Hybrid electric vehicle controller and method of controlling a hybrid electric vehicle

Abstract

Embodiments of the invention provide control means for a hybrid electric vehicle (HEV) operable to control first and second actuators of a vehicle to deliver motive torque to drive a vehicle, the control means being operable to control a vehicle to transition between a first mode in which a first actuator is substantially disconnected from a driveline of a vehicle and a second actuator delivers motive torque to drive a vehicle and a second mode in which a first actuator is connected to a driveline by means of a releasable torque transmitting means and the control means controls first and second actuators to deliver respective first and second actuator target torque split values to drive a vehicle thereby to provide a driver demanded drive torque, when a transition from the first mode to the second mode is required the control means being configured to control rotation of a first actuator by means of a speed control means towards a target first actuator speed and to control a releasable torque transmitting means to transition between an actuator disconnected condition and an actuator connected condition thereby to connect a first actuator to a driveline, the control means being further configured to ramp an amount of torque delivered by a first actuator towards a first actuator target torque split value, and to ramp an amount of torque delivered by a second actuator towards a second actuator target torque split value while retaining a total drive torque value provided to a vehicle substantially equal to a driver demanded torque, wherein the target first actuator speed is a speed greater than a speed at which a first actuator would rotate with a releasable torque transmitting means in the actuator connected condition.

Claims

1. A system for controlling an internal combustion engine and an electric machine of a hybrid electric vehicle (HEV) to deliver motive torque to drive the HEV, the system being operable to control the HEV to transition between: a first mode, in which the internal combustion engine is disconnected from a driveline of the HEV, and the electric machine delivers motive torque to drive the HEV; and a second mode, in which the internal combustion engine is connected to the driveline by a releasable torque transmitter, and the system controls the internal combustion engine and the electric machine to deliver respective internal combustion engine target torque split value and electric machine target torque split value to drive the HEV thereby to provide a driver demanded drive torque; wherein, when a transition from the first mode to the second mode is required, the system is configured: to control rotation of the internal combustion engine by a speed control towards a target internal combustion engine speed; and to control the releasable torque transmitter to transition between an engine disconnected condition and an engine connected condition thereby to connect the internal combustion engine to the driveline; the system being further configured, after the releasable torque transmitter has transitioned to the engine connected condition and the internal combustion engine is connected to the driveline, to: ramp an amount of torque delivered by the internal combustion engine towards the internal combustion engine target torque split value; ramp an amount of torque delivered by the electric machine towards the electric machine target torque split value while retaining a total drive torque value provided to the HEV equal to the driver demanded torque; wherein the target internal combustion engine speed is a speed greater than a speed at which the internal combustion engine would rotate with the releasable torque transmitter in the engine connected condition.

2. The system as claimed in claim 1 configured to ramp an amount of torque delivered by the internal combustion engine to become equal to the internal combustion engine target torque split value and to adjust an amount of torque delivered by the electric machine to become equal to the electric machine target torque split value once the releasable torque transmitter has assumed an engine connected condition.

3. The system as claimed in claim 1 configured to control an amount of motive torque delivered by the electric machine during a transition of the releasable torque transmitter between the engine disconnected condition and the engine connected condition such that a total drive torque delivered by the internal combustion engine and the electric machine remains equal to a value of the driver demanded torque.

4. The system as claimed in claim 1 wherein when a transition from the first mode to the second mode is required the speed control is controlled to control a speed of rotation of the internal combustion engine to approach a target speed.

5. The system as claimed in claim 4 further configured to control the internal combustion engine to maintain a speed of rotation thereof equal to a speed at which the internal combustion engine would rotate when in the engine connected condition while the releasable torque transmitter transitions from the engine disconnected condition to the engine connected condition.

6. The system as claimed in claim 1 configured to control the releasable torque transmitter to assume the engine connected condition when a speed of the internal combustion engine is substantially equal to a speed at which the internal combustion engine would rotate with the releasable torque transmitter in the engine connected condition.

7. The system as claimed in claim 1 wherein when a transition from the first mode to the second mode is required the system is configured to control a speed of rotation of the internal combustion engine by the speed control until the releasable torque transmitter is in the engine connected condition.

8. The system as claimed in claim 1 wherein an amount of torque delivered by the internal combustion engine is ramped from a value demanded by the speed control to the internal combustion engine target torque split value according to a ramp function, wherein the ramp function comprises one selected from a linear function and a non-linear function.

9. The system as claimed in claim 8 wherein the ramp function is responsive to a value of the driver demanded torque.

10. The system as claimed in claim 9 wherein the ramp function is arranged to increase a rate at which an amount of torque delivered by the internal combustion engine is ramped from the value demanded by the speed control to the internal combustion engine target torque split value responsive to a value of the driver demanded torque.

11. The system as claimed in claim 1 configured to ramp an amount of torque delivered by the internal combustion engine at a rate corresponding to a rate of change of the internal combustion engine target torque split value, a rate of change of torque delivered by the internal combustion engine being offset from that of the internal combustion engine target torque split value thereby to cause convergence of an amount of torque delivered by the internal combustion engine towards the internal combustion engine target torque split value.

12. The system as claimed in claim 1 configured to determine the internal combustion engine target torque split value and the electric machine target torque split value according to an energy management protocol based on one or more parameters of the HEV.

13. The system as claimed in claim 1 wherein the releasable torque transmitter comprises a clutch.

14. A method of controlling a hybrid electric vehicle (HEV) to deliver motive torque to drive the HEV by an internal combustion engine and an electric machine, the method comprising: controlling the HEV to transition between: a first mode, in which the internal combustion engine is substantially disconnected from a driveline of the HEV, and the electric machine delivers motive torque to drive the HEV, and a second mode, in which the internal combustion engine is connected to the driveline by a releasable torque transmitter, which controls the internal combustion engine and the electric machine to deliver respective internal combustion engine target torque split value and electric machine target torque split value to drive the vehicle and thereby to provide a driver demanded drive torque, when a transition from the first mode to the second mode is required, the method comprising controlling rotation of the internal combustion engine by a speed control towards a target rotational speed and controlling the releasable torque transmitter to transition between an engine disconnected condition and an engine connected condition, thereby to connect the internal combustion engine to the driveline, the method further comprising, after the releasable torque transmitter has transitioned to the engine connected condition and the internal combustion engine is connected to the driveline, ramping an amount of torque delivered by the internal combustion engine from a value demanded by the speed control to the internal combustion engine target torque split value, and ramping an amount of torque delivered by the electric machine from a current value to the electric machine target torque split value while retaining a total drive torque value provided to drive the HEV equal to the driver demanded drive torque, whereby the target rotational speed is a speed greater than a speed at which the internal combustion engine would rotate with the releasable torque transmitter in the engine connected condition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures in which:

(2) FIG. 1 is a schematic illustration of a hybrid electric vehicle (HEV) according to an embodiment of the invention;

(3) FIG. 2 shows an example of a plot of demanded torque from an internal combustion engine TQ.sub.e and a crankshaft integrated motor generator (CIMG) TQ.sub.c as a function of time during a transition from an electric vehicle (EV) mode to a parallel mode together with engine speed W.sub.e, CIMG speed W.sub.c, clutch state S.sub.c and engine on/off state S.sub.e for one embodiment of the invention;

(4) FIG. 3 is a flow chart of a method of transitioning from EV to parallel mode according to an embodiment of the invention; and

(5) FIG. 4 shows an example of a plot of TQ.sub.e and TQ.sub.c as a function of time during a transition from EV mode to parallel mode together with W.sub.e, W.sub.c, S.sub.c and S.sub.e in a vehicle according to a further embodiment of the invention.

DETAILED DESCRIPTION

(6) FIG. 1 shows a hybrid electric vehicle (HEV) 100 according to an embodiment of the present invention. The HEV 100 has an internal combustion engine 121 releasably coupled to a crankshaft integrated motor/generator (CIMG) 123 by means of a clutch 122. The clutch 122 has an input shaft 1221N coupled to a crankshaft of the engine 121 and arranged to rotate therewith. The clutch 122 also has an output shaft 122OUT coupled to the CIMG 123 and arranged to rotate therewith.

(7) The clutch 122 has a pair of plates 122A, 122B that are fixedly coupled to the input shaft 1221N and output shaft 122OUT respectively.

(8) The clutch 122 is operable to transition between an open condition and a closed condition. In the open condition the plates 122A, 122B are separated from one another such that substantially no torque is transferred from the input shaft 1221N to the output shaft 122OUT. In the closed condition the plates 122A, 122B are urged together such that torque applied to the input shaft 1221N by the engine 121 is transferred substantially directly to the output shaft 122OUT.

(9) The clutch 122 is operable to move the plates 122A, 122B towards one another as the clutch 122 transitions from the open condition to the closed condition whereby the amount of torque transferred from the input shaft 1221N to the output shaft 122OUT may be increased in a controlled manner.

(10) Similarly, the clutch 122 is operable to move the plates 122A, 122B away from one another as the clutch transitions from the closed condition to the open condition.

(11) The CIMG 123 is in turn coupled to an automatic transmission 124. The transmission 124 is arranged to drive a pair of front wheels 111, 112 of the vehicle 100 by means of a pair of front drive shafts 118. The transmission 124 is also arranged to drive a pair of rear wheels 114, 115 by means of an auxiliary driveline 130 having an auxiliary driveshaft 132, a rear differential 135 and a pair of rear driveshafts 139.

(12) A battery 150 is provided that may be coupled to the CIMG 123 in order to power the CIMG 123 when it is operated as a motor. Alternatively the battery 150 may be coupled to the CIMG 123 to receive charge when the CIMG 123 is operated as a generator, thereby to recharge the battery 150.

(13) The vehicle 100 is configured to operate in either one of a parallel mode and an electric vehicle (EV) mode.

(14) In the parallel mode of operation the clutch 122 is closed and the engine 121 is arranged to provide torque to the transmission 124. In this mode the CIMG 123 may be operated either as a motor or as a generator.

(15) In the EV mode of operation the clutch 122 is opened and the engine 121 is turned off. Again, the CIMG 123 is then operated either as a motor or as a generator. It is to be understood that the CIMG 123 may be arranged to act as a generator in EV mode in order to effect regenerative braking of the vehicle.

(16) The vehicle 100 has a controller 140 arranged to control the vehicle 100 to transition between the parallel and EV modes when required.

(17) In the present embodiment when a transition from EV mode to parallel mode is required the controller 140 is configured to start the engine 121 by means of a starter motor 121M and to control the speed of the engine 121 to match that of the output shaft 122OUT of the clutch 122 before closing the clutch 122. In the embodiment of FIG. 1 the speed of the output shaft 122OUT corresponds to that of the CIMG 123, W.sub.c. The controller 140 controls W.sub.e by reference to an output of an engine speed sensor 121S that provides a signal corresponding to the actual engine speed W.sub.e(t) at a given time t.

(18) In the embodiment of FIG. 1 the controller 140 controls the engine 121 to achieve the required W.sub.e (which may be referred to as a target engine speed W.sub.eT) by modulating the amount of torque TQ.sub.e that the controller 140 demands the engine 121 to provide.

(19) The controller 140 is arranged to employ a closed loop feedback control methodology in order to modulate TQ.sub.e to achieve the required value of W.sub.e. Thus, the controller 140 uses the signal from the speed measurement device 121S to calculate an engine speed error value e(t) which corresponds to a difference between the actual engine speed W.sub.e(t) and the target engine speed W.sub.eT.

(20) It is to be understood that it is desirable that the controller 140 controls the engine 140 to achieve W.sub.eT as quickly as possible and in a manner such that minimal overshoot of W.sub.eT occurs. Furthermore it is desirable to reduce oscillation of the engine about W.sub.eT to a minimum.

(21) As well as controlling the engine to start and to achieve a target speed W.sub.eT, in some embodiments the controller 140 is configured gradually to close the clutch 122 to connect the engine 121 to the CIMG 123 once the engine speed W.sub.e has achieved the target engine speed W.sub.eT.

(22) It is to be understood that the controller 140 is configured to maintain the engine speed at the target engine speed W.sub.eT as the clutch 122 is gradually closed.

(23) In some embodiments control of the engine speed W.sub.e is performed by means of an engine speed controller. The engine speed controller may be implemented by means of a software program run by the controller 140. Alternatively the engine speed controller may be provided by a separate controller, for example by a separate engine speed control module.

(24) Once the clutch is closed the controller 140 is arranged to control the engine 121 and CIMG 123 to deliver torque to the driveline according to a torque split determined by an energy management program (EMP). In other words, the EMP is configured to determine the relative amounts of torque to be delivered to the transmission 124 by the engine 121 and CIMG 123 respectively.

(25) Since torque to drive the vehicle 100 is provided substantially entirely by the CIMG 123 in EV mode it is to be understood that once the clutch 122 is closed the controller 140 will typically control the engine 121 and CIMG 123 such that the amount of torque provided to the transmission 124 by the engine 121 (TQ.sub.e) is increased and the amount of torque provided by the CIMG 123 (TQ.sub.c) is decreased (compared to that provided by the CIMG 123 when in EV mode) for a given value of driver demanded torque TQ.sub.d.

(26) However it is to be understood that the actual amounts of torque provided to the transmission 124 by the engine 121 and CIMG 123 will depend on the value of driver demanded torque TQ.sub.d.

(27) For example if TQ.sub.d is substantially zero, TQ.sub.e and TQ.sub.c may also be substantially zero. In some embodiments when TQ.sub.d is substantially zero the controller 140 may be configured to control the engine 121 to deliver a positive torque TQ.sub.e and the CIMG 123 to deliver a negative torque TQ.sub.c, the two torque values being substantially equal in magnitude so that the net torque to the transmission 124 is substantially zero. It is to be understood that the CIMG 123 may also be arranged to deliver a negative torque at other times when TQ.sub.d is non-zero.

(28) It is to be understood that when the CIMG 123 delivers a negative torque electric power may be generated by the CIMG 123 for storage in the battery 150.

(29) In the present embodiment the controller 140 is configured to control the engine 121 and CIMG 123 to change the amount of torque provided to the transmission 124 by each in a substantially linear manner from their value substantially at the instant of clutch closure to the values determined by the EMP.

(30) FIG. 2 shows a plot of W.sub.e and W.sub.c as a function of time together with the amounts of torque generated by each of the engine 121 (TQ.sub.e) and CIMG 123 (TQ.sub.c) and the total torque TQ.sub.t provided to the transmission 124 by the engine 121 and the CIMG 123 together. It is to be understood that TQ.sub.t is typically arranged to correspond to the driver demanded torque TQ.sub.d.

(31) FIG. 2 also shows the state of a clutch control signal S.sub.c being a signal by means of which the controller 140 controls the clutch 122 to open or close and the state of an engine start/stop control signal S.sub.e by means of which the controller 140 controls the engine 121 to start or stop.

(32) When the controller 140 determines that the clutch 122 should be open the controller 140 sets control signal S.sub.c=0. If the clutch 122 is closed a clutch controller (which may be provided by a transmission control module or TCM) controls the clutch 122 to open. When the controller 140 determines that the clutch 122 should be closed the controller sets control signal S.sub.c=1. If the clutch 122 is open the clutch controller controls the clutch 122 to close.

(33) Similarly, when the controller 140 determines that the engine 121 should be off it sets control signal S.sub.e=0 whilst when the controller 140 determines that the engine 121 should be on it sets control signal S.sub.e=1. An engine controller (not shown) then controls operation of the engine 121 and starter motor 121M accordingly.

(34) FIG. 2 illustrates control of the vehicle 100 during an example transition from EV to parallel mode.

(35) It can be seen that at time t.sub.0 in the example of FIG. 2 the clutch 122 is set to the open condition (S.sub.c=0) and the engine 121 is set to remain off (S.sub.e=0). The amount of torque developed by the engine 121 (TQ.sub.e) is therefore substantially zero.

(36) In contrast, the CIMG 123 is providing a torque TQ.sub.c to the transmission 124, TQ.sub.c being equal to TQ.sub.d.

(37) At time t.sub.1 the controller 140 controls the engine 121 to start by setting engine start/stop control signal S.sub.e=1.

(38) The controller 140 then controls the engine 121 to spin up to a speed corresponding to that of the CIMG 124 (W.sub.c) so that the clutch 122 may be closed without causing an undue decrease in NVH performance of the vehicle 100.

(39) Thus, at time t.sub.2 when W.sub.e is substantially equal to W.sub.c the controller 140 controls the clutch 122 to close by setting control signal S.sub.c=1 as described above. Whilst the clutch 122 is closing the controller 140 controls the engine 121 to maintain a speed corresponding to that of the CIMG 123, i.e. W.sub.e=W.sub.c.

(40) Once the clutch 122 is fully closed (at time t.sub.3) the controller 140 controls the engine 121 and CIMG 123 to deliver respective amounts of torque according to the torque split determined by the EMP.

(41) If at t.sub.3 the amounts of torque being delivered by the engine 121 and CIMG 123 are not equal to the required torque split determined by the EMP then the controller 140 controls the engine 121 and CIMG 123 gradually to change the amount of torque they are each providing to the transmission 124 according to a substantially linear ramp function although other functions are also useful.

(42) The controller 140 is arranged to perform this operation whilst maintaining a total torque TQ.sub.t delivered to the transmission 124 substantially equal to a value of driver demanded torque TQ.sub.d as determined by the controller 140.

(43) As can be seen from FIG. 2, between time t.sub.3 and t.sub.4 TQ.sub.e is controlled to change from a value TQ.sub.1 at time t.sub.3 to a value TQ.sub.2 at time t.sub.4.

(44) Similarly, TQ.sub.c is configured to change from a value TQ.sub.3 at time t.sub.3 to a value TQ.sub.4 at time t.sub.4.

(45) Throughout the period from t.sub.3 to t.sub.4 the controller 140 controls the engine 121 and the CIMG 123 such that the total torque TQ.sub.t provided to the transmission 124 corresponds to the value of driver demanded torque TQ.sub.d that the controller 140 determines is required to be provided to the transmission 124.

(46) From time t.sub.4 onwards the relative amounts of torque provided by the engine 121 and CIMG 123 are determined substantially entirely by the EMP and the controller 140 controls the engine 121 and CIMG 123 to provide torque accordingly.

(47) FIG. 3 is a flow chart of the method of controlling the vehicle 100 when a transition from EV to parallel mode is required as discussed above.

(48) At step S101 the controller 140 determines whether the EMP requires a transition from EV to parallel mode to be made.

(49) If at step S101 the controller determines that a transition from EV to parallel mode is required then at step S102 the controller 140 controls the engine 121 to start by providing an engine start control signal S.sub.e=1 to the engine 121.

(50) At step S103 the controller 140 controls the engine 121 to rotate at a speed matching that of the CIMG 123.

(51) At step S104 the controller determines whether the engine speed is equal to the CIMG speed. If the speeds are not equal the controller repeats step S103.

(52) If the speeds are equal then at step S105 the controller 140 controls the clutch 122 to close by setting clutch control signal S.sub.c=1. The controller 140 maintains the speeds of the engine 121 and CIMG 123 substantially equal throughout the period of clutch closure from time t.sub.2 to time t.sub.3.

(53) At step S106 the controller 140 determines whether the clutch 122 has actually closed. If the clutch is not closed the controller 140 waits for the clutch 122 to close. If the clutch 122 has closed the controller 140 proceeds to step S107.

(54) At step S107 the controller 140 controls TQ.sub.e and TQ.sub.c gradually to become equal to the values that the EMP determines they should provide.

(55) As discussed above the controller controls TQ.sub.c and TQ.sub.e gradually to change according to a linear ramp function whilst keeping the total torque TQ.sub.t delivered to the transmission 124 substantially equal to the driver demanded torque TQ.sub.d determined by the controller 140.

(56) As noted above other ramp functions are also useful.

(57) It is to be understood that embodiments of the invention have the advantage that a transition from EV mode to parallel mode may be made with improved NVH performance. This is because when the transition from EV to parallel mode is made and clutch closure has taken place, the amounts of torque delivered to the driveline of the vehicle 100 by the engine 121 and CIMG 123 are controlled to approach the values determined to be required according to the EMP in a gradual manner rather than an abrupt, ‘digital’ manner in which torque values are changed substantially instantaneously.

(58) If abrupt changes in the amounts of torque provided by the engine 121 and CIMG 123 are made, the risk exists that an undesirable acceleration and/or deceleration of the vehicle may take place. This is at least in part because the time taken for TQ.sub.c and TQ.sub.e to attain their respective EMP torque values may be different. Thus the CIMG 123 may attain its EMP value sooner than (or later than) the engine 121 resulting in TQ.sub.t rising above or falling below TQ.sub.d.

(59) In a variation on this embodiment of the invention, when a transition from EV to parallel mode is required the controller 140 is configured to control the engine 121 to achieve a target speed W.sub.T that is greater than W.sub.c rather than substantially equal to W.sub.c. The controller 140 may provide a signal to the clutch 122 to close as the speed of the engine 121 increases through W.sub.c. The controller 140 continues to control the engine 121 to achieve a speed equal to W.sub.T as the clutch closes.

(60) It is to be understood that closure of the clutch 122 has the effect of increasing the load experienced by the engine 121 and therefore decreasing W.sub.e for a given engine torque.

(61) Since in this alternative embodiment the controller 140 is seeking to control the engine 121 to achieve a speed that is greater than that of the CIMG 123 the amount of torque developed by the engine 121 will increase above that which would be developed if W.sub.eT were substantially equal to W.sub.c.

(62) This has the feature that upon completion of closure of the clutch 122 a difference between the respective amounts of torque required to be provided to the driveline by the engine 121 and CIMG 123 according to the EMP and the respective amounts of torque actually provided at the moment the clutch 122 is fully closed may be reduced.

(63) Accordingly, as the clutch 122 is closed TQ.sub.c is controlled to decrease as TQ.sub.e increases in order to main TQ.sub.t substantially equal to TQ.sub.d throughout the period of clutch closure.

(64) It is to be understood that in embodiments such as that described above in which W.sub.e is maintained substantially equal to W.sub.c during closure of the clutch 122 the amount of torque transferred to the transmission 124 by the engine 121 is in principle substantially zero throughout the period of clutch closure. Therefore it may not be necessary in such embodiments to adjust TQ.sub.c in order to compensate for torque provided to the transmission 124 by the engine 121.

(65) However in some such embodiments some torque may still be provided by the engine 121 to the transmission 124 and therefore it may be necessary to adjust TQ.sub.c in order to compensate for this.

(66) Once the clutch 122 is fully closed control of the engine 121 by the controller 140 transitions to control according to the EMP (rather than according to engine speed) in order to deliver a required proportion of TQ.sub.d as opposed to a required speed of rotation for clutch closure.

(67) The transition from ‘speed control’ to ‘EMP control’ is performed in a similar manner to that described above whereby the relative amounts of torque provided by the engine 121 and CIMG 123 are ‘blended’, allowing a relatively smooth change in TQ.sub.e and TQ.sub.c and allowing TQ.sub.t to remain substantially equal to TQ.sub.d during the course of the torque blending process.

(68) Thus, TQ.sub.e is controlled to transition to the value demanded by the EMP according to a substantially linear ramp function whilst TQ.sub.c is controlled to transition to the EMP demanded value in a complementary manner such that TQ.sub.t remains substantially equal to TQ.sub.d.

(69) FIG. 4 shows a plot of W.sub.e W.sub.c as a function of time (lower plot) and a corresponding plot (upper plot) of TQ.sub.d, TQ.sub.c and TQ.sub.e as a function of time for an embodiment in which W.sub.eT is greater than W.sub.c. Also shown (middle plot) are the states of control signals S.sub.e and S.sub.c.

(70) It can be seen that at time t.sub.0 the engine 121 is started (S.sub.e=1) and W.sub.e is controlled to increase towards a target engine speed W.sub.eT as shown in the lower plot. As W.sub.e passes through W.sub.c (i.e. when W.sub.e is equal to or greater than W.sub.c) at time t.sub.2, the controller 140 controls the clutch 122 to close (S.sub.c=1). As the clutch 122 closes the load applied to the engine 121 increases and W.sub.e is caused to decrease towards W.sub.c. As the clutch continues to close the load on the engine 121 increases still further until at the moment of clutch closure W.sub.e and W.sub.c are substantially equal.

(71) It is to be understood that S.sub.c may be set to a value of 1 before or after the value of W.sub.e passes through that of W.sub.c.

(72) As shown in the upper plot, TQ.sub.e increases relatively abruptly when the engine 121 is started before decreasing rapidly as W.sub.e approaches W.sub.eT.

(73) As the clutch 122 begins to close, a finite portion of TQ.sub.e is transmitted through the clutch 122 to the transmission 124. The controller 140 therefore controls TQ.sub.c to decrease in order to compensate for the torque transmitted to the transmission 124 from the engine 121 by the clutch 122 so that the total amount of torque TQ.sub.t delivered to the transmission 124 remains substantially equal to TQ.sub.d throughout the period of closure of the clutch 122.

(74) As noted with respect to the embodiment described above, when the clutch 122 is fully closed at time t.sub.3, a mismatch may exist between the actual values of TQ.sub.e and TQ.sub.c and the values demanded by the EMP.

(75) Thus the controller 140 controls TQ.sub.e and TQ.sub.c to change in a substantially linear manner from their values at time t.sub.3 towards the values demanded by the EMP. This transition takes place between time t.sub.3 and time t.sub.4 in the plot of FIG. 4.

(76) It is to be understood that because W.sub.eT is set to be higher than W.sub.c in this embodiment, the value of TQ.sub.e at the moment of clutch closure (time t.sub.3) is greater than in the case where W.sub.eT is substantially equal to W.sub.c. This is because the controller 140 attempts to make W.sub.e equal to W.sub.eT by increasing TQ.sub.e, but the torque loading applied by the clutch 122 as it is closed prevents W.sub.e from achieving W.sub.eT.

(77) Thus at the moment of clutch closure (when W.sub.e is substantially equal to W.sub.c<W.sub.eT), TQ.sub.e is higher than that value which would be required to maintain W.sub.e equal to W.sub.c. Thus the amount by which TQ.sub.e must be increased and TQ.sub.c must be decreased in order to attain the respective torque values required according to the EMP is lower than if W.sub.eT were equal to Wc.

(78) In some embodiments the target engine speed W.sub.eT during the transition from EV mode to parallel mode is responsive to the current value of W.sub.c. Thus W.sub.eT may be greater than W.sub.c by a fixed amount, for example a value from around 300-400 rpm greater than W.sub.c. Thus the value of W.sub.eT may track that of W.sub.c with a fixed offset.

(79) Alternatively W.sub.eT may be greater than W.sub.c by a fixed proportion of W.sub.c such as 10%, 20%, 30% or any other suitable proportion. In some embodiments W.sub.eT is not responsive to W.sub.c, W.sub.eT being a substantially fixed value regardless of the value of W.sub.c.

(80) It is to be understood that other arrangements are also useful.

(81) It is to be understood that in the embodiments described, W.sub.c is the speed at which the engine 121 will rotate when the clutch 122 is closed. This is the reason W.sub.eT is set equal to W.sub.c or the value of W.sub.c plus a prescribed value of (say) 400 rpm in the second embodiment described above.

(82) As noted above embodiments of the invention have the advantage that a transition from EV mode to parallel mode may be made with increased NVH performance. That is, an amount of noise, vibration and/or harshness of ride experienced by the driver of the vehicle 100 may be reduced. Alternatively or in addition, closure of the clutch 122 may be effected more rapidly for a given required NVH performance of the vehicle 100.

(83) Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

(84) Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

(85) Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.