What is the key difference between spark ignition and compression ignition engines?

The key difference between spark ignition and compression ignition engines lies in the method they employ to ignite the fuel-air mixture.

In spark ignition engines, such as those commonly found in gasoline-powered vehicles, a spark plug generates a spark that ignites the air-fuel mixture in the combustion chamber. This system relies on a relatively low compression ratio, which means that the fuel-air mixture is pre-mixed before entering the cylinder and ignited by the spark.

On the other hand, compression ignition engines, typically found in diesel engines, operate on a different principle. These engines achieve ignition by compressing the air to a very high pressure, which significantly raises its temperature. When the fuel is injected into this hot, high-pressure air, it ignites without the need for a spark. This method allows for a more efficient engine design, as it can utilize higher compression ratios and typically achieves better thermal efficiency.

While other options mention various components or fuel types, they do not accurately encapsulate the core operational principle that distinguishes these two types of engines. The fundamental difference in ignition methods fundamentally influences the design and operation of each engine type.