GB2208410A

GB2208410A - Control of i.c. engine fuel quantity or ignition timing to suppress vehicle drive train oscillations

Info

Publication number: GB2208410A
Application number: GB8718281A
Authority: GB
Inventors: John Philip Tonkin
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1987-08-01
Filing date: 1987-08-01
Publication date: 1989-03-30
Anticipated expiration: 2007-08-01
Also published as: GB2208410B; GB8718281D0

Abstract

In response to vehicle acceleration the quantity of injected fuel or the ignition timing is adjusted to reduce jerkiness during acceleration. The retardation or advance from a basic timing may occur above and below aceptable values of acceleration which values may be dependent upon throttle position, curve 28 providing for large and 30 for small throttle positions with the change over between control curves occurring at a predetermined throttle position. <IMAGE>

Description

ENGINE CALIBRATION The present invention relates to engine calibration, that is to say to the setting of ignition advance and mixture strength in dependence upon engine operating conditions.

It is known that the drive train in a vehicle can be the cause of certain oscillations during acceleration. These oscillations can be caused, for example, by softness in the mounting of the various elements of the drive train or by the shock absorbing springs in a clutch plate. The jerkiness in the movement of the vehicle resulting from such oscillations is naturally undesirable and the present invention seeks to mitigate this problem.

According to the present invention, there is provided an engine control system which includes means for sensing vehicle acceleration and means for varying the engine calibration in dependence upon the sensed acceleration so as to suppress drive train induced oscillations.

Preferably, the control system is operative to derate engine performance in response to excessive acceleration by retarding the ignition as such control can be effected rapidly but it is alternatively possible to vary the injection quantity in a fuel injected system in order to achieve the same aim of providing negative feedback.

It is preferred not to derate the engine performance during wide throttle positions as maximum torque is required by the driver and drive train jerkiness is a minor consideration under these conditions.

It is also preferred that the engine performance should not be derated at high engine speed, nor at very low vehicle speed, nor during fast throttle movement.

In the preferred embodiment of the invention, engine ignition timing is varied as a function of sensed acceleration in such a manner that within a first permissible band, corresponding to normal acceleration, ignition timing is not significantly modified, when the acceleration exceeds a first threshold ignition is retarded progressively with increasing accelerations and below a second lower threshold ignition is advanced progressively with reducing accelerations.

The thresholds may either be fixed thresholds or may vary with throttle position and the variations in threshold values may either be a continuous or a stepped function of the throttle position.

The invention is capable of being implemented by means of analogue and digital circuits. A conventional ignition system may be modified to include an element which introduces an acceleration dependent time delay in the spark timing but electronic systems lend themselves particularly simply to such modification since ignition timing is computed on a cycle by cycle basis and the invention may be implemented by a few additional lines of code in the control program. The addition required for implementation of the invention comprises sensing if certain operating conditions apply and using a look-up table to determine the correct ignition advance as a function of the sensed acceleration.

It is a particular advantage of the invention that computerised ignition systems already have available all the necessary input signals for implementation of the invention and accordingly the problem of drive train induced oscillations can be mitigated in such systems without any increase in hardware.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which Figure 1 is a graph showing the effect of drive train induced oscillations on vehicle speed and acceleration, and Figure 2 is a graph showing the spark advance setting as a function of acceleration for different positions of the throttle.

The lines designated 10 and 12 in Figure 1 represent respectively the variation of acceleration and speed with time when a vehicle is driven away smoothly from a stand still. The effect of oscillations caused by the drive train on the acceleration is indicated by the dotted line 14 and the effect on vehicle speed by the dotted line 16.

The amplitude of modulation of the acceleration curve is not insignificant when compared with magnitude of the steady acceleration and it is possible to detect excessive acceleration caused by jerkiness to provide a feedback signal which can be used to regulate the engine calibration. In general terms, to avoid jerkiness in the motion of the vehicle, the engine output power should be modulated in inverse proportion to the difference between the actual vehicle acceleration and the desired steady acceleration represented by the line 10. Thus in the shaded areas 18 of the graph, engine output torque should be reduced and in the shaded areas 20 the output torque should be increased. In practice, engine performance can be derated substantially by retarding the ignition timing but only slight improvement can be gained by advancing the ignition.Thus, reduction in output torque synchronised with the acceleration peaks is used to reduce the extent of modulation of the acceleration curve.

Because operation of the oscillation suppressing feedback control of the invention tends to reduce maximum power, there are occasions when it may be desirable to override such control so as not to interfere with the normal calibration of the engine, n-twithstanding jerkiness in the motion of the vehicle.

In particular, one may deduce from the certain operating parameters that the vehicle is being driven for high or maximum power and under such conditions it is not desirable to derate engine performance. Thus it is preferred to inhibit the feedback control whenever the throttle is wide open, when the engine speed is high and when the throttle has just been rapidly depressed.

It is also preferred to inhibit operation of the feedback control when the vehicle speed is very low for example less than 1 kph.

Assuming that none of the above conditions obtains, the preferred strategy for varying ignition advance with vehicle acceleration is shown in Figure 2. This graph shows two control curves 28 and 30, of which the first 28 determines the ignition advance at high throttle positions and the second 30 at small throttle positions. A change over from one control curve to the next occurs at a predetermined throttle position.

Each of the two curves 28 and 30 comprises a central dead band 28a, 30a during which the acceleration is within permissible limits and no attempt is made to modulate engine output power. The additional advance of the ignition timing approximates to zero for both curves but it is to be noted that the dead band 28a is wider than the dead band 30a for obvious reasons. Outside the dead band, the ignition is retarded (28b and 30b) in response to high acceleration and advanced (28c and 30c) in response to low acceleration.

The control strategy represented by the graphs in Figure 2 can readily be implemented using analogue or digital circuitry and in the case of a digital implementation, the circuit may be a dedicated control circuit or a suitably programmed engine management computer. A signal indicative of the acceleration can be derived by differentiating the speed signal or in the case of a digital circuit computing the difference between consecutive speed samples.

The acceleration signal may now be compared with a reference dependent upon the throttle positions to provide a signal for controlling a time delay element. The variable time delay element may, for example, be a flipflop circuit and in the case of a computer based implementation, the desired timing can be obtained from one of two permanently stored look-up tables which provide the ignition advance as a function of acceleration, the table being selected as a function of the prevailing throttle position.

It will be clear that alternative control strategies may be adopted and one need not select between two control curves as function of throttle position. Indeed, suitable algorithms may be developed to permit the spark advance/retard to be varied continuously as a function of throttle position or other engine control parameters.

While the invention has been described by reference to feedback control acting on the ignition system, it is alternatively possible to regulate engine performance by modify the injection quantity in a fuel injection system and this can apply to both spark ignited and diesel internal combustion engines. The essential feature of the invention is that performance is modulated in dependence upon acceleration in order to suppress drive train induced oscillations.

Claims

1. An engine control system comprising means for sensing vehicle acceleration and means for varying the engine calibration in dependence upon the sensed acceleration so as to suppress drive train induced oscillations.

2. An engine control system as claimed in claim 1, wherein the control system is operative to derate engine performance in response to excessive acceleration by retarding the ignition.

3. An engine control system as claimed in claim 1, wherein the control system is operative to derate engine performance in response to excessive acceleration by varying the quantity of fuel injected into the engine cylinders during each engine cycle.

4. An engine control system as claimed in any preceding claim, comprising means for inhibiting during selected operating conditions operation of the feedback control tending to suppress drive train induced oscillations.

5. An engine control system as claimed in claim 4, wherein the operation of the feedback control tending to suppress drive train induced oscillations is inhibited during any one or more of the following conditions, namely wide open throttle, high engine speed, low vehicle speed and rapid variation in throttle position.

6. An engine control system as claimed in claim or any claim appended thereto, wherein engine ignition timing is varied as a function of sensed acceleration in such a manner that within a first permissible band, corresponding to normal acceleration, ignition timing is not significantly modified, when the acceleration exceeds a first threshold ignition is retarded progressively with increasing accelerations and below a second lower threshold ignition is advanced progressively with reducing accelerations.

7. An engine control system as claimed in claim 6, wherein the thresholds vary with throttle position.

8. An engine control system substantially as herein described with reference to and as illustrated in the accompanying drawings.