Diesel cars don't require spark plugs. Gasoline cars rely on spark plugs to generate an electric spark, igniting the combustible mixture entering the cylinder and propelling the vehicle. Diesel cars, on the other hand, use compression ignition. Fresh air is compressed and heated in the cylinder, and the fuel injector injects diesel fuel at the right time. Under the high-temperature and high-pressure environment, the diesel mixes with the air and burns spontaneously. This unique ignition method means diesel cars don't require spark plugs.
Specifically, after a diesel engine draws in fresh air, it compresses it to just before top dead center, at which point the fuel injection system precisely injects diesel fuel into the cylinder. Under the extreme conditions of high pressure and temperature, the diesel and air rapidly mix and explode, producing work. The entire process occurs seamlessly, without the need for spark plugs to provide external ignition energy.
This compression ignition method makes the diesel engine relatively simple, eliminates the need for spark plugs, a vulnerable component, and reduces maintenance costs and complexity. Furthermore, thanks to this unique operating method, diesel engines are able to achieve higher compression ratios, resulting in better fuel economy and greater torque output.
In contrast, gasoline engines typically require one or two spark plugs per cylinder. Because gasoline has a high auto-ignition temperature and is difficult to burn on its own under normal compression conditions, the engine must rely on the spark generated by the spark plug to ignite the combustible mixture in the cylinder at the right moment for smooth engine operation.