Compression starts once the air inlet phase finishes. Dependent on the type of engine, this happens differently. In 2-stroke engines, compression occurs when the inlet ports in the cylinder liner are closed by the piston. In a 4-stroke engine, compression occurs after the inlet valves have closed.
Once compression starts, pressure and temperature both increase in the cylinder while the volume is decreasing. Eventually, fuel will be admitted, and combustion will commence, but at this stage, we are only interested in making sure there are no problems with the pure compression of the admitted air. There should be no effects from combustion at this stage, so issues will be to do with leaks or the amount of air admitted.
Most diesel engines are turbocharged and have pressurised air admitted to the cylinders. There can be problems downstream of the cylinder itself in the inlet manifold, the intercooler or the turbocharger. Such issues usually affect all cylinders equally, but comparisons with previous tests can often be used to identify these. On V engines, it is quite common to have different boost pressure (or scavenge pressure as it is often called, especially on 2-strokes) on each bank of the engine.
The graph’s best area to compare compressions is just before the first cylinder has fired or at TDC if all the cylinders fire after TDC.
In this example, firing is after TDC so that compression can be compared at TDC; this is very healthy with just over 1.5% variation.
This engine fires ahead of TDC (mostly), so we look at compression just forward of the cylinder’s firing point that fires first. Here the variation is just under 30% which is much too high. Ideally, variation would be under 5% on an engine like this one. 2-stroke main engines should aim for less than 3%.
Most of the compression loss here is clearly due to cylinder 5. The cause, in this case, was a leaking exhaust valve, and the crew were good enough to provide a picture showing the damage. The leak is so significant that the compression peak pressure occurs ahead of TDC.
The example may seem quite extreme but shows how the Diesel Doctor can quickly identify problems like this.
To learn more about the Diesel Doctor, click here.