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Flat-six engine
A flat-six engine, also known as a horizontally opposed-six, is a six-cylinder piston engine with three cylinders on each side of a central crankshaft. The most common type of flat-six engine is the boxer-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 an inline-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 central carburetor is used, and duplication of the inlet and outlet connections for water-cooled engines.
The first production flat-six engine was in the 1904 Wilson-Pilcher 18/24 HP car. The most notable use of flat-six engines is the Porsche 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 the flat-four engine.
A boxer-style flat-six engine is able to have perfect primary and secondary 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 a boxer configuration, a flat-six engine does not have a rocking 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.
These characteristics result in low vibration for flat-six engines (as in straight-six engines), especially when compared with the imbalances that are present in V6 engines with a 90-degree bank angle.
Early flat-six engines include the Franklin O-265 which began production in 1940, and the Lycoming O-435 which began production in 1942. Several manufacturers use the letter O in their model codes for flat-layout engines as a designation for “opposed” cylinder layouts. The Franklin O-335 was used in the Bell 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.
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Flat-six engine
A flat-six engine, also known as a horizontally opposed-six, is a six-cylinder piston engine with three cylinders on each side of a central crankshaft. The most common type of flat-six engine is the boxer-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 an inline-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 central carburetor is used, and duplication of the inlet and outlet connections for water-cooled engines.
The first production flat-six engine was in the 1904 Wilson-Pilcher 18/24 HP car. The most notable use of flat-six engines is the Porsche 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 the flat-four engine.
A boxer-style flat-six engine is able to have perfect primary and secondary 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 a boxer configuration, a flat-six engine does not have a rocking 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.
These characteristics result in low vibration for flat-six engines (as in straight-six engines), especially when compared with the imbalances that are present in V6 engines with a 90-degree bank angle.
Early flat-six engines include the Franklin O-265 which began production in 1940, and the Lycoming O-435 which began production in 1942. Several manufacturers use the letter O in their model codes for flat-layout engines as a designation for “opposed” cylinder layouts. The Franklin O-335 was used in the Bell 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.