How does a diesel engine differ from a gasoline engine?

The primary difference between a diesel engine and a gasoline engine is the method of ignition. While gasoline engines rely on a spark-ignition system, diesel engines are compression-ignition engines.12

Key distinctions include:

• Ignition Process: Diesel engines induce only air into the cylinder during the intake stroke, compressing it until it reaches a temperature high enough to spontaneously ignite the fuel upon injection. Gasoline engines typically compress a premixed air-fuel mixture and require an electric spark to initiate combustion.12
• Compression Ratios: Because diesel engines do not compress an air-fuel mixture, they are not limited by the preignition (knocking) issues that affect high-compression spark-ignition engines. Consequently, diesel engines operate at higher compression ratios (typically 14:1 to 22:1), contributing to superior thermal efficiency.1
• Efficiency and Cost: Diesel engines generally offer better fuel efficiency and lower operating costs per horsepower-hour, though they are typically heavier and more expensive to manufacture than gasoline engines of equivalent power.12
• Emissions: Diesel engines often produce higher levels of particulate matter and nitrogen compounds compared to gasoline engines.1