DISI INJECTION TIMING STRATEGY

Abstract

A control strategy for a DISI engine fitted with a catalytic converter, for reducing the light off time of the catalyst during cold starts, which comprises setting the ignition timing for each engine cylinder and injecting at least a proportion of the fuel, the timing of the injection in relation to the ignition timing being varied as a function of an engine operating condition.

Description

FIELD OF INVENTION

This invention relates to direct injection spark ignition engines and more particularly to the timing of the injection of fuel to improve emissions.

BACKGROUND AND SUMMARY OF THE INVENTION

With a view to meeting ever more stringent emissions legislation, rapid catalyst light-off is increasingly desirable. In order to achieve this in direct injection spark ignition (DISI) engines, significant levels of spark retard may be used to generate the necessary exhaust gas temperatures.

In attempts of prior art DISI engines to retard ignition timing, undesirable emissions and less than ideal combustion placed a limit on the degree of retard that could be achieved, and thus the amount of accelerated catalyst heating.

With a view to mitigating the foregoing disadvantage, the present invention provides a control strategy for a DISI engine fitted with a catalytic converter, for reducing the light off time of the catalyst during cold starts, which comprises setting the ignition timing for each engine cylinder and injecting at least a proportion of the fuel, the timing of the injection in relation to the ignition timing being varied as a function of an engine operating condition.

In one embodiment of the present invention the ignition timing occurs during the expansion stroke.

In yet another embodiment of the present invention, the timing of the injection in relation to the ignition timing is measured from the end of the injection to the spark timing.

In an alternative embodiment, an initial proportion of fuel may be injected prior to the secondary injection described above.

Alternatively, the initial injection of fuel may occur during the intake or compression strokes.

In yet another embodiment, the timing of the secondary is injection may be a function of any one or more of exhaust gas temperature, engine speed, engine coolant temperature, engine oil temperature, throttle position, ambient air temperature, ambient air pressure, intake air pressure, intake air temperature and compression ratio.

It will be appreciated that features of the invention are susceptible to being combined in any combination without departing from the scope of the invention as defined by the accompany claims.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, embodiments of the present invention will now be described with reference to the accompanying drawings wherein:

FIG. 1 is schematic section through a cylinder of a DISI engine;

FIG. 2 is a graph showing cylinder pressure versus crank angle in a DISI engine operated according to a first embodiment of the present invention; and

FIG. 3 is a similar graph showing cylinder pressure in a DISI engine operated according to a second embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT(S)

FIG. 1 shows a cylinder of a DISI engine 10. This is much the same as a conventional spark ignition engine with the addition of a high-pressure fuel injector 12 spraying fuel directly into the cylinder. The injector 12 is controlled by an injector driver 22 that receives a signal from a central processing unit 14 (CPU). The mixture within the cylinder is ignited by a conventional spark plug 18, controlled by an ignition system 20, again triggered by the CPU 14. Exhaust gases are then fed through an exhaust system to a catalytic converter 16.

The catalyst needs to be operated at a minimum temperature before it is able to treat the exhaust gases. The period of time between the engine starting and the catalyst achieving full operating temperature is known as the light-off period. During this period the vehicle emissions are undesirably high. It is therefore important to reduce this period to the shortest time possible.

This is achieved by igniting the mixture in the cylinder as late as possible into the expansion stroke. FIGS. 2 and 3 show the in-cylinder pressure throughout the four stroke cycle. The regions marked A, B, C and D represent the exhaust, intake, compression and expansion strokes respectively. The small drop early in the intake stroke represents the opening of the inlet valve at which point the pressure drops as fresh air is sucked in. At some point during the induction, compression and expansion stoke, prior to ignition of the mixture, fuel is injected via direct injector 12. During the compression stroke the pressure rises as the piston approaches TDC. After peak pressure the piston travels down again during the expansion stroke and as the pressure falls the spark plug ignites the mixture at the point Ig causing the pressure to climb to a second peak before dropping and the cycle restarting. (This does not, of course, prevent the spark from being fired during the compression stroke).

By retarding the ignition point Ig well into the expansion stroke, there is not sufficient time for the flame to ignite the air and fuel mixture within the cylinder. Unburned fuel and air therefore exit the cylinder with the exhaust gases. This serves to raise the temperature of the exhaust gases since the energy normally provided by combustion of the mixture in the cylinder has not been imparted to the downward movement of the piston. This function is not dependent on ignition occurring in the expansion stroke. There are situations when the same temperature increase can be achieved by retarding the ignition timing relative to minimum spark for best torque (MBT) whilst it still remains in the compression stroke.

The amount of ignition retard is dependent upon combustion stability and the combustion concept which is related to cylinder design and geometry.

A result of retarding the ignition is that the engine develops less power. To compensate for this, more fuel is injected to maintain power whilst still providing unburned mixture for heating the exhaust gases and the catalyst 16.

In DISI engines, the timing of the injection of fuel is critical to achieve good exhaust gas heat flux whilst minimizing fuel flow rate and exhaust gas emissions. The relationship between the injection and ignition timing is therefore important. Due to the expressed desire to vary the ignition timing in order to accelerate catalyst light-off, the present invention provides for varying the injection timing in relation to the ignition timing based on an operating condition of the engine. The relative timing of the injection and ignition is measured in degrees of crank angle from the end of the injection as the length of the injection will itself vary during different engine operating conditions.

The relationship between injection and ignition timing need not be constant. It is foreseeable that the crank angle separation could vary depending on many factors, including but not limited to exhaust gas temperature, engine speed, engine coolant temperature, engine oil temperature, throttle position, ambient air temperature, ambient air pressure, intake charge pressure, compression ratio among others. The important point remains that as ignition timing varies, the crank angle separating injection timing and ignition timing is maintained in a predetermined functional relationship, regardless of whether the spark is provided in the compression or expansion strokes. This separation angle is represented by the arrows in FIGS. 2 and 3.

FIG. 3 shows a second and preferred embodiment of the invention in which the injection of fuel is split into two smaller injections. This is because a single larger injection can lead to the mixture being too rich and can restrict the degree of retard achievable thereby limiting the heat flux to the exhaust.

Split injection allows an overall weaker mixture to be ignited. Fuel injected at Inj1 mixes to produce a homogeneous yet weak charge, while the latter injection Inj2 gives a localised rich mixture in the region of the spark to allow ignition to occur. In this embodiment, it is the second injection to create a localized rich mixture that is timed in predetermined relation to the variable ignition timing. While the fuel is shown as being injected at Inj1 during the compression stroke, there is no reason why it could not equally have been injected prior to that during the intake stroke.

Claims

1. A method for reducing light off time of a catalyst coupled downstream of a DISI engine, comprising:

setting the ignition timing for each engine cylinder and injecting at least a proportion of the fuel, the timing of the injection in relation to the ignition timing being varied as a function of an engine operating condition.

2. The method as claimed in claim 1, wherein the ignition timing occurs during the expansion stroke.

3. The method as claimed in claim 2, wherein the timing of the injection in relation to the ignition timing is measured from the end of the injection to the spark timing.

4. The method as claimed in claim 3, wherein an initial proportion of fuel is injected prior to a secondary injection.

5. The method as claimed in claim 4, wherein the initial injection of fuel occurs during the intake or compression strokes.

6. The method as claimed in claim 5, wherein the timing of the secondary is injection further a function of any one or more of exhaust gas temperature, engine speed, engine coolant temperature, engine oil temperature, throttle position, ambient air temperature, ambient air pressure, intake air pressure, intake air temperature and compression ratio.

7. A method for reducing light off time of a catalyst coupled downstream of a DISI engine, comprising:

injecting fuel and air into each engine cylinder; and

retarding ignition timing such that at least a portion of said injected fuel and air exits said engine cylinders unburned thereby raising engine exhaust gas temperature.