Hemispherical combustion chamber
internal combustion engine in which the roof of each cylinder's combustion chamber is of hemispherical form
A hemispherical combustion chamber is a combustion chamber in the cylinder head of an internal combustion engine with a domed "hemispheric" shape. An engine featuring this type of hemispherical chamber is known as a hemi engine. In practice, shapes less than a full hemisphere are typically employed, as are variations (or faceting in parts) of a true hemispheric profile. The primary advantage of such shapes are increased compression (leading to greater power) and very large intake and exhaust valves (allowing better flow of intake and exhaust gasses, also resulting in improved volumetric efficiency and greater power); the primary disadvantages are complex valve trains (caused by valves being placed opposite one-another in a head) and expense (of machining the heads and pistons, and additional valve train components).
While hemispherical combustion chambers are still found in the 2000s multi-valve arrangements (of four and even five valves per cylinder) and the popularity of overhead cam (including double overhead cam) arrangements have altered the traditional trade-offs in employing "hemi heads".
History
Hemispherical combustion chambers were introduced on some of the earliest automotive engines, shortly after the viability of the internal combustion engine was first demonstrated. Their name reflects the domed recess in a cylinder head and correspondingly shaped top of a piston enclosing a space that approximates a half of a sphere (hemi- + -sphere + -ical), although in practice the actual enclosed space 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 by the 1907 Fiat 130 HP Grand Prix racer. The Peugeot Grand Prix car of 1912 and the Alfa Romeo Grand Prix car of 1914 were both four-valve engines, and Daimler and Riley were also using hemispherical combustion chambers at the time. Beginning in 1912, Stutz used four-valve engines, conceptually anticipating modern car engines.
The public source identifies “Hemispherical combustion chamber” as internal combustion engine in which the roof of each cylinder's combustion chamber is of hemispherical form. This brief keeps that definition visible, then builds a research path around Hemispherical, combustion and chamber.
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This entry incorporates text from “Hemispherical combustion chamber” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.