What are the main differences between diesel and gasoline engines?

Diesel engines use compression ignition (fuel ignites from heat of compression) while gasoline engines use spark ignition. Diesels have higher compression ratios (16-22:1 vs 10-14:1), producing higher thermal efficiency and torque. Diesel fuel has higher energy density but engines are heavier and more expensive. Gasoline engines rev higher and are quieter. Diesels excel in efficiency and towing; gasoline engines offer smoother, quicker response.