Which ignition type describes most diesel engines?

• A. Spark
• B. Compression
• C. Electrical
• D. Ionization

Answer: (B)

Compression ignition is how most diesel engines start combustion. In these engines, air is squeezed to a very high pressure and temperature. When fuel is injected into this superheated, high-pressure air, it ignites automatically without a spark. That is the defining feature of diesel operation: ignition occurs from the heat of the compressed air, not from an electrical spark.

Spark ignition, used in most gasoline engines, relies on a spark plug to ignite a premixed air-fuel mixture, which isn't how diesels operate. Electrical ignition would imply sparking to start combustion, which diesel engines typically don’t use. Ionization relates to detecting or analyzing charged particles in the mixture, not the method that starts combustion. Some diesel engines use glow plugs to warm the air for easier starting in cold weather, but the actual ignition mechanism remains compression heating of the air.