What best describes the main difference between diesel and gasoline engines?

• A. Diesel engines are compression-ignition engines; air is compressed, which heats and causes injected fuel to ignite.
• B. Diesel engines rely on spark ignition to ignite fuel.
• C. Gasoline engines rely on compression to ignite fuel without a spark.
• D. Both engines ignite fuel only with an external ignition source.

Answer: (A)

The main idea is how ignition happens. Diesel engines rely on compression ignition: air is drawn in and compressed to a very high temperature, then fuel is injected and ignites from the heat—no spark plug needed. Gasoline engines use spark ignition: the air–fuel mixture is ignited by a spark from a spark plug. Because diesel relies on the heat from compression, it uses much higher compression ratios and does not need a spark, while gasoline engines require a spark and therefore operate with lower compression to prevent premature ignition. The other statements mix up how ignition works: diesel doesn’t use a spark; gasoline doesn’t ignite purely by compression; and neither engine relies on an external ignition source.