A Hemi engine is an internal combustion engine in which the roof of each cylinder's combustion
chamber is of hemispherical form.
History Hemispherical combustion chambers, which had
been used for centuries in mortars and cannons, 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; they were used by the
Belgian car maker Pipe in 1905 and the 1907 Fiat 130 HP Grand Prix racer. 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, conceptually anticipating modern car engines.
Other examples include the BMW double-pushrod design, the Peugeot 403, the Toyota T engine
and Miller racing engines, and the Jaguar XK engine.
Technology and implementation A hemispherical head gives an efficient combustion
chamber with minimal heat loss to the head, and allows for two large valves. However,
a hemi-head allows no more than two valves per cylinder, and these large valves are necessarily
heavier than in a multi-valve engine. 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 ration 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. 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.
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 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 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. These have pushed the hemi
head out of favor in the modern era.
Usage 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 in the 1950s, the second, developed for NASCAR
in 1964 and produced through the early 1970s, and finally the "new HEMI" in the early 2000s.
The original design is pretty much the same except for sensors and fuel injection. Currently,
Chrysler produces 3 variations of the Hemi engine: a 5.7 L that makes 370 horsepower
and 395 lb-ft. of torque, a 6.4 L that makes 485 horsepower and 475 lb-ft of torque, and
a supercharged 6.2 L that makes 707 horsepower and 650 lb-ft 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 302hp they came out as 215 120hp 245
160hp 265(std) 203hp and Chrysler-rootes group also inherited the
humber supersnipe 3 litre hemi six making 130hp
Ford Ardun heads for the Ford flathead were perhaps
the first use of a hemispherical head on a readily available American V8. 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 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 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. 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 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. Dynamometer results of the day showed the SOHC Hemi producing almost 700 hp in
crate form. The overhead cams meant that it was not as rpm-limited as the Chrysler Hemis
were with their pushrods and heavy and complex valvetrains.
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.
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.
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, 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". 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.
Aston Martin
Aston Martin's famous DOHC V8 used a hemispherical chamber during the late 1960s through the
late 1980s. Each cam controlled one set of valves, either a bank of intake valves or
a bank of exhaust valves. The Aston Martin V8 5.3 L produced 315 hp.
Mitsubishi Mitsubishi produced several hemi engines including
the 'Orion', 'Astron', and 'Saturn' units. Alfa Romeo
Alfa Romeo has produced many successful hemi-head engines throughout the years. 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 of
all time. Part of this praise is likely because the hemispherical heads on the original 2-valve
engine allowed for an almost completely straight exhaust port, resulting in a less diluted
or muddied engine sound, allowing Alfa Romeo to use quieter stock exhausts without losing
much of their distinct and beloved race-bred engine noise.
Porsche Porsche has made extensive use of hemi-head
engines, including the air-cooled flat-6 engine in Porsche 911 models from 1963 to 1999. The
1973 2.7 L version generated 56 hp per naturally aspirated litre of piston displacement.
Jaguar Jaguar used this head design, beginning in
1949, on the legendary XK engines, which powered cars ranging from the Le Mans winning D-Type
to the XJ6. Lancia
Both the Lancia V4 engine and the Lancia V6 engine used hemispherical chambers.
Lotus Lotus used hemispherical chambers in some
of their engines. The relatively large valves possible in such a chamber allowed large volumes
of air-fuel mixture to enter and exit the chamber quickly; not always completely combusted.
In the modern emissions-era, the hemi chamber has begun to fade away from continuing development.
The hemispherical combustion chamber is the simplest, and easiest design to understand.
As such, it has served for more than a century as the basic design in combustion engines,
from which all other improvements and engineering developments derive. As the engineering involved
in new engines has improved and evolved, the true hemispherical chamber has morphed and
twisted into more sophisticated and complex designs that are meant to extract more power,
with lower emissions, from any given combustion event.
Design evolution in modern engines
Many of today's engines use active combustion chambers designed to tumble and swirl the
fuel/air mix within the chamber for the most efficient combustion event possible. These
active chambers usually look like kidney beans or two merged small 'hemi' areas surrounded
by flat quenching areas over the pistons. By the end of the 1970s, development of engines
utilizing true hemispherical chambers had ceased in the U.S.(Continuing in places like
Italy, on Alfa Romeos for example); it had been gradually displaced by newer emissions
friendly engine designs. Today,"hemi" is more of a trademark than a description of a combustion
chamber.
References