The Dodge Hemi engine operates similarly to other internal combustion engines but uses a hemispherical combustion chamber that differentiates it from many other passenger car engines. The engine's name comes from this feature.
The original Chrysler FirePower engine that evolved into the Hemi engines used in Dodge vehicles was introduced in 1951, a time when so-called "flathead" engines remained the predominant design. Flathead designs use a cylinder that is flat near the spark plug, resulting in a cylinder akin to a soup can in shape. In contrast, the use of a hemispherical combustion chamber in FirePower and Hemi engines results in a cylinder that resembles a rounded bullet in shape, with the point of the bullet near the valves and spark plug. The additional volume allows for a more efficient offset arrangement of the engine's valves as compared to flathead engines, enabling better airflow to the engine and more power for the same engine displacement. However, this design also results in more complex and expensive engine valve designs.
While hemispherical combustion chambers were not invented by Chrysler or Dodge, the expense involved in engineering the complicated valves required for such engines limited their use to racing cars and military aircraft. However, the FirePower engine became the first to use this type of cylinder design for mass-produced passenger cars. Though other car companies have since developed motors with similar combustion chamber designs, Dodge continues to use the Hemi designation for marketing as of 2015.