WO2001007782A1

WO2001007782A1 - Engine spark timing management

Info

Publication number: WO2001007782A1
Application number: PCT/GB2000/001814
Authority: WO
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Global Technologies, Inc.
Priority date: 1999-07-24
Filing date: 2000-05-12
Publication date: 2001-02-01
Also published as: GB2352478A; GB9917319D0

Abstract

A method is disclosed for reducing the light-off time of a catalytic converter in an exhaust system of a multi-cylinder spark ignition engine. The spark timing for all the engine cylinders is retarded to such an extent that combustion gases enter the exhaust system before their combustion is complete to generate heat in the exhaust system for heating the catalytic converter. To compensate for the resulting engine instability, the spark timing of individual cylinders is advanced from the retarded setting in a periodic sequence such that higher output torque pulses are produced periodically by the engine to sustain a stable engine speed.

Description

Engine Spark Timing Management

Field of the invention

The present invention relates to an engine management system that controls the spark timing of a multi-cylinder spark ignition engine and in particular to a system that employs delayed spark timing during cold idling conditions to reduce catalyst light-off time.

Background of the invention

It has previously been proposed to resort to delayed spark timing to reduce catalyst light-off time. The effect of delaying the spark is to increase the exhaust gas temperature because the combustion of the gases that is initiated in the combustion chambers is not completed until after the burning gases enter the exhaust system. For complete combustion to occur in the exhaust system, it is important that there should be oxygen present in the exhaust gases and for this reason the mixture in the combustion chamber must either be stoichiometric or lean.

Normally, during cold idling conditions, the engine runs better with a rich mixture and using a stoichiometric or lean mixture could cause instability. Retarding the spark aggravates this problem, particularly since the engine during this time is producing hardly any output power and is only just capable of ticking over.

The need to maintain engine stability limits the extent to which the ignition timing can be retarded and therefore limits the rate at which the catalytic converter can be heated to its light-off temperature. Object of the invention

The present invention therefore seeks to provide an engine management system m which the ignition can be retarded sufficiently to provide a marked improvement m the speed of light-off of a catalytic converter in the exhaust system while maintaining engine instability within acceptable limits.

Summary of the invention

According to the present invention, there is provided a method of reducing the light-off time of a catalytic converter in an exhaust system of a multi-cylinder spark ignition engine, which comprises setting a retarded spark timing for all the engine cylinders, the spark retard being such that combustion gases enter the exhaust system before their combustion is complete, and advancing the timing of individual cylinders from the retarded setting m a periodic sequence such that higher output torque pulses are produced periodically by the engine to sustain a stable engine speed.

The effect of the periodic advancing of the spark timing is to produce a stable average engine output torque comprised of a series of stronger torque pulses from the earlier firing cylinders spaced in a constantly repeating pattern between groups of weaker torque pulses from the retarded cylinders, the latter generating exhaust heat but little output power.

According to another aspect of the invention, there is provided an engine management system for controlling the spark timing of a multi-cylinder spark ignition engine the system, which system comprises means for setting a retarded spark timing for all the engine cylinders, the spark retard being such that combustion gases enter the exhaust system before their combustion is complete, and means for advancing the timing of individual cylinders from the retarded setting in a periodic sequence such that higher output torque pulses are produced periodically by the engine to sustain a stable engine speed.

In the invention, for a constant throttle setting and fixed fuel calibration, the stronger torque pulses from the earlier firing cylinders are sufficient to mask any instability so that the engine will feel smooth and stable even though some cycles in between the stronger torque pulses may be unstable. In effect, the engine note will resemble that of an engine with fewer cylinders.

It is important to ensure that it is not always the same cylinder or the same small group of cylinders that provides the stronger torque pulses . This is because it is important that all cylinders should heat up at the same rate, that their emissions should be matched and that their spark plugs should all be kept clean. This cycling of the high torque cycles between all the engine cylinders can be achieved by ensuring that the periodicity of the earlier firing torque producing cycles and the firing pattern of the cylinders do not share a common factor and are not therefore harmonically related to one another.

This can be understood from the following table below which shows as an example of the firing pattern for a four cylinder engine operated with a High/Low torque periodicity of one m three,

1 3 4 2 1 3 4 2 1 3 4 2 1 3 4 2 1 3 4 2 1 H L L H L L H L L H L L H L L H L L H L L

In this example, the h gh torque pulses are produced in sequence by all four cylinders 1 2 4 3 and all the cylinders will heat up at the same rate. The same would apply if the pattern of high and low torque timings were two in three 1 3 4 2 1 3 4 2 1 3 4 2 1 3 4 2 1 3 4 2 1 H H L H H L H H L H H L H H L H H L H H L

only in this case the engine would fire more regularly but the heating effect of the catalyst would be reduced.

On the other hand, with a six cylinder engine and a periodicity of one in three, one would obtain the following table

1 4 2 5 3 6 1 4 2 5 3 6 1 4 2 5 3 6 1 4 2 5 3 6 H L L H L L H L L H L L H L L H L L H L L H L L

This time it will be seen that only cylinders 1 and 5 ever produce high torque pulses and the other cylinders are permanently operated with cold start retard.

The same problem is experienced with a six cylinder engine and a spark timing periodicity of one in four, where one would obtain the table

1 4 2 5 3 6 1 4 2 5 3 6 1 4 2 5 3 6 1 4 2 5 3 6 H L L L H L L L H L L L H L L L H L L L H L L L

Here, only cylinders 1 3 2 produce high torque pulses and one end of the engine block would end up hotter than the other end. In both these cases, the number of cylinders and the spark advance periodicity share a common factor.

The preferred periodicity for a six cylinder engine is five but one could use a periodicity of seven, eleven or thirteen, these being numbers that do not share a common factor with six. With any of these periodicities, it is necessary to have more than one higher torque pulse in each repeat pattern as otherwise the high torque pulses would be too few and far between to maintain smooth engine operation. If approximately half of the engine torque pulses are higher torque pulses, then the engine will sound like an engine w th naif the number of cylinders, which in the case of a four cylinder or six cylinder engine is perfectly acceptable during idling. A spark advance periodicity of three in seven (i.e. H L H L H L L) will work for all common engines with three, four, five, six, eight, ten and twelve cylinders and is preferred because it will enable the engine management system to operate on all common platforms .

The spark retard for improving catalyst light off must be disabled either when the vehicle is driven off or when the catalyst reaches its light-off temperature. During disablement, it is preferred that there should not be a sudden power surge as the engine switches between operating modes . This can be achieved by gradually reducing the spark retard for all the cylinders or by increasing the number of cylinders producing higher torque in the spark advance repeat pattern. Thus, one can progress from a three in seven periodicity, to four then five then six m seven until all cylinders are returned to normal spark timing.

It is an important advantage of the invention that it can be implemented entirely in software and does not require any additional costly hardware to achieve fast catalyst light-off. Despite its low cost, the system can achieve catalyst warm-up rates that compare favourably with those obtained using electrically heated catalysts.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawing, which is a flow chart of a spark timing management system. Detailed description of the preferred embodiments

The hardware construction of an engine management system is known per se and need not therefore be described in detail within the present context. Essentially, a microcomputer is connected to receive a variety of input signals representative of such engine parameters as throttle pedal position, engine load, engine temperature, the angular position of the crankshaft etc and is programmed to produce output signals for setting for each cylinder the spark timing, the fuel injection quantity, the fuel injection timing etc. The present invention is concerned not with the hardware but with the control algorithm used to reduce catalyst light-off time, the algorithm being represented by the flow chart m the drawing.

Prior to sending eacn instruction to generate one or more sparks m an engine cylinder, it is first determined m block 10 if the engine is warm. If so, the catalytic converter will already have achieved light-off and the spark timing is set in block 12 to its normal value, which will itself depend on certain parameters such as engine load, engine speed and air-fuel ratio.

Spark retard for improved catalyst light-off as used m the present invention affect engine output power and can only be used when the engine is idling. It is therefore next decided in block 14 if the engine is idling. If the engine is under load, then the spark timing is again set in block 12 in the normal manner but if the engine is found to be idling and cold then the program proceeds to block 16 where spark timing for all engine cylinders is retarded.

The degree of spark retard set in block 16 is such that charge combustion is only started in the cylinder and is not completed until after the burning gases enter the exhaust system. This generated heat n the exhaust gases but as the spark is not fired until the piston is well advanced into the power stroke, the amount of power and output torque produced by the combustion is very small. This lack of power adversely affects engine stability and places limits on the extent of spark retard that can be employed.

In the present invention, it is possible to set a value of spark retard that would normally cause unacceptable engine instability and to compensate for this lack of torque by periodically allowing different cylinders to fire with a less retarded spark timing setting. In the example illustrated the cylinders fire with a periodicity of three in seven following the repeating sequence L H L H L H L. In other words, the high and low torque cycles nearly alternate with each other simulating the operation of an engine with approximately half (or more precisely three sevenths) the number of cylinders. The reason for not using a periodicity of say two or four with a four cylinder engine is that this would result in some of the cylinders being operated permanently m the low torque spark retard mode, which is to be avoided for the reasons given previously, namely variation m temperature, emissions and spark plug fouling between cylinders.

To achieve the three in seven periodicity, the spark events are counted in block and the count is reset by the loop 20, 22 every time the count reaches a value of seven. Within each pattern of seven spark events, selected ones are advanced while others are left retarded. In the illustrated embodiment, the even counts are advanced in blocks 24 and 26 to produce higher torque pulses whereas for the odd counts the spark timing is left retarded to generate more heat in the exhaust system.

Instead of just deciding in decision block 24 if the count is odd or even, one could set up a different selection criterion to vary the repetition pattern. For example, to achieve a periodicity of five in seven it could be decided in block 24 if the count matches one of the numbers 1 2 3 5 and 6. Such a repetition pattern would yield more power but less exhaust heat.

The possibility to vary the selection criteria in block 24 allows the engine to be returned from a catalyst heating mode to a power producing mode in a gradual manner. Hence it is possible to reduce the number of cylinders in each repeat pattern that are operated in low torque mode gradually until all cylinders are fired normally.

Because of the regular higher torque pulses, the engine can operate with a steady speed albeit sounding like an engine with fewer cylinders. However, as this effect will only ever occur during cold idling, it will be barely perceptible to the driver.

Claims

1. A method of reducing the light-off time of a catalytic converter in an exhaust system of a multi-cylinder spark ignition engine, which comprises setting a retarded spark timing for all the engine cylinders, the spark retard being such that combustion gases enter the exhaust system before their combustion is complete, and advancing the timing of individual cylinders from the retarded setting in a periodic sequence such that higher output torque pulses are produced periodically by the engine to sustain a stable engine speed.

2. A method as claimed in claim 1, wherein the number of the engine cylinders and the periodicity of the spark timing sequence do not share a common factor.

3. A method as claimed in claim 2, wherein the spark timing of the cylinders is advanced with a periodicity of seven, the spark timing of three or four cylinders in each seven consecutive engine firing events being set to generate a higher torque pulse.

4. An engine management system for controlling the spark timing of a multi-cylinder spark ignition engine the system, which system comprises means for setting a retarded spark timing for all the engine cylinders, the spark retard being such that combustion gases enter the exhaust system before their combustion is complete, and means for advancing the timing of individual cylinders from the retarded setting in a periodic sequence such that higher output torque pulses are produced periodically by the engine to sustain a stable engine speed.