Why don't diesel engines have spark plugs?
Diesel engines use the heat of compression to ignite fuel instead of a spark. Diesel engines have very high compression ratios, typically 16 to 1 and higher. When the piston comes up on the compression stroke, therefore, it creates very high temperatures in the combustion chamber. A diesel engine has no carburetor or throttle like a gasoline engine, so only air is drawn into the cylinders. The fuel is then injected directly into the combustion chamber through a mechanical fuel injector. When it hits the hot air, it ignites and burns just like gasoline ignited by a spark. It's a relatively simple design that delivers high fuel efficiency (typically 30 to 40% higher than a gasoline engine of equal displacement) and low carbon monoxide emissions (but a lot of soot).