Final answer:
In the four-stroke compression ignition engine (diesel engine), ignition occurs without the aid of a spark, due to the high temperature achieved by the adiabatic compression of air.
Step-by-step explanation:
In the four-stroke compression ignition engine, also known as a diesel engine, ignition takes place without the aid of a spark. Instead, the ignition process relies on the high temperature caused by adiabatic compression of air within the cylinder. When the air is compressed, its temperature rises significantly. At the point of maximum compression, diesel fuel is injected into the cylinder. The temperature of the compressed air is high enough to ignite the diesel fuel without the need for a spark, leading to combustion. This is in contrast to gasoline engines, which use a spark plug to initiate combustion.
To elaborate based on provided figures, in a gasoline engine, fuel is normally ignited by a spark plug . However, in diesel engines, air is compressed to a point where its temperature is above the ignition temperature of fuel, at which point fuel is added and spontaneously ignites. This is a fundamentally thermal process, where the sharp increase in temperature leads to a spike in pressure, thereby pushing the piston and performing work.