What is a key difference between diesel engines and gasoline engines?

• A. Spark ignition
• B. Compression ignition
• C. Two-stroke
• D. Carburetor engine

Answer: (B)

The key idea is how the air–fuel mixture is ignited. Gasoline engines use spark ignition: a timed spark from a spark plug ignites a premixed air–fuel charge. Diesel engines use compression ignition: air is compressed to a very high temperature, and fuel is injected into that hot, compressed air where it auto-ignites without a spark. This fundamental difference in ignition method is what sets diesel apart from gasoline.

The other options describe aspects that aren’t the defining difference. Spark ignition is the method used by gasoline engines, not diesel. The cycle (two-stroke) is independent of whether ignition is by spark or compression. And a carburetor is about how fuel is mixed with air, a point that’s largely superseded in modern gasoline engines by fuel injection, but it doesn’t define the ignition method.