, of the downstream shaft rotational speed, and it is therefore possible to accurately estimate the rigidity of the torsion element even when a manufacturing error and/or a chronological change occurs in the torsion element. In addition, because a resonance influence component caused by the influence of a resonance due to torsion of the torsion element on the output shaft rotational speed is calculated based on the thus estimated rigidity of the torsion element, the output shaft rotational speed and the downstream shaft rotational speed, and the occurrence of the misfire in the internal combustion engine is determined based on a rotational speed for determination that is obtained by subtracting the calculated resonance influence component from the output shaft rotational speed, it is possible to accurately determine the occurrence of a misfire in the internal combustion engine even when the resonance due to torsion of the torsion element occurs. A fifth aspect of the invention relates to a torsion-element rigidity estimation method. The torsion-element rigidity estimation method is a torsion-element rigidity estimation method that estimates a rigidity of a torsion element that is interposed between an output shaft of a multi-cylinder internal combustion engine and a downstream shaft. The torsion-element rigidity estimation method extracts a frequency component caused by a resonance due to torsion of the torsion element from an output shaft rotational speed that is the rotational speed of the output shaft, extracts a frequency component caused by the resonance from a downstream shaft rotational speed that is the rotational speed of the downstream shaft, and estimates a rigidity of the torsion element based on a value obtained by comparing amplitudes of both of the extracted frequency components and on the output shaft rotational speed.

According to the above aspect, a frequency component caused by a resonance due to torsion of the torsion element is extracted from an output shaft rotational speed that is the rotational speed of the output shaft, a frequency component caused by the resonance is extracted from a downstream shaft rotational speed that is the rotational speed of the downstream shaft, and the rigidity of the torsion element is estimated based on a value obtained by comparing amplitudes of both of the extracted frequency components and on the output shaft rotational speed. Thus, it is possible to more properly estimate the rigidity of the torsion element even when a manufacturing error and/or a chronological change occurs in the torsion element.

DETAILED DESCRIPTION OF EMBODIMENT

Modes for carrying out the invention will be described below using embodiments.FIG. 1is a configuration diagram showing an outline of a configuration of a hybrid car20in which a misfire determination system for an internal combustion engine according to an embodiment of the invention is installed. As shown inFIG. 1, the hybrid car20of this embodiment includes: an engine22; a three-axis power distribution/integration mechanism30that is connected to a crankshaft26, which serves as an output shaft of the engine22, through a damper28, which serves as a torsion element; a motor MG1capable of generating electricity that is connected to the power distribution/integration mechanism30; a speed reduction gear35fixed to a ring gear shaft32athat is connected to the power distribution/integration mechanism30; a motor MG2connected to the speed reduction gear35; and an electronic control unit70for a hybrid system (hereinafter referred to as the hybrid ECU70), which controls the whole vehicle. An electronic control unit24for an engine, which mainly controls the engine22, a crank position sensor140, which detects the rotational position of the crankshaft26of the engine22, described later, and rotational position detection sensors43,44, which detect the rotational positions of the motors MG1, MG2, function as the misfire determination system for an internal combustion engine of this embodiment.

The engine22is an eight-cylinder internal combustion engine capable of outputting mechanical power using hydrocarbon fuel, such as gasoline or light oil, for example. As shown inFIG. 2, in the engine22, air cleaned by an air cleaner122is taken in through a throttle valve124, the intake air and gasoline are mixed by injecting gasoline from a fuel injection valve126provided for each cylinder, the mixture is taken into a combustion chamber through an intake valve128and explosively combusted by the electric spark of an ignition plug130, and the reciprocation motion of a piston132that is pushed down by the energy of the combustion is converted into the rotational motion of the crankshaft26. The exhaust gas from the engine22is discharged into the outside through a purification device (three-way catalyst)134that removes harmful components, such as carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx).

The engine22is controlled by the electronic control unit24for an engine (hereinafter, referred to as the engine ECU24). The engine ECU24is a microprocessor including a CPU24aas a main component, and includes, in addition to the CPU24a, a ROM24bfor storing processing programs, a RAM24cfor temporarily storing data, and input and output ports and a communication port (not shown). Supplied to the engine ECU24through the input port are signals from various sensors for detecting status values of the engine22, that is, a signal indicating the crank position (crank angle CA) from the crank position sensor140for detecting the rotational position (crank angle CA) of the crankshaft26, a signal indicating the coolant temperature from a coolant temperature sensor142for detecting the temperature of coolant of the engine22, a signal indicating the cam position from a cam position sensor144for detecting the rotational position of a cam shaft for opening/closing the intake valve128and exhaust valve for taking and discharging gas into and from the combustion chamber, a signal indicating the throttle position from a throttle valve position sensor146for detecting the position of the throttle valve124, a signal indicating the intake air amount Q from an air flow meter148that is attached in an intake pipe, a signal indicating the temperature of the intake air from a temperature sensor149that is also attached in the intake pipe, a signal indicating the air/fuel ratio AF from an air/fuel ratio sensor135a, and a signal indicating oxygen concentration from an oxygen sensor135b. On the other hand, output from the engine ECU24through the output port are various control signals for driving the engine22, that is, for example, a drive signal to be sent to the fuel injection valve126, a drive signal to be sent to a throttle motor136for adjusting the position of the throttle valve124, a control signal to be sent to an ignition coil138that is integrated with an igniter, and a control signal to be sent to a variable valve timing mechanism150capable of varying the open/close timing of the intake valve128. The engine ECU24communicates with the hybrid ECU70, and controls the operation of the engine22based on control signals from the hybrid ECU70, and at the same time, outputs data concerning the operational status of the engine22as needed. The above-described crank position sensor140is an electromagnetic pickup sensor having a timing rotor that is fixed so as to rotate in synchronization with the crankshaft26and 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 sensor140generates a shaped wave every time the crankshaft26rotates 10 degrees. The engine ECU24calculates, as a rotational speed Ne of the engine22, the rotational speed during each 30-degree rotation of the crankshaft26based on the shaped waves received from the crank position sensor140.

The power distribution/integration mechanism30includes a sun gear31that is an external gear, a ring gear32that is an internal gear arranged concentrically with the sun gear31, a plurality of pinion gears33that mesh with the sun gear31and the ring gear32, and a carrier34that rotatably and revolvably supports the plurality of pinion gears33. The power distribution/integration mechanism30is thus constructed in the form of a planetary gear mechanism that effects differential operation in which the sun gear31, the ring gear32and the carrier34are used as rotary elements. In the power distribution/integration mechanism30, the crankshaft26of the engine22is connected, through the damper28, to a carrier shaft34athat is connected to the carrier34, the motor MG1is connected to the sun gear31, and the speed reduction gear35is connected to the ring gear32through the ring gear shaft32a. The power distribution/integration mechanism30distributes the mechanical power that is input from the engine22through the carrier34, to the sun gear31side and the ring gear32side according to the gear ratio when the motor MG1functions as an electric generator, and on the other hand, the power distribution/integration mechanism30integrates the mechanical power that is input from the engine22through the carrier34and the mechanical power that is input from the motor MG1through the sun gear31and outputs the integrated mechanical power to the ring gear32side when the motor MG1functions as an electric motor. The mechanical power output to the ring gear32is ultimately output to driving wheels63a,63bof the vehicle through the ring gear shaft32a, a gear mechanism60and a differential gear62.

The motors MG1and MG2are known synchronous generator/motors that operate as electric generators and electric motors, and exchange electric power with a battery50through inverters41,42. Electric power lines54that connect the battery50and the inverters41,42are a positive bus and a negative bus that are shared by the inverters41,42, so that the electric power generated by one of the motor MG1and the motor MG2can be used by the other motor. Thus, the battery50is charged by the electric power generated by the motor MG1or the motor MG2and is discharged when electric power falls short. When the input and output of electric power between the motors MG1, MG2are balanced, the battery50is neither charged nor discharged. Driving of the motor MG1and driving of the motor MG2are both controlled by an electronic control unit40for motors (hereinafter, referred to as the motor ECU40). Supplied to the motor ECU40are signals required to control driving of the motors MG1, MG2, that is, for example, signals from the rotational position detection sensors43,44for detecting the rotational positions of the rotors of the motors MG1, MG2, and signals indicating the phase currents applied to the motors MG1, MG2that are detected by current sensors (not shown). Switching control signals are output from the motor ECU40to the inverters41,42. The motor ECU40communicates with the hybrid ECU70, and controls driving of the motors MG1, MG2based on control signals from the hybrid ECU70, and at the same time, outputs data concerning the operational status of the motors MG1, MG2to the hybrid ECU70as needed. The rotational position detection sensors43,44each include a resolver. The motor ECU40calculates the rotational speeds Nm1, Nm2of the motors MG1, MG2every predetermined time period (every 50 μs or every 100 μs, for example) based on the signals from the rotational position detection sensors43,44.

The battery50is controlled by the electronic control unit52for a battery (hereinafter, referred to as the battery ECU52). Supplied to the battery ECU52are signals required to control the battery50, that is, a signal indicating the voltage across terminals of the battery50that is output from a voltage sensor (not shown) placed between the terminals of the battery50, a signal indicating the charging/discharging electric current that is output from a current sensor (not shown) attached to one of the electric power lines54connected to the output terminals of the battery50, and a signal indicating the battery temperature Tb that is output from a temperature sensor51attached to the battery50. The battery ECU52outputs data concerning conditions of the battery50to the hybrid ECU70via communication as needed. The battery ECU52also 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 battery50.

The engine ECU70is a microprocessor including a CPU72as a main component, and includes, in addition to the CPU72, a ROM74for storing processing programs, a RAM76for temporarily storing data, and input and output ports and a communication port (not shown). Supplied to the hybrid ECU70through the input port are an ignition signal from an ignition switch80, a signal indicating the shift position SP from a shift position sensor82that detects the position of a shift lever81, a signal indicating the accelerator pedal operation amount Acc from an acceleration pedal position sensor84that detects the amount of depression of an accelerator pedal83, a signal indicating the brake pedal position BP from a brake pedal position sensor86that detects the amount of depression of a brake pedal85, and a signal indicating the vehicle speed V from a vehicle speed sensor88. The hybrid ECU70is connected to the engine ECU24, the motor ECU40, and the battery ECU52via the communication port, and exchanges various control signals and data with the engine ECU24, the motor ECU40, and the battery ECU52.

In the hybrid car20of the embodiment constructed as described above, the required torque that should be output to the ring gear shaft32ais calculated based on the accelerator pedal operation amount Acc corresponding to the amount of depression of the accelerator pedal83by a driver and on the vehicle speed V, and operation of the engine22, operation of the motor MG1, and operation of the motor MG2are controlled such that the required mechanical power corresponding to the required torque is output to the ring gear shaft32a. Modes for controlling operation of the engine22, the motor MG1, and the motor MG2include: a torque conversion operation mode in which operation of the engine22is controlled such that the mechanical power corresponding to the required mechanical power is output from the engine22, and operation of the motor MG1and operation of the motor MG2are controlled such that all the mechanical power output from the engine22is subjected to the torque conversion performed by the power distribution/integration mechanism30, the motor MG1, and the motor MG2and is output to the ring gear shaft32a; a charge/discharge operation mode in which operation of the engine22is 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 battery50, and operation of the motor MG1and operation of the motor MG2are controlled such that all of or part of the mechanical power output from the engine22is subjected to the torque conversion performed by the power distribution/integration mechanism30, the motor MG1and the motor MG2, and the required mechanical power is output to the ring gear shaft32a, which involves charging/discharging of the battery50; and a motor operation mode in which operation control is performed such that operation of the engine22is stopped and the mechanical power corresponding to the required mechanical power is output from the motor MG2to the ring gear shaft32a.

Next, an operation performed to determine whether there is a misfire in one of cylinders of the engine22mounted on the hybrid car20of the embodiment constructed as described above will be described.FIG. 3is a flow chart showing an example of a misfire determination process performed by the engine ECU24. This routine is repeatedly performed every predetermined time period.

When the misfire determination process is performed, the CPU24aof the engine ECU24acquires a rotational speed Nj(CA) for determination (step S100), and performs a process of calculating a 30-degree rotation time T30(CA) that is required for the crankshaft26to rotate 30 degrees based on the reciprocal of the acquired rotational speed Nj(CA) for determination (step S110). 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 damper28from the rotational speed Ne of the engine22. The rotational speed Nj(CA) for determination is calculated in a process of calculating the rotational speed for determination shown inFIG. 4as an example. For convenience of explanation, the process of calculating the rotational speed Nj(CA) for determination will be described later.

Next, the difference (T30(ATDC30)−T30(ATDC90)) between the 30-degree rotation time T30(ATDC30) 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 (ATDC30) and T30(ATDC90) at the point 90 degrees after the same top dead center (ATDC90) is calculated as a time difference TD30(step S120), and the difference (difference between two time differences TD30s, the latter of which is calculated 360 degrees after the point at which the former is calculated)(TD30−TD30(360 degrees ago)) between the calculated time difference TD30and the value calculated as the time difference TD30360 degrees before the point at which the current time difference TD30is calculated, is calculated as a determining value J30(step S130). The calculated determining value J30is compared with a threshold value Jref (step S140). When the determining value J30is greater than the threshold value Jref, it is determined that there is a misfire in the subject cylinder (step S150), and the misfire determination process is exited. When the determining value J30is 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 piston132due to combustion (explosion) in the engine22, it should be understood that the time difference TD30has a negative value when the combustion (explosion) 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 J30becomes a value close to zero, and on the other hand, when there is a misfire in the subject cylinder, the determining value J30becomes a positive value greater than the absolute value of the time difference TD30of the cylinder in which the combustion is normal. Accordingly, when a value close to the absolute value of the time difference TD30of 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 inFIG. 4, the CPU24aof the engine ECU24acquires, every 30 degrees of rotation of the crankshaft, the crank angle CA, the rotational speed Ne(CA) of the engine22, and the rotational speed on the power distribution/integration mechanism30side of the damper28, that is, a rotational speed Nd(CA) downstream of the damper, which is the rotational speed of the carrier shaft34a(step S200). Of the rotational speeds Ne of the engine22each calculated by the engine ECU24every time the crankshaft26rotates 30 degrees based on the shaped waves sent from the crank position sensor140, the rotational speed at the crank angle CA is acquired as the rotational speed Ne(CA) of the engine22. Of the rotational speeds calculated by the hybrid ECU70in the downstream-of-damper rotational speed calculation process shown inFIG. 5, the rotational speed at the crank angle CA is acquired as the downstream-of-damper rotational speed Nd(CA) via communication. Next, a method of calculating the downstream-of-damper rotational speed Nd(CA) will be described with reference toFIG. 5.

In the process of calculating the downstream-of-damper rotational speed Nd, as shown in the downstream-of-damper rotational speed calculation process shown inFIG. 5, the CPU72of the hybrid ECU70acquires the rotational speeds Nm1, Nm2of the motors MG1, MG2(step S300), and calculates the downstream-of-damper rotational speed Nd according to the following equation (1) using the acquired rotational speeds Nm1, Nm2of the motors MG1, MG2, the gear ratio ρ of the power distribution/integration mechanism30(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 gear35(step S310). Then, the calculated downstream-of-damper rotational speed Nd is transmitted to the engine ECU24(step S320), and this process is exited. The rotational speeds Nm1, Nm2that are calculated based on the signals from the rotational position detection sensors43,44are acquired via communication.
Nd=[Nm2·Gr+ρ·Nm1]/(1+ρ)  (1)

Returning back toFIG. 4, when the crank angle CA, the rotational speed Ne(CA) of the engine22, and the downstream-of-damper rotational speed Nd(CA) are acquired (step S200), the torsion angle θd(CA) of the damper28is calculated according to the following equation (2) using the rotational speed Ne(CA) of the engine22and the downstream-of-damper rotational speed Nd(CA) (step S210). A noise-containing resonance influence component Nden(CA) containing low-frequency noise is then calculated as the influence of resonance of the damper28on the rotational speed of the engine22, using a constant ratio (K/J) that is the ratio between the spring constant K of the damper28and a moment of inertia J on the engine22side of the damper28and the calculated torsion angle θd(CA) (step S220). The spring constant K that is estimated in a spring constant estimation process shown inFIG. 6is acquired and used in this embodiment. Next, a method of calculating the spring constant K of the damper28will be described with reference toFIG. 6.
θd(CA)=∫{Ne(CA)−Nd(CA)}dt(2)
Nden(CA)=(K/J)·∫θd(CA)dt(3)

In the spring constant estimation process shown inFIG. 6, the CPU24aof the engine ECU24acquires the rotational speed Nd of the engine22and the downstream-of-damper rotational speed Nd (step S400), determines whether the engine22is in steady operation (step S410), and determines whether the acquired rotational speed Ne of the engine22is within a predetermined rotational speed range determined by an upper limit value NH and a lower limit value NL (step S420). Determination as to whether the engine22is in steady operation can be made by determining that the engine22is in steady operation, provided that variation in the rotational speed Ne of the engine22and the load is small for a predetermined period of time (a few seconds, for example). The upper limit value NH and the lower limit value NL, which are used in determining whether the rotational speed Ne of the engine22is within the predetermined rotational speed range, are set as the upper limit value and the lower limit value of the rotational speed range in which the resonance due to torsion of the damper28is caused. For example, the lower limit value NL is set to 1000 rpm or 1500 rpm, and the upper limit value NH is set to 2000 rpm or 2500 rpm. When it is determined that the engine22is not in steady operation (NO in step S410), or it is determined that the rotational speed Ne of the engine22is not within the range determined by the upper limit value NH and the lower limit value NL (No in step S420), it is determined that the situation is not suitable to estimate the spring constant K. In this case, the spring constant K that is estimated in the preceding cycle of the spring constant estimation process is used as the spring constant K (step S430), and the process is exited. On the other hand, when it is determined that the engine22is in steady operation (YES in step S410), and that the rotational speed Ne of the engine22is within the range determined by the upper limit value NH and the lower limit value NL (YES in step S420), a rotational speed FNe after filtering is calculated by passing the acquired rotational speed Ne of the engine22through 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 S440). The band-pass filter extracts, from the rotational speed Ne of the engine22and the downstream-of-damper rotational speed Nd, the frequency components caused by the resonance due to torsion of the damper28.FIG. 7shows an example of the band-pass filter. Assuming that the resonance due to torsion of the damper28occurs in the cycle in which misfires occur, that is, the cycle in which the crankshaft26rotates twice (half of the rotational cycle), when the rotational speed Ne of the engine22is 1000 rpm, a filter that does not attenuate 8 Hz, which is the resonance frequency, and significantly attenuates (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. 8shows an example of the rotational speed Ne of the engine22and the after-filtering rotational speed FNe.

After the after-filtering rotational speeds FNe, FNd are calculated, an amplitude ratio ΔA(=ANe/ANd) that is the ratio of an amplitude ANe of the after-filtering rotational speed FNe to an amplitude ANd of the after-filtering rotational speed FNd is calculated (step S450). A correction coefficient “s” is then set based on the calculated amplitude ratio ΔA and the rotational speed Ne of the engine22(step S460), the value obtained by multiplying a nominal value Kn of the spring constant by the set correction value “s” is set as the spring constant K (step S470), and the process is exited. An example of the relation between the amplitude ANe of the after-filtering rotational speed FNe and the amplitude ANd of the after-filtering rotational speed FNd is shown inFIG. 9. Because the crankshaft26of the engine22is connected to the carrier shaft34athrough the damper28, the fluctuation in torque of the crankshaft26induces the resonance due to torsion of the damper28, and the resonance in turn brings about fluctuation in rotation of the carrier shaft34aand the crankshaft26. Thus, by detecting the ratio between the amplitude of the rotational speed Ne of the engine22and the amplitude of the downstream-of-damper rotational speed Nd, it is possible to estimate the gain characteristics of the damper28, that is, the spring constant K. In this embodiment, the rotational speed Ne of the engine22and the downstream-of-damper rotational speed Nd contain much noise, and it is therefore not easy to detect the actual amplitude ratio of the rotational speeds Ne, Nd. Thus, the spring constant K is estimated by detecting the amplitude ratio ΔA of the after-filtering rotational speeds FNe, FNd after converting the rotational speed Ne of the engine22and the downstream-of-damper rotational speed Nd into the after-filtering rotational speeds FNe, FNd having smooth sinusoidal waveforms with low noise with the use of the band-pass filter that extracts the frequency components caused by the resonance due to torsion of the damper28. In this embodiment, the relation between the amplitude ratio ΔA of the after-filtering rotational speeds FNe, FNd, the rotational speed Ne of the engine22, and the correction coefficient s is determined and stored as a map in the ROM74in advance, and the correction coefficient s is set by, when the amplitude ratio ΔA and the rotational speed Ne of the engine22are given, deriving the corresponding correction coefficient s from the map.FIG. 10shows an example of the map. As shown inFIG. 10, the correction coefficient s is set such that the lower the amplitude ratio ΔA is, the lower the spring constant K is. In this way, even when a manufacturing error and/or a chronological change occurs in the damper28, it is possible to calculate the noise-containing resonance influence component Nden(CA) with the use of a proper spring constant K.

As described above, in the spring constant estimation process shown inFIG. 6, the spring constant K is estimated using the amplitude ratio ΔA of the after-filtering rotational speeds FNe, FNd that are obtained by extracting the frequency components caused by the resonance due to torsion of the damper28from the rotational speed Ne of the engine22and the downstream-of-damper rotational speed Nd. Because resonance occurs in the cycle in which the engine22misfires, the spring constant estimation process may be such that it is performed only when the determining value J30is close to the threshold value Jref and therefore there is a possibility that the engine22is misfiring.

Returning again toFIG. 4, after the noise-containing resonance influence component Nden is calculated in this way, 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 S230), 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 engine22(step S240). With regard to the high-pass filter, it suffices that the cut-off frequency is set so that the resonance frequency of the damper28is not attenuated, while the band of frequencies lower than the resonance frequency is attenuated. 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 damper28from the rotational speed detected by the crank position sensor140and calculated, that is, the rotational speed Ne of the engine22that is the rotational speed subjected to the influence of the resonance due to torsion of the damper28. 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 engine22. Thus, when the misfire determination in the engine22is performed using the rotational speed Nj(CA) for determination, it is possible to accurately determine the occurrence of a misfire in the engine22even when the resonance due to torsion of the damper28is occurring.

According to the misfire determination system for an internal combustion engine that is mounted on the hybrid car20of the above-described embodiment, the after-filtering rotational speeds FNe, FNd are calculated by passing the rotational speed Ne of the engine22and the downstream-of-damper rotational speed Nd on the downstream side of the damper28through a band-pass filter that extracts, from the rotational speed Ne of the engine22and the downstream-of-damper rotational speed Nd, the frequency components caused by the resonance due to torsion of the damper28, and the spring constant K of the damper28is estimated based on the amplitude ratio ΔA of the calculated after-filtering rotational speeds FNe, FNd and on the rotational speed Ne of the engine22. Then, the resonance influence component Nde(CA) is calculated using the rotational speed Ne(CA) of the engine22, the downstream-of-damper rotational speed Nd(CA), the constant ratio (K/J) that is the ratio between the spring constant K of the damper28and the moment of inertia J on the engine22side of the damper28. Then, 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 engine22, and the occurrence of a misfire in the engine22is determined based on the rotational speed Nj(CA) for determination. Thus, even when a manufacturing error and/or a chronological change occurs in the damper28, it is possible to accurately estimate the spring constant K, and it is possible to calculate the resonance influence component Nde(CA) based on the spring constant K that is estimated in this way. In addition, it is possible to more accurately determine the occurrence of a misfire in the engine22even when there is a resonance due to torsion of the damper28by determining the occurrence of a misfire in the engine22based 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 engine22.

Although, in the misfire determination process performed in the misfire determination system for the internal combustion engine mounted on the hybrid car20of the above-described embodiment, it is not assumed that vibration control for controlling the vibration due to the fluctuation in torque of the ring gear shaft32aconnected to the axle shaft side is performed using the motors MG1, MG2, it is possible to determine the occurrence of a misfire in the engine22with the use of the above-described misfire determination process even when the vibration control is performed using the motors MG1, MG2.

In the misfire determination system for an internal combustion engine mounted on the hybrid car20of the embodiment, the spring constant K is not estimated when the engine22is not in steady operation. However, although the accuracy is slightly reduced, the misfire determination system may be configured such that the spring constant K is estimated even when the engine22is not in steady operation.

Although, in the misfire determination system for an internal combustion engine mounted on the hybrid car20of the embodiment, the spring constant K is estimated using the amplitude ratio ΔA(=ANe/ANd) that is the ratio of the amplitude ANe of the after-filtering rotational speed FNe to the amplitude ANd of the after-filtering rotational speed FNd, the spring constant K may be estimated using the difference (=ANd−ANe) between the amplitude ANd of the after-filtering rotational speed FNd and the amplitude ANe of the after-filtering rotational speed FNe.

In the misfire determination system for an internal combustion engine mounted on the hybrid car20of the embodiment, the torsion angle θd (CA) of the damper28is calculated based on the rotational speed Ne(CA) of the engine22and the downstream-of-damper rotational speed Nd(CA) on the downstream side of the damper28, the noise-containing resonance influence component Nden(CA) is calculated based on the spring constant K of the damper28, the constant ratio (K/J), and the torsion angle θd(CA), the calculated noise-containing resonance influence component Nden(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 engine22, and the occurrence of a misfire in the engine22is 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 engine22and the downstream-of-damper rotational speed Nd(CA) on the downstream side of the damper28. 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 system for an internal combustion engine mounted on the hybrid car20of the embodiment, the downstream-of-damper rotational speed Nd is calculated based on the rotational speeds Nm1, Nm2of the motors MG1, MG2, a rotational speed sensor may be provided for the carrier shaft34ato directly detect the rotational speed of the carrier shaft34a, and the detected rotational speed may be used as the downstream-of-damper rotational speed Nd.

In the misfire determination system for an internal combustion engine mounted on the hybrid car20of 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 damper28that is calculated with the use of the rotational speed Ne(CA) of the engine22and 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 damper28and the moment of inertia J on the engine22side of the damper28. However, the component obtained by reflecting, in the spring force term of the damper28calculated according to the equation (3), a gain g and a phase β, which are influences of the damping force term of the damper28on 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 inFIG. 11. In this rotational-speed-for-determination calculation process, after the torsion angle θd(CA) of the damper28is 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 damper28(step S222), the gain g and the phase β that are influences of the damping force term of the damper28on the spring force term Nk according to the following equations (4) and (5) based on the rotational speed Ne(CA) of the engine22(step S224), 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 S226). 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 S230), 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 engine22(step S240). Next, the gain g and the phase β will be described that are influences of the damping force term of the damper28on the spring force term Nk.

Assume that the rotational angular velocity of the damper28that is a component that exerts an influence on the crankshaft26is ωe-damp, the rotational angular velocity of the crankshaft26is ωe, the rotational angular velocity of the shaft on the downstream side of the damper28is ωinp, the rotational angle of the crankshaft26is θe, the rotational angle of the shaft downstream of the damper28is θinp, the spring constant of the damper28is Kdamp, the constant of the damping force term of the damper28is Cdamp, and the moment of inertia on the engine22side of the damper28is Ie. Then, the component ωe-damp that is an influence of the damper28on the crankshaft26can 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.

When it is assumed that the frequency of misfires when there is a misfire in one of the cylinders of the engine22is f, the amplitude of the torsional angular velocity of the damper28is α, and the torsional angular velocity of the damper28is 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 of the equation (9) with the third line thereof, the above equations (4) and (5) can be obtained.
(ωinp−ωe)=α·sin(2πf)  (8)

When it is assumed that misfires consecutively occur in one of the cylinders of the engine22, a misfire occurs per two rotations of the crankshaft26, and the frequency f of misfires can be calculated as f=Ne/120 using the rotational speed Ne of the engine22. Thus, the gain g and the phase β that are influences of the damping force term of the damper28on the spring force term Nk can be calculated by substituting, into the equations (4) and (5), the values of the frequency f of misfires that is calculated using the rotational speed Ne of the engine22, the spring constant Kdamp obtained by multiplying the constant ratio (K/J) by the moment of inertia J empirically obtained in advance, for example, and the constant Cdamp empirically obtained in advance, for example. Needless to say, the rotational speed Ne of the engine22and the downstream-of-damper rotational speed Nd can be replaced by the rotational angular velocity ωe of the crankshaft26and the rotational angular velocity ωinp of the shaft downstream of the damper28by multiplying the rotational speed Ne(CA) of the engine22and 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 damper28on 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 engine22.

The misfire determination system for an internal combustion engine mounted on the hybrid car20of the embodiment acquires, every 30 degrees of rotation of the crankshaft, the crank angle CA, the rotational speed Ne(CA) of the engine22, and the rotational speeds Nm1(CA), Nm2(CA) of the motors MG1, MG2, 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) for determination may be calculated every 10 degrees or 5 degrees of rotation of the crankshaft.

In the misfire determination system for an internal combustion engine mounted on the hybrid car20of the embodiment, the occurrence of a misfire in the engine22is determined by determining the 30-degree rotation time T30(CA) from the rotational speed Nj(CA) for determination, calculating the time difference TD30, which is the difference between the 30-degree rotation time T30(ATDC30) 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 (ATDC30) and T30(ATDC90) at the point 90 degrees after the same top dead center (ATDC90), and calculating the determining value J30, which is the difference in the time differences TD30, the latter of which is calculated 360 degrees after the point at which the former is calculated. However, any calculation method may be used to determine the occurrence of a misfire in the engine22as long as the occurrence of a misfire in the engine22is determined using the rotational speed Nj(CA) for determination.

Although, in the misfire determination system for an internal combustion engine mounted on the hybrid car20of the embodiment, the occurrence of a misfire in one of the cylinders of the 8-cylinder engine22is determined, the number of cylinders is not limited as long as the system determines the occurrence of a misfire in one of the cylinders of a multi-cylinder engine, that is, for example, the system determines the occurrence of a misfire in one of the cylinders of a 6-cylinder engine, or the occurrence of a misfire in one of the cylinders of a 4-cylinder engine.

Although, in the misfire determination system for an internal combustion engine mounted on the hybrid car20of the embodiment, the occurrence of a misfire in the engine22in a system in which the motor MG2is connected to the ring gear shaft32athrough the speed reduction gear35is determined, the occurrence of a misfire in the engine22in a system in which the motor MG2is connected to the ring gear shaft32athrough a transmission instead of the speed reduction gear35may be determined. Alternatively, the engine22, in which the occurrence of a misfire is determined, may have a configuration in which the motor MG2is directly connected to the ring gear shaft32awithout the speed reduction gear35or the transmission interposed therebetween.

The misfire determination system for an internal combustion engine mounted on the hybrid car20of the embodiment determines the occurrence of a misfire in the engine22of the vehicle provided with the power distribution/integration mechanism30and the motor MG2, the power distribution/integration mechanism30connected to the crankshaft26of the engine22through the damper28, which serves as a torsion element, and connected to the ring gear shaft32aand the rotary shaft of the motor MG1, the motor MG2connected to the ring gear shaft32athrough the speed reduction gear35. 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, and therefore, the engine22, in which the occurrence of a misfire is determined, may have a configuration in which the mechanical power from the motor MG2is transmitted to the axle (the axle connected to wheels64a,64binFIG. 12) different from the axle (the axle connected to the wheels63a,63b) to which the ring gear shaft32ais connected, as illustrated by a hybrid car120of a modification shown inFIG. 12. Alternatively, as illustrated by a hybrid car220of a modification shown inFIG. 13, the engine22, in which the occurrence of a misfire is determined, may be provided with a double-rotor generator230that has an inner rotor232connected to the crankshaft26of the engine22through the damper28and an outer rotor234connected to the axle side on which the mechanical power is output to the driving wheels63a,63b, and that transmits part of the mechanical power from the engine22to the axle side and converts the remaining mechanical power into electric power. In this case, the motor MG2may be either connected to the axle side through the speed reduction gear35or the transmission, or connected to the axle side without the speed reduction gear35or 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 sensor140that detects the rotational position of the crankshaft26and the engine ECU24that calculates, as the rotational speed Ne of the engine22, the rotational speed during each 30-degree rotation of the crankshaft26based on the shaped waves received from the crank position sensor140are an example of the “output-shaft rotational-speed detection section”. The rotational position detection sensors43,44that detect the rotational positions of the rotors of the motors MG1, MG2, the motor ECU40that calculates the rotational speeds Nm1, Nm2of the motors MG1, MG2based on the signals from the rotational position detection sensors43,44, and the hybrid ECU70that calculates the downstream-of-damper rotational speed Nd, which is the rotational speed of the carrier shaft34a(an example of the downstream shaft) downstream of the damper28based on the rotational speeds Nm1, Nm2of the motors MG1, MG2are an example of the “downstream shaft rotational-speed detection section”. An example of the “rigidity estimation section” is the engine ECU24that performs the spring constant estimation process shown inFIG. 6in which the after-filtering rotational speeds FNe, FNd obtained by extracting the frequency components caused by the resonance due to torsion of the damper28from the rotational speed Ne of the engine22and the downstream-of-damper rotational speed Nd with the use of the bandpass filter are calculated, and the spring constant K of the damper28is estimated based on the amplitude ratio ΔA of the calculated after-filtering rotational speeds FNe, FNd and the rotational speed Ne of the engine22. An example of the “resonance-influence component calculation section” is the engine ECU24that performs the steps of S200to S230shown inFIG. 4in which: the engine ECU24calculates the torsion angle θb of the damper28according to the equation (2) using the rotational speed Ne of the engine22and the downstream-of-damper rotational speed Nd; the engine ECU24calculates the noise-containing resonance influence component Nden(CA) containing low-frequency noise as the influence of resonance of the damper28on the rotational speed of the engine22using a constant ratio (K/J) that is the ratio between the spring constant K of the damper28and a moment of inertia J on the engine22side of the damper28and the torsion angle θd; and the engine ECU24calculates 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 section” is the engine ECU24that performs the process of S240shown inFIG. 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 engine22, and that also performs the misfire determination process shown inFIG. 3in which the occurrence of a misfire in the engine22is determined using the rotational speed Nj(CA) for determination. The motor MG2that outputs power to the carrier shaft34aside downstream of the damper28, that is, the downstream ring gear shaft32a, through the speed reduction gear35is an example of the “electric motor”. The power distribution/integration mechanism30connected to the carrier shaft34adownstream of the damper28and to the axle-side ring gear shaft32aand the motor MG1connected to the sun gear31of the power distribution/integration mechanism30are an example of the “electric power/mechanical power input/output device”. The relations between the main components of the embodiment 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 merely 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 system for an internal combustion engine mounted on the hybrid car20, the invention may be applied to the misfire determination system 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 system for an internal combustion engine mounted on a vehicle other than cars, or a mobile object, such as a boat, a ship, or an aircraft, and may also be applied to the misfire determination system for an internal combustion engine installed in a fixed facility. Instead of the form of the misfire determination system for an internal combustion engine or the vehicle in which the system is installed, the invention may be implemented in the form of a misfire determination method for an internal combustion engine, in the form of a system for estimating the rigidity of a torsion element, which corresponds to the damper28, or in the form of a method of estimating the rigidity of a torsion element. When the invention is implemented in the form of a system for estimating the rigidity of a torsion element or in the form of a method of estimating the rigidity of a torsion element, instead of applying the invention to a misfire determination system for an internal combustion engine or to a misfire determination method for an internal combustion engine, the invention can be used, for example, to identify the cylinder in which combustion is weak. This is because, when the engine22is idling and there is unevenness in combustion between the cylinder in which combustion is strong and the cylinder in which combustion is weak, the cycle of the unevenness is the same as the misfiring cycle (the cycle in which the crankshaft26rotates twice).

While best modes for carrying out the invention have been described using the embodiments, 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.