Hemi engine

A Hemi engine (from "hemisphere") is an internal-combustion engine in which the combustion chambers are of hemispherical form. Chrysler Corporation has registered "HEMI" as a trademark in the United States [United States Patent and Trademark Office trademark registration number 1741153 (1992-12-22)] , though they were neither the hemi engine's inventors nor the first to commercialize hemi engines.

Hemispherical combustion chambers, which had been used for centuries in mortars and cannon [ [http://www.webcitation.org/5V8sQDdCA "Gibbons Artillery Manual"] ] , were introduced on some of the earliest automotive engines, shortly after proving the concept of internal combustion engines themselves.

Technology & implementation

With the hemispherical combustion chamber design, the intake and exhaust valves are usually on opposite sides of the chamber, unlike the in-line valve arrangement common to most engines with wedge-shaped combustion chambers. Therefore, significant challenges in the commercialization of hemi engines revolved around the design of the valve actuation, and how to make it effective, efficient, and reliable at an acceptable cost. Early in Chrysler's development of their 1950s hemi engine, the head was referred to in company advertising as the "Double Rocker" head. [ [http://www.webcitation.org/5V4ga42bv A history of the origins of the American Hemi] ]

Benefits & drawbacks

Although a wedge-head design offers simplified valve actuation, it usually requires the air/fuel mixture to make sharp turns en route to and from the chamber. With a hemispherical chamber, larger valves are possible and a straighter, less restrictive flow path can be provided for the air/fuel mixture. This improves engine breathing. Placing the spark plug near the center of the chamber aids in achieving complete combustion of the fuel/air mixture, though it is not mandatory.

Drawbacks of the hemispherical chamber such as increased production cost, high relative weight (25% heavier than a comparable wedge head per Chrysler's engineers [cite book
last = Mueller
first = Mike
authorlink =
title = American Horsepower: 100 years of Great Car Engines
publisher = MBI Publishing
date = 2006
pages = p. 112, 113
doi =
isbn = 139780760323274] ), poor low-rpm performance characteristics and difficulty meeting emissions standards have pushed the hemi head out of favor.

History & usage

Hemispherical cylinder heads have been used since at least 1903; they were used by the Belgian car maker Pipe in 1905 [ [http://www.webcitation.org/5V4ga42bv "A history of the origins of the American Hemi"] ] ] , the Peugeot Grand prix Car of 1912, the Alfa Romeo GP car of 1914, Daimler, and Riley. Stutz built four valve engines, conceptually anticipating modern car engines. The BMW double push rod design, taken over by Bristol Cars, the Peugeot 403 and the Toyota T engine are other well known hemi engines. Harry Arminius Miller racing engines were also a notable exampleCite web|url=http://www.ddavid.com/formula1/miller.htm|title=Miller 91|accessdate=2008-01-08|work=ddavid.com] .

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 (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. The "Hemi" engine introduced in 2002 by DaimlerChrysler had a combustion chamber featuring valve and spark plug locations markedly different from the 426ci Hemi engine of muscle car fame. The current-production "Hemi" V8 with its pinched chamber, does not have true hemispherical combustion chambers despite the name. Rather, it bears a closer resemblance to the mid-1950s Polyspherical chamber, which Chrysler engineers developed as a lower-cost alternative head for their V8 engines. The Polyspherical head needed less metal and was narrower due to using only one rocker shaft. This saved costs in material, space and warranty claims and allowed it to be used in smaller vehicles. Chrysler's Australian-market Hemi-6 of 1970-80 had partial-spherical hemi chambers, though they were only 35% of a sphere.

Ford

Hemi Heads for Ford V8
1947 through today - Top to Bottom:
1) Ardun Head for Flathead
2) M/T Heads for FE
3) Ford SOHC Heads for FE
4) Ford 3V Head for Calliope
5) Ford "Semi Hemi" Head for 429SCJ

Ardun heads for the Ford Flathead were perhaps the first use of a hemispherical head on a readily available American V8. [cite book
last = Mueller
first = Mike
authorlink =
title = American Horsepower: 100 years of Great Car Engines
publisher = MBI Publishing
date = 2006
pages = p. 42
doi =
isbn = 139780760323274] 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 Auto in3cm3|425|0 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 [http://www.mustangandfords.com/featuredvehicles/mufp_0508_jack_hazelgren_1963_galaxie_lightweight/427_sohc.html ""63 Galaxie Lightweight""] , "Mustangs and Fords magazine", August 2005] 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 its power levels it was banned from NASCAR races, though allowed in certain drag racing classesFact|date=February 2008. After the NASCAR ban, Ford continued to produce the 427 SOHC for several years and sold it over the counter to racers and others [ [http://www.webcitation.org/5VGWr5bHY "History of Ford 427 SOHC"] ] . Dynamometer results of the day showed the SOHC Hemi producing almost 700 hp (522 kW) in crate form (100 hp per liter), before any modifications by the purchaser [cite book
last = Guide Editors
first = Consumer
authorlink =
title = Muscle Car Chronicles
publisher = Publications International
date = 2005
pages = p. 168
doi =
isbn = 1-4127-1201-7] . The overhead cams meant that it was not as rpm-limited as the Chrysler Hemis were with their pushrods and heavy and complex valvetrains [cite book
last = Genat
first = Robert
authorlink =
title = Hemi: The Ultimate American V-8
publisher = MBI Publishing
date = 2007
pages = p. 14
doi =
isbn = 978-0-7603-2747-0] .

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 [ [http://en.wikipedia.org/wiki/
] ] . 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" [cite book
last = Guide Editors
first = Consumer
authorlink =
title = Muscle Car Chronicles
publisher = Publications International
date = 2005
pages = p. 214
doi =
isbn = 1-4127-1201-7] .
In the 1970s, Ford designed and produced a smallblock engine with hemispherical heads to address the growing concerns about fuel economy. Unfortuately, even with an ahead-of-its-time GDI system feeding a stratifed charge chamber, [ [http://www.time.com/time/magazine/article/0,9171,947023-4,00.html "Detroit's "Total Revolution""] , "TIME magazine", March 19 1979] [ [http://www.caranddriver.com/columns/8130/the-steering-column.html "Will gasoline direct injection finally make it?"] , Csaba Csere, "Car and Driver", June 2004] 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.

Aston Martin

Aston Martin's famous DOHC (4 cams) V8 used hemispherical chambers as well during the late 1960s through the late 1980s, until Ford purchased Aston Martin and funded the development of new engines, with modern combustion chambers. The new engines increased the horsepower using less displacement: Aston Martin V8 5.3 L (5340 cc/325 in³) with 315 hp (235 kW) replaced by the Ford V8 Vantage 4.3 L (4300 cc) quad-cam 32-valve V8 with 380 hp (283 kW).

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 2.4 L version generated 56 hp per naturally-aspirated litre of piston displacement. [ [http://www.webcitation.org/5VGaK6J2L "Porsche 911 Technical Specifications"] ] .

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 sedan [ [http://www.webcitation.org/5VFPjVlBW "A history of the Jaguar DOHC"] ] .

Mitsubishi

Mitsubishi produced several hemi engines including the 'Orion', 'Astron', and 'Saturn' units.

upersession 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. [cite book
last = Genat
first = Robert
authorlink =
title = Hemi: The Ultimate American V-8
publisher = MBI Publishing
date = 2007
pages = p. 13
doi =
isbn = 978-0-7603-2747-0] These active chambers usually look like kidney beans or two merged small 'hemi' areas surrounded by flat quenching areas over the pistons. [ [http://www.webcitation.org/5VFQnTclL "Quench Tumble and Swirl"] ] By the end of the 1970s, development of engines utilizing true hemispherical chambers had ceased around the world; it has been supplanted and dramatically improved upon by newer designs. Today, "hemi" is little more than a copyrighted word that bears little meaning, descriptively, for the engines designated as such.

References