GB2291118A - System for measuring i.c. engine cylinder peak pressures - Google Patents

Abstract

A multi-cylinder engine plenum chamber 26, 30 has pressure tappings 24 connected to each of combustion chambers 12 and a one-way valve 22 in each pressure tapping allows gas flow from the combustion chambers to the chamber when the pressure in the respective combustion chamber exceeds the pressure in the plenum chamber. A discharge outlet 38 allows gas flow out of the chamber 26, 30 at a rate equal to the average gas flow rate into the chamber from all the combustion chambers and a pressure sensor 50 measures the chamber pressure. A tube 34 containing orifices 32 provides a pressure drop to the outlet 38. A valve 28 between the chamber portions 26 and 30 may be closed briefly to provide measurement of individual combustion chamber peak pressures. <IMAGE>

Description

System for Measuring Cylinder Peak Pressure Field of the invention The present invention relates to the measurement of the cylinder peak pressures in an internal combustion engine.

Background of the invention Many engine control concepts make use of the cylinder peak pressure as the input control parameter. Examples of such systems include lean burn and high EGR calibration for combustion stability, pressure limit control, misfire and knock detection, and adaptive transient fuel calibration control.

Conventionally, pressure sensors have been mounted directly in the combustion chambers but such sensors are difficult to install and have not proved durable because of their direct exposure to the hostile environment in the combustion chamber. Also a separate pressure sensor is required for each cylinder making it expensive for application in multi-cylinder engines.

Object of the invention The present invention seeks to provide a system that allows a single sensor to be used to measure the peak pressures of several cylinders of an internal combustion engine and that avoids direct exposure of the sensor to gases within the combustion chambers.

Summary of the invention According to the present invention, there is provided a combustion pressure measuring system for a multi-cylinder internal combustion engine comprising a plenum chamber, pressure tappings connecting each of a plurality of combustion chamber of the engine to the plenum chamber, a one-way valve in each pressure tapping to allow gas flow from the combustion chambers to the plenum chamber when the pressure in the respective combustion chamber exceeds the pressure in the plenum chamber, a discharge opening for allowing gas flow out of the plenum chamber at a rate equal to the average gas flow rate into the plenum chamber from all the combustion chambers, and a pressure sensor mounted in the plenum chamber for measuring the pressure in the plenum chamber.

In the preferred embodiment of the invention, one-way ball valves are press-fitted to the walls of the combustion chambers and are connected by small passages, or pressure tappings, to a small volume plenum chamber that has a gas leakage path that presents a very high air resistance, being formed for example by a series of orifices or a long capillary tube. A pressure sensor is mounted in the plenum chamber to measure the equilibrium pressure in the plenum chamber which is supplied by the high pressure gases in the combustion chamber only during a short time interval spanning the pressure peaks in each combustion cycle. The equilibrium pressure in the plenum chamber is an indirect measure of the peak pressure in the combustion chamber.

Furthermore, because gas is supplied to the plenum chamber intermittently but the discharge is substantially constant, the pressure will undergo fluctuations related to the pressure pulses from the individual cylinders. Such fluctuations will be affected by the total volume of the plenum chamber and the discharge rate from the plenum chamber and these can be determined empirically to optimise the average and instantaneous pressure readings from the sensor.

The pressure sensor of the invention can be easily installed and can be a low cost and durable device because it is now mounted in a remote location from the combustion chamber.

Furthermore, one sensor will suffice to measure the pressure peaks coming from all the engine cylinders.

The leakage gas flow from the plenum chamber may also be put to good use by directing it through air-assisted fuel injectors to improve fuel atomisation. Even when not used to assist in fuel preparation, it is desirable to return the discharged gases to the intake system of the engine to avoid interfering with the calibration of the engine and unnecessary environmental pollution.

Brief description of the drawings The invention will now. be described further, by way of example, with reference to the accompanying drawings, in which the single figure shows section through a cylinder head of an engine incorporating a system for sensing the peak pressures in the combustion chambers.

Detailed description of the preferred embodiment In the drawing, a cylinder head 10 is shown for an engine having four cylinders, each with four valves. A small tapping 24 is drilled into the wall of each combustion chamber 12, the end of the tapping 24 within the combustion chamber having a countersunk recess 20 within which there is fitted a one-way ball valve 22. All the four tappings 24 are connected to one another by a drilling 26 that has the form of a blind bore, its open end being sealed by a pressure sensor 50. A second blind bore 30 of larger volume is formed in the cylinder head 10 and is connected to the drilling 26 by a normally open shut-off valve 28. The bore 30 and the drilling 26 together constitute a plenum chamber within which combustion gases supplied by the combustion chambers when they are near their peak pressure are stored.

The plenum chamber has at theXopen end of the bore 30, which is a discharge opening, a flow restrictor comprising a long tube 34 having several separator plates each formed with a small orifice 32. This arrangement forms a series of chambers within which the pressure is progressively reduced.

Gas from the chamber at the lowest pressure is discharged through a tube 36 within a plug 40 closing the end of the bore 30. The end of the tube 36 opens within the intake system of the engine and its end may be formed with a nozzle 38 in an air-assisted fuel injector to assist fuel preparation.

In operation, gases are intermittently admitted into the plenum chamber during pressure peaks when the one-way valves 22 are open and the pressure in the plenum chamber rises.

With increasing pressure, the rate of discharge also rises and ultimately an equilibrium is reached when the flows into and out of the plenum chamber averaged over an engine cycle are equal to one another. If a high pressure is reached within the plenum chamber then there will be a high discharge rate and in a steady state this can only be achieved by a high peak pressure within the combustion chambers. The pressure in the plenum chamber averaged over the engine combustion cycles of all the cylinders is therefore a true indication of the peak pressure in the combustion chambers. Thus the single pressure sensor 50 positioned outside the combustion chambers can still provide an indication of the averaged peak pressure for all the combustion chambers.

It is important in tis context to ensure that the discharge from the plenum chamber should be fairly small so that the one-way valves should only be open for a very short period straddling the pressure peaks. This avoids loss of power from the engine and also ensure that the pressure in the plenum chamber is very close to the peak pressures in the combustion chamber. It is also desirable to minims the volume of the plenum chamber so that when the peak pressures in the combustion chambers drop, the small discharge is still sufficient to allow the pressure in the plenum chamber to follow without an excessive delay.

Because gases are discharged from the plenum chamber as a uniform flow whereas gases enter the plenum chamber spasmodically from the individual combustion chambers, there will be pressure surges within the plenum chamber as the individual cylinders fire. The measurement of these pressure fluctuations would provide an indication of the peak combustion pressure within each individual cylinder.

In control systems requiring only an indication of average peak pressure, for example pressure limit control, the output signal of the sensor 50 can be filtered and used directly. However the pressure measurement though related to peak pressure is also affected by the discharge rate through the nozzle 38. If an absolute pressure measurement is required, a calibration can be effected by briefly closing the shut-off valve 28 during steady state operation.

If this is done the pressure would rapidly rise to the true peak pressure on account of the small volume of the drilling 26 and the absence of any discharge flow. After calibration the shut-off valve 28 can be re-opened to resume normal operation.

The pressure sensor can also be used in control systems in which smoother engine operation is achieved by balancing the peak pressures in the different combustion chambers. The sensor output signal will contain a fluctuating or AC component in which each individual pressure peak can be attributed to an individual cylinder from a knowledge of the firing sequence of the engine as derived from the distributor or a crankshaft position sensor. The value of the pressure peak can be affected by altering the ignition timing or the fuel supplied to the individual cylinders and it is possible for the control system to modify such parameters in such a sense as to reduce the difference between the pressure peaks of the individual firing cylinders.

It is in practice important to equalise the pressure peaks even when measuring averaged pressure because unless the pressure peaks are equalised, one of the cylinders may be contributing significantly more than others to the gas flow into the plenum chamber and the high pressure from that cylinder will be disproportionately represented in the averaged pressures because its one-way valve 22 will remain open for a longer time than the valves of the other cylinders.

Once a steady equalisation of the pressure peaks from all the combustion chambers has been achieved, the output signal of the pressure sensor 50 can provide other valuable data regarding the operation of the engine.

For example, should abnormally low pressure peaks be sensed for any individual cylinder on an irregular basis, this will indicate the possibility of misfire. Similarly abnormally high pressure peaks for an individual cylinder will indicate pre-ignition or knocking. Measurement of the variance of successive peak pressures over a period of time can also be detected to indicate engine instability during steady state operation. Similar analysis of the pressure peaks occurring during the transition from one steady state to another can also indicate inaccuracy in the transient control strategy of the engine, it being desirable to achieve a smooth and gradual change in the pressure peak values while changing from one steady state to another.

Claims (14)

1. A combustion pressure measuring system for a multi-cylinder internal combustion engine comprising a plenum chamber, pressure tappings connecting each of a plurality of combustion chambers of the engine to the plenum chamber, a one-way valve in each pressure tapping to allow gas flow from the combustion chambers to the plenum chamber when the pressure in the respective combustion chamber exceeds the pressure in the plenum chamber, a discharge opening for allowing gas flow out of the plenum chamber at a rate equal to the average gas flow rate into the plenum chamber from all the combustion chambers, and a pressure sensor mounted in the plenum chamber for measuring the pressure in the plenum chamber.

2. A combustion pressure measuring system as claimed in claim 1, wherein each pressure tapping comprises a drilling through the wall of the combustion chamber and a countersunk recess at its end within the combustion chamber into which recess a one-way valve is press fitted.

3. A combustion pressure measuring system as claimed in claim 2, wherein the one-way valve comprises a ball and a cap for retaining the ball within the countersunk recess, the cap having a communication hole that is sealed off by the ball when the pressure in the plenum chamber is greater than the pressure in the combustion chamber.

4. A combustion pressure measuring system as claimed in any preceding claim, wherein the discharge opening in the plenum chamber is connected to a flow restrictor formed of a series of orifices, a long capillary tube or a pressure regulator.

5. A combustion pressure measuring system as claimed in any preceding claim, wherein a shut-off valve is provided in series with the discharge opening of the plenum chamber.

6. An engine management system comprising a combustion pressure measuring system as claimed in any preceding claim, wherein the instantaneous signal derived from the pressure sensor is averaged to give a mean pressure value indicative of the average peak pressure in all the combustion chambers.

7. An engine management system comprising a combustion pressure measuring system as claimed in any of claims 1 to 5, wherein the fluctuating components of the signal derived from the pressure sensor is analysed to provide an indication of the peak pressure in each of the combustion chambers.

8. An engine management system as claimed in claim 7, comprising means for correlating the pressure peaks with engine timing information synchronised with the firing order of the cylinders.

9. An engine management system as claimed in claim 8, wherein under steady state operating condition, at least one parameter affecting the peak combustion pressure in each individual cylinder is varied in dependence upon the measured peak pressure for that cylinder, in a sense to equalise the peak pressures for all the cylinders.

10. An engine management system as claimed in claim 9, wherein means are provided to indicate the combustion stability of each cylinder by monitoring the variation in the peak pressures in successive cycles for the respective cylinders.

11. An engine management system as claimed in claim 9 or 10, wherein means are provided to sense misfire by sensing an abnormal drop in the peak pressure of each cylinder.

12. An engine management system as claimed in claim 9, 10 or 11, wherein means are provided to sense an abnormal rise in the peak pressure from a cylinder, whereby to identify knocking within the combustion chamber of the cylinder.

13. An engine management system as claimed in any of claims 9 to 12, wherein means are provided for identifying inaccurate open loop engine transient calibration during the transition between two steady state operating conditions, by monitoring the progressive change in the peak pressures of the engine combustion chambers.

14. A combustion pressure measuring system for an internal combustion engine constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.