Hemi

A hemispherical combustion chamber is a type of combustion chamber in a reciprocating internal combustion engine with a domed cylinder head. The hemispherical shape provides a number of advantages over a reverse-flow cylinder head but comes up short in others, particularly in carbureted engines. An engine featuring this type of hemispherical chamber is known as a hemi engine.

Hemispherical combustion chambers were introduced on some of the earliest automotive engines, shortly after proving the concept of internal combustion engines themselves. Their name reflects the concept of a domed cylinder head and the top of the piston enclosing a space that approximates a half of a sphere ( + + ), although in practice it is generally less than half.

Hemispherical cylinder heads have been used since at least 1901; they were used by the Belgian car maker Pipe in 1905 and the 1907 Fiat 130 HP Grand Prix racer. The Peugeot Grand Prix Car of 1912 and the Alfa Romeo Grand Prix car of 1914 both were four valve engines also, Daimler, and Riley were also using hemispherical combustion chambers. Stutz, beginning in 1912, used four-valve engines, conceptually anticipating modern car engines. Other examples include the BMW double-pushrod design (adopted by Bristol Cars), the Peugeot 403, the Toyota T engine and Toyota V engine (Toyota's first V8 engine), and Miller racing engines, and the Jaguar XK engine.

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