GB2312970A

GB2312970A - Diesel engine control

Info

Publication number: GB2312970A
Application number: GB9609869A
Authority: GB
Inventors: Garon Nigel Heslop; Malcolm Daniel Macgowan Dale; Gavin Fraser Mccall
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1996-05-11
Filing date: 1996-05-11
Publication date: 1997-11-12
Also published as: DE69701285D1; GB9609869D0; WO1997043534A1; EP0897465B1; EP0897465A1; ES2142160T3; DE69701285T2

Abstract

A method of varying the quantity of fuel injected into the combustion chambers of a diesel engine during each combustion cycle in dependence upon the position of a demand pedal and the load on the engine, in which at least for demand pedal depressions not exceeding a preset limit the fuel is varied within an idling operating range as a first function of load to regulate the engine speed 44 at a substantially constant idling speed value and within a part load operating range as a second function of load 40 to allow the engine speed to reduce gradually with increased engine load, the idling speed value regulated within the idling operation range being itself dependent upon the position of the demand pedal. The method may involve determining the quantity of fuel required to maintain the engine idling at a speed dependent on accelerator pedal position, determining 40 the quantity of fuel required to allow the engine speed to vary with load in accordance with engine speed and accelerator depression via a speed droop controller, and supplying the engine with the higher of the two quantities 42.

Description

DIESEL ENGINE CONTROL The present invention relates to the control of a diesel engine.

Diesel engines differ from spark ignited engines in that the load is determined by the quantity of fuel injected into the engine rather than by the air intake mass. In a motor vehicle, the quantity of fuel injected in any cycle is set by a controller in dependence upon the position of a demand pedal. A controller is required because the injection quantity is not related uniquely to the demand pedal position but depends also upon the load on the engine.

When the driver takes his foot completely off the demand pedal to set the engine to idling speed, the engine is not expected to stall even if it is under load. If the vehicle is in gear, the idle setting is expected to make the vehicle move forward, even up a hill. When the demand pedal is depressed, then the driver immediately expects the vehicle to move faster. Also if the demand pedal is held stationary in a part load position, the driver normally expects the vehicle to speed up when travelling down a hill and to slow down when travelling up a hill.

The fuelling strategy described above is represented by the graphs in Figure 1 in which the fuel injection quantity expressed in milligrams of fuel per engine stroke is plotted against engine speed, the different graphs representing different demand pedal depressions. The vertical line 10 at the engine speed of 800 rpm represents the idle control.

During idle control, regardless of load the fuel injection quantity is adjusted to prevent the engine from stalling and the fuel injection quantity is automatically varied with load to hold the engine speed at 800 rpm. The inclined lines 12a, 12b and 12c correspond to constant demand pedal depressions of 20%, 40% and 60%, respectively. The inclination represents what is sometimes termed the speed droop. As the engine speed increases on account of reduced load, so is the fuel injection quantity reduced. The reduction is not however sufficient to counteract fully the increase in speed and there is some change in engine speed as the load is varied. In the converse manner, if the load is increased, the fuel injection quantity is increased but again not enough to maintain the vehicle speed constant.

The pedal depression scheme described above and represented by the diagram of Figure 1 has certain disadvantages because it can sometimes result in a dead band that is disconcerting to the driver. To explain this effect more clearly, let it be assumed in Figure 1 that the vehicle is allowed to creep up a hill under idle control with the driver's foot completely off the demand pedal. The fuel injection quantity required to keep the engine at idling speed could be anywhere along the vertical line 10 and it will be supposed. just as an example, that the fuel injection quantity is set at the point designated A in the drawing. The driver now puts his foot down on the demand pedal hoping to make the vehicle move slightly faster, the pedal now moving to the 10% demand position. The variation of fuel injection quantity with engine speed for a 10% demand pedal depression is represented by the line 12d in the drawing and it will be noted that for all speeds near the idling speed, the injection quantity would be decreased giving rise to a reduced engine speed. This of course cannot be allowed to occur so that the greater fuel injection quantity set by the idle control continues to be supplied to the engine and the engine speed does not change.

Not until the demand pedal is depressed beyond the 20% position does the vehicle commence to travel any faster.

Thus the first 20% of movement of the demand pedal has no effect and the width of this dead band increases with the load on the engine when it is at its idling speed.

The present invention seeks to provide a diesel engine control strategy that avoids a dead band and allows the engine always to respond immediately to a change in the position of the demand pedal.

According to a first aspect of the present invention, there is provided a method of varying the quantity of fuel injected into the combustion chambers of a diesel engine during each combustion cycle in dependence upon the position of a demand pedal and the load on the engine, wherein for each position of the demand pedal, the fuel is varied within an idling operating range as a first function of load to regulate the engine speed at a substantially constant idling speed value and within a part load operating range as a second function of load to allow the engine speed to reduce gradually with increased engine load, the idling speed value regulated within the idling operation range being itself dependent upon the position of the demand pedal.

The invention differs from prior art proposals in that instead of a unique idling speed common to all positions of the demand pedal, the regulated idling speed is increased with increased demand. If the driver depresses the demand pedal while the engine is idling, the engine will idle at a higher speed and thereby avoid the dead band within which demand pedal position has no effect on engine speed.

In accordance with a second aspect of the invention, there is provided a diesel engine regulator for setting the quantity of fuel injected into the combustion chambers of the engine during each combustion cycle, comprising a demand pedal, means for sensing the engine speed, an idle speed controller responsive for determining the quantity of fuel required to maintain the engine idling at a speed that varies as a function of the demand pedal position only and is substantially independent of engine load, a speed droop controller for determining as a function of the demand pedal depression and engine speed the quantity of fuel required to allow the engine speed to vary with load for any given demand pedal depression, and means for setting the fuel injection pump of the engine to deliver to the combustion chambers of the engine the higher of the fuel quantities determined by the two controllers.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which Figure 1 is diagram showing the variation of the fuel quantity with engine speed for different demand pedal position in a prior art diesel engine controller, Figure 2 is a block diagram of a controller suitable for implementing the method of the present invention, Figure 3 shows the operation of the processor in the block diagram of Figure 2 in order to carry out the present invention, and Figure 4 is a diagram generally similar to that of Figure 1 illustrating the control strategy employed in the present invention.

The present invention is most easily understood from a comparison of Figures 1 and 4. Each constant demand pedal position line in Figure 4 (at least those lower than the 50% demand position line), for example the line 32 for a 20% demand pedal position is formed of two distinct parts. The first part 32a is an idle control line which is very steep and maintains a substantially constant idle speed and a second part 32b which corresponds to a speed droop characteristic that allows some variation of the engine speed with the engine load. Where Figures 1 and 4 differ is that, in accordance with the invention, there is a different idling speed maintained for each position of the demand pedal, the idling speed increasing with increased demand pedal position. As a result, the engine will idle at a higher speed as the demand pedal is depressed. When the engine is idling, therefore, changes in the position of the demand pedal will be reflected in different regulated idling speeds without the presence of an objectionable dead band.

The method of the invention is implemented by providing two separate controllers, the first determining the fuel quantity required to maintain the minimum idling speed for the position of the demand pedal and the second determining the quantity of fuel to achieve the desired speed droop characteristic. The fuel pump is set in accordance with the higher of the two fuel quantities.

The speed droop controller can be a P (proportional) or a PD (proportional-differential) controller but it is preferred to use a look-up table. The idle speed controller on the other hand is preferably a PI (proportional-integral) or a PID controller. Such a controller may be implemented in the form of a variable gain amplifier with feedback but in the described embodiment both controllers are implemented in software using a programmable processor.

Referring to Figure 2, the position of the demand pedal is sensed by a demand pedal position sensor 20 which produces an analogue signal. The latter signal is converted into a digital signal by an A/D converter 22 before being applied to a processor 26 and provides the processor with an indication of pedal position. The processor 26 also receives a digital signal from a crankshaft position sensor 24. The crankshaft position sensor 24 produces pulses that coincide with instants when a crankshaft passes through predetermined angular positions. These pulses are processed in the processor 26 to generate an actual engine speed signal. Depending on the error or difference between the desired and actual speeds of the engine, the processor sends a signal to the fuel pump 28 to vary the fuel quantity injected into the combustion chambers of the engine during each operating cycle and thereby changes the speed of the engine.

The operation of the processor 26 can best be understood from the flow chart of Figure 3. The processor consists of an idle speed regulator 52 and a speed droop regulator 40 arranged to operate in parallel on the two signals indicative of the desired and actual engine speeds.

Within the idling speed regulator 52, the pulses from the crankshaft position sensor are processed in a block 46 to calculate the actual engine speed. The demand pedal position signal from the A/D converter is used in block 44 to calculate or look-up a desired engine idling speed. This idling speed will vary with pedal depression in the manner shown in Figure 4. In the block designated 48, the actual engine speed is subtracted from the desired engine speed to produce an error signal that is applied to a PI controller 50. The PI controller has an output signal composed of the sum of two components, the first related to instantaneous value of the error signal and the second to the integral of the error signal. The effect of using a PI controller is that a fuel quantity is computed that maintains the engine speed within a very narrow band centred on the desired value, hence the steepness in Figure 4 of the idling curve sections 34b.

The second regulator 40 could be a second controller, this time without an integral component, but in the illustrated embodiment of the invention is constituted as a look-up table addressed by the demand pedal position signal and the actual speed calculated within the block 46 of the idle speed regulator. The quantity of fuel derived from the look-up table is sufficient to allow the engine speed to vary with load for any given demand pedal position along the speed droop curves 34a shown in Figure 4. The higher of the fuel quantities set by the two regulators 52 and 50 is selected in the block 42 and used to control the fuel injection pump so that as the engine slows down to its idling speed, the regulator 52 takes over from the regulator 40 and conversely as the engine speed increases, the speed droop regulator 40 takes over from the idle speed regulator 52.

Claims

1. A method of varying the quantity of fuel injected into the combustion chambers of a diesel engine during each combustion cycle in dependence upon the position of a demand pedal and the load on the engine, wherein at least for demand pedal depressions not exceeding a preset limit the fuel is varied within an idling operating range as a first function of load to regulate the engine speed at a substantially constant idling speed value and within a part load operating range as a second function of load to allow the engine speed to reduce gradually with increased engine load, the idling speed value regulated within the idling operation range being itself dependent upon the position of the demand pedal.

2. A diesel engine regulator for setting the quantity of fuel injected into the combustion chambers of the engine during each combustion cycle, comprising a demand pedal, means for sensing the engine speed, an idle speed controller responsive for determining the quantity of fuel required to maintain the engine idling at a speed that varies as a function of the demand pedal position only and is substantially independent of engine load, a speed droop controller for determining as a function of the demand pedal depression and engine speed the quantity of fuel required to allow the engine speed to vary with load for any given demand pedal depression, and means for setting the fuel injection pump of the engine to deliver to the combustion chambers of the engine the higher of the fuel quantities determined by the two controllers.

3. A diesel engine regulator as claimed in claim 2, wherein the idle speed controller is a closed loop controller having a proportional and an integral component, the proportional component determining the fuel quantity required to minimise the error between the actual and desired engine speeds and the integral component determining the fuel quantity required to minimise the time integral of the error between the actual and desired engine speeds.

4. A diesel engine regulator as claimed in claim 2 or 3, wherein the second controller comprises a look up table wherein the cell addresses in the table correspond to the demand pedal depression and the prevailing engine speed.

5. A diesel engine regulator as claimed in any of claims 2 to 4, wherein the two controllers are constituted by a programmed microprocessor.

6. A diesel engine regulator constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.