For: Chrysler Hemi engine

Hemispherical combustion chambers, which had been used for centuries in mortars and cannon, were introduced on some of the earliest automotive engines, shortly after proving the concept of internal combustion engines themselves.

Technology & implementation

With the hemispherical combustion chamber design, the intake and exhaust valves are usually on opposite sides of the chamber, unlike the in-line valve arrangement common to most engines with wedge-shaped combustion chambers. Therefore, significant challenges in the commercialization of hemi engines revolved around the design of the valve actuation, and how to make it effective, efficient, and reliable at an acceptable cost. Early in Chrysler's development of their 1950s hemi engine, the head was referred to in company advertising as the Double Rocker head.

Benefits & drawbacks

Although a wedge-head design offers simplified valve actuation, it usually requires the air/fuel mixture to make sharp turns en route to and from the chamber. With a hemispherical chamber, larger valves are possible and a straighter, less restrictive flow path can be provided for the air/fuel mixture. This improves engine breathing. Placing the spark plug near the center of the chamber aids in achieving complete combustion of the fuel/air mixture, though it is not mandatory.

Drawbacks of the hemispherical chamber such as increased production cost, high relative weight (25% heavier than a comparable wedge head per Chrysler's engineers), poor low-rpm performance characteristics and difficulty meeting emissions standards have pushed the hemi head out of favor.

History & usage

Hemispherical cylinder heads have been used since at least 1903; they were used by the Belgian car maker Pipe in 1905], the Peugeot Grand prix Car of 1912, the Alfa Romeo GP car of 1914, Daimler, and Riley. Stutz built four valve engines, conceptually anticipating modern car engines. The BMW double push rod design, taken over by Bristol Cars, the Peugeot 403 and the Toyota T engine are other well known hemi engines. Harry Arminius Miller racing engines were also a notable example .

Chrysler

Perhaps the most widely known proponent of the hemispherical chamber design is the Chrysler Corporation. Chrysler became identified primarily by trademarking the "Hemi" name and then using it extensively in their advertising campaigns beginning in the 1960s. Chrysler has produced three generations of such engines: the first (the Chrysler FirePower engine) in the 1950s, the second (the 426 Hemi), developed for NASCAR in 1964 and produced through the early 1970s, and finally the "new Hemi" in the early 2000s. The "Hemi" engine introduced in 2002 by DaimlerChrysler had a combustion chamber featuring valve and spark plug locations markedly different from the 426ci Hemi engine of muscle car fame. The current-production "Hemi" V8 with its pinched chamber, does not have true hemispherical combustion chambers despite the name. Rather, it bears a closer resemblance to the mid-1950s Polyspherical chamber, which Chrysler engineers developed as a lower-cost alternative head for their V8 engines. The Polyspherical head needed less metal and was narrower due to using only one rocker shaft. This saved costs in material, space and warranty claims and allowed it to be used in smaller vehicles. Chrysler's Australian-market Hemi-6 of 1970-80 had partial-spherical hemi chambers, though they were only 35% of a sphere.