Aflat-six engine (also known as ahorizontally opposed-six engine) is asix-cylinderpiston engine with three cylinders on each side of a centralcrankshaft. The most common type of flat-six engine is theboxer-six engine, where each pair of opposed cylinders moves inwards and outwards at the same time. An alternative configuration for flat engines is a 180-degree V engine, where both cylinders move to the right then the left at the same time.
The advantages of the flat-six layout are good engine balance (for reduced vibration), a low center of gravity, short length (compared with aninline-six engine) and being well suited to air-cooling. The disadvantages are a large width (which can limit the maximum steering angle when used in a front-engined car), a large intake manifold being required when a centralcarburetor is used, and duplication of the inlet and outlet connections for water-cooled engines.[1]
The first production flat-six engine was in the 1904Wilson-Pilcher 18/24 HP car. The most notable use of flat-six engines is thePorsche 911 sports car, which has used flat-six engines continuously since 1963. Several other car manufacturers, including Subaru, have produced flat-six engines at times. Flat-six engines have also occasionally been used in motorcycles, and commonly in general aviation aircraft, along with theflat-four engine.
Aboxer-style flat-six engine is able to have perfectprimary andsecondary balance. As in other six-cylinder engines, the overlapping of the power strokes of the different cylinders (with a firing interval of 120 degrees in a four-stroke engine) reduces the pulsating of the power delivery relative to that of similar engines with fewer cylinders.
In aboxer configuration, a flat-six engine does not have arocking couple. The symmetry of the arrangement, in which one bank of three cylinders is mirrored by the other bank, means that there is no net force from any given mirrored cylinder pair along the axis of movement of the pistons. So not only is there no net primary or secondary reciprocating effect, there is no net turning force - moment - that would try to rotate the engine back and forth about its engine mountings. (The moment is not quite zero in practice because each cylinder pair is offset slightly from the other, or else they would clash at the crankshaft.) In this regard, the six cylinder boxer engine is the same as a flat-four boxer engine, which also does not experience any rocking couple.[2]
These characteristics result in low vibration for flat-six engines (as instraight-six engines), especially when compared with the imbalances that are present inV6 engines with a 90-degree bank angle.
Early flat-six engines include theFranklin O-265 which began production in 1940, and theLycoming O-435 which began production in 1942. Several manufacturers use the letterO in their model codes for flat-layout engines as a designation for “opposed” cylinder layouts. TheFranklin O-335 was used in theBell 47 light helicopter.
Flat engines largely replaced the historically more popular radial engines in small aircraft after World War II because they were less expensive to manufacture. The smaller frontal area compared with a radial engine also results in less drag, although the radial design offers improved cooling.
Some aircraft have used flat-six engines originally designed for cars. ThePorsche PFM 3200 engine, produced from 1985 to 1991, was based on the engine used in the Porsche 911 sports car. Several examples of thePietenpol Air Camper homebuilt monoplane aircraft have used the air-cooled engine from theChevrolet Corvair compact car.[3] Compared with the engines used in cars, flat-six engines used by helicopters have large displacements and are low revving, producing more torque and less power.[4]
A notable recent flat-six aero-engine is the 4-litreside-valveBelgianD-Motor LF39, a modular variant of the 2.7-litreflat-fourD-Motor LF26. Although the side-valve format has long been abandoned for most automotive applications because its combustion chamber is a bar to high enginerpm, the massively over-square (1.295:1) D-Motor is a very simple, low-revving, compact, reliable lightweight aero-engine (without the heavy (and bulky) complication ofohv valve-gear).[5]
The flat-six engine is often associated with thePorsche 911, a rear-engined sports car which has used flat-six engines exclusively since 1963. The engines wereair-cooled until 1999, when Porsche started usingwater-cooled engines.[6]
Other Porsche models that use flat-six engines are the 1970–1972Porsche 914/6 (mid-engine), the 1986–1993Porsche 959 (rear-engine), and the 1996–2021Porsche Boxster/Cayman (mid-engine).
The first car to use a flat-six engine was the 1904–1907Wilson-Pilcher 18/24 HP,[7] which used an engine based on a flat-four engine with two cylinders added. The Wilson-Pilcher was a front-engined car with the crankshaft in-line with the chassis and the cylinders between the chassis rails. Reports on this car quote it as being "remarkably silent and smooth running" and "almost total absence of vibration".[8]
Two American manufacturers briefly produced cars with flat-six engines—the 1948Tucker 48 (water-cooled, based on the Franklin O-335) and the 1960–1969Chevrolet Corvair (air-cooled).[9] Both are rear-engined cars with rear-wheel drive. Chevrolet produced over 1.8 million Corvairs of various types and utilized a turbocharger on some models; one of the first uses of a turbocharger on a mass-produced automobile.
Japanese manufacturer Subaru produced water-cooled flat-6 engines from 1988–1996 and 2000–2019.[10] Their first flat-6 engine, theSubaru ER27, was added to the model range of theSubaru XT coupe. In 1991, theSubaru EG33 engine was used in theSubaru Alcyone SVX, the replacement for the Subaru XT.[11] Following a hiatus of flat-6 engine production for four years, theSubaru EZ30 engine was introduced in the 2000Subaru Legacy/Outback. TheSubaru EZ36 engine was introduced in 2007 and was produced until 2019. All engines were used in front-engine cars with all-wheel drive, plus the XT and Alcyone SVX were also available with front-wheel drive.
Most motorcycles use engines with four or fewer cylinders, however theHonda Gold Wing touring motorcycle has used a water-cooled flat-six engine since 1988.[12] Initially, the engine had a displacement of 1.5 L (92 cu in), until it was enlarged to 1.8 L (110 cu in) in 2001.[13]: 208
TheHonda Valkyrie F6C (1997–2003) was a cruiser based on the GL1500 Gold Wing.[13]: 198, 218 The limited edition 2004 Valkyrie Rune was based on the GL1800.[13]: 218, 220
As the years passed, Bernard Pietenpol taught himself how to repair televisions and radios. By 1960, Chevrolet introduced the six cylinder air-cooled Chevrolet Corvair. This would be the last engine Bernard would experiment with in powering his Air Camper. In 1966 Bernard completed his first Corvair powered Air Camper, then another in 1970 which was dubbed "the Last Original".
