The purpose of this test is to evaluate the performance of each cylinder based on the electrical load placed on the starter motor during the compression stroke.
The waveform (Fig.1) shows the current draw at the starter motor whilst the motor is turning the engine.
The initial high current spike is what is required to overcome static friction. This drops to the cranking level and then each peak in the sawtooth section of the waveform represents a piston compression stroke.
It is the comparative height of the peaks that indicates the relative compression of each cylinder.
The second waveform example (Fig.2) shows the complete loss of compression on one cylinder.
It is essential to the running of the engine that it has sufficient compression. The compression provided by the rising piston will be determined by the swept volume being compressed into the combustion area, the compression ratio. The compression is also determined by the effectiveness of the seal between the cylinder wall and the piston, this seal is maintained by the piston rings. The same applies to the seating of both the inlet and exhaust valves.
Piston rings are made of centrifugally spun cast iron, which produces a radial pressure, forming the seal. Cast iron is also used for its excellent self lubricating properties.
If a relative compression waveform highlights a problem, it will be necessary to perform a compression test. A typical diesel compression can range from 19 bar (275 psi) to 34 bar (495 psi). This pressure tends to be slightly lower on vehicles with indirect injection and higher on direct injection systems. The diesel engine relies upon compression to generate the heat which is required to ignite the atomised fuel. Any reduction in compression will reduce the heat generated and compromise the combustion process. A low compression will eventually result in the cylinder not firing. It is therefore imperative that the valve clearances (when adjustable) are set to manufacturer's specifications.
A low compression can be caused by:
All readings should be similar. If one is lower than the others a wet test can be performed by squirting a small amount of oil into the cylinder and re-testing the compression. The inclusion of the oil improves the seal between the piston and the bore, so if the compression is regained the fault lies within the piston rings. If very little difference has been made, the fault lies within the valves.
There should not be more than 25% difference between the highest and lowest compression readings.
The amperage required to crank the engine depends on many factors including, engine capacity, number of cylinders, oil viscosity, starter and circuit condition and of course the compression in the cylinders.
The current for a typical 4 cylinder diesel engine is about 200 to 300 amps. The compressions can be compared against each other by monitoring the current required to push each cylinder up on its compression stroke. It is therefore important that the current draw on each cylinder is equal.
Given the inherent difficult access for pressure testing on most diesel engines this PicoScope test is a very useful tool to help decisions to be made regarding further time consuming work.
NOTE; When conducting compression tests on a diesel engine ensure that the appropriate gauge is used (diesels have much higher compression than petrol engines). Also make certain that the fuel supply to the injectors is stopped by electrically isolating the fuel cut-off solenoid.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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