Relative compression - petrol

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.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

1. Breakout leads.
2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

• When testing a sensor, it is desirable to gain access at the control module.
• When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

1. Note: You will need to disable the fuel supply to allow the engine to crank sufficiently to get a suitable pattern for comparison.
2. Connect the high amp current clamp into Channel A. on your PicoScope, switch on and zero the current clamp.
3. Position the current clamp onto one of the battery leads (+) or (-) observe current direction arrow on the current clamp, incorrect positioning will invert your waveform.
4. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
5. Select GO or press the space bar to see live data.
6. Hold the throttle open and crank the engine for around 3 seconds to capture your waveform.
7. After cranking use the WAVEFORM BUFFER and ZOOM tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform shows the following:

The current drawn by the starter motor indicates the starter motor torque required to turn the engine; the higher the current, the higher the torque.

An initial high current is required to overcome static friction and the inertia of the engine components.

Once rotating, the friction and inertia forces are reduced and the in-cylinder forces acting on the piston dominate: thus, each peak in the oscillating cranking waveform represents the starter motor torque required to overcome a piston's compression stroke.

Each of the peaks and troughs in the steady-state cranking phase should be comparable, with equal amplitudes. Any periodic differences, e.g. occurring every 4 cycles, indicate an in-cylinder pressure anomaly.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Relative Compression Current.

Further guidance

This PicoScope test is a very useful tool to help decisions to be made regarding further time consuming work.

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 low compression can be caused by.

• An ineffective seal between the cylinder and the piston.
• Poor seating of the inlet and exhaust valves.
• Broken or seized piston rings.
• Incorrect camshaft timing.
• An obstructed induction tract.

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 compressions can be compared against each other by comparing 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.

GT004-4

Suitable accessories