Hybrid vehicle

Abstract

Hybrid vehicle (10), comprising: an internal combustion engine (20); an electric motor (40) which is driven by an electric current; a starter motor (26) for starting the internal combustion engine (20); a speed detector (27) for detecting the speed of the internal combustion engine (20), a first power source (71) and a second power source (72); and a switching unit (60) for switching a current supply state between the first power source (71), the second power source (72) and the electric motor (40), wherein the electric motor (40) and the internal combustion engine (20) are connected to each other by means of a wrap-around gear mechanism (21, 41, 42) which has an endless element, in such a way that power can be transferred reciprocally between them and the internal combustion engine (20) rotates together with the electric motor (40) during a rotation of the electric motor (40), the hybrid vehicle (10) further comprises: a control unit (50) configured to control the internal combustion engine (20), the electric motor (40) and the starter, wherein the control unit (50) performs electric driving in which the operation of the internal combustion engine (20) is suspended in order to perform driving using the power of the electric motor (40), and the internal combustion engine (20) is started by driving the starter (26) and injecting fuel when the speed of the internal combustion engine (20) decreases to or below a first threshold value during electric driving, where the first threshold is a rotational speed that is higher than a rotational speed in a stopped state, wherein the switching unit (60) comprises: a first state in which a current is supplied to the electric motor (40) from the first power source (71), and a second state in which the electric motor (40) is supplied with current from the second power source (72), and wherein the control unit (50) switches the switching unit (60) from the second state to the first state when the speed of the internal combustion engine (20) decreases to or below a second threshold higher than the first threshold during electric driving in a state in which the switching unit (60) is in the second state.

Description

BACKGROUND OF THE INVENTION 1. Technical Field

[0001] This invention relates to a hybrid vehicle. 2. State of the art methods

[0002] A hybrid vehicle described in JP 2004-124 914 A was previously known as a conventional hybrid vehicle. The hybrid vehicle described in JP 2004-124 914 A includes an anomaly detection device that determines that a power transmission device is anomalous in a case where the internal combustion engine revolutions per minute (RPM), detected by an internal combustion engine speed detection device, are less than or equal to a predetermined value, and an anomaly of a motor generator is not detected by an electric motor anomaly detection device, even if a motor generator control device transmits a signal to activate the motor generator.Furthermore, the hybrid vehicle described in JP 2004-124 914 A includes a starter-start device that activates a starter for restarting an engine when the anomaly detection device detects that the power transmission device is anomalous. BRIEF SUMMARY OF THE INVENTION

[0003] However, if a power transmission system anomaly occurs while the hybrid vehicle is operating using the motor-generator's power, the hybrid vehicle described in JP 2004-124914A cannot detect the anomaly. For this reason, there are concerns that the hybrid vehicle described in JP 2004-124914A might stop against the driver's intent if a power transmission system anomaly occurs.

[0004] This invention was conceived with regard to the aforementioned problem, and one object of this invention is to provide a hybrid vehicle capable of preventing the vehicle from stopping against the intention of a driver, even if an endless element is cut.

[0005] According to aspects of this invention, a hybrid vehicle is provided which includes: an internal combustion engine, an electric motor driven by an electric current, a starter for starting the internal combustion engine, a speed detector for detecting the speed of the internal combustion engine, a first power source and a second power source, and a switching unit for switching a power supply state between the first power source, the second power source and the electric motor, wherein the electric motor and the internal combustion engine are connected to each other by means of a wrap-around transmission mechanism having an endless element, such that power can be transferred reciprocally between them and the internal combustion engine rotates together with the electric motor when the electric motor is rotating, wherein the hybrid vehicle further includes a control unit configured to control the internal combustion engine,to control the electric motor and the starter, wherein the control unit performs electric driving in which operation of the internal combustion engine is suspended in order to perform driving using the power of the electric motor, and the control unit starts the internal combustion engine by driving the starter and injecting fuel when a speed of the internal combustion engine during electric driving falls to or below a first threshold, wherein the first threshold is a speed higher than a speed in a stopped state, wherein the switching unit has: a first state in which current is supplied to the electric motor from the first power source, and a second state in which current is supplied to the electric motor from the second power source, and wherein the control unit switches the switching unit from the second state to the first state.when the rotational speed of the combustion engine decreases to or below a second threshold higher than the first threshold during electric driving in a state where the switching unit is in the second state.

[0006] As described above, according to this invention described above, it is possible to prevent a vehicle from stopping contrary to the intention of a driver, even if an endless element is cut. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a diagram of a configuration of a hybrid vehicle according to an embodiment of this invention; Fig. Figure 2-1 is a diagram illustrating a first state in which a current is supplied to a lead-acid battery from an integrated starter generator (ISG) and a current is supplied to a Li-battery load from a Li-battery in a switching unit of the hybrid vehicle according to an embodiment of this invention; Fig. Figure 2-2 is a diagram illustrating a second state in which the ISG receives current from a Li-battery and the Li-battery load receives current from the lead-acid battery in the switching unit of the hybrid vehicle according to an embodiment of this invention; and Fig. Figure 3 is a flowchart describing the operation of an electronic control unit (ECU) of the hybrid vehicle according to an embodiment of this invention. DESCRIPTION OF EXECUTION FORMS

[0007] A hybrid vehicle according to embodiments of this invention is a hybrid vehicle comprising: an internal combustion engine, an electric motor driven by an electric current, a starter for starting the internal combustion engine, and a speed detector for detecting the rotational speed of the internal combustion engine, wherein the electric motor and the internal combustion engine are connected to each other by means of a wrap-around transmission mechanism having an endless element, such that power can be transferred reciprocally between them and the internal combustion engine rotates together with the electric motor when the electric motor is rotating, wherein the hybrid vehicle further comprises a control unit configured to control the internal combustion engine, the electric motor, and the starter, wherein the control unit performs electric driving in which the operation of the internal combustion engine is suspended.to enable driving using the power of the electric motor, and the control unit starts the combustion engine by driving the starter and injecting fuel when the rotational speed of the combustion engine during electric driving falls to or below a first threshold, wherein the first threshold is a rotational speed higher than the rotational speed in a stopped state. In this way, according to embodiments of this invention, the hybrid vehicle can prevent the vehicle from stopping against the driver's intention, even if an endless element is cut. EXECUTION FORMS

[0008] The following is a description of the hybrid vehicle according to the embodiments of this invention with reference to the drawings. Fig. 1 to Fig. Figure 3 shows diagrams for a description of the hybrid vehicle according to the embodiments of this invention.

[0009] As in Fig. As illustrated in Figure 1, a hybrid vehicle 10 comprises an engine 20, a transmission 30, a wheel 12, and an electronic control unit (ECU) 50, which comprehensively controls the hybrid vehicle 10. The engine 20 in this embodiment is an internal combustion engine according to this invention. The electronic control unit 50 in this embodiment is a control unit according to this invention.

[0010] The engine 20 has a plurality of cylinders. In this embodiment, the engine 20 is configured to perform a series of four strokes, comprising an intake stroke, a compression stroke, a power stroke, and an exhaust stroke for each cylinder. The engine 20 includes an intake pipe 22 for introducing air into a combustion chamber (not shown).

[0011] A throttle valve 23 is provided in the inlet pipe 22, and the throttle valve 23 regulates the amount (the amount of intake air) of air flowing through the inlet pipe 22. The throttle valve 23 includes an electronically controlled throttle valve that is opened and closed by means of an electric motor (not shown). The throttle valve 23 is electrically connected to the electronic control unit 50, and the degree of throttle valve opening is controlled by the electronic control unit 50.

[0012] In the engine 20, each cylinder is equipped with an injector 24, which injects fuel through an inlet port (not shown) into the combustion chamber, and a spark plug 25, which ignites an air-fuel mixture in the combustion chamber. The injector 24 and the spark plug 25 are electrically connected to the electronic control unit 50. The fuel injection quantity and timing of the injector 24, as well as the ignition timing and discharge of the spark plug 25, are controlled by the electronic control unit 50.

[0013] The motor 20 is equipped with a crankshaft angle sensor 27, which detects engine revolutions per minute (RPM) based on the rotational position of a crankshaft 20A and transmits a detection signal to the electronic control unit 50. The crankshaft angle sensor 27 constitutes a speed detector according to this invention.

[0014] The transmission 30 converts a rotation transmitted by the motor 20 to drive the wheel 12 via a drive shaft 11. The transmission 30 includes a torque converter, a gear mechanism, and a differential mechanism (not shown).

[0015] The torque converter amplifies the torque by converting the rotation transmitted by the motor 20 into torque using a working fluid. A converter clutch (not shown) is provided in the torque converter. When the converter clutch is disengaged, power is transferred via the working fluid back and forth between the motor 20 and the transmission mechanism. When the converter clutch is engaged, power is transferred directly between the motor 20 and the transmission mechanism via the converter clutch.

[0016] The transmission mechanism incorporates a continuously variable transmission (CVT) and performs gear changes automatically in a stepless manner, using a set of pulleys around which a metal belt is wound. Changes in the gear ratio within the transmission 30 and the engagement or disengagement of the torque converter clutch are controlled by the electronic control unit 50.

[0017] The transmission mechanism can correspond to an automatic transmission (a so-called stepped automatic transmission), which performs step-by-step shifting using a planetary gear mechanism. The differential mechanism is connected to a right and a left drive shaft 11 and transmits power shifted by the transmission mechanism to the right and left drive shafts 11, thus enabling differential rotation.

[0018] Alternatively, the transmission 30 can correspond to an automated manual transmission (AMT). An AMT is an automatic transmission that performs automatic shifting by adding an actuator to a manual transmission, which includes a parallel shaft transmission mechanism. If the transmission 30 corresponds to the AMT, a single-plate dry clutch is provided in the transmission 30 instead of the torque converter.

[0019] Alternatively, the transmission 30 can be a dual-clutch transmission (DCT). A DCT is a type of stepped automatic transmission and has two rows of gears, each with its own clutch.

[0020] The hybrid vehicle 10 includes an accelerator pedal opening degree sensor 13A, and the accelerator pedal opening degree sensor 13A detects an actuation degree of an accelerator pedal 13 (hereinafter referred to simply as an "accelerator pedal opening degree") and transmits a detection signal to the electronic control unit 50.

[0021] The hybrid vehicle 10 includes a brake stroke sensor 14A, and the brake stroke sensor 14A detects an actuation extent of a brake pedal 14 (hereinafter referred to simply as a "brake stroke") and transmits a detection signal to the electronic control unit 50.

[0022] The hybrid vehicle 10 includes a vehicle speed sensor 12A. The vehicle speed sensor 12A detects a vehicle speed based on the rotational speed of the wheel 12 and transmits a detection signal to the electronic control unit 50. The detection signal from the vehicle speed sensor 12A is used by the electronic control unit 50 or another control unit to calculate a slip ratio of each wheel 12 with respect to the vehicle speed.

[0023] The hybrid vehicle 10 includes a starter 26. The starter 26 includes an electric motor (not shown) and a drive gear attached to a rotating shaft of the electric motor. Meanwhile, a disc-shaped drive plate is attached to an end section of the crankshaft 20A of the motor 20, and a ring gear is provided on an outer circumferential section of the drive plate. The starter 26 drives the electric motor in response to a command from the electronic control unit 50 and rotates the ring gear by engaging the drive gear and the ring gear to start the motor 20. In this way, the starter 26 starts the motor 20 through the transmission mechanism that includes the drive gear and the ring gear.

[0024] The hybrid vehicle 10 includes an ISG 40. The ISG 40 is a rotating electric machine that integrates a starter for starting the motor 20 and a generator for producing an electric current. The ISG 40 functions as a generator, producing current from an external power source, and as an electric motor, producing power by being supplied with an electric current. The ISG 40 is an electric motor according to this invention.

[0025] The ISG 40 is connected to the motor 20 by a wrap-around gear mechanism comprising a pulley 41, a crank pulley 21, and a belt 42, and transmits power reciprocally to and from the motor 20. More specifically, the ISG 40 has a rotating shaft 40A, and the pulley 41 is attached to the rotating shaft 40A. The crank pulley 21 is attached to the other end of the crankshaft 20A of the motor 20. The belt 42 is wound as an endless element around the crank pulley 21 and the pulley 41. A gear and a chain can be used as the wrap-around gear mechanism. In this case, the endless element corresponds to the chain.

[0026] The ISG 40 is driven as an electric motor to rotate the crankshaft 20A, thereby starting the engine 20. In this embodiment of the hybrid vehicle 10, the ISG 40 and the starter 26 form a starting device for the engine 20. The starter 26 is primarily used for cold starts of the engine 20 based on a starting operation by the driver, and the ISG 40 is primarily used to restart the engine 20 from an idle stop.

[0027] The ISG 40 can perform a cold start of the engine 20. However, the hybrid vehicle 10 includes the starter motor 26 for a reliable cold start of the engine 20. For example, there may be a case where, due to an increase in the viscosity of the lubricating oil during winter in a cold region, a cold start of the engine 20 using the ISG 40 is difficult, or there may be a case where the ISG 40 is defective. Considering such a case, the hybrid vehicle 10 includes both the ISG 40 and the starter motor 26 as starting devices.

[0028] Power generated by the ISG 40 is transferred to the wheel 12 via the crankshaft 20A of the motor 20, the gearbox 30 and the drive shaft 11.

[0029] Furthermore, the rotation of the wheel 12 is transmitted to the ISG 40 via the drive shaft 11, the gearbox 30 and the crankshaft 20A of the motor 20 and is used for regeneration (electricity generation) in the ISG 40.

[0030] Therefore, the hybrid vehicle 10 can not only drive using only the power (engine torque) of the engine 20 (hereinafter also referred to as internal combustion engine driving), but can also perform driving to assist the engine 20 using the power of the ISG 40 (electric motor torque).

[0031] Furthermore, the hybrid vehicle 10 can drive by using only the power of the ISG 40 (hereinafter also referred to as electric driving) in a state where the operation of the engine 20 is suspended by switching from fuel injection to non-injection. During electric driving, the engine 20 rotates together with the ISG 40.

[0032] In this way, the hybrid vehicle 10 is included in a parallel hybrid system that can drive using at least one of the power of the engine 20 and the power of the ISG 40.

[0033] The hybrid vehicle 10 includes a lead-acid battery 71 as a primary power source and a lithium-ion battery 72 as a secondary power source. Both the lead-acid battery 71 and the lithium-ion battery 72 are made of rechargeable secondary batteries. The number of cells, etc., in the lead-acid battery 71 and the lithium-ion battery 72 is specified to generate an output voltage of approximately 12 V.

[0034] The lead-acid battery 71 is made from a lead storage battery using lead for one electrode. The Li-ion battery 72 is made from a lithium-ion secondary battery, which is discharged and charged by the exchange of lithium ions between a positive and a negative electrode.

[0035] In comparison to the Li-battery 72, the lead-acid battery 71 has the characteristic that a higher current can be discharged in a short period of time.

[0036] The Li-ion battery 72 has the characteristic that it can repeat more charge and discharge cycles compared to the lead-acid battery 71. Furthermore, the Li-ion battery 72 can be charged in a shorter time compared to the lead-acid battery 71. In addition, the Li-ion battery 72 has the characteristic of offering higher output power and higher energy density compared to the lead-acid battery 71.

[0037] A state-of-charge detector 71A is provided in the lead-acid battery 71. The state-of-charge detector 71A detects a voltage at the terminals, an ambient temperature, or an input / output current of the lead-acid battery 71 and sends a detection signal to the electronic control unit 50. The electronic control unit 50 uses the voltage at the terminals, the ambient temperature, or the input / output current of the lead-acid battery 71 to detect a state of charge (SOC).

[0038] The lithium battery 72 is equipped with a state-of-charge (SOC) detector 72A. This detector senses a voltage at the terminals, an ambient temperature, or an input / output current of the lithium battery 72 and sends a detection signal to the electronic control unit 50. Using the voltage at the terminals, the ambient temperature, or the input / output current of the lithium battery 72, the electronic control unit 50 determines the state of charge. The SOCs of the lead-acid battery 71 and the lithium battery 72 are managed by the electronic control unit 50.

[0039] The hybrid vehicle 10 includes a lead-acid battery load 16 and a Li-ion battery load 17 as electrical loads.

[0040] The lead-acid battery load 16 is an electrical load primarily supplied with current by the lead-acid battery 71. The lead-acid battery load 16 includes a stability control device to prevent the vehicle from skidding, a control device for an electric power steering system (not shown) that electrically assists the steering force of a steered wheel, a headlight, a blower fan, etc. Furthermore, the lead-acid battery load 16 includes a windshield wiper (not shown) and an electrically driven cooling fan that blows cooling air towards a radiator (not shown). The lead-acid battery load 16 is either an electrical load that consumes more electrical current than the lithium-ion battery load 17 or is a temporarily used electrical load.

[0041] The lithium-ion battery load 17 is an electrical load primarily supplied with electrical current by the lithium-ion battery 72. The lithium-ion battery load 17 also includes lamps and gauges from an instrument panel, as well as a vehicle navigation system (not shown). The lithium-ion battery load 17 consumes less current than the lead-acid battery load 16.

[0042] The hybrid vehicle 10 includes a switching unit 60, and the switching unit 60 switches the power supply state between the lead-acid battery 71, the lithium-ion battery 72, the lead-acid battery load 16, the lithium-ion battery load 17, and the ISG 40. The switching unit 60 includes a mechanical relay, a solid-state relay (also known as an SSR), etc., and is controlled by the electronic control unit 50.

[0043] Power cables 61, 62, 63, and 64 are connected to the switching unit 60. Power cable 61 connects the switching unit 60, the lead-acid battery 71, the lead-acid battery load 16, and the starter 26 in parallel. Power cable 62 connects the switching unit 60 and the lithium-ion battery 72. Power cable 63 is connected to the switching unit 60 and the lithium-ion battery load 17. Power cable 64 connects the switching unit 60 and the ISG 40.

[0044] Therefore, the lead-acid battery load 16 and the starter 26 are continuously supplied with current from the lead-acid battery 71. However, in this embodiment, the current supply state is switched such that the lithium-ion battery load 17 is selectively supplied with current from either the lithium-ion battery 72 or the lead-acid battery 71. Furthermore, the current supply state is switched such that the ISG 40 is selectively supplied with current from either the lithium-ion battery 72 or the lead-acid battery 71.

[0045] In Fig. 2-1 and Fig. 2-2 The switching unit 60 includes the switches SW1, SW2, SW3 and SW4. In a closed state, the switches SW1, SW2, SW3 and SW4 form an electrical connection state, and in an open state, they form a disconnect state.

[0046] Switch SW1 connects power cable 61 to power cable 64 or disconnects power cable 61 from power cable 64. Therefore, switch SW1 connects lead-acid battery 71 to ISG 40 or disconnects lead-acid battery 71 from ISG 40.

[0047] Switch SW2 connects power cable 61 to power cable 63 or disconnects power cable 61 from power cable 63. Therefore, switch SW2 connects lead-acid battery 71 to lithium-ion battery load 17 or disconnects lead-acid battery 71 from lithium-ion battery load 17.

[0048] Switch SW3 connects power cable 62 to power cable 64 or disconnects power cable 62 from power cable 64. Therefore, switch SW3 connects Li-battery 72 to ISG 40 or disconnects Li-battery 72 from ISG 40.

[0049] Switch SW4 connects power cable 62 to power cable 63 or disconnects power cable 62 from power cable 63. Therefore, switch SW4 connects Li-battery 72 to Li-battery load 17 or disconnects Li-battery 72 from Li-battery load 17.

[0050] The switching unit has a first state, which is in Fig. Figure 2-1 illustrates this. In the first state, switches SW1 and SW4 are closed, and switches SW2 and SW3 are open. When the switching unit 60 is in the first state, current is supplied to the lead-acid battery 71 from the ISG 40, and current is supplied to the lithium-ion battery load 17 from the lithium-ion battery 72.

[0051] The switching unit 60 has a second state, which is in Fig. Figure 2-2 illustrates this. In the second state, switches SW1 and SW4 are open, and switches SW2 and SW3 are closed. When the switching unit 60 is in the second state, current is supplied to the ISG 40 from the Li-ion battery 72, and current is supplied to the Li-ion battery load 17 from the lead-acid battery 71.

[0052] The electronic control unit 50 includes a computer unit that has a central processing unit (CPU), random access memory (RAM), read-only memory (ROM), flash memory that stores backup data etc., an input port and an output port.

[0053] The ROM of the computer unit stores a program, along with various constants and characteristic maps, to cause the computer unit to function as the electronic control unit 50. That is, in this embodiment, these computer units function as the electronic control unit 50 when the CPU executes a program stored in the ROM, using the RAM as a working area.

[0054] The input port of the electronic control unit 50 is connected to various sensors, including the crankshaft angle sensor 27, the accelerator pedal opening degree sensor 13A, the brake stroke sensor 14A, the vehicle speed sensor 12A, and the charge state detectors 71A and 72A, which are described above. In this embodiment, the motor 20 and the ISG 40 are connected to each other by the belt 42 in such a way that power can be transferred reciprocally between them. The electronic control unit 50 indirectly detects the rotational speed of the ISG 40 by means of the crankshaft angle sensor 27.

[0055] The output terminal of the electronic control unit 50 is connected to various control targets, which include different devices such as the throttle valve 23, the injector 24, the spark plug 25, the switching unit 60, the ISG 40, the starter 26, etc. The electronic control unit 50 controls the various control targets based on information received from the various sensors.

[0056] When a predefined condition for electric driving is met, the electronic control unit 50 causes the ISG 40 to perform electric driving in order to power the hybrid vehicle 10. The condition for electric driving includes, for example, a condition that the state of charge (SOC) of the lead-acid battery 71 and the lithium-ion battery 72 is greater than a predefined value, a condition that the accelerator pedal opening degree is "0", etc.

[0057] The electronic control unit 50 puts the switching unit 60 into the second state during electric driving, which is in Fig. Figure 2-2 illustrates this. The ISG 40 is powered during electric driving using current from the Li-battery 72.

[0058] If the belt 42 is cut during electric driving, the rotation of the ISG 40 will not be transferred to the motor 20, and thus the hybrid vehicle 10 will stop contrary to the driver's intention.

[0059] Therefore, in this embodiment, the electronic control unit 50 monitors the combustion engine speed during electric driving and, based on the result of a comparison between the combustion engine speed and threshold values ​​N1 and N2, performs an operation as described below, thereby preventing the vehicle from stopping against the driver's intention.

[0060] The threshold value N1 is a rotational speed higher than the rotational speed in a stopped state. The rotational speed in the stopped state is 0 rpm. In this invention, the threshold value N1 corresponds to a first threshold value. In this embodiment, the threshold value N1 is set to 200 rpm.

[0061] The threshold value N2 is set to be higher than the first threshold value N1. A second threshold value is a rotational speed that is higher than or equal to a lower limit of a rotational speed at which the engine 20 can be started by means of fuel injection. In this invention, the threshold value N2 corresponds to the second threshold value. In this embodiment, the threshold value N2 is set to 600 rpm.

[0062] When the combustion engine speed decreases to or below the threshold N2 (600 rpm) during electric driving, the electronic control unit 50 switches the switching unit 60 from the second state to the first state.

[0063] When the speed of the engine 20 increases due to the switching unit 60, which switches from the second state to the first state, the electronic control unit 50 starts the engine 20 by means of fuel injection and carries out internal combustion engine driving, in which driving is carried out using the power of the engine 20.

[0064] When the combustion engine speed decreases to or below N1 (200 rpm) during electric driving, the electronic control unit 50 starts the engine 20 by driving the starter 26 and injecting fuel.

[0065] Referring to a flowchart that is in Fig. Figure 3 illustrates a vehicle stop prevention operation using the electronic control unit 50 of the hybrid vehicle 10, which is configured as described above.

[0066] In Fig. 3. In step S1, the electronic control unit 50 determines whether the combustion engine speed decreases to or below the threshold value N2 during electric driving. The electronic control unit 50 repeats step S1 until it determines that the combustion engine speed decreases to or below the threshold value N2.

[0067] If the combustion engine speed decreases to or below the threshold value N2 in step S1, the electronic control unit 50 switches a battery, which supplies current to the ISG 40 from the lithium battery 72, to the lead-acid battery 71 in step S2. Furthermore, the electronic control unit 50 performs a fuel injection into the engine 20 in step S2.

[0068] When the combustion engine speed is increased by supplying current from the lead-acid battery 71 to the ISG 40 in step S2, the electronic control unit 50 detects that the belt 42 is not cut and that a decrease in the output power of the SOC of the Li-battery 72 etc. is a cause.

[0069] When the combustion engine speed is increased by performing a fuel injection in step S2, the electronic control unit 50 performs combustion engine driving.

[0070] In step S3, the electronic control unit 50 determines whether 0.5 seconds have elapsed since the combustion engine speed dropped to or below the threshold value N1. If the combustion engine speed has not dropped to or below the threshold value N1, or if 0.5 seconds have not elapsed since the combustion engine speed dropped to or below the threshold value N1, the electronic control unit 50 returns to step S1.

[0071] If 0.5 seconds have elapsed since the combustion engine speed in step S3 has decreased to or below the threshold value N1, the electronic control unit 50 drives the starter 26 and starts the engine 20 in step S4. Afterwards, the electronic control unit 50 performs combustion engine driving.

[0072] As described above, the electronic control unit 50 in the hybrid vehicle 10 according to this embodiment performs electric driving, during which the operation of the engine 20 is suspended in order to perform driving using the power of the ISG 40. Furthermore, if the speed of the engine 20 decreases to or below the threshold value N1 during electric driving, the electronic control unit 50 starts the engine 20 by driving the starter 26 and injecting fuel. The threshold value N1 is a speed that is higher than the speed in a stopped state.

[0073] In this way it is possible to start the engine 20 by driving the starter 26 and injecting fuel when the speed of the engine 20 decreases to or below the threshold N1 during electric driving.

[0074] For this reason, it is possible to avoid a decrease in the speed of motor 20 to 0 rpm corresponding to the speed in the stopped state, which causes the engine to stall, and to continue driving using the drive torque of motor 20.

[0075] As a result, it can be prevented that the vehicle stops contrary to the driver's intention, even if the endless element is cut.

[0076] Furthermore, in the hybrid vehicle 10 according to this embodiment, the electronic control unit 50 switches the switching unit 60 from the second state to the first state when the speed of the engine 20 decreases to or below the threshold N2, which is higher than the threshold N1, during electric driving in a state in which the switching unit 60 is in the second state.

[0077] In this way, if the speed of motor 20 does not increase when the switching unit 60 is switched to the first state, it can be determined that a broken belt 42 is the cause. Furthermore, if the speed of motor 20 does increase when the switching unit 60 is switched to the first state, it can be determined that a decrease in the state of charge (SOC) of the lithium battery 72 is the cause.

[0078] As a result, it is possible to determine whether a decrease in the speed of the motor 20 is caused by a cut in the belt 42.

[0079] Furthermore, in the hybrid vehicle 10 according to this embodiment, in a case where the speed of the engine 20 increases when the switching unit 60 is switched from the second state to the first state, the electronic control unit 50 starts the engine 20 by means of a fuel injection and carries out combustion engine driving, in which driving is carried out by means of the power of the engine 20.

[0080] In this way, it is possible to generate electricity using the ISG 40 by driving with the internal combustion engine, since the belt 42 is not cut when the engine speed 20 increases as the switching unit 60 switches from the second state to the first state. Therefore, it is possible to prevent over-discharging of the lead-acid battery and the lithium-ion battery 72, which serve as power sources.

[0081] Furthermore, the lead-acid battery 71 in the hybrid vehicle 10 according to this embodiment has the property that, compared to the lithium-ion battery 72, a higher current can be discharged in a short period of time. In addition, the lithium-ion battery 72 has the property that it can repeat more charging and discharging cycles compared to the lead-acid battery 71.

[0082] In this way it is possible to create an adequate current supply state according to the situation, since the properties of the lead-acid battery 71 and the Li-ion battery 72 differ from each other.

[0083] Furthermore, the threshold N2 in the hybrid vehicle 10 according to this embodiment is a speed that is higher than or equal to a lower limit of a speed at which the engine 20 can be started by means of fuel injection.

[0084] In this way, the motor 20 can be started by means of fuel injection if the speed of the motor 20 drops to or below the threshold value N2 during electric driving. Therefore, it is possible for the driver not to perform a starting operation using an ignition key, etc., to restart the motor 20, which has stopped rotating.

[0085] Although embodiments of this invention have been described, it is evident that a person skilled in the art can make modifications without departing from the scope of this invention. Any and all such modifications and equivalents are to be included in the appended claims.

Claims

[1] Hybrid vehicle (10), comprising: an internal combustion engine (20); an electric motor (40) driven by a current; a starter (26) for starting the internal combustion engine (20); a speed detector (27) for detecting a speed of the internal combustion engine (20), a first current source (71) and a second current source (72); and a switching unit (60) for switching a power supply state between the first power source (71), the second power source (72) and the electric motor (40), wherein the electric motor (40) and the internal combustion engine (20) are connected to each other by means of a belt transmission mechanism (21, 41, 42) having an endless element such that power can be mutually transmitted between them and the internal combustion engine (20) rotates together with the electric motor (40) during rotation of the electric motor (40), wherein the hybrid vehicle (10) further comprises: a control unit (50) configured to control the internal combustion engine (20), the electric motor (40) and the starter, wherein the control unit (50) performs electric driving in which operation of the internal combustion engine (20) is suspended in order to perform driving by means of the power of the electric motor (40), and starts the internal combustion engine (20) by driving the starter (26) and performing fuel injection when a speed of the internal combustion engine (20) decreases to or below a first threshold value during electric driving, wherein the first threshold value is a speed higher than a speed in a stopped state, wherein the switching unit (60) comprises: a first state in which the electric motor (40) is supplied with a current from the first current source (71), and a second state in which the electric motor (40) is supplied with a current from the second current source (72), and wherein the control unit (50) switches the switching unit (60) from the second state to the first state when the rotational speed of the internal combustion engine (20) decreases to or below a second threshold value that is higher than the first threshold value during electric driving in a state in which the switching unit (60) is in the second state. [2] The hybrid vehicle (10) according to claim 1, wherein, in a case where the rotational speed of the engine (20) increases when the switching unit (60) is switched from the second state to the first state, the control unit (50) starts the engine (20) by means of fuel injection and performs engine driving performed by means of the power of the engine (20). [3] Hybrid vehicle (10) according to claim 2, wherein the first current source (71) has the property that a higher current can be discharged in a short period of time compared to the second current source (72), and the second power source (72) has the property that it can repeat more charging and discharging processes compared to the first power source (71). [4] Hybrid vehicle (10) according to one of claims 1 to 3, wherein the second threshold value is a speed higher than or equal to a lower limit of a speed at which the internal combustion engine (20) can be started by means of a fuel injection.

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