The present invention relates to a method for correcting the ignition advance in a transient state of an internal-combustion engine propelling a motor vehicle. More particularly, the invention relates to an application of this method to the smoothing out of jerks in the speed of the engine on changing the speed-reducing gear ratio of a gearbox mounted on the output shaft of the engine.

After changing this gear ratio in the gearbox, jerks are at present observed in the speed of the engine during the recoupling of the transmission to the output shaft of the engine through the clutch and the gearbox. In Figure 1 of the attached drawing, the change with time in the absolute pressure P at the induction manifold of an internal-combustion engine and that of the speed R of this engine during such a gear-ratio change are represented. Also represented is the change with time in the ignition-advance angle A of the engine (with respect to top dead centre) as is conventionally established by an ignition-advance computer in which this advance is "mapped" as a function of the speed R of the engine and the induction pressure P so as to optimise the torque provided by the engine. As represented in this Figure 1, prior to passing to a higher gear ratio for example, the speed of the engine begins to increase because of the action of the driver on the accelerator pedal, the pressure at the induction manifold increasing at the same time. The driver then disengages the clutch, changes the gear ratio and re-engages the clutch, the speed falling during these phases (by 1000 to 1200 rpm for example) until the moment (t _x ) when the driver reaccelerates in order to increase the speed once again. As is visible in Figure 1, this acceleration is accompanied by an

oscillation or "surging" of the speed R, which is progressively damped out in order to tend towards a steady state, this oscillation of the speed being manifested by "jerks" which are detrimental to the driving comfort of the vehicle.

In order to eliminate these jerks, a transient correction of the ignition-advance angle has been proposed which is introduced briefly at the start of the reacceleration actuated by the driver, this correction aiming substantially to reduce this advance in order thus to diminish the torque provided by the engine and to facilitate the transition. The correction has a fixed value or is a function of the speed and/or of the pressure at the induction manifold and may be steadily decremented. This solution, which is only implemented when a pressure on the accelerator pedal is detected, has been shown to be insufficient to reduce satisfactorily the jerks observed during the reacceleration.

The object of the present invention is therefore to reduce these jerks effectively, so that the latter do not disturb the driving comfort.

This objective of the invention, together with others which will emerge on reading the present description, has been achieved by means of a method for correcting the ignition advance in a transient state of an internal-combustion engine propelling a motor vehicle, this advance normally being calculated as a function of the speed and of the pressure at the induction manifold of the engine. According to the invention, an entry into a transient state of the engine is detected and the ignition advance is then corrected as a function of the time derivative of the induction pressure, substantially until leaving the transient state.

As will be seen below, taking into account the derivative with respect to time of the induction pressure in order transiently to correct the ignition advance normally calculated as a function of the speed and of this induction pressure of the engine, for example, makes

it possible substantially to attenuate the oscillations in the speed of the engine which are observed on reacceleration which follows a gear-box speed-reducing gear-ratio change for example, thus substantially improving the driving comfort of the vehicle.

For the implementation of this method in a device for digitally controlling the operation of the engine where the induction pressure is sampled, the transient- state advance correction (TACi) at the sampling instant (i) is obtained from the expression:

Pi

where P ₁₇ P _i . ₁ are samples of the induction pressure and k is a factor between 0 and 1.

According to another implementation of the method according to the invention, in such a sampled system, the transient-state advance correction (TAC at the sampling instant (i) is obtained from the expression:

Pi - PΠL

TACi x k

where Pi is a sample of the induction pressure, k is a factor and Pitii is a filtered average pressure obtained by the recurrence formula:

Pπii = Pirii. _! + (Pi - Pπii.i) x k' where k' is a factor between 0 and 1.

Other characteristics and advantages of the present invention will emerge on reading the description which follows and on examining the attached drawing in which:

- Figure 1 represents the time graphs of the absolute pressure at the induction manifold, of the speed and of the ignition-advance angle in an internal- combustion engine equipped with a device for controlling the ignition-advance angle as a function of this pressure

and of this speed, graphs such as these having been discussed in the preamble of the present description, and - Figure 2 is similar to Figure 1 and illustrates the advance correction method according to the invention. The ignition-advance angle A of an internal- combustion engine propelling a motor vehicle, for example, is regulated by mechanical or electronic means so as to optimise the torque provided by the engine at the various gear ratios available, in the steady state. The laws for controlling this angle usually take into account the absolute pressure P at the induction manifold (or the inducted-air flow rate) and the speed R of the engine. Nowadays electronic control devices are commonly used in which these laws are "mapped" as a function of these variables, as has been seen above.

During a transient-state operation of the engine, due for example to a change in the speed-reducing gear ratio of the gearbox, which change is executed by the driver of the vehicle, the fluctuations represented in Figure 1 are observed for the ignition advance A, which fluctuations roughly follow those of the speed of the engine, which speed is affected by exaggerated oscillations when the driver accelerates after execution of the gear-ratio change (as from the instant t _x ) , these oscillations generating "jerks" which lower the level of driving comfort.

According to the present invention, the amplitude of these jerks is substantially reduced, in the transient state of the operation of the engine, by a subtractive correction TAC of the ignition-advance angle A normally established by an electronic device for calculating and controlling this angle, this correction being a function of the derivative with respect to time of the induction pressure during this transient state. Various means may be used for determining the presence of a transient state requiring the application of the correction according to the invention.

It is possible, for example, to command the application of the correction TAC when the absolute value of the derivative of the induction pressure with respect to time exceeds a predetermined threshold. As a variant, it is possible to trigger the advance correction TAC with the aid of a sensor which is sensitive to the position or to the position variation of an accelerator pedal of the vehicle. It is also possible to apply the advance correction TAC under the control of a sensor which is sensitive to the state of a clutch member disposed in the transmission of the vehicle.

Current devices for electronically controlling the ignition and/or the injection of fuel into an internal-combustion engine use as input data variables such as, especially, the absolute pressure at the induction manifold and the speed of the engine, variables which are sampled and digitised. In such a device, the transient-state advance correction (TACi) ^ ^s calculated according to the invention at the sampling instant i from the expression:

' Pi - Pi-!

TACi = x k

where Pi, P^ are samples of the induction pressure and k is a factor between 0 and 1.

It appears that this expression indeed introduces the derivative of the pressure P in a digital form, because of the presence of the difference (P _t - P^) in the pressures observed at the end of a sampling period of fixed duration.

This correction TAC is represented in Figure 2 where it appears that the latter effectively follows the variations of the slope of the graph of the induction pressure during the transient phase which coincides with the continuous lowering of the speed R of the engine and which is over when the driver (at the instant t' _x ) presses on the accelerator again.

Such a transient phase, and the speed fluctuations which it causes, may result especially, as has been seen earlier, from manoeuvres executed by the driver in order to change the speed-reducing gear ratio of a gearbox associated with the engine. The advance correction TAC according to the invention is applied at the moment when the engine is decoupled from the transmission by an action of the driver on a pedal controlling a clutch included in this transmission. If the oscillations which are observed on reacceleration of the engine by the driver, after executing the gear-ratio change in the case of a vehicle for which the ignition advance is conventionally controlled (Figure 1, after the instant t _x ) are compared with those in the case of the same vehicle after introduction of the ignition-advance correction TAC according to the invention (Figure 2, after the instant t' _j j, it is clear that the amplitude of the speed oscillations then observed is greatly reduced by the implementation of the invention (by approximately half) . It appears that the correction TAC subtracted from the normally calculated advance (see Figure 1) gives, in the transient phase, the advance profile A observed in Figure 2 where this profile reveals a complete cancellation of the advance coinciding with the steep-slope variations of the induction pressure. The two major reductions in the engine torque caused by these cancellations of the advance (from non-corrected values of the order of 30 to 40° ahead of top dead centre, as illustrated in Figure 2) prepare the engine for the reacceleration required by the driver which is then put into operation without any appreciable jerks.

As an alternative form of the implementation of the method according to the invention described hereinabove, it is possible to obtain the correction TAC _r from the expression:

TACi = i where Pi is a sample of the absolute pressure at the induction manifold, k is a factor and Pπii is a filtered average pressure. The filtering of this average pressure is obtained by the recurrence formula: where k' is a factor between 0 and 1. This procedure makes it possible to diminish the sensitivity to small pressure variations of the correction method according to the invention.

Of course, the invention is not limited to the embodiments described and represented which have been given merely by way of example. Thus the invention is not limited to the smoothing-out of jerks observed on changing the speed-reducing gear ratio of the gearbox. It is also applicable to any transient state resulting from controlled accelerations or decelerations, for example with the aid of the accelerator pedal. A position sensor for this pedal, such as a sensor which is sensitive to the "foot up"/"foot down" transition, in either direction, may serve to detect the presence of a transient state. As a variant, the existence of such a state may result from observing the variations of the position of this pedal if the latter is associated with a potentiometric sensor. Various additional measures may then be taken to ensure the correct operation of these variants for implementing the method according to the invention. Thus, so as not to lower the level of the driving agreeableness by too abrupt an advance reduction, it is preferable to delay the activation of the correction (for example by a few crankshaft revolutions) . Such a delay is not annoying from the point of view of driving agreeableness to gear-ratio changes. Likewise, setting time limits for the advance correction is necessary during the reacceleration phase in order not to maintain a zero or reduced advance for too long, which

would otherwise lower the level of the engine torque. The next step is a progressive return to the nominal advance, in stages of a few degrees per crankshaft revolution- for example. Finally, in order not to disrupt the operation of the vehicle at start-up, the correction according to the invention may be inhibited for as long as the vehicle is not moving.

Furthermore, although the invention is described as incorporated into a device for regulating the ignition advance operating from signals provided by induction- pressure and engine-speed sensors, the invention may naturally be extended to devices in which the pressure sensor is replaced by an inducted-air flow-rate sensor.