In which type of engine is fuel ignited through heat and pressure?

Fuel is ignited through heat and pressure in a diesel engine, which operates on the principle of compression ignition. In this type of engine, air is compressed to a high pressure and temperature, and then diesel fuel is injected into the combustion chamber. The heat generated from the compression is sufficient to ignite the fuel without the need for a spark plug, which is a hallmark of diesel engines.

This process allows diesel engines to have a higher thermal efficiency compared to those that rely on spark ignition, as the compression ratios can be significantly higher. Diesel engines are particularly known for their durability and fuel efficiency, making them suitable for heavy-duty vehicles and machinery.

In contrast, spark ignition engines rely on a spark plug to ignite a fuel-air mixture. Rotary engines, characterized by their unique design and operation, do not use traditional piston and cylinder mechanics, while small engines can refer to a variety of engine types, often including those with either spark or compression ignition. None of these other options utilize the heat and pressure method of ignition that defines a diesel engine.