Updated on 19 October, 2021
There are essentially two kinds of compression checks for engines. The first is a regular compression check which uses a single gauge to show the maximum instantaneous compression developed by that particular cylinder. A second compression check uses the engine turning at idle speed to produce the maximum compression in that cylinder for the complete compression stroke, where both exhaust and intake ports are shut. The purpose of a compression check is to show the engine's capability in reducing compression and increase horsepower. If an engine is producing less horsepower than it should be doing, then this could indicate a serious problem with one of the components.
A compression stroke occurs when the piston rod moves in a linear direction through the piston and cylinder combination. There are three main contributing forces to this movement; namely, compression pressure, induced velocity, and axial force. The compression pressure is caused by the ambient temperature, the ambient atmospheric pressure, and the density of the fuel/oxygen mixture. The induced velocity is caused by the induction of air into the combustion chamber. Lastly, there is the axial force, which is caused by the rotation of the crankshaft around its axis, and is therefore only caused when the piston remains in motion against the rotor.
There are a number of different ways to measure engine compression, including multiple cylinders of similar length and the piston rod position. Multiple cylinder measurements are typically done with a portable v-notch piston gauge, while single cylinder gauges are used for small engines (usually less than a liter and under) with a tapered output stroke width. The performance of any compression test depends upon the efficiency of the combustion process and the level of compression pressure generated during ignition.
The gauge used to determine engine compression pressure is a sealed piston cylinder head, which can be placed in either the intake manifold or combustion chamber. The maximum achievable pressure is noted and a value is calculated as a function of the input variable (the stroke width), output variable (the piston rod displacement and airflow), and the input variable (the ambient temperature). All the four components will influence the calculations, resulting in a reliable measurement that is repeatable.
As per manufacturers, a gauge for compression testing should have a minimum and a maximum pressure reading. The minimum pressure is selected to minimize the time for the piston to cool between compression strokes. The maximum pressure reading is for situations where very high temperatures are experienced in the combustion chamber. In case of cylinder pressure readings exceeding the predetermined maximums, it is advisable to reduce the power or increase the flow rate to compensate for increased temperature.
There are several instances where an engine compression leak may occur. Firstly, if a cylinder valve is rotated during startup, it may lead to a compression leak. Secondly, when the pistons reach full compression during shutdown the valve may lock up resulting in a leakage. Thirdly, when the pistons are shut, it may leak after the chamber has cooled down causing a compression leak.
Gas engine compression testing is important to identify potential problems in cylinders and cylinder valves. It provides engineers with data on cylinder performance, and a good test program can help them identify possible issues before they interfere with normal operations. A wide variety of tests are available. It is essential to follow guidelines provided by the manufacturer carefully to avoid damaging the engine or damaging the testing equipment. Following the test procedures meticulously will ensure that the readings do not change beyond the established range.
It is essential to adjust the spark plug settings. Spark plugs have a lower compression pressure than pistons so when they are tightened, they pull down on the gaskets which will decrease compression pressure. It is recommended to have the spark plugs tightened before the engine is started. After the fuel filter has been replaced, and the air filter removed, the new spark plug holder should be installed and its spark plug holes aligned with the holes on the piston. Tighten the bolts and screws tightly, then turn on the ignition and check the readings.