GB2343967A

GB2343967A - Deriving fuel supply control algorithms for each engine cylinder to maintain balanced air/fuel ratio

Info

Publication number: GB2343967A
Application number: GB9825484A
Authority: GB
Inventors: Benjamin James Bradshaw; Alan George Wilson
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1998-11-21
Filing date: 1998-11-21
Publication date: 2000-05-24
Also published as: GB9825484D0

Abstract

A control method for use in controlling the operation of an engine comprising the steps of operating an engine under given speed/load conditions, modifying the fuel level applied to each cylinder of the engine to substantially balance the air fuel ratios of the cylinders and using the modified fuel levels to derive a target value of a fuel balancing algorithm for each cylinder. The invention also relates to a method of controlling an engine comprising the steps of using a fuel balancing algorithm, and controlling the quantity of fuel supplied to each cylinder to ensure that the output of the algorithm matches the target value for the speed/load condition at which the engine is operating.

Description

CONTROL METHOD This invention relates to a method of controlling the operation of an internal combustion engine, and in particular to a method whereby the level of particulate emissions produced by the engine, in use, can be reduced.

It is known to use a fuel correction or balancing algorithm to adjust the quantity of fuel supplied to each cylinder of an engine to reduce variations in engine speed between the firing stroke of one cylinder and that of a successive cylinder. By reducing such speed variations, engine vibration and noise can be reduced. It is usually assumed that there are no cylinder specific differences between the cylinders other than the amount of fuel injected for a given fuel demand. Unfortunately, cylinder specific differences do exist, and the use of the fuel balancing algorithm may make certain engine operating parameters worse.

For example, the mass airflow rate to the cylinders of an engine may be unequal due for example, to the geometry of the intake manifold and the location of a turbocharger or supercharger if either of these devices is fitted.

It is known that the in-cylinder pressure throughout the compression and power strokes is dependent upon the in-cylinder pressure at the start of the compression stroke. This is partly due to the direct relationship between initial pressure and pressure during the compression and power strokes and partly due to a secondary effect whereby the ignition delay and proportion of fuel burnt in the pre-mixed phase of the combustion cycle are dependent upon the in-cylinder pressure. If the mass airflow results in unequal in-cylinder pressures between the cylinders, this will result in cylinder specific speed variations, and a conventional fuel balancing algorithm can be used to calculate fuel correction factors to compensate for the mass airflow variations.

Unequal mass airflow results in unequal air fuel ratios between the cylinders, which can result in the richer cylinders emitting a significantly large quantity of particulates. This is undesirable and is the reason for attempting to balance the air fuel ratios between cylinders instead of balancing cylinder specific engine speeds.

These effects are illustrated in Figures 1 and 2. Figure 1 illustrates the output of a fuel balancing algorithm for an engine in which cylinder 1 is supplied with 10% additional air compared to the remaining cylinders, all four cylinders being supplied with the same quantity of fuel, Figure 1 illustrating the effect both when the engine is operating under high load conditions and when it is operating under low load conditions. Figure 1 also illustrates the air fuel ratio for each cylinder at high and low load conditions. The spread of the air fuel ratios at high load is 2.6, the spread being 13.2 at low load.

If the algorithm output is used to modify the fuelling applied to each cylinder to reduce cylinder specific speed variations to zero, then the fuelling levels which should be applied, at high and low load, are shown in Figure 2. Figure 2 also shows the air fuel ratios for each cylinder. It will be noted that the spread of air fuel ratios at high load has increased to 4.1, the spread at low load having increased to 35.

In general, for a given average air fuel ratio, the cylinders supplied with richer fuel mixtures produce higher levels of particulate emissions than the cylinders supplied with leaner mixtures, and the greater the spread of the air fuel ratio, the greater the levels of particulate emissions. It is clear from the description hereinbefore and Figures 1 and 2 that, although the levels of noise and vibrations produced by an engine can be improved by using a fuel balancing algorithm, this may be at the expense of increased levels of particulate emissions.

It is an object of the invention to provide a control method whereby the levels of particulate emissions produced by an engine can be reduced.

According to the present invention there is provided a control method for use in controlling the operation of an engine comprising the steps of: operating an engine under given speedAoad conditions; modifying the fuel level applied to each cylinder of the engine to balance the air fuel ratios of the cylinders ; and using the modified fuel levels to derive a target value of a fuel balancing algorithm for each cylinder.

The method is conveniently repeated over a range of speed/load conditions to derive target algorithm values over the range of conditions.

It will be appreciated if, in subsequent operations, the fuelling levels applied to the cylinders are modified in accordance with the target algorithm value rather than to those levels at which cylinder specific speed variations are minimised, the levels of particulate emissions can be reduced.

The invention also relates to a method of controlling an engine comprising the steps of using a fuel correction algorithm, and controlling the quantity of fuel supplied to each cylinder to ensure that the output of the algorithm matches the target algorithm values described hereinbefore for the speed and load conditions at which the engine is operating.

The invention will further be described, by way of example, with reference to the accompanying drawings, in which: Figures 1 and 2 are graphs illustrating the algorithm output and fuelling levels for an engine where a conventional fuel balancing algorithm is used; Figure 3 is a view similar to Figure 2 but showing the fuelling levels where the air fuel ratios of the cylinders have been balanced; and Figure 4 is a view similar to Figure 1 illustrating the target fuel balancing algorithm values necessary to achieve the balanced air fuel ratios.

As discussed herein before, the graphs shown in Figures 1 and 2 relate to an engine in which the quantity of air supplied to cylinder 1 is 10% greater than that supplied to the remaining three cylinders. In accordance with the present invention, the quantity of fuel supplied to each cylinder of the engine is modified until the air fuel ratios for each of the cylinders are equal. The left hand side of Figure 3 illustrates the fuelling levels for each cylinder under high load when the engine is operating at 1000 rpm, the right hand part of Figure 3 illustrating the fuelling levels for each cylinder where the engine is operating at low load. As expected, in each case, cylinder 1 which is supplied with a greater quantity of air must also be supplied with a greater quantity of fuel than the remaining three cylinders in order to ensure that the air fuel ratios of the cylinders under given engine speed and load conditions are equal.

As discussed hereinbefore, it will be appreciated that by balancing the quantity of fuel supplied to each cylinder to ensure that the air fuel ratios for the cylinders are equal, the levels of particulate emissions produced, in use, can be reduced.

Once the fuelling quantities for each cylinder have been determined for a range of engine speed and load conditions, this information can be used to derive a target algorithm output value for each cylinder for each speed/load condition. When the engine is operated at any given speed and load, rather than using the fuel balancing algorithm to produce fuel correction values whereby cylinder specific speed variations can be compensated for, the algorithm is used to produce fuelling correction values which are used to control the operation of the engine to ensure that the air fuel ratios of the cylinders of the engine are balanced, or substantially balanced throughout the operation of the engine. Figure 4 illustrates the algorithm output values necessary to achieve the fuelling levels illustrated in Figure 3 in which the air fuel ratios are balanced. In use of the engine, the quantity of fuel supplied to each cylinder is modified in such a manner as to ensure that, for the speed/load condition at which the engine is operating, the output of the fuel balancing algorithm is as shown in Figure 4 or the equivalent of Figure 4 for that speed/load.

It will be appreciated, by comparing Figure 1 with Figure 4, that in order to balance the air fuel ratios of the cylinders of the engine, it is necessary to drive the engine in a manner in which the cylinder specific speed variations are not minimise. It will therefore be appreciated that the reduction or minimisation of emission of particulates is achieved at the expense of minimising engine speed variations, and hence at the expense of vibration and noise.

It will be appreciated that in order to put the invention into effect, the air flow to each cylinder of an engine over a range of engine speeds must be measured. This is conveniently achieved by running a number of nominally identical engines on a test bed and measuring the rate of airflow to the cylinders over a range of engine speeds. Exhaust emissions may also be measured. In use, when the speed engine/load conditions are such that the exhaust emissions would be acceptable without using the air fuel ratio balancing algorithm, then the algorithm may be disabled, and fuel control used to balance engine speed instead, if desired.

Once it has been determined under which circumstances the algorithm should be used, parameters such as the engine speed, in-cylinder pressure or another parameter from which the air fuel ratio can be inferred are identified, and the fuel quantities applied to the engines under test are modified until the balanced conditions of air fuel ratios are achieved for a range of engine speeds and/or loads. The values of the parameters or the equivalent algorithm outputs are stored as target values for use in subsequent control of the engine. Alternatively, the target values may be derived using a computer model.

Claims

CLAIMS 1. A control method for use in controlling the operation of an engine comprising the steps of; operating an engine under given speed/load conditions; modifying the fuel level applied to each cylinder of the engine to substantially balance the air fuel ratios of the cylinders; and using the modified fuel levels to derive a target value of a fuel balancing algorithm for each cylinder.
2. The method as claimed in Claim 1, comprising the further step of repeating the method over a range of speed/load conditions to derive the target value of the fuel balancing algorithm over the range of conditions.
3. The method as claimed in Claim 2, including the step of measuring the airflow to each cylinder over the range of conditions.
4. The method as claimed in Claim 3, including the step of operating a number of nominally identical engines on a test bed and measuring the rate of airflow to each cylinder over the range of conditions.
5. A method of controlling an engine comprising the steps of using a fuel balancing algorithm, and controlling the quantity of fuel supplied to each cylinder to ensure that the output of the algorithm substantially matches the target value derived from the method of any of Claims 1 to 4 for the speed/load condition at which the engine is operating.
6. The method as claimed in Claim 5, including the step of measuring exhaust emissions and disabling the fuel balancing algorithm when the exhaust emissions are at an acceptable level for the speed/load condition at which the engine is operating.
7. The method as claimed in Claim 6, comprising the step of using a fuel control method to balance engine speed when the fuel balancing algorithm is disabled.
8. A control method for use in controlling the operation of an engine substantially as hereinbefore described with reference to Figures 3 and 4.
9. A method of controlling an engine substantially as hereinbefore described with reference to Figures 3 and 4.