puristusluvut

Puristusluvut, often translated as "compression ratios," are a fundamental concept in the design and operation of reciprocating engines, particularly internal combustion engines. They represent the ratio between the volume of the cylinder when the piston is at the bottom of its stroke (bottom dead center, or BDC) and the volume when the piston is at the top of its stroke (top dead center, or TDC). Mathematically, it is expressed as the total cylinder volume at BDC divided by the cylinder volume at TDC.

A higher puristusluku generally leads to greater thermal efficiency, meaning more of the fuel's energy is converted into mechanical work. This is because a greater compression of the air-fuel mixture before ignition results in a more powerful expansion of the combustion gases. However, excessively high puristusluvut can lead to engine knocking or pre-ignition, where the fuel-air mixture ignites prematurely due to the high temperatures and pressures, potentially damaging the engine.

The optimal puristusluku for an engine is a compromise between efficiency, power output, and the fuel's resistance to knocking (octane rating). Different engine designs, such as gasoline, diesel, and Atkinson cycle engines, have distinct ranges of puristusluvut tailored to their specific operating principles and fuel types. For instance, diesel engines typically have much higher puristusluvut than gasoline engines because they rely on compression ignition rather than spark ignition. Engineers carefully select and tune puristusluvut during engine development to achieve the desired balance of performance and reliability.