A hemispherical combustion chamber is a type of combustion chamber in a reciprocating internal combustion engine with a domed cylinder head notionally in the approximate shape of a hemisphere (in reality usually a spheric section thereof). An engine featuring this type of hemispherical chamber is known as a hemi engine.
Hemispherical combustion chambers were introduced on some of the earliest automotive engines, shortly after the viability of the internal combustion engine was first demonstrated. Their name reflects the domed cylinder head and the top of the piston enclosing a space that approximates a half of a sphere ( hemi- + -sphere + -ical ), although in practice the actual enclosed space is generally less than half.
Hemispherical cylinder heads have been used since at least 1901; [1] they were used by the Belgian car maker Pipe in 1905 [2] and by 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 were both four-valve engines, and Daimler and Riley were also using hemispherical combustion chambers at the time. Beginning in 1912, Stutz 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 Toyota V engine (Toyota's first V8 engine), Miller racing engines, and the Jaguar XK engine. [5]
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 to the difficulty in arranging the 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 yields a lower compression ratio unless a smaller chamber is utilized.
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, [2] which normally requires the use of either a dual rocker system, or dual camshafts to operate the inlet and exhaust valves. 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] Ford's CVH (Compound Valve Hemispherical) engine of the 1980s solved the problem by way of utilising a complex geometry of the valve angle combined with a cam-in-head configuration that allowed hemispherical arranged valves to be operated by a single camshaft and without the need for two rocker shafts.
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 axes. 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 causes 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.
Flame temperatures are very high, leading to excessive NOx output which may require exhaust gas recirculation and other emission control measures to meet modern standards. 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 had pushed the hemi head out of favor in the modern era, until Chrysler's 2003 redesign that has proven popular.
 SOHC Cutaway showing cross-flow design, |
 The hemi engine requires parts |
 A major drawback of the hemi design is evident here— |
Alfa Romeo has produced 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. [7] 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.
Aston Martin's famous DOHC (4 cams) V8 used a hemispherical chamber during the late 1960s through to 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 (5340 cc/325 in3) produced 315 hp (235 kW).
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.
Ardun heads for the Ford flathead were perhaps the first use of a hemispherical head on a readily available American V8. [8] 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 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 [9] 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. [9] 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 [10] 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. [11] Dynamometer results of the day showed the SOHC Hemi producing almost 700 hp (522 kW) in crate form (100 hp per liter). [12] The overhead cams meant that it was not as rpm-limited as the Chrysler Hemis were with their pushrods and heavy and complex valvetrains. [13]
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. [14] 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. [15]
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, [16] [17] 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 (combustion chamber). Post 1986 the cylinder head of this engine was reworked to heart-shaped lean-burn combustion chambers, and used in low-performance models not benefiting from multipoint fuel injection - 1.4, 1.6, 1.8 in Europe, though was still referred to colloquially as the CVH.
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. [18]
The Lamborghini V12, designed in 1963 and produced for more than 50 years, used hemispherical chambers.
The Lancia V4 engine and Lancia V6 engine both used hemispherical combustion chambers.
Lotus has used hemispherical chambers in some of their engines (see photo to right). 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.
Hemispherical chambers were a feature of the M102 engine introduced in 1980, which together with the crossflow head design promoted greater efficiency over the M115 engine it replaced. [19]
The MG factory in Abingdon-on-Thames produced a Twin-Cam variant of the pushrod MGA 1600 MkI MG MGA line from 1958 to 1960. The original push rod 1588 cc cast iron block was crowned with a cast aluminum twin cam cylinder head which included one inlet and exhaust valve each, per cylinder. This configuration enabled a cross flow "hemi head" circulation within the combustion chamber, that enabled improved breathing (volumetric efficiency) as well as utilization of enlarged valves and domed pistons. Early versions proved fragile on the street and in competition due to pre-ignition (detonation), and oil loss, which led to decreasing the compression ratio from 9.1. to 8.3 with redesigned pistons. It was a successful update but sales dropped off so rapidly the company halted Twin Cam production and used the matching chassis for some MGAs, with push rod engines, known as the MGA 1600 MkI and MkII DeLuxe models.
Mitsubishi produced several hemi engines including the 'Orion', 'Astron', and 'Saturn' units.
Nissan's Z, VG (SOHC version only) and VQ engines use hemispherical combustion chambers. The Z and VG are true hemispherical while the VQ uses a compound pent-roof shape.
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. [20]
Toyota Motor Corporation's V family of engines were a longitudinally mounted V8 engine design. They were used from the 1960s through the 1990s. The V family engine was used in the prestigious Toyota Century. Toyota had worked with Yamaha to produce the first Japanese full aluminum alloy block engine. The V Family is often referred to as the Toyota HEMI as the engine features a similar cylinder head design to those found on the Chrysler's Hemi, even though most of the engine design is completely different.
The V 2.6L engine was first used in the Crown Eight from 1964 to 1967 as part of the second generation Crown range. Thereafter the Crown Eight was replaced by the upmarket Toyota Century.
The 3V, 4V and 5V engines were used in the Toyota Century up until 1997 when it got a complete redesign and gained the 5.0 L 1GZ-FE V12.
The V series engines, like several Toyota Motor Corporation engines (e.g. 2T-C, 2M, 4M etc.) at the time had a hemispherical combustion chamber. The spark plugs were located at the top of the combustion chamber.
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. It has served for more than a century as the basic design in combustion engines, from which many other improvements and engineering developments derive.[ citation needed ] 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.
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. [21] These active chambers usually look like kidney beans or two merged small 'hemi' areas surrounded by flat quenching areas over the pistons. [22]
In an internal combustion engine, the cylinder head sits above the cylinders and forms the roof of the combustion chamber. In sidevalve engines, the head is a simple sheet of metal; whereas in more modern overhead valve and overhead camshaft engines, the cylinder head is a more complicated block often containing inlet and exhaust passages, coolant passages, valves, camshafts, spark plugs and fuel injectors. Most straight engines have a single cylinder head shared by all of the cylinders and most V engines have two cylinder heads.
The Ford FE engine is a Ford V8 engine used in vehicles sold in the North American market between 1958 and 1976. The FE was introduced to replace the short-lived Ford Y-block engine, which American cars and trucks were outgrowing. It was designed with room to be significantly expanded, and manufactured both as a top-oiler and side-oiler, and in displacements between 332 cu in (5.4 L) and 428 cu in (7.0 L).
The Ford 385 engine family is a series of big-block V8 engines designed and manufactured by Ford Motor Company. The family derives its name from the 3.85-inch (98 mm) crankshaft stroke of the 460 cubic-inch V8 introduced in 1968. A 429 cu in (7.0 L) version was also introduced the same year, with a 370 cu in (6.1 L) variant appearing in 1977.
The Ford Kent is an internal combustion engine from Ford of Europe. Originally developed in 1959 for the Ford Anglia, it is an in-line four-cylinder overhead-valve–type pushrod engine with a cast-iron cylinder head and block.
The Ford CVH engine is a straight-four automobile engine produced by the Ford Motor Company. The engine's name is an acronym for either Compound Valve-angle Hemispherical or Canted Valve Hemispherical, where "Hemispherical" describes the shape of the combustion chamber. The CVH was introduced in 1980 in the third generation European Escort and in 1981 in the first generation North American Escort. Engines for North America were built in Ford's Dearborn Engine plant, while engines for Europe and the UK were built in Ford's then-new Bridgend Engine plant in Wales.
The Ford 335 engine family was a group of engines built by the Ford Motor Company between 1969 and 1982. The "335" designation reflected Ford management's decision to produce an engine of that size with room for expansion during its development. This engine family began production in late 1969 with a 351 cu in (5.8 L) engine, commonly called the 351C. It later expanded to include a 400 cu in (6.6 L) engine which used a taller version of the engine block, commonly referred to as a tall deck engine block, a 351 cu in (5.8 L) tall deck variant, called the 351M, and a 302 cu in (4.9 L) engine which was exclusive to Australia.
The Ford 302 H.O. is a high-performance "small block" 302 cu in (4.9 L) V8 engine manufactured by Ford Motor Company. The original version of this engine was used in the 1969 and 1970 Boss 302 Mustangs and Cougar Eliminators and was constructed by attaching heads designed for the planned 351 Cleveland to a Ford small block. The construction was aided by the two engines sharing a cylinder head bolt pattern, though the Boss heads had to have their coolant passages slightly modified.
The LA engines are a family of pushrod OHV small-block 90° V-configured gasoline engines built by Chrysler Corporation. They were factory-installed in passenger vehicles, trucks and vans, commercial vehicles, marine and industrial applications from 1964 through 2003. Their combustion chambers are wedge-shaped, rather than polyspherical, as in the predecessor A engine, or hemispherical in the Hemi. LA engines have the same 4.46 in (113 mm) bore spacing as the A engines.
The Chrysler Hemi-6 engine is a family of inline six-cylinder petrol engines produced by Chrysler Australia in three piston displacements and multiple configurations. Hemi-6 engines were installed in Australian-market Chrysler Valiants from 1970 through 1981. It was also installed in the Valiants closely related variants, the Chrysler VIP, the Chrysler by Chrysler & the Valiant Charger.
An overhead camshaft (OHC) engine is a piston engine in which the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion chamber in the engine block.
An overhead valve (OHV) engine, sometimes called a pushrod engine, is a piston engine whose valves are located in the cylinder head above the combustion chamber. This contrasts with flathead engines, where the valves were located below the combustion chamber in the engine block.
In automotive engineering a multi-valve or multivalve engine is one where each cylinder has more than two valves. A multi-valve engine has better breathing and may be able to operate at higher revolutions per minute (RPM) than a two-valve engine, delivering more power.
Variable displacement is an automobile engine technology that allows the engine displacement to change, usually by deactivating cylinders, for improved fuel economy. The technology is primarily used in large, multi-cylinder engines. Many automobile manufacturers have adopted this technology as of 2005, although the concept has existed for some time prior to this.
A flathead engine, also known as a sidevalve engine or valve-in-block engine, is an internal combustion engine with its poppet valves contained within the engine block, instead of in the cylinder head, as in an overhead valve engine.
The Chrysler Hemi engines, known by the trademark Hemi or HEMI, are a series of American V8 gasoline engines built by Chrysler with overhead valve hemispherical combustion chambers. Three different types of Hemi engines have been built by Chrysler for automobiles: the first from 1951 to 1958, the second from 1964 to 1971, and the third beginning in 2003. Although Chrysler is most identified with the use of "Hemi" as a marketing term, many other auto manufacturers have incorporated similar designs. The engine block and cylinder heads were cast and manufactured at Indianapolis Foundry.
The Toyota S Series engines are a family of straight-4 petrol or CNG engines with displacement from 1.8 L to 2.2 L produced by Toyota Motor Corporation from January 1980 to August 2007. The series has cast iron engine blocks and aluminium cylinder heads.
A valvetrain or valve train is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture into the combustion chamber, while the exhaust valves control the flow of spent exhaust gasses out of the combustion chamber once combustion is completed.
The intake/inlet over exhaust, or "IOE" engine, known in the US as F-head, is a four-stroke internal combustion engine whose valvetrain comprises OHV inlet valves within the cylinder head and exhaust side-valves within the engine block.
A V8 engine is an eight-cylinder piston engine in which two banks of four cylinders share a common crankshaft and are arranged in a V configuration.
A Heron cylinder head, or simply Heron head, is a design for the combustion chambers of the cylinder head on an internal combustion piston engine, named for engine designer S.D.Heron. The head is machined flat, with recesses only for inlet and exhaust valves, spark plugs, injectors and so on. The combustion chamber itself is contained within a dished depression in the top of the piston. The Heron head is suitable for petrol and diesel engines, for ohv and ohc valve-gear, and for small and large engine displacement capacities.