A Hemi engine (from hemisphere) is an internal-combustion engine in which the combustion chambers are of hemispherical form.
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History
Hemispherical combustion chambers, which had been used for centuries in mortars and cannon,[1] 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;[2] they were used by the Belgian car maker Pipe in 1905,[3] 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 example.[4]
Technology and implementation
With the hemispherical combustion chamber design, the intake and exhaust valves are usually on opposite sides of the chamber, allowing for the combustion mixture to flow directly across the chamber, commonly referred to as "cross-flow" heads. 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.[5]
Benefits and 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[6]), poor low-rpm performance characteristics and difficulty meeting emissions standards have pushed the hemi head out of favor.
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 (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 twin spark plug locations markedly different from the second generation 426ci version. 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
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 cu in (6,965 cc) 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 it was banned from NASCAR races, though allowed in certain drag racing classes. After the NASCAR ban, Ford continued to produce the 427 SOHC for several years and sold it over the counter to racers and others.[9] Dynamometer results of the day showed the SOHC Hemi producing almost 700 hp (522 kW) in crate form (100 hp per liter).[10]. The overhead cams meant that it was not as rpm-limited as the Chrysler Hemis were with their pushrods and heavy and complex valvetrains[11].
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.[12] 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[13].
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 stratified charge chamber,[14][15] 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 an updated version of a hemispherical chamber called a pentroof during the late 1960s through the late 1980s. A pentroof chamber, in order to fit 2 additional valves, has its 4 "corners" pushed out into a cloverleaf and its sparkplug located in the center of the chamber at the highest point...a hemispherical hip roof with a sparkplug at the peak. The Aston Martin V8 5.3 L (5340 cc/325 in³) produced 315 hp (235 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.[16].
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[17].
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 (even in its latter 24v forms) of all time. Part of this praise is likely due to the fact that 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 noises.
Lotus
Along with many other manufacturers, some listed above, many others not, the hemi chamber was chosen where power at high-rpm's was desired. The 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 which is why the hemi chamber has faded away in the new emissions-era, but, for racing and other pursuits it served.
Supersession 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.[18] These active chambers usually look like kidney beans or two merged small 'hemi' areas surrounded by flat quenching areas over the pistons.[19] By the end of the 1970s, development of engines utilizing true hemispherical chambers had ceased around the world; it had been gradually displaced by dramatically improved newer engine designs. Today, "hemi" is more of a trademark than a description of a combustion chamber.
References
- ^ "Gibbons Artillery Manual"
- ^ CURTIS BOAT & WOODWORKING CO.. 1901 Hemi engine by Truscott Launch and Engine Company, St Joeseph, MI. Designed by HEMI inventor, Allie Ray Welch, Chelsea Manufacturing Company, Chelsea, MI.. CurtisBoat.com. 2009-09-25. URL:http://www.curtisboat.com/hemi_prototype.html. Accessed: 2009-09-25. (Archived by WebCite® at http://www.webcitation.org/5k4B8p3aw)
- ^ "A history of the origins of the American Hemi"
- ^ "Miller 91". ddavid.com. http://www.ddavid.com/formula1/miller.htm. Retrieved 2008-01-08.
- ^ A history of the origins of the American Hemi
- ^ Mueller, Mike (2006). American Horsepower: 100 years of Great Car Engines. MBI Publishing. pp. 112, 113. ISBN 139780760323274.
- ^ Mueller, Mike (2006). American Horsepower: 100 years of Great Car Engines. MBI Publishing. pp. 42. ISBN 139780760323274.
- ^ a b ""63 Galaxie Lightweight"", Mustangs and Fords magazine, August 2005
- ^ "History of Ford 427 SOHC"
- ^ Guide Editors, Consumer (2005). Muscle Car Chronicles. Publications International. pp. 168. ISBN 1-4127-1201-7.
- ^ Genat, Robert (2007). Hemi: The Ultimate American V-8. MBI Publishing. pp. 14. ISBN 978-0-7603-2747-0.
- ^ Ford Calliope
- ^ Guide Editors, Consumer (2005). Muscle Car Chronicles. Publications International. pp. 214. ISBN 1-4127-1201-7.
- ^ "Detroit's "Total Revolution"", TIME magazine, March 19, 1979
- ^ "Will gasoline direct injection finally make it?", Csaba Csere, Car and Driver, June 2004
- ^ "Porsche 911 Technical Specifications"
- ^ "A history of the Jaguar DOHC"
- ^ Genat, Robert (2007). Hemi: The Ultimate American V-8. MBI Publishing. pp. 13. ISBN 978-0-7603-2747-0.
- ^ "Quench Tumble and Swirl"
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