Variable Compression
Model diesels have variable compression ratios. This variable compression is achieved by a "contra-piston," at the top of the cylinder, which can be adjusted by a screwed "T-bar". The swept volume of the engine remains the same, but as the volume of the combustion chamber at top dead center is changed by adjusting the contra-piston, the compression ratio (swept volume + combustion chamber / combustion chamber) changes accordingly. There also exists model diesels with fixed compression heads. A few examples are the British 5cc OWAT or the American DRONE. These engines have a fixed compression ratio, but the compression can be altered slightly with the use of more or less lubrication oil in the fuel. Also the engine's load has an impact on the compression and ignition timing. Larger propellers give a longer combustion time and so higher compression. Smaller propellers reduce the load, and therefore the compression pressure build up, which changes the ignition timing to earlier.
Read more about this topic: Carbureted Compression Ignition Model Engines
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