Below are some facts about the hemi.
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"Hemi engine" redirects here. For the Chrysler engine, see
Chrysler Hemi engine.
"Hemi" redirects here. For the prefix "hemi-", see
Prefix.
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Hemi- in Wiktionary, the free dictionary.
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.
Contents
[
hide]
History[edit]
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;
[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
Miller racing engines, and the
Jaguar XK engine.
[5]
Technology and implementation[edit]
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 the difficulty in arranging 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 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 centre, resulting in a combustion chamber in the shape of the thick peel of half an orange.
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] 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[edit]
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 makes 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.
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.
Usage[edit]
Chrysler[edit]
Main article:
Chrysler Hemi engine
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. Currently, Chrysler produces 5 variations of the Hemi engine: a 5.7 L that, depending on configuation, makes between 360 and 395 horsepower and 390-407 lb*ft (529-552 N*m) of torque, a 6.4 L (also known as the 392 cubic inch) that makes 485 horsepower and 475 lb·ft (644 N·m) of torque, a (different) 6.4L that makes 410 horsepower and 425 lb·ft (576 N·m) of torque, and a supercharged 6.2 L (commonly known as the Hellcat) that makes 707 horsepower and 650 lb·ft (881 N·m) of torque.
Chrysler also released a Hemi 6 in Australia with a non full hemispherical chamber but the top of the line 4.3 265cubic inch motor made 302 hp (225 kW). They came out as 215 120 hp 245 160 hp 265(std) 203 hp.
Ford[edit]
Ardun heads for the
Ford flathead were perhaps the first use of a hemispherical head on a readily available American
V8.
[7] First offered in 1947 as an aftermarket product, these heads converted the
Ford flathead to overhead valves operating in a hemispherical chamber. Zora
Arkus-
Duntov, who later worked for GM and was a major force behind the development of the
Chevrolet Corvette, and his brother Yura, were the "AR" "DUN" of "Ardun."
Ford produced an engine with two overhead cams (one cam per head) and hemispherical chambers in the mid-1960s. The engine, displacing 425 cubic inches and belonging to the
FE family of
Ford engines, was known as the "427
SOHC"; it was also known as the
Cammer. It was basically a set of SOHC hemi heads that bolted onto Ford's
FE engine block. The 1964 engine was designed in 90 days of intensive engineering effort
[8] for use in racing. The 427 SOHC used the side oiler engine block modified slightly to deal with the missing in-block cam among other
OHC issues.
[8] Because of their power levels, and the fact that Chrysler had showed Bill France that a DOHC 426 Hemi was in the works, it was banned from
NASCAR races, though allowed in certain drag racing classes. After the NASCAR ban, Ford continued to produce the SOHC, selling it over the counter to racers and others
[9] who used it to power many altered-wheelbase A/FX Mustangs and supercharged
Top Fuel dragsters.
Connie Kalitta,
Pete Robinson, and
"Snake" Prudhomme all used the engine in their Top Fuel racers. In 1967 Connie Kalitta's SOHC-powered "Bounty Hunter" won Top Fuel honors at AHRA, NHRA and NASCAR winter meets, becoming the only "triple crown" winner in drag racing history.
[10] Dynamometer results of the day showed the SOHC Hemi producing almost 700 hp (522 kW) in crate form (100 hp per liter).
[11] The overhead cams meant that it was not as rpm-limited as the Chrysler Hemis were with their pushrods and heavy and complex valvetrains.
[12]
Later Ford engine designs with hemispherical chambers included the
Calliope, which used two in-block cams, arranged one over the other, to drive 3 valves per hemispherical chamber.
[13] The pushrods activating the valves from the top camshaft were almost horizontal. In 1968, Ford brought out a completely new engine family called the
385-series. This engine's heads used a modified form of the hemispherical chamber called
Semi-Hemi.
[14]
In the 1970s, Ford designed and produced a
small-block engine with hemispherical heads to address the growing concerns about fuel economy. 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. This plus the cost of the valve actuation systems, along with the cost of the high pressure pump needed to deliver fuel directly into the chamber, as well as the
gilmer belt drive system needed to drive the pump, made further development pointless at the time. Most 1980s 4-cylinder Fords used the
Ford CVH engine, CVH meaning "Compound Valve, Hemispherical (Head)". In 2007, taking advantage of the improvements in equipment, computer controls, and chamber design during the intervening years, Ford introduced its non-hemi
Ecoboost line of direct-injected engines.
.
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