Hemispherical combustion chamber
Hemi (from "hemisphere") or "crossflow cylinder head" is a design of internal-combustion engines in which the cylinder head's combustion chamber is of hemispherical form. The term, "Hemi engine" is a trademark of Chrysler Corporation, though the concept is used by many manufacturers.
The BMW double push rod design, taken over by Bristol Cars, the Peugeot 403 and the Toyota T engine are other well known examples. Harry Arminius Miller racing engines were a more notable example. Stutz had built four valve engines, resembling modern car engines. Chrysler became synonymous with the "Hemi" by building them in such large numbers.
With the hemispherical combustion chamber design, the intake/exhaust valves are not parallel as in the wedge head; although this makes operating the valves more complicated, it allows larger valves, and better flow of air through the head (although the inlet and exhaust valves are not simultaneously open and there is no continuous flow). The spark plug in the center of the chamber makes for better ignition of the fuel/air mixture. These aspects help make the hemi-type engine more efficient and powerful, and less prone to engine knock.
The hemispherical cylinder head increases the engine's efficiency through reduced thermal energy loss and increased airflow through the engine. (A hemisphere has the lowest surface area to volume ratio, meaning the most space for combustion while losing the least amount of energy to the engine walls.) Drawbacks such as increased production cost have meant that it has been a rare design. Placing the intake on the opposite side of the engine also reduces the air intake temperature and increases efficiency.
Hemispherical cylinder heads have been used in some engines since they were first used by the Belgian car maker Pipe in 1905.[citation needed] Most applications have been in higher-priced luxury or sporting vehicles, because the hemi design is more expensive to build. Among first applications with hemispherical heads were also Peugeot Grand prix Car 1912 and Alfa Romeo GP car in 1914.
Perhaps the best-known proponent of the Hemi design has been the Chrysler Corporation, which has produced three generations of such engines: the first (the Chrysler FirePower engine) in the 1950s; the second (the 426 Hemi) from the mid 1960s through the mid 1970s; and finally in the early 2000s. Chrysler has used the word "Hemi" extensively in its advertising, to the extent that the word is indelibly associated with Chrysler in North America.
Porsche has also been a notable user of the Hemi design, generating up to 86 hp per liter displacement on production cars (1973 2.4 L 911S), and even more on racing engines (906 Carrera engine). Jaguar used this head design as well on the legendary XK engines, which powered cars ranging from the Le Mans winning D-Type to the XJ6 sedan. Chrysler supplied engine blocks and heads to Rolls Royce in the 1960s, that although heavily re engineered, were hemis.
Other manufacturers used the hemispherical design before World War II, including Daimler and Riley.
Porsche Hemi engines
When Porsche introduced the 911, it had a 2.0 L flat-6 engine, with hemispherical heads. The basic design did not change much until 1996, when Porsche moved to water cooling.
some notable engines designed and used by Porsche in both customer production car and Race cars:
| Year | Displacement | Power | Power per Litre | Notes |
|---|---|---|---|---|
| 1964 | 2.0 L | 130 hp (96 kW) at 6100 rpm | 75 hp (56 kW)/L | |
| 1966 | 2.0 L | 210 hp (157 kW) | 105 hp (78 kW)/L | Carrera 6 race |
| 1966 | 2.0 L | 160 hp (119 kW) at 6600 rpm | 80 hp (60 kW)/L | S |
| 1970 | 2.2 L | 180 hp (134 kW) at 6800 rpm | 81 hp (60 kW)/L | S |
| 1971 | 4.9 L | 630 hp (470 kW) | 128 hp (95 kW)/L | 917 race |
| 1973 | 5.4 L | 1100 hp (820 kW) at 7800 rpm | 203 hp (151 kW)/L | 917/30 race, forced induction |
| 1973 | 2.4 L | 190 hp (142 kW) at 6500 rpm | 79 hp (59 kW)/L | S |
| 1974 | 2.2 L | 500 hp (373 kW) | 227 hp (169 kW)/L | RSR T race, forced induction |
| 1976 | 3.0 L | 260 hp (194 kW) | 86 hp (64 kW)/L | turbocharged |
The classic Hemi engines of the 1960s featured true hemispherical heads, and splayed (perpendicular) valves.
Mitsubishi hemispherical engines
Beginning production in 1977 was the 'Orion' engine used in the first Mitsubishi Colt. The 4G13 was very small, a 1.3 L motor. The spark plugs were not in the centre of the dome, but slightly off to the exhaust side. The "hemi" was used in other Mitsubishi vehicles, the largest being the 4G16 'Saturn' (101 hp, 75 kW @6000 rpm and 110 ft·lbf (149 N·m) @ 4000 rpm) installed in Hyundai Pony (1974-1987) and Hyundai Stellar (1983-1986) and the smallest being the 4G12, a 1.2 L engine, (Pony all years) was used in European and Middle Eastern Pony cars, wagons, and pickups. They are I4, crossflow aluminum head, chain-driven 8 valve SOHC units.