What initiates the ignition process in a four-stroke engine?

The ignition process in a four-stroke engine begins when the air/fuel mixture is ignited inside an enclosed cylinder. In this context, the four-stroke cycle involves four distinct phases: intake, compression, power, and exhaust. During the intake stroke, a mixture of air and fuel is drawn into the cylinder. This mixture is then compressed during the compression stroke, raising its temperature and pressure.

When the piston reaches the top of the compression stroke, it is at this moment that the spark plug generates a spark, igniting the compressed air/fuel mixture. This ignition is what leads to the rapid expansion of gases, which drives the piston down during the power stroke. The entire process relies on the combustion of the air/fuel mixture within the confined space of the cylinder to produce power.

By understanding this sequence, it becomes clear that while other factors like the spark from the ignition coil and the high compression contribute to the ignition process, it is specifically the ignition of the air/fuel mixture in the cylinder that initiates the power generation in a four-stroke engine. Additionally, an external flame source is not used in modern engines, as the ignition is solely managed by the spark plug and the properties of the mixture itself.