1. History

2. Technology and implementation

3. Benefits and drawbacks

4. Usage
    Alfa Romeo  Aston Martin  Chrysler  Ford  Jaguar  Lancia  Lotus  Mercedes Benz  MG  Mitsubishi  Porsche  Toyota 

5. Design evolution in modern engines

6. References

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.

History

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 (hemi- + -sphere + -ical), although in practice it is generally less than half.

Hemispherical cylinder heads have been used since at least 1901;^[1] they were used by the Belgian car maker Pipe in 1905^[2] and the 1907 Fiat 130 HP Grand Prix racer.^[3] 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,^[4] 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.^[5]

Technology and implementation

A hemispherical head ("hemi-head") gives an efficient combustion chamber with minimal heat loss to the head, and allows for two large valves. However, a hemi-head usually allows no more than two valves per cylinder due to the difficulty in arranging the valve gear for four valves at diverging angles, and these large valves are necessarily heavier than those in a multi-valve engine of similar valve area, as well as generally requiring more valve lift. The intake and exhaust valves lie on opposite sides of the chamber and necessitate a "cross-flow" head design. Since the combustion chamber is virtually a hemisphere, a flat-topped piston yields a lower compression ratio unless a smaller chamber is utilized. There is a misconception that all hemi engines have dome pistons. This is not true. Some of the modern factory hemi engines that have high compression ratios such as Chrysler's 2009 5.7L engine which is 10.5:1 have dome pistons and some of Chrysler's 5.7 engines have flat top pistons. Typically, high compression ratios use 92 - 93 octane gasoline. Theoretically, if the engine required a low compression ratio due to the fact that it is supercharged, flat top pistons can be used or if the goal is to use a lower octane gasoline, flat top pistons can also be used. There are many hemi engines with flat top pistons. Desired compression ratio is key in determining piston shape and combustion chamber size along with many other factors such as

Combustion chamber volume,

Piston compression height,

Piston valve relief volume,

Piston dome volume,

Piston dish volume,

Head gasket's thickness,

and Piston-to-deck clearance.

Another example of a hemi flat top piston is the after market Manley Chrysler 6.1L Hemi Flat Top Piston.

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.{{citation needed|date=September 2014}} This complexity was referenced early in Chrysler's development of their 1950s hemi engine: the head was referred to in company advertising as the Double Rocker head.^[2]

Benefits and drawbacks

Although a wedge-head design offers simplified valve actuation, it usually does so by placing the valves side by side within the chamber, with parallel stem axis. This can restrict the flow of the intake and exhaust into and out of the chamber by limiting the diameters of valve heads to total no more than the bore of the cylinder in a two valve per cylinder arrangement. With a hemispherical chamber with splayed valve stem angle, this limitation is increased by the angle, making the total valve diameter size possible to exceed the bore size within an overhead valve configuration. See IOE engine for another method.

Also, the splayed valve angle causes the valve seat plane to be tilted, giving a straighter flow path for the intake and exhaust to/from the port exiting the cylinder head. Engineers have learned that while increasing the valve size with straighter port is beneficial for increasing the maximum power at high rpm, it slows the intake flow speed, not providing the best combustion event for emissions, efficiency, or power in the normal rpm range.

Domed pistons are commonly used to maintain a higher mechanical compression ratio, which tend to increase the flame propagation distance, being also detrimental to efficient combustion, unless the number of spark plugs per cylinder is increased.

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. This is one of the main reasons for its decline in popularity.

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]). These have pushed the hemi head out of favor in the modern era.