Motorcycle engines without spark plugs typically use a different type of ignition system called a compression ignition system. One such example is a diesel engine, which is commonly found in motorcycles and other vehicles. Here's how it works:
Compression: Just like in a traditional gasoline engine, the piston compresses the air-fuel mixture inside the combustion chamber. However, in a compression ignition system, only air is initially drawn into the combustion chamber.
Fuel Injection: After the air is compressed, fuel is injected into the combustion chamber at the end of the compression stroke. The fuel is atomized into small droplets, which mix with the hot, compressed air.
High Compression and Heat: The compressed air-fuel mixture is subjected to extremely high compression ratios, typically between 14:1 to 22:1. This compression causes the air-fuel mixture to heat up significantly.
Self-Ignition (Autoignition): Unlike a spark plug in a gasoline engine, where a spark ignites the air-fuel mixture, in a compression ignition system, the mixture ignites spontaneously due to the high temperature and pressure. This phenomenon is known as autoignition or self-ignition.
Combustion: Once the air-fuel mixture ignites, it rapidly combusts, generating a powerful force that pushes the piston downward. This downward motion is converted into rotational motion, which drives the motorcycle.
Exhaust: After the power stroke, the exhaust gases are expelled from the combustion chamber through the exhaust valves. The process then repeats with the piston moving back up to compress a fresh charge of air for the next cycle.
Compression ignition engines, such as diesel engines, are known for their fuel efficiency and torque output. They rely on the high compression ratio and self-ignition of the air-fuel mixture, eliminating the need for spark plugs. It's worth noting that not all motorcycles use compression ignition engines, as many still rely on spark-ignition gasoline engines for their operation.