Fuel switching for dual fuel engine

The present invention generally relates to a dual fuel internal combustion engine selectively combusting first fuel and second fuel, and more particularly relates to switching fuel for a dual fuel internal combustion engine selectively combusting gasoline and gaseous hydrogen.

In recent years, for a lower tailpipe emission from internal combustion engines, dual fuel engines using together gaseous fuel, such as compressed natural gas, liquidized propane gas, or compressed hydrogen, and gasoline have been developed. For example, in Japanese patent application publication no. 03-26835, a dual fuel engine using together gasoline and gaseous hydrogen is described. Also, disclosed in Japanese patent application publication no. 11-311136 is a hybrid automotive vehicle using an electric motor as well as a dual fuel engine using bi-fuel of gasoline and gaseous hydrogen to drive the vehicle.

In the dual fuel engine using together gaseous hydrogen and gasoline, when fuel to be injected into the engine cylinders or combustion chambers is switched from gasoline to gaseous hydrogen, after the hydrogen injection, ignition may occur earlier than a desired ignition timing, and high temperature and high pressure blow back to an engine intake system may occur. In particular, unburned carbon or other matters remaining in the engine combustion chamber after the gasoline combustion, as a firing origin, may cause the injected hydrogen to combust in the intake system (so called back fire), or residual gas originating from gasoline adhered to the inside of intake system may abnormally combust.

Therefore, this invention is intended to overcome the disadvantages of prior approaches and to provide an improved fuel switching for dual fuel engine particularly avoiding the occurrence of backfire or abnormal combustion in the fuel switching from one fuel to another, such as from gasoline to gaseous hydrogen.

This object is solved according to the invention by the features of the independent claims. Preferred embodiments of the invention are subject of the dependent claims.

In one aspect of the present invention, the disadvantages of prior approaches are overcome by a method for controlling a vehicle powertrain. The vehicle powertrain has an internal combustion engine combusting first fuel in a first mode or second fuel in a second mode and generating driving torque to vehicle wheels. The method comprises supplying first fuel into combustion chambers of the internal combustion engine in the first mode, determining to switch from the first mode to the second mode based on vehicle driving condition, stopping to supply the first fuel into the combustion chambers when it is determined to switch to the second mode, suspending fueling into the combustion chambers for a predetermined period, and resuming the fueling with the second fuel after the predetermined period.

In another aspect of the invention, a control system for a vehicle powertrain is provided comprising an internal combustion engine combusting first fuel in a first mode or second fuel in a second mode and generating driving torque to vehicle wheels and a fuel supply system configured to selectively supply the first fuel or the second fuel into combustion chambers of the internal combustion engine, wherein the control system is configured to control the fuel supply system to supply the first fuel into combustion chambers of the internal combustion engine in the first mode, determine to switch from the first mode to the second mode based on vehicle driving condition, stop to supply the first fuel into the combustion chambers when it is determined to switch to the second mode, suspend fueling into the combustion chambers for a predetermined period, and resume the fueling with the second fuel after the predetermined period.

In still another aspect of the present invention, a vehicle powertrain is provided comprising an internal combustion engine combusting first fuel in a first mode or second fuel in a second mode and generating driving torque to vehicle wheels, a fuel supply system configured to selectively supply the first fuel or the second fuel into combustion chambers of the internal combustion engine, and a control system. The control system is configured to control the fuel supply system to supply the first fuel into combustion chambers of the internal combustion engine in the first mode, determine to switch from the first mode to the second mode based on vehicle driving condition, stop to supply the first fuel into the combustion chambers when it is determined to switch to the second mode, suspend fueling into the combustion chambers for a predetermined period, and resume the fueling with the second fuel after the predetermined period.

In further another aspect of the present invention, there is provided a computer program product comprising data representing instructions executable by a computer to control a vehicle powertrain having an internal combustion engine combusting first fuel in a first mode or second fuel in a second mode and generating driving torque to vehicle wheels. The storage medium comprises the instructions to supply the first fuel into combustion chambers of the internal combustion engine in the first mode, determine to switch from the first mode to the second mode based on vehicle driving condition, stop to supply the first fuel into the combustion chambers when it is determined to switch to the second mode, suspend fueling into the combustion chambers for a predetermined period, and resume the fueling with the second fuel after said predetermined period.

In still further another aspect of the present invention, there is provided a computer readable storage medium having stored data representing instructions executable by a computer to control a vehicle powertrain having an internal combustion engine combusting first fuel in a first mode or second fuel in a second mode and generating driving torque to vehicle wheels. The storage medium comprises the instructions to supply the first fuel into combustion chambers of the internal combustion engine in the first mode, determine to switch from the first mode to the second mode based on vehicle driving condition, stop to supply the first fuel into the combustion chambers when it is determined to switch to the second mode, suspend fueling into the combustion chambers for a predetermined period, and resume the fueling with the second fuel after said predetermined period.

According to the above aspects of the invention, by suspending the fueling into the combustion chambers upon switching the fueling modes, the combustion chamber of the engine and the intake system is scavenged so that the occurrence of backfire or the abnormal combustion can be prevented.

In one embodiment of the present invention, the engine speed may be prevented from falling beyond a certain value during the fueling suspension, so that a stable driving state can be sustained.

In alternative embodiment of the present invention, the vehicle powertrain may further comprises an electric motor generating driving torque to the vehicle wheels, and the driving torque from the electric motor may be increased during the fueling suspension, so as to prevent the engine speed from falling beyond the certain value and output torque of the vehicle powertrain from falling.

In a further alternative embodiment of the present invention, the vehicle powertrain may further comprise the electric motor and a battery storing and supplying electricity to the electric motor. It may be determined whether output torque currently generated by the internal combustion engine can be compensated by the electric motor with the supplied electricity from the battery during said fueling suspension. If it is determined so, output torque from the electric motor may be increased when the fueling is stopped. If it is determined not, the engine speed is increased before the fueling is stopped. In accordance with this embodiment, the engine speed can be prevented from falling beyond the certain value so as to sustain the stable driving state.

In a further alternative embodiment of the present invention, the vehicle powertrain may further comprise the electric motor, the battery and an electric generator driven by said internal combustion engine and generating electricity to selectively supply it to the electric motor and the battery. After the engine speed is increased, the electric generator may recover and transform kinetic energy of the internal combustion engine into electricity during the engine speed falling, and the electric motor may be driven with the electricity supplied from the electric generator and the battery. In accordance with this embodiment, the output torque from the vehicle powertrain can be prevented from falling.

In the embodiments of the present invention, the first fuel may be gasoline, alcohol such as ethanol, natural gas such as compressed natural gas and liquidized propane gas, or other hydrocarbon fuels or a mixture thereof. The second fuel may be hydrogen such as gaseous hydrogen, or gaseous fuel such as compressed natural gas and liquidized propane gas or a mixture thereof.

Also in the embodiments of the present invention, the engine may still rotate during the fueling suspension so as to scavenge the combustion chamber and the intake system so that the occurrence of backfire or abnormal combustion can be prevented.

The advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings. It should be understood that even though embodiments are separately described, single features thereof may be combined to additional embodiments. <ul id="ul0001" list-style="none" compact="compact"><li>Figure 1 is a schematic diagram of a vehicle powertrain having an internal combustion engine combusting bi-fuel, a battery, an electric motor and an electric generator, adopted for an embodiment of the present invention;</li><li>Figure 2 is a graphical representation of a transmission including a power split mechanism of the vehicle powertrain shown in Figure 1;</li><li>Figure 3 is a collinear diagram of the rotational speeds of planetary carrier, sun gear and ring gear respectively corresponding to the rotational speeds of engine, generator and motor;</li><li>Figure 4 is a time chart showing changes in gasoline supply, hydrogen supply, motor torque and engine speed during the fuel switching when the motor torque is utilized to compensate the engine output torque;</li><li>Figure 5 is a collinear diagram depicting a relationship between the rotational speeds of planetary carrier, sun gear and ring gear when the rotational speed of planetary gear is increased while the rotational speed of ring gear is fixed;</li><li>Figure 6 is a time chart showing changes in gasoline supply, hydrogen supply and engine speed during the fuel switching when the engine speed is increased; and</li><li>Figure 7 is a flowchart showing the fuel switching process from gasoline to gaseous hydrogen in accordance with the present invention.</li></ul>

Referring to Figure 1, it is a schematic representation of a powertrain 1 having a dual fuel internal combustion engine 2 using two different energy sources such as bi-fuel of gasoline and gaseous hydrogen or selectively combusting them, an electric drive motor 10, and its control system including an electronic control unit (hereinafter referred to ECU) 20. The powertrain 1 basically consists of the engine 2, a power split mechanism 7, an electric generator 8, an electric drive motor 10 and a reduction gear set 16. Power from the engine 2 is divided (preferably selectively dividable) by the power split mechanism 7 into at least two, one of which is transferred to the generator 8 and the other of which is transferred to the reduction gear set 16. The reduction gear set 16 comprises a differential gear, and is attached to at least one axis 17, one or more ends of which have vehicle driving wheels 18.

The engine 2 is a dual fuel engine particularly using together gaseous hydrogen and gasoline and (selectively) combust them to generate driving torque. The engine 2 is shown having a hydrogen injector 3 and gasoline injector 4 coupled thereto for delivering preferably gaseous hydrogen and gasoline respectively into the combustion chamber as controlled by the ECU 20, preferably in proportion of pulse width of signal from ECU 20. Although the injectors 3 and 4 are shown coupled to the engine 2 itself to directly inject fuel into the combustion chamber, they may be coupled to an intake system 19 to indirectly deliver fuel into the combustion chamber. Also one of the injectors 3 and 4 may be for the direct injection while the other may be the indirect (or port) injection. The injectors 3 and 4 are connected respectively to hydrogen supply system 5 and gasoline supply system 6 which store high pressure gaseous hydrogen and gasoline and supply them under control of the ECU 20 through the signals to the respective injectors 3 and 4 from the ECU 20. The intake system 19 has a throttle valve 13 to adjust amount of air inducted into the combustion chamber of the engine 2. The throttle valve 13 is operated by a throttle valve actuator 14 under control of the ECU 20.

The powertrain 1 also has a battery 12 which stores electric power as driving source of the electric motor 10. The battery 12 supplies electric power at least to the drive motor as well as stores electric power generated by the electric generator 8. Occasionally, the electric power from the battery 12 may be supplied to the electric generator 8, which functions as an electric motor (in addition or alternatively to the electric motor 10) then. Further, an inverter circuit 9 is provided which performs transformation between AC current of the generator 8 and/or the electric motor 10 and DC current of the battery 12 and controls electric current flow between the generator 8, the motor 10 and the battery 12. Alternatively, the electric power generated by the generator 8 may be directly used as a driving source of the motor 10 through the inverter circuit 9 rather than stored in the battery 12.

The ECU 20 is a generic control apparatus and performs operational control of various components such as the powertrain 1, the injectors 3 and 4, the electric generator 8, the inverter circuit 9, the electric motor 10, the battery 12, the throttle valve 13, and others (not specifically illustrated), based on information detected from one or more of various onboard sensors (not specifically illustrated, but, e.g. an airflow sensor, a water temperature sensor, an engine rotational speed sensor, an intake air temperature sensor, a battery charge sensor, etc.). The ECU 20 comprises therein a microcomputer (not shown) which performs processing such as correction, computation, determination, etc. in the various operational control, and includes a microprocessor and one or more memories that store data including that representing instructions executable by the microprocessor elements.

Now, the power split mechanism 7 will be described in detail. Figure 2 shows a schematic representation of various components of a power transmission mechanism within the powertrain 1 including the power split mechanism 7. In this embodiment, the power split mechanism 7 comprises a planetary gear set, where its rotational axis of planetary carrier 7a is connected to the engine 2 and the engine output is transmitted through pinion gears 7b carried on the planetary carrier 7a to a sun gear 7c and a ring gear 7d. A rotational axis of the sun gear 7c is connected to the generator 8, which generates electric power or electricity with mechanical power transmitted from the sun gear 7c. On the other hand, the ring gear 7d is engaged with an intermediate gear 11 positioned in an outside proximity, through which power from the ring gear 7d is transmitted to the reduction gear 16, then eventually to the vehicle wheels 16 through the axles 17 shown in Figure 1. The intermediate gear 11 also engages with an output gear 10a directly connected to a rotational axis of the electric motor 10, power from which is transmitted through the intermediate gear 11 to the reduction gear 16, then eventually to the vehicle wheels 18, along with the power from the ring gear 7d.

Further, although not specifically illustrated, there is a clutch mechanism between a rotational axis of the planetary carrier 7a and the engine 2. Then, the engine output may or may not be transmitted to the power split mechanism 7 during running of the engine 2, depending on a state of this clutch mechanism (engaged state or disengaged state).

Now, characteristic of the power split mechanism 7 will be described. Figure 3 shows a rotational speed based collinear diagram of the planetary carrier 7a, the sun gear 7c and the ring gear 7d of the power split mechanism 7. As is clear from this diagram, between rotational speeds of the respective gears there is a substantially linear relationship or dependency or a relationship which always aligns them together on one substantially straight line and respectively on fixed vertical lines (see points A, B and C) due to the mechanical principle of planetary gear set. The engaging mechanism of the gears shown in Figure 2 causes a rotational speed of the planetary carrier 7a to vary corresponding to the engine rotational speed and a rotational speed of the ring gear 7d to vary corresponding to rotational speed of the electric motor 10 and the vehicle wheels 18 (vehicle speed). The opposite is similar, that is, the engine rotational speed varies substantially corresponding to the rotational speed of planetary carrier 7a, the rotational speed of generator 8 varies substantially corresponding to the rotational speed of sun gear 7c, and the rotational speeds of electric motor 10 and the vehicle wheels 18 vary substantially corresponding to the rotational speed of ring gear 7d. Further, the engaging mechanism of the gears shown in Figure 2 causes the rotational speed of ring gear 7d to vary corresponding to various parameters such as the rotational speed of the reduction gear 16 and output torque in addition to the rotational speed of motor 10 and the vehicle speed.

In the power split mechanism 7, corresponding to change in rotational speed of either one of the planetary carrier 7, the sun gear 7c and the ring gear 7d, rotational speeds of the rest of the gears vary while keeping the substantially linear relationship of the rotational speeds of gears substantially aligned together on the one line and respectively substantially on the fixed vertical lines. For example, in a state where the rotational speed of sun gear 7c is fixed to be substantially constant, as the rotational speed of ring gear 7d is increased, the rotational speed of planetary gear 7a increases, while as the rotational speed of ring gear 7d is decreased, the rotational speed of planetary gear 7a decreases. Also, in a state where the rotational speed of ring gear 7d is fixed to be substantially constant, as the rotational speed of sun gear 7c is increased, the rotational speed of planetary gear 7a increases, while as the rotational speed of sun gear 7c is decreased, the rotational speed of planetary carrier 7a decreases. Further, in a state where the rotational speed of planetary carrier 7a is fixed to be substantially constant, as the rotational speed of ring gear 7d is increased, the rotational speed of sun gear 7c decreases, while as the rotational speed of ring gear 7d is decreased, the rotational speed of sun gear 7c increases.

Now, a basic control strategy on the powertrain 1 will be described. In the powertrain 1, if powertrain load required in accordance with vehicle driving condition is relatively small, a clutch mechanism between the engine 2 and the power split mechanism 7 is disengaged, and electric power is supplied from the battery 12 to the electric motor 10, thereby, the wheels 18 are driven only with motor output by the electric motor 10. Also if the powertrain load is at a mid or intermediate level, electric power supply from the battery 12 to the electric motor 10 is stopped, and the clutch mechanism is set engaged, thereby, the wheels 18 are driven only with engine output. In this case simultaneously electricity generation is performed by the generator 8 driven with a part of the mechanical power transmitted from the engine 2 through the power split mechanism 7. The generated electric power or electricity preferably is charged into the battery 12. Further, if the powertrain load is relatively high, the wheels 18 are driven with the motor torque by the electric motor 10 powered by the electric power supplied from the battery 12 in addition to the engine output transmitted through the engaged clutch mechanism.

Continuing to describe the basic control strategy on the powertrain 1, during vehicle braking, braking energy may be transmitted from the wheels 18 through the axle 17, reduction gears 16, etc. to the electric motor 10, and in this case, the drive motor 10 can be caused to act as a regenerative brake with electricity generation function able to transform the braking energy (mechanical or kinetic energy) into electric power or electricity. The electric power transformed from the braking energy preferably is charged into the battery 12.

Referring now to the fuel switching between gaseous hydrogen and gasoline performed by the engine 12, if a specific (predetermined or predeterminable) switching condition is met (e.g. if remaining amount of gaseous hydrogen in the hydrogen supply system 5 decreases below a predetermined or predeterminable amount, and/or if it is determined by a car navigation system that the vehicle has exited out of a city to suburbs), fuel supplied to the engine 2 through the injectors 3 and 4 is automatically switched from gaseous hydrogen to gasoline. On the other hand if the specified switching condition is not met (e.g. if the remaining amount of gaseous hydrogen exceeds the predetermined or predeterminable amount and/or if it is determined by the car navigation system that the vehicle has entered a city from suburbs), the fuel is automatically switched from gasoline to gaseous hydrogen. Alternatively the fuel switching may be performed in accordance with a desire of vehicle driver. In particular, a hardware switch arranged such as on a vehicle instrument panel or a software switch arranged on a display screen such as for the vehicle navigation switch may be provided for the driver to switch fuel between gasoline and hydrogen. In other words, the fuel supply is selectively switched based on a specified switching condition (based on a detected state of the vehicle and/or on a switch operation by a user) between states in which a first fuel (e.g. gasoline) and/or a second different fuel (e.g. hydrogen) is supplied.

In this embodiment, in the fuel switching from gasoline to gaseous hydrogen where vehicle speed or the engine speed is above the specified (predetermined or predeterminable) level or where it is accelerated, after the gasoline supply is stopped, the fueling into the cylinders of engine 2 is suspended until the gaseous hydrogen supply is commenced or the fueling is resumed. While the fueling is suspended, the engine 2 is still rotated with inertia of the engine 2, the generator 8, the motor 10 and other rotational components of the powertrain 1. By rotating the engine 2 with no fuel supply, fresh air is inducted into and pumped out of the combustion chamber and the intake system 19 of engine 2 to scavenge the combustion chamber and the intake system particularly with the fresh air so as to have no gasoline left there, thereby preventing or substantially reducing occurrence of backfire or abnormal combustion at a beginning of gaseous hydrogen supply.

Conventionally, if the fueling into the engine is suspended, the engine speed will decrease and enough output torque to keep the vehicle speed at the beginning of fuel switching can not be sustained, because only the inertia of powertrain 1 is not enough. Then a driving state may become unstable. To avoid such a situation, it is desirable to suppress the decrease of engine speed during the fueling suspension period. In other words, it is desirable to maintain the engine speed at the end of fuel switching to be substantially equal or close to the speed at the beginning of fuel switching, and/or to substantially maintain the output torque to maintain the vehicle speed at the beginning of fuel switching.

To realize the above, 1) the electric drive motor 10 is activated, and/or 2) the engine speed is increased just before the fueling suspension. Before the fueling suspension, the engine load state and state of charge (SOC) of the battery 12 is detected, and either of the methods is appropriately selected and performed depending on capacity of the motor 10 determined based on the various conditions detected. In particular, at first currently generated engine power is determined based at least on the engine load and engine rotational speed (e.g. as detected by one or more of the various onboard sensors). Then, maximum power which the electric motor 10 can currently generate is determined or retrieved preferably based on a rated output and a current speed of the electric motor 10. If the current engine power is substantially equal to or less than the maximum motor power, and if the SOC of battery 12 is higher than a specified (predetermined or predeterminable) amount (e.g. when it is in fully charged state), it is determined that the capacity of electric motor 10 is high enough, in other words, that it is possible to maintain the output torque at the beginning of fuel switching with operation of the electric motor 10, then the electric motor 10 is operated. On the other hand, if the engine load exceeds the maximum motor power, and/or the SOC of battery 12 is lower than the specified (predetermined or predeterminable) amount even though the current engine power is below the maximum motor power, it is determined that the capacity of drive motor is not high enough, or that it is impossible to keep the output torque at the beginning of fuel switching with the operation of drive motor 10, then just before the fueling suspension, preferably the throttle valve 13 and/or the gasoline injector 4 are controlled in order to increase the engine speed.

Meanwhile, the engine load state preferably may be detected based on such as opening of the throttle valve 13, intake airflow rate and/or intake manifold pressure, and on the other hand, the SOC of battery 12 preferably may be detected by using known methods such as integrating electric current balance while correcting it with battery temperature or electric voltage.

Figure 4 shows a graphical representation conceptually depicting changes in various parameters (gasoline supply, hydrogen supply, motor output, engine rotational speed) in a case where the electric motor 10 is operated upon fuel switching from gasoline to gaseous hydrogen. During the fueling suspension period, if the electric motor is operated to sustain output torque to keep the vehicle speed at the beginning of fuel switching, the drive motor 10 is operationally controlled so that motor output can be obtained to establish rotational speed of the ring gear 7d corresponding to the vehicle speed at the beginning of fuel switching. At this time, the rotational speed of sun gear 7c is maintained to be substantially constant by driving it with the electricity from the battery 12 to compensate negative torque from the engine 2. So, a rotational speed of the planetary carrier 7a at the beginning of fuel switching is substantially sustained based on the characteristic of power split mechanism 7 shown in Figure 3 by maintaining the speed of sun gear 7c, thereby, the engine rotational speed at the beginning of fuel switching will be substantially sustained during the fueling suspension period. In this case, upon the fuel switching from gasoline to gaseous hydrogen, points A, B and C in Figure 3, which are respectively representing the rotational speeds of ring gear 7d, planetary carrier 7a and sun gear 7c, do not move.

On the other hand, upon the fuel switching from gasoline to gaseous hydrogen through the fueling suspension period, if the current engine power exceeds the maximum motor power such as where the vehicle is driving at high speed, or if the SOC of battery 12 is lower than the specified (predetermined or predeterminable) amount even though the current engine power is below the maximum motor power, the engine rotational speed is increased while output torque of powertrain 1 is sustained to substantially keep the vehicle speed. Thereby, during the fueling suspension period, engine rotational speed decrease beyond the engine rotational at the beginning of fuel switching is suppressed. Figure 5 is a collinear diagram based on the rotational speeds of planetary carrier 7a, sun gear 7c and ring gear 7d just after engine rotational speed has been increased from the condition shown in Figure 3, while the output torque is substantially sustained. As is clear from this diagram, if it is determined that the capacity of drive motor 10 is not high enough, the engine speed preferably is compulsorily increased to substantially sustain the output torque preferably by controlling the throttle valve 13 to be more opened, the gasoline injector 4 to inject more fuel and/or the generator 8 to generate less electricity while rotational speed of the ring gear 7d is kept at the point A, and accordingly the rotational speed of planetary carrier 7a which is in proportion with the engine rotational speed increases from the point B to a point B'. Also, accompanied with this, rotational speed of the sun gear 7c increases from the point C to a point C', based on the characteristic of power split mechanism 7 described above. If output torque of the ring gear 7d is increased due to the engine speed increase operation, the excessive torque is absorbed by using the motor 10 for electricity generation so as to maintain the rotational speed of ring gear 7d at the point.

Subsequently, once the fueling is decreased or suspended, the engine rotational speed is gradually decreasing, and according to this, the rotational speed of planetary carrier 7a decreases from the point B' to the point B. During this fueling decrease or suspension, the combustion chamber and the intake system 19 of the engine 2 are scavenged with the engine 2 rotating with no fuel supplied as described above. At this time, the rotational speed of sun gear 7c gradually decreases from the point C' to the point C, as well. So, while the rotational speeds of planetary carrier 7a and sun gear 7c are decreasing, the inertia of the engine 2 and/or the generator 8 is supplemented to the output of the electric motor 10 through the power split mechanism 7. The inertia of the engine 2 and/or the generator 8 may be utilized through the sun gear 7c for electricity generation at the generator 8. The electricity generated through it is supplemented to the electric power to drive the motor 10 in addition to that from the battery 12. At a time when the rotational speed of planetary carrier 7a has decreased generally to the point B, or when the engine rotational speed has decreased to engine rotational speed in the gasoline operation, hydrogen is supplied and the hydrogen operation is commenced.

Figure 6 shows a graphical representation conceptually depicting changes in various parameters (gasoline supply, hydrogen supply, generated electricity from generator 8, output of motor 10 and engine rotational speed) in a case where the engine speed is increased just before the fueling suspension upon the fuel switching from gasoline to gaseous hydrogen. As can be seen from this diagram, although the engine rotational speed decreases during the fueling suspension period, engine rotational speed decrease beyond the engine rotational speed in the gasoline operation is substantially suppressed by compulsorily increasing the engine rotational speed just before gasoline is stopped to be supplied.

Figure 7 is a flow chart for a routine of the fuel switching process from gasoline to gaseous hydrogen out of various control processes executed by the ECU 20. In this routine, at first it is determined which one of the hydrogen operation mode where gaseous hydrogen is inducted into the engine 2 and the gasoline operation mode where gasoline is inducted into the engine 2 is a current operation mode (block #11). If the result is the hydrogen operation mode, various steps at the hydrogen operation mode are executed (block #23), then subsequently it is returned to the main routine.

On the other hand, if as a result of the block #11 the operation mode is the gasoline mode, subsequently it is determined whether the switching condition from gasoline to gaseous hydrogen is met (block #12). In particular, when a remaining amount of gasoline is less than the predetermined amount, or when there is a desire of vehicle driver for the hydrogen operation, and if the remaining amount of hydrogen is sustained enough to maintain a stable driving state, it is determined that the switching condition is met and the routine proceeds to a block #14. On the other hand, if it is determined that the switching condition is not met, it is notified to the driver that a switching at that point of time is impossible, then it is returned to the main routine.

It is determined or discriminated at a block #14 which vehicle state is given, preferably it is determined which one of decelerating, stopping and cruising or acceleration the vehicle state is. If as a result of the block #14 the vehicle is decelerating, the routine proceeds to the block #15 where fueling (gasoline supply) is stopped (fuel cut), the engine 2 is scavenged during the fueling suspension period, then a switching from gasoline to gaseous hydrogen is performed after the scavenging. During the deceleration, it is not necessary to substantially maintain the engine speed and the output torque, or to suppress decrease of engine speed such as operation of the drive motor 10 or increase of the engine speed. Then the routine proceeds to a block #21.

Also, if it is determined that the vehicle is stopping as a result of the block #14, normal switching from gasoline to gaseous hydrogen is performed at a block #16. During stopping, as engine rotational speed is low or engine rotational speed is zero, a possibility that backfire or abnormal combustion may occur is low, then switching from gasoline to gaseous hydrogen is performed preferably through no fueling suspension. Then the routine proceeds to the block #21.

Further, if it is determined that the vehicle is accelerating or cruising as a result of the block #14, the routine proceeds to a block #17 where the currently generated power of the engine 2 is determined and compared to the maximum power obtainable from the motor 10 as described above. If as a result of the block #17 it is determined the current engine power exceeds the maximum motor power, considering that it is impossible or very difficult to maintain the output torque at the beginning of fuel switching by only using the operation of the drive motor 10, the routine proceeds to a block #20, where, as described above, just before the fueling is stopped, engine speed is increased to sustain higher engine speed, the engine 2 is scavenged during the fueling suspension period, at the same time the electric motor 10 is driven with the electricity from the generator 8 and the battery 12, then after the scavenging, the fueling is resumed by commencing to supply gaseous hydrogen. Then the routine proceeds to the block #21.

On the other hand, if it is determined at the block #17 that the current engine power is below the maximum motor torque, then at a block #18 it is determined whether the battery 12 is in a fully charged state. If it is determined at the block #18 that the battery 12 is not in the fully charged state, considering that it is impossible or very difficult to maintain the output torque at the beginning of fuel switching, the routine proceeds to the block #20, where the above described switching from gasoline to gaseous hydrogen is performed. On the other hand, if it is determined at the block #18 that the battery 12 is in the fully charged state or close thereto (or the charged state of the battery 12 is higher than a specific threshold), considering that it is possible to maintain the output torque at the beginning of fuel switching with the operation of the drive motor 10, the routine proceeds to a block #19, where the drive motor 10 is temporarily operated associated with the fueling suspension, during this fueling suspension period the engine 2 is scavenged, and after the scavenging the fueling is resumed by commencing to supply gaseous hydrogen. Then the routine proceeds to the block #21.

At the block #21, associated with fuel switching from gasoline to gaseous hydrogen, a throttle opening characteristic to accelerator position (torque characteristic) is changed. Then the routine proceeds to a block #22, where preferably an indication is made for notifying the driver or operator that gasoline operation is being done at that point of time. For example, this indication may be illumination of a dedicated indication lamp on the instrument panel, or a notation of "Hydrogen Operation" on a display screen for the car navigation system. Then the routine for the fuel switching process from gasoline to gaseous hydrogen is completed and returned to the main routine.

As is clear from the above description, according to the present invention, in the powertrain 1 having the engine 2 using bi-fuel e.g. of gasoline and gaseous fuel, upon the fuel switching from gasoline to gaseous hydrogen, by suspending fueling and scavenging the engine 2, the occurrence of backfire or abnormal combustion can be substantially prevented, and further, during the fueling suspension period, by suppressing decrease of engine speed beyond that in the gasoline operation or at the beginning of fuel switching, a stable driving state can be sustained. Further, since, as means for suppressing the decrease of engine rotational speed, either of driving the motor and increase of engine speed is selected depending on the engine load state or the SOC of battery 12, during the fueling suspension period to prevent occurrence of backfire or abnormal, a stable driving state can be certainly sustained.

It is needless to say that this invention is not limited to the illustrated embodiment and that various improvements and alternative designs are possible without departing from the substance of this invention as claimed in the attached claims. While in the embodiment above the engine is using together gasoline and gaseous hydrogen, the fuel is not limited to this combination and other fuels such as compressed natural gas and liquidized propane gas may be used in stead of the gaseous hydrogen. Further other fuels such as ethanol and compressed natural gas may be used in stead of the gasoline.