Hemispherical combustion chamber

A hemispherical combustion chamber is a type of combustion chamber with a domed cylinder head. The hemispherical shape provides some advantages in an internal combustion engine. An engine featuring this type of chamber is known as a hemi engine, as with the Chrysler Hemi engine.

History

Hemispherical combustion chambers were introduced on some of the earliest automotive engines, shortly after proving the concept of internal combustion engines themselves.

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.

Chrysler Hemi engine

The Chrysler Hemi engine, known by the trademark Hemi, is a series of V6 and V8 engines built by Chrysler with a hemispherical combustion chamber. Three different types of Hemi engines have been built by Chrysler for automobiles: the first (known as the Chrysler FirePower engine) from 1951 to 1958, the second from 1964 to 1971, and the third beginning in 2003. Although Chrysler is most identified with the use of "Hemi" as a marketing term, many other auto manufacturers have incorporated similar designs.

During the 1970s and 1980s, Chrysler also used the Hemi name for their Australian-made Hemi-6 Engine and applied it to the 4-cylinder Mitsubishi 2.6L engine installed in various North American market vehicles.

Hemispherical engines

A hemispherical cylinder head ("hemi-head") gives an efficient combustion chamber with an excellent surface-to-volume ratio, with minimal heat loss to the head, and allows for two large valves. However, a hemi-head allows no more than two valves per cylinder, and these large valves are necessarily heavier than in a multi-valve engine. The intake and exhaust valves lie on opposite sides of the chamber and necessitate a "cross-flow" head design. Since the combustion chamber is a partial hemisphere, a flat-topped piston would yield too low a compression ratio unless a very long stroke is used, so to attain desired compression ratio the piston crown is domed to protrude into the head at top dead center, resulting in a combustion chamber in the shape of the thick peel of half an orange.

Chrysler Hemi-6 Engine

The Chrysler Hemi-6 engine is a family of inline six-cylinder petrol engines produced by Chrysler Australia in three piston displacements and multiple configurations. Hemi-6 engines were installed in Australian-market Chrysler Valiants from 1970 through 1981.

Development and release

Chrysler Corporation in the US had been working since 1966 on an inline 6-cylinder engine, called the D-engine, to replace the Slant 6 (G-engine) in Dodge trucks, but abandoned the effort. Chrysler Australia wanted a new six-cylinder engine for use in the Australian Chrysler Valiant, and so Chrysler USA sent a prototype engine to Chrysler Australia's engineers to continue developing the D-engine. The first 245 cu in (4.0 L) variant was released for the 1970 model year in the VG-model Valiant.

In a major coup for the company, Chrysler Australia's ad agency, the Young & Rubicam Advertising Agency in Adelaide, South Australia, secured the services of British racing driver Stirling Moss to promote the new Hemi-6 (245 cui) in 1969. The agency managed to fly Moss to Adelaide in secret for the advertising campaign, surprising Chrysler Australia's executives. Young & Rubicam's parent company were also the advertising agents for the Chrysler Corporation in the US.