Hybrid vehicle

Abstract

A hybrid vehicle is disclosed, in which the drive torque is controlled according to a target, and the engine operating point and the battery charging rate are controlled thereby as targeted to improve the fuel consumption rate of the hybrid vehicle as a whole An engine torque estimating unit estimates the engine torque based on the motor current, and an engine output correction unit.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a hybrid vehicle having an engine and a motor, or in particular to a hybrid vehicle in which the fuel consumption can be improved by controlling the operating point of the engine and the battery charging rate according to a target.

[0002] One system intended to reduce engine fuel consumption is a hybrid vehicle utilizing the driving force of the motor. Various types of such a system have been proposed and include the series type and the parallel type. For example, JP-A-7-135701 discloses a system in which two motors and one planetary gear are used so that the engine driving force is input to the planetary gear, and the motor is controlled to drive the vehicle by the driving force obtained from the output shaft of the planetary gear. Part of the energy of the engine is derived from a generator (which is one of the motors) generating power while the motor coupled to the output shaft of the generator delivers a driving force as an assistance. Thus, the engine is always driven efficiently in a high torque area while at the same time providing the shift function.

[0003] A method of controlling the driving torque of the hybrid vehicle is described in JP-A-8-207601 in which the torque of the generator is calculated and the torque of the motor on the output shaft is corrected by the calculated torque of the generator. According to this method, the vehicle driving torque is not affected greatly by variations in the engine output and therefore the drivability can be improved.

[0004] In order to suppress the change in the charging condition of the battery connected to the generator or the motor, on the other hand, JP-A-10-243503 discloses a method in which the motor torque command or the target engine speed is corrected in accordance with the current value of the battery. This method can maintain the normal condition of the battery and therefore can prevent the deterioration of the battery. Also, the battery charging rate can be controlled as scheduled.

[0005] Of all the methods described above, the method of correcting the output of the motor according to the estimated torque value can control the vehicle drive torque as intended and therefore can improve the drivability. Nevertheless, the change in motor output may cause unexpected charge and discharge of the battery, often leading to the deviation from the optimum schedule for charging the battery, resulting in a deteriorated fuel consumption rate.

[0006] According to the method of correcting the motor output or the target engine speed in accordance with the battery current or the like, on the other hand, the battery deterioration can be prevented and the optimum schedule can be followed. However, the required driving output cannot be produced often adversely affecting the drivability.

[0007] In any of the methods described above, the engine operating point, if deviated from the target, is not corrected, thereby posing the problem that the engine deviates from the optimum operating point and the fuel consumption rate is deteriorated.

SUMMARY OF THE INVENTION

[0008] The object of the invention is to provide a hybrid vehicle n which the efficiency and the fuel consumption can be improved by controlling the engine operating point and the battery charging rate as intended without adversely affecting the drivability.

[0009] In order to achieve the aforementioned object, according to the invention, there is provided a hybrid vehicle comprising an engine for generating the energy for driving the vehicle, a transmission for transmitting the driving force to the wheels by changing the rotational speed of the engine, at least a motor for changing the wheel driving force, a battery for supplying power to the motor, and a drive control unit for calculating and outputting an operation command value for the engine and the motor based on the drive information including the accelerator angle, wherein the drive control unit includes an engine output correcting mechanism for correcting the operation command value for the engine based on the difference between the engine operation command value and the torque generated by the engine thereby to maintain an optimum engine operating point.

[0010] The optimum operating point is defined as a point on or near a curve associated with the best fuel consumption rate of the engine including the efficiency of the transmission and the motor.

[0011] According to another aspect of the invention, there is provided a hybrid vehicle comprising an engine for generating the energy for driving the vehicle, a transmission for transmitting the driving force to the wheels by changing the rotational speed of the engine, at least a motor for changing the wheel driving force, a battery for supplying power to the motor, means for determining a target engine torque, means for calculating the torque generated by the engine, and means for correcting the engine output based on the difference between the target engine torque value and the engine torque value calculated by the engine torque calculation means.

[0012] According to still another aspect of the invention, there is provided a hybrid vehicle comprising an engine for generating the energy for driving the vehicle, transmission for transmitting the driving force to the wheels by changing the rotational speed of the engine, at least a motor for changing the wheel driving force, a battery for supplying power to the motor, means for determining a target engine output value instead of the engine torque, means for calculating the output of the engine, and means for correcting the engine output based on the difference between the target engine output value and the calculated engine output value.

[0013] According to yet another aspect of the invention, there is provided a hybrid vehicle comprising an engine for generating the energy for driving the vehicle, a transmission for transmitting the driving force to the wheels by changing the rotational speed of the engine, at least a motor for changing the wheel driving force, a battery for supplying power to the motor, a battery management unit for determining a target current value of the battery, means for detecting the battery current, and means for correcting the engine output based on the difference between a target battery current and a detected battery current value, wherein the battery management unit produces a schedule for the battery charging rate based on the navigation information and determines the target battery current value based on the difference between the detected value of the battery charging rate and the scheduled battery charging rate.

[0014] The engine output correcting means can correct the output by controlling the throttle opening degree or correcting the target engine output value.

[0015] According to this invention, an optimum engine operating point can be maintained while producing the target vehicle driving torque and also the battery charging rate can be kept as scheduled for an improved fuel consumption rate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a diagram showing an example of a configuration of a driving system of a hybrid vehicle according to this invention.

[0017] FIG. 2 is a diagram showing a configuration of a drive control unit shown in FIG. 1.

[0018] FIG. 3 is a diagram showing a configuration of an engine output correcting unit shown in FIG. 2.

[0019] FIG. 4 is a diagram for explaining the engine operating point.

[0020] FIG. 5 is a diagram showing a configuration of another example of the hybrid vehicle driving system according to this invention.

[0021] FIG. 6 is a diagram showing a configuration of the drive control unit shown in FIG. 5.

[0022] FIG. 7 is a diagram showing another configuration of the drive control unit of the hybrid vehicle driving system according to the invention.

[0023] FIG. 8 is a diagram showing still another configuration of the drive control unit according to this invention.

[0024] FIG. 9 is a diagram showing a configuration of the hybrid vehicle of another type according to the invention.

[0025] FIG. 10 is a diagram showing yet another configuration of the drive control unit according to the invention.

DESCRIPTION OF THE EMBODIMENTS

[0026] FIG. 1 shows a hybrid vehicle in which the tires 3a, 3b are rotated by use of the energy of an engine through a drive shaft 2. This hybrid vehicle including a planetary gear train A4 and a planetary gear train B5 as a differential mechanism each including a sun gear, a planetary gear and a ring gear. The sun gears are driven by a motor A8 and a motor B9 controlled by power converters 10, 11, respectively. The battery 12 is used for supplying the energy required by these motors or storing the energy generated in these motors at the time of deceleration braking. Each planetary gear is fastened to the same input shaft, and the driving torque of the engine 1 is divided into two or more planetary gears. The ring gears, on the other hand, are coupled to a common output shaft through gears having different gear ratios. The torque output from the two planetary gear trains are combined into a vehicle drive torque &tgr;v. As a result, it is possible to secure an acceleration or deceleration of the vehicle as intended by the driver. By controlling the torque &tgr;a, &tgr;b and the speeds &ohgr;a, &ohgr;b of the motors A8, B9 for driving the sun gears, on the other hand, the vehicle drive torque &tgr;v and the engine speed &ohgr;e can be regulated. The drive control unit 31 calculates and outputs the engine throttle opening degree command value &thgr;t, the speed command value &ohgr;ar of the motor A and the torque command value &tgr;br of the motor B using predetermined functions and data according to predetermined processing steps based on the information including the accelerator angle &thgr;a, the vehicle speed &ohgr;v, the torque command value &tgr;ar of the motor A and the currents Ia, Ib of the motors A, B. The drive control unit 31 is configured with a microcomputer including a CPU, a RAM, a ROM, input/output control means and various programs stored in the ROM.

[0027] A specific method of controlling the motors A8, B9 is described below. In the system shown in FIG. 1, equations (1) to (4) below hold.

&ohgr;e=Kp&ohgr;a+Ka&ohgr;v  (1)

&ohgr;e=Kp&ohgr;b+Kb&ohgr;v  (2)

&tgr;e=(&tgr;a+&tgr;b)/Kp  (3)

&tgr;v=(Ka&tgr;a+Kb&tgr;b)/Kp  (4)

[0028] where &ohgr;e, &ohgr;v, &ohgr;a, &ohgr;b are the engine speed, the vehicle speed, the rotational speed of the motor A and the rotational speed of the motor B, respectively, and &tgr;e, &tgr;a, &tgr;b, &tgr;v the engine torque, the torque of the motor A, the torque of the motor B and the vehicle drive torque, respectively. Characters Kp, Ka, Kb are constants relating to the gear ratio.

[0029] Using this relation, the following equation (5) is obtained from equation (1).

&ohgr;ar=(&ohgr;er−Ka&ohgr;v)  (5)

[0030] where &ohgr;er is the target engine speed, &ohgr;v the detected vehicle speed and &ohgr;ar the rotational speed setting of the motor A.

[0031] By controlling the rotational speed of the motor A based on this equation, the engine can be driven at the desired operating point and the desired change gear ratio is obtained.

[0032] Also, let &tgr;vr be the target vehicle drive torque, and &tgr;a be the output torque of the motor A, and from equation (4), the following relation holds.

&tgr;br=(Kp&tgr;vr−Ka&tgr;a)/Kb  (6)

[0033] Assuming that &tgr;br determined from equation (6) is a torque setting of the motor A, the desired vehicle drive torque can be obtained.

[0034] By controlling the motors according to equations (5) and (6), the engine speed can be controlled to the desired change gear ratio or the target vehicle drive torque can be generated. Equation (6) contains no engine torque &tgr; e, and therefore even when the engine torque undergoes a change, the vehicle drive torque can be controlled as targeted by controlling the two motors.

[0035] The drive control unit 31, which is for realizing the aforementioned control operation, calculates and outputs the engine throttle opening command value &thgr;t, the speed command value &ohgr;ar of the motor A and the torque command value &tgr;br of the motor B based on the information including the accelerator angle &thgr;a, the vehicle speed &ohgr;v, the torque command value &tgr;ar of the motor A and the currents Ia, Ib of the motors A, B. The throttle opening command value &thgr;t is sent to the throttle control unit 13, the motor A speed command value &ohgr;ar to the motor A control unit 14, and the motor B torque command value &tgr;br to the motor B control unit 15 thereby to actually control the engine and the motors.

[0036] The motor A control unit 14, based on the difference between the speed command value &ohgr;ar and the speed detection value &ohgr;a, produces the torque command value &tgr;ar in such a manner as to eliminate the difference by the proportional integral control or the like thereby to control the power converter 10. Also, the torque command value &tgr;ar involved is sent to the drive control unit 31.

[0037] Now, the configuration of the drive control unit 31 will be explained with reference to FIG. 2.

[0038] First, the target drive torque determining unit 21 determines a target drive torque &tgr;vr of the vehicle based on a map predetermined from the accelerator angle &thgr;a and the vehicle speed &ohgr;v.

[0039] In the overall control unit 22, the engine output and the change gear ratio are determined based on the target drive torque &tgr;vr and the vehicle speed &ohgr;v, and the engine operating point X (the target engine speed &ohgr;er, the target torque &tgr;er) is calculated. In the process, the operating point is determined in such a manner as to enable the engine to operate in an area as efficient as possible.

[0040] The engine control unit 23 determines the throttle opening command value &thgr;to in accordance with the target engine speed &ohgr;er and the target torque &tgr;er determined in the overall control unit 22. The correction value &Dgr;&thgr;t determined in the engine output correction unit 27 is added to &thgr;to to obtain &thgr;t, and a command is issued to the throttle control unit 13.

[0041] The motor A control unit 24 calculates the speed command value &ohgr;ar determined in equation (5) based on the target engine speed &ohgr;er determined in the overall control unit 22 and the actual measurement &ohgr;v of the vehicle speed and issues a speed command to the motor A control unit 14.

[0042] The motor B control unit 25 calculates the torque command value &tgr;br of the motor B by substituting &tgr;ar into &tgr;a of equation (6) based on the target drive torque &tgr;vr of the vehicle and the torque command value &tgr;ar of the motor A8 sent from the overall control unit 22, and issues a command to the motor B control unit 15.

[0043] The engine torque estimation unit 41 determines an estimated engine torque &tgr;e by the following method from the armature currents Ia, Ib of the motors A8, B9.

[0044] First, the input torque &tgr;ai of the motor A8 is calculated based on the following equation from the armature current Ia of the motor A8.

&tgr;ai =Pn&thgr;Iq+Pn(Ld−Lq)IdIq   (7)

[0045] where &thgr; is the magnetic fluxes interlinking the armature, Pn the number of poles, Id, Iq the Ia components along d and q axes, respectively, and Ld, Lq inductances of the armature winding along d and q axes, respectively.

[0046] Then, the output torque &tgr;a is calculated from the relation of equation (8)

&tgr;a=&tgr;ai−Ja(d&ohgr;a/dt)  (8)

[0047] where Ja is the inertia of the motor A8, and d&ohgr;a/dt the change rate of the rotational speed. The change rate of the rotational speed can be calculated from the difference of the rotational speed &ohgr;a or the like. A simple method of this calculation is to ignore the term of the change rate of the rotational speed and regard the input torque as an output torque.

[0048] This is also the case with the motor B, for which the output torque &tgr;b is calculated from the armature current Ib. The motor output torque &tgr;a and &tgr;b thus calculated are substituted into equation (3) thereby to determine the estimated engine torque &tgr;e.

[0049] The engine output correction unit 42 calculates the throttle opening correction value &Dgr;&thgr;t in accordance with the difference between the target engine torque &tgr;er and the estimated engine torque &tgr;e. The correction value is determined in such a manner as to increase the throttle opening in the case where the estimated torque is smaller, and to decrease the throttle opening in the case where the estimated torque is larger. By doing so, the engine output can be controlled so that the engine torque approaches the target torque.

[0050] FIG. 3 shows an example configuration for the proportional integral control. By setting the gains Kp and Ki appropriately, the control operation can be performed to eliminate the difference between the target torque and the estimated torque rapidly.

[0051] Now, the effect of correcting the engine output will be explained with reference to FIG. 4. Assume that the overall control unit 22 has determined a target operating point of the engine at point X on the best fuel consumption curve including the transmission efficiency and the motor efficiency based on a given target drive torque &tgr;vr and the vehicle speed &ohgr;v. The engine control unit 23 controls the throttle valve to attain the operating point at point X. In view of the fact that the engine characteristics change with the atmospheric pressure or the like, however, the target output may fail to be achieved. For example, the actual torque may deviate to point Y. By the way, the engine speed &ohgr;e, which can be controlled accurately by controlling the speed of the motor A, is assumed not to develop any deviation.

[0052] Once the actual operating point deviates from the target operating point in this way, the optimum operating point is missed, and therefore the fuel consumption rate may deteriorate. Also, the control method described above is intended to secure the required drive torque from the motor regardless of the engine torque, and therefore the engine output deviation from the target leads to an unexpected discharge or charge of the battery power. In the case of FIG. 4, for example, the engine output runs short and therefore the motor output increases correspondingly, resulting in the battery being discharged. A protracted situation of this battery discharge will cause the battery charging rate to deviate from the target value and therefore the need arises for an unexpected charging operation, thereby leading to an overall deterioration of the efficiency.

[0053] The present invention is intended for a control operation in which the target engine operating point is set at or in the vicinity of point X (optimum operating point) on the total best fuel consumption rate curve including the efficiency of the transmission and the motor. The use of this method can correct this engine torque deviation and restore the operating point at or in the vicinity of point X. Thus, the engine operates at the optimum operating point and the battery charging rate undergoes no unexpected change, thereby preventing the deterioration of the fuel consumption rate.

[0054] In the aforementioned example, the output torque is calculated from the armature current of the motor. As an alternative, an estimated engine torque value &tgr;e can be determined by substituting the torque command values &tgr;ar, &tgr;br directly into &tgr;a, &tgr;b in equation (3). In such a case, the estimation is possible using a simple method without using the motor current at the sacrifice of the likelihood of an estimation error being developed.

[0055] A similar effect is attained also by attaching a torque detector to the engine output shaft and using the output of the torque detector as an estimated torque value. In such a case, the detection accuracy is improved as compared with the estimation based on the motor torque.

[0056] Also, instead of correcting the output based on the deviation from the target engine torque value as in the aforementioned case, a similar effect can be obtained by a method of detecting the deviation from the target engine output. The engine output is determined as the product of the engine torque and the engine speed. In FIG. 2, therefore, this method can be accomplished by adding the engine speed information.

[0057] Now, another example configuration of the drive control unit will be explained with reference to FIGS. 5 and 6. In this example, the distance to be covered up to the destination constituting the navigation information, the current Ic flowing in the battery 12 and the battery charging rate SOC are input to the drive control unit 32.

[0058] The battery management unit 43 first produces a schedule for the battery charging rate based on the navigation information. In the case where a mountainous road and an ascending slope are in the way ahead, the battery charging rate is set to a larger value to provide a sufficient torque assistance by the motor. In the case where a descending road ahead is forecast, on the contrary, the battery charging rate is set to a smaller value to provide a sufficient regenerative braking. Also, in the case where an urban area is nearing and a low-speed run on the motor is expected, the battery charging rate is increased.

[0059] Then, the battery charging rate schedule thus prepared is compared with the current battery charging rate, and based on the difference, a target power value Pcr to be charged to (or discharged from) the battery is determined. At the same time, the target value Icr of the charge (or discharge) current for the battery is calculated. The target battery current value Icr is calculated by solving the quadratic equation (9), for example.

Pcr=IcVo+Ic2R  (9)

[0060] where vo is the electromotive force of the battery, and R the internal resistance of the battery. As for the signs attached to Pcr and Icr, the plus sign is defined as indicating the charging and the minus sign as indicating the discharge.

[0061] The overall control unit 22 determines the engine output and the change gear ratio based on the target drive torque &tgr;vr, the vehicle speed &ohgr;v and the target battery power Pcr, and calculates the engine operating point (target engine speed &ohgr;er, the target engine torque &tgr;er). In the case where the target battery power value Pcr is positive (charging), the target engine output value is the sum of the output for driving the vehicle and the output for charging the battery. In the case where the target value Pcr is negative (discharge), on the other hand, the target engine output is decreased correspondingly.

[0062] As described above, by correcting the target engine output value as required for the charge or discharge of the battery, the battery charging rate can be managed as targeted. In view of the aforementioned fact that the engine characteristics are subjected to various changes, however, the target output is not always produced. In the case where the target X is missed and the point Y is reached instead, as shown in FIG. 4, for example, the engine output decreases and the discharge increases correspondingly. Also, the loss occurring in the motor may change depending on the prevailing conditions. As a result, it may be that the target battery current cannot be secured, often making it impossible to manage the battery charging rate to the target value. In such a case, the correction is carried out by the engine output correction unit 44 as described below.

[0063] In the engine output correction unit 44, the throttle opening correction value &Dgr;&thgr;t is calculated based on the difference between the target battery current value Icr and the detected current value Ic. In the case where the detected value is smaller, the throttle valve opening is increased to increasing the charging rate, while in the case where the detected value is larger, the throttle opening value is reduced. As a result, the engine operating point is corrected from point Y to point X, for example, in FIG. 4 thereby making it possible to control the battery current toward the target value. As a configuration of the control system, the proportional integral control similar to that shown in FIG. 3 can be used. By doing so, the engine output, even if it deviates from the target value, can be corrected so that the battery current attains the target value.

[0064] Now, another configuration example of the drive control unit 32 will be explained with reference to FIG. 7. In this example, the engine output correction unit 45 outputs the battery power correction value &thgr;Pcr but not the throttle opening correction value &Dgr;&thgr;t based on the difference between the target battery current value Icr and the detected battery current value Ic, and adds the battery power correction value &Dgr;Pcr to the target battery power value Pcr output from the battery management unit. In the case where the detected battery current value is smaller than the target battery current value, the target battery power value is corrected upward. By doing so, the target engine output value is corrected and therefore the engine output is indirectly corrected, thereby making it possible to control the battery current as targeted.

[0065] Further, an example configuration with the cases of FIGS. 2 and 7 combined is shown in FIG. 8. In this case, the engine output correction unit 42 outputs the throttle opening correction value &Dgr;&thgr;t based on the estimated engine torque value, and the engine output correction unit 45 outputs the battery power correction value &Dgr;Pcr based on the detected battery current value. With this configuration, the engine is always kept at the optimum operating point, while at the same time controlling the battery current to the target value.

[0066] The foregoing description concerns the case in which the schedule for battery charging rate is prepared by the battery management unit 43 using the navigation information. The present invention is applicable, however, also to the case where the charging rate is managed simply by setting the upper and lower limits thereof without using the navigation information. In such a case, too, the battery charge and discharge can be controlled to the target by correcting the engine output while always securing the vehicle driving force.

[0067] Now, an explanation will be given of the case in which the invention is applied to an ordinary hybrid vehicle other than shown in FIG. 1.

[0068] FIG. 9 shows a hybrid vehicle comprising an engine 1, a transmission 17, a motor 16 for changing the drive torque, and a power converter 11 and a battery 12 for driving the motor. The drive control unit 30 outputs the engine throttle opening command value &thgr;t, the change gear ratio command value r, the motor torque command value &tgr;r based on the information including the accelerator angle &thgr;a and the vehicle speed &ohgr;v. The throttle opening command value &thgr;t is sent to the throttle control unit 13, the change gear ratio command r to the transmission control unit 19, and the motor torque command &tgr;r to the motor control unit 15.

[0069] The configuration of the drive control unit 34 will be explained with reference to FIG. 10. The target torque determining unit 21, the overall control unit 22, the engine control unit 23 and the battery management unit 43 are similar to the corresponding parts shown in FIG. 6. The transmission control unit 27 calculates the change gear ratio command value r from the target engine speed &ohgr;er determined by the overall control unit 22 and the actual measurement &ohgr;v of the vehicle speed, and issues a command to the transmission control unit 19. The motor control unit 26 calculates the torque required of the motor for assistance, from the target drive torque &tgr;vr of the vehicle and the target engine torque &tgr;er, and outputs a motor torque command &tgr;r.

[0070] The engine output correction unit 44 operates similarly to the case of FIG. 6 and calculates the throttle opening correction value &Dgr;&thgr;t based on the difference between the target battery current value Icr and the detected battery current value Ic output from the battery management unit 43. In the case where the detected value is smaller, the throttle opening value is increased to increase the charging rate, while in the case where the detected value is larger, the throttle opening is decreased. As a result, the battery current can be controlled to approach the target value.

[0071] There is also a method for correcting the battery current which may be different from the target value in response to a command from the motor. This method, however, employed is at the risk of failing to achieve the target drive torque of the vehicle. In this method, if the motor is controlled to produce the target drive torque and the battery current is corrected on the engine side, the battery charging rate can be managed while at the same time producing a target drive torque.

[0072] It will thus be understood from the foregoing description that according to this invention, the engine output is corrected while maintaining a target vehicle driving force, thereby making it possible to control the engine operating point and the battery charging rate to the target, thereby improving the fuel consumption rate of the vehicle as a whole.

Claims

1. A hybrid vehicle comprising:

an engine for generating the energy for driving the vehicle;

a transmission for changing the engine speed and transmitting the driving force to the wheels;

at least a motor for increasing/decreasing the wheel driving force;

a battery for supplying power to said motor; and

a drive control unit for calculating and outputting an operation command for said engine and said motor based on the operation information including the accelerator angle;

wherein said drive control unit includes an engine output correction mechanism for correcting the operation command value for said engine and maintaining an optimum operating point of said engine.

2. A hybrid vehicle comprising:

an engine for generating the energy for driving the vehicle;

a transmission for changing the engine speed and transmitting the driving force to the wheels;

a motor for increasing/decreasing the wheel driving force;

a battery for supplying power to said motor;

means for determining a target torque value of said engine;

means for calculating the engine torque generated by said engine; and

means for correcting the output of said engine based on the difference between the target value of the engine torque and the engine torque calculated by said engine torque calculation means.

3. A hybrid vehicle according to

claim 1,

wherein the torque generated by said engine is calculated based on the detected torque value of said motor or a toque command value.

4. A hybrid vehicle according to

claim 2,

wherein said torque generated by said engine is calculated based on the detected torque value of said motor or a torque command value.

5. A hybrid vehicle comprising:

an engine for generating the energy for driving the vehicle;

a transmission for changing the engine speed and transmitting the driving force to the wheels;

a motor for increasing/decreasing the wheel driving force;

means for determining a target output value of said engine;

means for calculating the output generated by said engine; and

means for correcting the output of said engine based on the difference between the target engine output value and the engine output value calculated by said engine output calculation means, thereby maintaining an optimum operating point of said engine.