Inside the cylinders of a diesel engine, the mixture of fuel and air is compressed to about 1/20 of its original volume. Although diesel engines have no spark plugs to ignite this mixture, it ignites anyway because of what?
Compression results in heating of the mixture. The heating is enough to cause ignition.
The diesel fuel in an engine ignites due to a process called compression ignition. In a diesel engine, the air-fuel mixture is compressed to a high pressure and temperature inside the cylinder. This compression causes the air to get highly heated.
To understand how compression ignition works, it is important to know that diesel fuel has a higher ignition temperature compared to gasoline. In a gasoline engine, a spark plug initiates the ignition by creating a spark to ignite the fuel-air mixture.
In a diesel engine, however, the air-fuel mixture is compressed to such a high pressure and temperature that it exceeds the ignition temperature of the diesel fuel. When the piston reaches the top of its compression stroke, the fuel is injected into the cylinder at a precise moment, forming a fine spray or mist. The high temperature and pressure in the cylinder cause the fuel to ignite spontaneously upon contact with the hot air.
In summary, the mixture of fuel and air in a diesel engine ignites due to compression ignition, where the high pressure and temperature reached during compression surpass the ignition temperature of the diesel fuel, causing it to ignite without the need for a spark plug.