How does the fuel in a gas engine ignite?

The ignition of fuel in a gas engine occurs through combustion caused by a spark. In a typical gasoline engine, the process begins when an air-fuel mixture is drawn into the combustion chamber. As the piston moves up in the cylinder, this mixture is compressed, which increases the temperature and pressure. Once the piston reaches the top of its stroke, a spark plug emits a spark that ignites the highly compressed air-fuel mixture, causing an explosive combustion. This rapid expansion of gases from the combustion is what drives the piston down and ultimately powers the engine.

The other methods presented are not used in standard gasoline engines. Compression alone typically applies to diesel engines, where the fuel ignites due to the heat generated from high compression without a spark. A flame from the exhaust is not a viable method for igniting the fuel; rather, it indicates a fault in the combustion process. Finally, mixing with coolant does not contribute to fuel ignition but is specifically for regulating engine temperature. Thus, the process involving a spark plug is what distinguishes gasoline engines and is the correct mechanism for ignition in this context.