GB2149535A

GB2149535A - Internal combustion engine fuel supply and tuning

Info

Publication number: GB2149535A
Application number: GB08427813A
Authority: GB
Inventors: Anthony Osborne Dye
Original assignee: Epicam Ltd
Current assignee: Epicam Ltd
Priority date: 1983-11-02
Filing date: 1984-11-02
Publication date: 1985-06-12
Also published as: GB8329252D0; GB8427813D0

Abstract

A method for adjusting the fuel:air ratio supplied to an internal combustion engine comprises measuring cylinder pressure during each of a sample of engine cycles and obtaining a measure of the extent of variation of the combustion process, e.g. the extent of intercycle variation in said pressure at a corresponding stage in each cycle, comparing the said measure to a corresponding standard and adjusting the fuel:air ratio to cause said measure to approach the standard. Carried out continuously the method may be used for the management of the fuel supply to an engine. Alternatively, the method may be used for tuning an engine. <IMAGE>

Description

SPECIFICATION Internal combustion engine fuel supply and tuning The present invention relates to apparatus and methods for tuning or managing the fuel supply to an internal combustion engine, in particular by adjustment of the fuel:air ratio thereof.

It is recognised that one of the factors necessary to the correct running of an internal combustion engine is the provision of the correct fuel:air mixture in the cylinders under all operating conditions, or as near the optimum mixture as can be managed.

In tuning an internal combustion engine, the fuel:air ratio must therefore be adjusted. However, the fuel:air ratio present in the cylinders is difficult or impossible to measure directly. Present methods of adjusting fuel:air ratios are far from satisfactory and tend to concentrate on the idle mode of engine operation because of the practical difficulty of making tests and adjustments to an engine under normal load conditions, particularly where the engine is a vehicle engine.

Increasingly, fuel supply to an engine is being controlled by electronic, microprocessor based engine management systems. To govern fuel:air ratio correctly, these systems need a method of measuring the fuel:air ratio actually achieved in the cylinders.

It is known that there are substantial and measurable variations in the peak cylinder pressure from cycle to cycle during engine operation. These derive from a cycle to cycle variation, or intercyclic variation, in the burn-time for the fuel charge. I have now appreciated that there is a useful correlation between such burn-time or pressure variations and the fuel:air mixture supplied to each engine cylinder under given operating conditions and that the extent of intercyclic variation in fuel burn-time and hence in cylinder pressure can be used as an indirect measure of the fuel:air mixture and hence as a tuning or engine management aid.

The present invention now provides a method for adjusting the fuel:air ratio supplied to an internal combustion engine which method comprises measuring cylinder pressure during each of a sample of engine cycles and obtaining a measure of the extent of variation of the combustion process, e.g. the extent of variation in said pressure, from cycle to cycle, comparing the said measure to a corresponding standard and adjusting the fuel:air ratio to cause said measure to approach the standard.

The measurement of cylinder pressure in each cycle may be a measure of maximum cylinder pressure.

More preferably, the cylinder pressure during each cycle is continuously measured.

Preferably, the measure of the extent of variation in combustion from cycle to cycle is derived from measurements of pressure during at least 40 cycles, e.g. about 100 cycles.

Where the engine is a multi-cylinder engine, pressure measurements are preferably carried out simultaneously on each cylinder and a measure of the extent of variation in combustion, e.g. in pressure, from cycle to cycle is derived for each cylinder and compared to a respective standard.

Preferably, the standard or, in the case of a multi-cylinder engine, the standards are obtained by carrying out a corresponding measure of pressures during the operation of a similar engine in a known good state of tune and obtaining a standard for the extent of variation from cycle to cycle to be expected from that type of engine when the fuel:air ratio is ideal.

Preferably, in methods of tuning an engine which derives its fuel:air ratio from a carburettor, the process of adjusting the air:fuel ratio to cause the measure to approach the standard comprises temporarily applying a progressive amount of enrichment or leaning to the fuel:air mixture without disturbing carburettor settings, thus determining the extent of enrichment or leaning eventually required, and then adjusting the settings of the carburettor to produce that extent of enrichment or leaning.

Preferably, the temporary enrichment or leaning is produced by applying positive or negative gas, air, pressure respectively above the fuel in the float chamber of the carburettor.

Preferably, said temporary enrichment or leaning is carried out automatically under computer control to produce an optimum match between the standard and the obtained measure. The computer may then produce a read out indicating the necessary adjustments to the carburettor settings to produce to same match.

Preferably, the above procedures are carried out with the engine under normal operating load conditions. For instance, in the case of a vehicle engine, the procedures are carried out under normal level cruising conditions, most preferably on a normal road, as opposed to rolling road. For a motor car, the ideal conditions are cruising on a level road at about a steady 90 km/hr in substantially neutral wind conditions.

The invention includes apparatus for use in such methods comprising means for measuring cylinder pressure during operation of an internal combustion engine, means for deriving from pressure measurements on a series of engine cycles a measure of the extent of variation of the combustion process, e.g. said pressure, from cycle to cycle, and means for adjusting the fuel:air ratio.

Preferably, the means for adjusting the fuel:air ratio is means for applying a temporary enrichment or leaning without alteration of the carburettor settings.

Alternatively, the means for adjusting the fuel: air ratio may be a variable jet of a carburettor.

Preferably, the apparatus comprises computer means connected to produce automatically the derived temporary enrichment and leaning.

Preferably, the means for adjusting the fuel:air ratio without altering carburettor settings comprises an adjustable source of negative and positive gas pressure, e.g. air pressure, and means for applying said gas pressure to the float chamber of a carburettor.

The invention includes in a second aspect apparatus for use in tuning an internal combustion engine comprising an adjustable source of negative and positive gas pressure, e.g. air pressure, and means for applying said gas pressure to the float chamber of a carburettor.

Such apparatus may comprise a pump having a positive and negative pressure output, an adjustable valve connected to both said outputs and having an outlet connected to a pressure inlet of a carburettor float chamber, the valve being adjustable to apply selectively a desired amount of positive or negative pressure through the outlet thereof.

The apparatus may include a carburettor float chamber cap connected to said valve outlet but modified by closure or omission of the air vent normally provided in such chamber caps.

The method of the invention may be used for the management of the fuel supply of an engine during normal running. Measurements of the extent of variations may be monitored, preferably continuously, and compared to a standard value for the engine under the operating conditions momentarily obtaining, such an engine speed and load.

The divergence from the standard may be used to control a carburettor or fuel injection system to adjust the fuel:air ratio to that ideal at the time.

The analysis of the intercyclic variations and the holding of the standards may be conducted by microprocessor circuitry in a manner similar to the methods employed in presently known engine management systems.

The standard value for the intercyclic parameter or parameters being monitored may be calculated continously from data stored in or fed by sensors of engine conditions to the microprocessor circuitry according to an algorithm for the engine in question.

In all the above described variations of the invention parameters other than peak cylinder pressure may be utilised. For instance, the crank angle at which peak cylinder pressure occurs will vary from cycle to cycle and the fuel:air ratio may be adjusted to keep this angle constant or, more preferably, within predetermined limits. The standard value or limit values for the angle may if desired be determined based on engine operating parameters in an engine management system or may be predetermined values used in an engine tuning method.

For multi-carburettor engines, the method and apparatus described above may employ an equivalent number of means for adjusting the fuel:air ratio.

The invention will be illustrated by the following description of a specific embodiment thereof with reference to the accompanying drawings in which: Figure 1 is a schematic layout of the components of one apparatus according to the invention; Figure 2 is a graph showing the relationship between intercyclic variations in combustion and fuel: air ratio; and Figure 3 is a schematic layout of apparatus according to the second aspect of the invention.

As shown in Figure 1 apparatus according to the invention for use in tuning an engine having up to four cylinders comprises a pressure transducer 1 adapted for communication with the interior of each cylinder.

These are conveniently provided in communication with the cylinders by means of spark plugs each having a piezoelectric pressure transducer therein or associated therewith and communicating with the cylinder by means of a small bore through the spark plug. Such spark plug mounted transducers are commercially available.

Signals from the transducers are amplified by charge amplifiers 2 in a known manner.

The outputs from each amplifier 2 pass to a multiplexer 3.

Although not essential, the apparatus includes means 4 for signalling the instantaneous position of the engine crank and means for signalling characteristics of the ignition spark discharge for instance derived from the current in the coil.

The multiplexed signals pass to an analogue to digital convertor G and then to a computer 7.

Computer 7 has outlets to a visual display unit 8, a printer 0, a storage device such as a tape deck 10 and communicates with an operator control panel 11.

In operation the computer receives pressure signals, in digital form from each transducer. In a basic mode of seperation, the computer simply derives the peak value of pressure for each cycle for each cylinder and stores these. Alternatively, instead of the maximum pressure being selected from digital data by the computer, it may be picked up whilst in the form of an analogue signal and only such maximum pressure values may be converted to digital form.

In more sophisticated modes of operation the computer may store a pressure profile for each cycle for each cylinder and by combining this profile with signals of the crank position may derive a measure of the burn time for the charge in each cycle or a profile of the rate of burn of the charge.

A measure of the extent of variation of any of these may be derived but for the sake of illustration only peak pressure measurement will be described.

From a sample of say 100 cycles for each cylinder, the computer may derive a coefficient of variation in peak pressure by deriving the mean peak pressure Pm and the standard deviation C and expressing the ratio O/Pm as a percentage.

The computer may display the coefficient for each cylinder so that an operator may compare these with coefficients obtained previously with a similar engine in the desired state of tune and act accordingly.

The standards to which comparison may be made may be held in the computer memory. They may be displayed for operator comparison with the test values or else compared by the computer and a recommended action displayed. Alternatively, as described below, the computer may make the comparison and carry out steps to improve the tune of the engine on the basis thereof.

The ignition system should be correctly set up before this tuning process commences. However, the computer may be set to monitor the signals from means 5 and to indicate if the ignition spark system appears to be to some degree unsatisfactory. The ignition system should be rectified before the test is continued.

Figure 2 shows a typical plot of the coefficient of variation against air:fuel ratio. The position to be aimed at on this graph might for instance be that of the intersection with dotted line 12. This point will generally reflect the optimum fuel economy to be achieved with the engine in question. Generally this will not coincide with the minimum coefficient of variation.

The position of the desired point on such a graph may vary from vehicle model to vehicle model.

In a preferred apparatus according to the invention, computer 7 provides an output to apparatus as shown in Figure 3, this apparatus comprises a small electric powered air pump 13 providing a positive pressure through outlet 14 and a negative pressure through outlet 15. These communicate with a motorised valve 16 adapted to respond to signals from the computer 7 to produce a controlled degree of positive or negative pressure at valve outlet 17. Outlet 17 is in communication with the cap of the float chamber for the carburettor of the engine under test. If appropriate the computer may control a number of valves 16 which may be served by one or several pumps 13.

The carburettor float chamber cap has its normal air vent sealed or connected to valve outlet 17. Under computer control air pressure is applied to the carburettor to enrich or lean the fuel:air mixture.

The effect of such a change on the coefficient of variation is monitored and the computer adjusts the direction and extent of the changes in ratio to obtain the best match to a set of standard values of coefficients for each cylinder stored in memory.

When this is achieved the computer produces a read out of the total change made to the ratio which may then serve as a basis for carburettor adjustment by changing of jets or resetting of control screws as appropriate. The engine may then be retested to check the accuracy of the adjustment.

It is an advantage of the apparatus described above that the whole apparatus may be made small enough to fit in a normal motor car, e.g. on the passenger seat, so that the computer readings may all be taken and recorded on tape or printer during actual road use under realistic driving conditions such as make the greatest difference to fuel consumption. For instance, the fuel:air ratio may be set to the ideal value for travel at 90 km/hr on the level.

By the use of the apparatus according to the second aspect of the invention, the consequences to engine performace of making a particular fuel:air ratio adjustment may be seen without making any alteration to the existing carburettor settings, thus avoiding any difficulty in returning to the starting state of the engine should it not be possible to improve it.

The invention is not limited to the specific features described with reference to the drawings and many modifications and variations thereof fall within the scope of the invention.

For instance instead of measuring maximum pressures to caiculate the coefficient of variation one might take the pressure at any constant point in the combustion or a function of the pressure with time as the basis for calculating such a coefficient.

Claims

1. A method for adjusting the fuel:air ratio supplied to an internal combustion engine which method comprises measuring cylinder pressure during each of a sample of engine cycles and obtaining a measure of the extent of variation of the combustion process from cycle to cycle, comparing the said measure to a corresponding standard and adjusting the fuel:air ratio to cause said measure to approach the standard.

2. A method as claimed in claim 1 wherein the variation of the combustion process measured is the variation in cylinder pressures between corresponding stages in succeeding cycles.

3. A method as claimed in claim 2 wherein the measure of cylinder pressure in each cycle is a measure of maximum cylinder pressure.

4. A method as claimed in claim 1 wherein the standard has been obtained by carrying out a corresponding measure of pressures during the operation of a similar engine in a known good state of tune and obtaining a standard for the extent of variation from cycle to cycle to be expected from that type of engine when the fuel:air ratio is ideal.

5. A method as claimed in claim 1 wherein the engine derives its fuel:air ratio from a carburettor, and wherein the process of adjusting the air:fuel ratio to cause the measure to approach the standard comprises temporarily applying a progressive amount of enrichment or leaning to the fuel:air mixture without disturbing carburettor settings, thus determining the extent of enrichment or leaning eventually required, and then adjusting the settings of the carburettor to produce that extent of enrichment or leaning.

6. A method as claimed in claim 1 for the management of the fuel supply of an engine during normal running comprising making said measurements of the extent of variations during normal running of the engine, comparing the measured variations to a standard value for the engine under the operating conditions momentarily obtaining and periodically or continuously adjusting the fuel:air ratio on the basis of said measure.

7. A method as claimed in claim 6 wherein standard value for the intercyclic variation being measured is calculated continuously from data stored in or fed by sensors of engine conditions to microprocessor circuitry according to an algorithm for the engine in question.

8. Apparatus for use adjusting the fuel:air ratio supplied to an internal combustion engine comprising means for measuring cylinder pressure during operation of an internal combustion engine, means for deriving from pressure measurements on a series of engine cycles a measure of the extent of variation of the combustion process from cycle to cycle, and means for adjusting the fuel:air ratio.

9. Apparatus for use in tuning an internal combustion engine comprising an adjustable source of negative and positive gas pressure and means for applying said gas pressure to the float chamber of a carburettor.

10. Apparatus as claimed in claim 9 comprising a pump having a positive and negative pressure output, an adjustable valve connected to both said outputs and having an outlet connected to a pressure inlet of a carburettor float chamber, the valve being adjustable to apply selectively a desired amount of positive or negative pressure through the outlet thereof.