Compression ratio refers to the volume, or amount, of an air and fuel mixture that the combustion chamber in a combustion engine can hold when it is empty and at its largest size compared to the volume it holds when the mixture is compressed down to the smallest size possible. This ratio applies to both internal combustion engines, such as those found in modern-day vehicles, and seldom-used external combustion engines. Diesel- and gas-powered engines alike each have a compression ratio, though the design of the diesel engine encourages a higher compression ratio. Engines with higher compression ratios are generally considered better because they create more power while still maintaining efficiency.
To calculate the compression ratio of an engine, an engineer would first calculate the volume a cylinder in the engine can hold when the piston is at the bottom of the cylinder. During one stroke of the engine, the piston moves from the bottom up toward the top and compresses the air-fuel mixture. After finding the volume of the cylinder when the piston is down, and therefore not compressed yet, the engineer would then need to calculate the volume when the piston is up and the air-fuel mixture has been compressed. A ratio such as 13:1, for example, means that the engine holds 13 times more volume when the piston is down than when it is compressed. The amount of air-fuel mixture does not change, but rather is simply pressed into a significantly smaller space to create a large explosion.
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Diesel engines use compression to create the temperature at which diesel oil ignites the air-fuel mixture that creates the necessary power to drive the vehicle forward. High compression ratios in gas engines often cause a problem known as engine knocking. Diesel engines, on the other hand, are designed for high compression in order to function. A ratio of 13:1 is considered high in a gas engine while a diesel engine can range from 14:1 up to 23:1 depending on the type.
High compression ratios cause more power by compressing the air and fuel even tighter than average and thus creating a more forceful explosion. The tight packing of the air-fuel mixture helps both air and fuel to blend better and when the explosion occurs more of the mixture evaporates. More evaporation is a sign of higher thermal efficiency, meaning the engine performs better without using too much extra energy to gain this power.
The disadvantage of a higher compression ratio in a gas engine is the possibility of engine knocking or pinging. This occurs when a larger explosion than desired occurs and causes the piston to move upward or downward too quickly. A loud knocking noise results and, if not fixed, continuous engine knocking can permanently damage the engine. Cars using gas with a higher octane rating or a knocking sensor can use higher compression ratios, but still cannot match the high ratio of a diesel engine.