Hemispherical combustion chamber
Hemispherical cylinder heads have been used since at least 1901;[1] they were used by the Belgian car maker Pipe in 1905[2] and by the 1907 Fiat 130 HP Grand Prix racer.
The intake and exhaust valves lie on opposite sides of the chamber and necessitate a "cross-flow" head design.
Significant challenges in the commercialization of engines utilizing hemispherical chambers revolved around the design of the valve actuation, and how to make it effective, efficient, and reliable at an acceptable cost, [2] which normally requires the use of either a dual rocker system, or dual camshafts to operate the inlet and exhaust valves.
Flame temperatures are very high, leading to excessive NOx output which may require exhaust gas recirculation and other emission control measures to meet modern standards.
Other drawbacks of the hemispherical chamber include increased production cost and high relative weight (25% heavier than a comparable wedge head according to Chrysler's engineers[6]).
SOHC Cutaway showing cross-flow design, hemispherical shape of the chamber, center position of the overhead cam with flanking supports for the rocker shafts.
Upper photos of double rocker system for a pair of Hemi heads and its complex piston casting.
Arguably one of their most beloved examples is Giuseppe Busso's original 2.5-liter V6, which has been cited by some as one of the best and most distinctive sounding production engines (even in its later 24v forms) of all time.
BMW became a worldwide marque on the strength of its responsive yet durable SOHC hemi-head inline-4 M10 engine, most famously made in a 2 L (122 cu in) displacement in its 2002 sports sedan of the 1960s and 1970s.
[11][dubious – discuss] It used the side oiler engine block modified to replace an in-block cam with an idler shaft driving the distributor and oil pump, and accommodate other overhead camshaft issues.
In 1968, Ford brought out the completely new 385-series engine family, which used a modified Semi-Hemi[clarify] form of the hemispherical chamber.
Unfortunately, even with an ahead-of-its-time direct fuel injection system feeding a stratified charge chamber,[15][16] the hemi's emissions could not be made clean enough for compliance with regulations.
Post 1986 the cylinder head of this engine was reworked to heart-shaped lean-burn combustion chambers, and used in low-performance models not benefiting from multipoint fuel injection - 1.4, 1.6, 1.8 in Europe, though was still referred to colloquially as the CVH.
The hemi-head Jaguar XK engine, introduced in 1949, powered cars ranging from the Le Mans winning D-Type to the XJ6.
Early versions proved fragile on the street and in competition due to pre-ignition (detonation), and oil loss, which led to decreasing the compression ratio from 9.1. to 8.3 with redesigned pistons.
It was a successful update but sales dropped off so rapidly the company halted Twin Cam production and used the matching chassis for some MGAs, with pushrod engines, known as the MGA 1600 MkI and MkII DeLuxe models.
In the modern emissions-era, the hemi-chamber has morphed into more sophisticated and complex designs that are able to extract more power with lower emissions from any given combustion event.
