GB2584866A

GB2584866A - Compression testing method for internal combustion engines

Info

Publication number: GB2584866A
Application number: GB1908714.7A
Authority: GB
Inventors: Mougel Valentin; Randazzo Stephane
Original assignee: Delphi Automotive Systems Luxembourg SA
Current assignee: BorgWarner Luxembourg Automotive Systems SA
Priority date: 2019-06-18
Filing date: 2019-06-18
Publication date: 2020-12-23
Also published as: GB201908714D0

Abstract

A method of compression testing with respect to one or more cylinders of an internal combustion engine, said engine including a starter motor, comprising: a) cranking the engine; b) subsequently during an angular window, measuring instantaneous speed; c) determining the value of the maximum instantaneous speed in said angular window; d) determining the value of the minimum instantaneous speed in said angular window; e) determining a difference value between the results of c) and d) to give a delta speed; said difference value being indicative of the degree of compression (according to the relationship of Figure 5). Step e) may include determining the maximum pressure generated during said angular window from the difference value computed at e). The maximum pressure may be determined from a pre-stored relationship relating maximum pressures to said difference values. The angular window may cover the compression and/or expansion phases or portions thereof. The difference values may be compared with respect to a plurality of cylinders. There may be no ignition and/or injection of fuel.

Description

COMPRESSION TESTING METHOD FOR INTERNAL COMBUSTION ENGINES

TECHNICAL FIELD

This relates to a method of testing compression in one or more cylinders of an internal combustion engine.

BACKGROUND OF THE INVENTION

Compression testing is used to evaluate engine health by measuring the cylinder pressure at the end of the compression phase/stroke (or the maximum pressure over a the complete cycle). This information allows evaluation of e.g. air leaks inside the cylinder which can be caused by e.g. a broken valve, a damaged piston or damaged piston ring.

Compression testing is currently performed using specific tools. Generally a specialized tool, which measures pressure, is plugged (i.e. screwed) in place of a spark plug, and then the engine is run (e.g. starter). The maximum pressure measured at the end of the compression phase elicits details on problems in the compression for that cylinder. Such a procedure however, has to be done for each cylinder. This means that it required to unplug the spark plug, connect the testing tool, run the engine starter, for each cylinder. There is also the problem that the tool may be improperly installed and air could leak which could lead to false measurement and diagnosis.

Another method consists of a technique which involves measuring an average engine speed in two adjustable time windows. Then, one of the two average speed, or the difference between both, or the average of both (selection via calibration) are stored for each cylinder. This is performed for all cylinders. Then the results obtained for all cylinder are compared to determine the extent to which there is imbalance. For this method, it is not strictly an estimation of maximum cylinder pressure during compression phase, so it is difficult to determine if the engine damaged just based on a recombined speed interpretation. The problem of time consuming and tool usage are solved by the second method but the main expected output is not delivered In all these cases the measurements have to be done for each cylinder. It means that it is required to unplug spark plug, connect the tool, run the engine starter, cylinder by cylinder.

Then, the tool could be improperly installed and air could leak.

It is an object to overcome these problems and provide a method of compression testing which does not require removal of spark plugs and the use of a special tool in their pace when testing.

SUMMARY OF THE INVENTION

In one aspect is provided a method of compression testing with respect to one or more cylinders of an internal combustion engine, said engine including a starter motor, comprising: a) cranking the engine; b) subsequently during an angular window, measuring instantaneous speed; c) determining the value of the maximum instantaneous speed in said angular window; d) determining the value of the minimum instantaneous speed in said angular window; e) determining a difference value between the results of c) and d); said difference value being indicative of the degree of compression.

The method may include the further step of determining the maximum pressure generated during said angular window from the difference value computed at e).

Said maximum pressure may be is determined from a pre-stored relationship relating maximum pressures to said difference values.

Said angular window may cover the compression and/or expansion phases or portions thereof.

The method may include comparing said difference values with respect to a plurality of cylinders.

There is preferable no ignition and/or injection of fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 shows a plot of cylinder pressure for a particular cylinder against crank angle; Figure 2 shows a chart showing plots of instantaneous engine speed against crank angle with respect to a particular cylinder; Figure 3 shows plots of cylinder pressure as well as corresponding instantaneous engine speed for six test cases, A, B, C, D, E, F; Figure 4 illustrates how the values of delta speed, is defined and can be calculated; Figure 5 shows the correlation between maximum pressure during the compression and the value of delta speed

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The problem of compression testing is solved by estimating the end of compression pressure (via ECU function) by simply running the engine with the starter motor and processing engine speed data. Such compression pressure estimation processing can be provided by the Engine Control Unit (ECU).

In an example of the methodology, the instantaneous crank speed is measured when the engine starter is used to drive the engine, when it is powered. This is processed to give information on the compression (pressure).

Figure 1 shows a plot of cylinder pressure for a particular cylinder against crank angle. Crank angle of 0 degrees is Top Dead Centre (TDC). As can be seen pressure rises to a peak around TDC and then falls again. This shows a plot when the engine is run by initiating the starter (motor), and there is no ignition.

Figure 2 shows a chart showing plots of instantaneous engine speed against crank angle with respect to a particular cylinder. The plot is shown in a angular window some time shortly after the starter motor is powered up. The crank angle is relative to Top Dead Centre for the respective cylinder under test so the plots shown are generally over an angular window spanning the end of the compression phase.

The plots are in respect of different test cases. In the tests, in place of the usual spark plugs, screw elements were screwed into the spark plug threads with different screws in place of spark plug. Some of the screws had holes of different sizes to create an air leak and thus simulate a problem/leaky cylinder in terms of lack of normal compression. Plot 1 shows no leak, plot 2 shows low leak conditions (the screw element had a hole diameter of 1mm) and plot 3 shows high leak conditions (the screw element had a hole diameter 2mm). The plots 1, 2, 3 had equivalent compression pressures (e.g. maximum pressures) of 19, 15 and 8 bar respectively.

As can be seen the instantaneous speed drops towards the TDC and thereafter rises again. There may be two minima with a local maximum therebetween.

Figure 3 shows plots of cylinder pressure as well as corresponding instantaneous engine speed for six test cases, A, B, C, D, E, F. Reference numerals 4A, 4B, 4C, 4D, 4E, 4F refer to the respective instantaneous speeds in the respective six test cases (A,B,C,D,E,F) and 5A, 5B, SC, SD, 5E, SF the respective cylinder pressures; all against crank angle. Test A was a normal cylinder with no leaks, Test B, C, D, E, F had screws in place of plugs with deliberate leaks by having hole of 1, 1.4, 1.6, 1.8, and 2mm respectively.

This tests were done on an engine dyno. The procedure was as follows: Open the throttle at 100%; run the engine's starter and record cylinder pressure and engine speed. . In methods according to the invention there is preferably no ignition and or no injection. The engine need only to be driven by the starter.

The inventors have determined that the (e.g. maximum pressure) in such tests and thus the degree of compression, is related to delta speed (A speed) which is defined as: A speed = max (instantaneous) speed -min (instantaneous) speed in the appropriate angular window.

Figure 4 illustrates how the values of delta speed, is defined and can be calculated, The plot again shows instantaneous engine speed against crank angle after powering the motor with the starter.

The delta speed (A speed) is computed as the difference between maximum and minimum speed during an appropriate angular window e.g. covering the expansion phase or the compression and expansion phase; i.e. a short time slot around the TDC. The skilled person would readily be aware of how to select appropriate angular windows. The window may span the TDC and include all or a portion of the compression and expansion phases. The time window would normally span from about 72 degrees before TDC with a length of one segment (720 degrees / number of cylinder i.e. 240 degrees for a 3 cylinders engine or 180 degrees for a 4 cylinders engine)..

In figure for the minimum instantaneous speed (MIN) is found at point X, and the maximum (MAX) is found at point Y This delta speed is a measure or indication of the pressure applied on the piston at the end of compression phase. So the speed analysis calculates the delta between the minimum an the maximum speed during expansion phase. This delta is used to estimate the maximum pressure during compression.

Figure 5 shows the correlation between maximum pressure during the compression and the value of delta speed i.e. a correlation between the delta speed and the maximum pressure inside the combustion chamber (figure 2). The black dots represent test results and the inventors have determined that there is clear correlation between A speed and the maximum pressure. A correlation curve is shown. In the example, this is a second degree polynomial function e.g. y = - 0.0015x2+0.3805x-5.987 and where y= max. pressure and x = Aspeed. The value of R2 is about.99 Method According to a method a test is performed where the engine's starter motor is run/activated and the instantaneous engine speed is measured over an appropriate angular window. The value of A speed is determined, and used to provide a value indicative of the compression e.g. the maximum pressure in the compression stroke in such a test. This value of A speed on its own is a measure of compression so need not necessarily be converted to a maximum pressure.

The value of A speed can be can be compared with e.g. stored thresholds or those obtained for other cylinders, in order to determine whether there is a significant lack of compression in one or more cylinders.

In refined embodiments the value of Aspeed can be converted to maximum pressure using look up tables or e.g. formula similar to that described above. Again the value of maximum pressure can be compared with e.g. stored thresholds or those obtained for other cylinders.

The robustness of the technique was tested versus battery voltage (from 101/ to 14V). In refined examples, compensation for atmospheric pressure is provided. The (raw) results of A speed/or max. pressure determined therefrom are varied somewhat i.e. tweaked to take into account variations of air pressure. Examples provided results which are a real and quick estimation of the cylinder pressure. The advantage of this invention compared to the prior art (usual method) is the time saving and repeatability of the measurement. A further advantage of the method is that it provides directly an estimation of the end of compression pressure value (instead of an engine speed which doesn't help in most of the case) to evaluate the engine integrity quickly.

Claims

CLAIMS1. A method of compression testing with respect to one or more cylinders of an internal combustion engine, said engine including a starter motor, comprising: a) cranking the engine; b) subsequently during an angular window, measuring instantaneous speed; c) determining the value of the maximum instantaneous speed in said angular window; d) determining the value of the minimum instantaneous speed in said angular window; e) determining a difference value between the results of c) and d); said difference value being indicative of the degree of compression.
2. A method as claimed in claim 1 including the further step of determining the maximum pressure generated during said angular window from the difference value computed at e).
3 A method as claimed in claim 2 where said maximum pressure is determined from a pre-stored relationship relating maximum pressures to said difference values.
4. A method as claimed in claims 1 to 3 wherein said angular window cover the compression and/or expansion phases or portions thereof
5. A method as claimed in claims 1 to 4 including comparing said difference values with respect to a plurality of cylinders.
6. A method as claimed in claim 1 to 5 where there is no ignition and/or injection of fuel.