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Flat-four engine
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Flat-four engine
A flat-four engine (also known as a horizontally opposed-four engine or boxer engine) is a four-cylinder piston engine with two banks of cylinders lying on opposite sides of a common crankshaft. The most common type of flat-four engine is the boxer-four engine, each pair of opposed pistons moves inwards and outwards at the same time.
A boxer-four engine has perfect primary and secondary balance, however, the two cylinder heads means the design is more expensive to produce than a straight-four engine. There is a minor, secondary unbalanced rotational torque pulse in the plane of the pistons, when a piston pair at one end of the engine is at TDC and the other pair at BDC. The TDC pair creates a torque greater than the BDC pair, so the net unbalanced torque pulse is the difference. The difference in TDC vs BDC inertial forces is explained in the Engine balance section. Boxer-four engines have been used in cars since 1897, especially by Volkswagen and Subaru. They have also occasionally been used in motorcycles and frequently in aircraft. Cessna and Piper use flat four engines from Lycoming and Continental in the most common civil aircraft in the world - the Cessna 172, and Piper Cherokee, while many ultralight and LSA planes use versions of the Rotax 912.
In a flat-four engine, each pair of opposing pistons successively moves inwards together and outwards together. The advantages of the boxer-four layout are perfect secondary vibration (resulting in minimal vibration), a low centre of gravity, and a short engine length. Flat-four engine have successfully used air cooling, although air-cooled engines are noisier and have a lower power output than an equivalent engine with liquid cooling. In light aircraft, where lightness is of primary importance, air-cooling has traditionally been common.
The downsides of boxer-four engines (compared with inline-four engines) are their extra width, the increased costs associated with having two cylinder heads instead of one, and the long exhaust manifold required to achieve evenly spaced exhaust pulses. Due to these factors, straight-four engines are more common in cars than are flat-four engines, and V6 engines are often used where larger displacements are required.
The equal and opposing forces generated in a boxer-four engine result in perfect secondary balance (unlike the unbalanced vertical forces produced by inline-four engines). Boxer-four engines are therefore better suited to displacements above 2.0 L (122 cu in), since they do not require balance shafts to reduce the secondary vibration.
In a boxer engine, each cylinder is slightly offset from its opposing pair due to the distance between the crankpin journals. This offset gives rise to a slight rocking couple, but any resulting vibration is normally insufficient to require balance shafts.
As with all four-stroke engines of four cylinders or fewer, the lack of overlap in the power strokes results in a pulsating delivery of torque to the flywheel, causing a torsional vibration along the crankshaft axis. Such vibration, if excessive, may be minimised using a harmonic damper.
The typical firing order for a boxer-four engine is for the left bank of cylinders to ignite one after another, followed by the right bank of cylinders (or vice versa), with the firing interval evenly spaced at 180 degrees. Traditionally, the exhausts from the two cylinders on each bank were merged, with the resulting uneven exhaust pulses causing a characteristic "flat-four burble" exhaust sound.
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Flat-four engine AI simulator
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Flat-four engine
A flat-four engine (also known as a horizontally opposed-four engine or boxer engine) is a four-cylinder piston engine with two banks of cylinders lying on opposite sides of a common crankshaft. The most common type of flat-four engine is the boxer-four engine, each pair of opposed pistons moves inwards and outwards at the same time.
A boxer-four engine has perfect primary and secondary balance, however, the two cylinder heads means the design is more expensive to produce than a straight-four engine. There is a minor, secondary unbalanced rotational torque pulse in the plane of the pistons, when a piston pair at one end of the engine is at TDC and the other pair at BDC. The TDC pair creates a torque greater than the BDC pair, so the net unbalanced torque pulse is the difference. The difference in TDC vs BDC inertial forces is explained in the Engine balance section. Boxer-four engines have been used in cars since 1897, especially by Volkswagen and Subaru. They have also occasionally been used in motorcycles and frequently in aircraft. Cessna and Piper use flat four engines from Lycoming and Continental in the most common civil aircraft in the world - the Cessna 172, and Piper Cherokee, while many ultralight and LSA planes use versions of the Rotax 912.
In a flat-four engine, each pair of opposing pistons successively moves inwards together and outwards together. The advantages of the boxer-four layout are perfect secondary vibration (resulting in minimal vibration), a low centre of gravity, and a short engine length. Flat-four engine have successfully used air cooling, although air-cooled engines are noisier and have a lower power output than an equivalent engine with liquid cooling. In light aircraft, where lightness is of primary importance, air-cooling has traditionally been common.
The downsides of boxer-four engines (compared with inline-four engines) are their extra width, the increased costs associated with having two cylinder heads instead of one, and the long exhaust manifold required to achieve evenly spaced exhaust pulses. Due to these factors, straight-four engines are more common in cars than are flat-four engines, and V6 engines are often used where larger displacements are required.
The equal and opposing forces generated in a boxer-four engine result in perfect secondary balance (unlike the unbalanced vertical forces produced by inline-four engines). Boxer-four engines are therefore better suited to displacements above 2.0 L (122 cu in), since they do not require balance shafts to reduce the secondary vibration.
In a boxer engine, each cylinder is slightly offset from its opposing pair due to the distance between the crankpin journals. This offset gives rise to a slight rocking couple, but any resulting vibration is normally insufficient to require balance shafts.
As with all four-stroke engines of four cylinders or fewer, the lack of overlap in the power strokes results in a pulsating delivery of torque to the flywheel, causing a torsional vibration along the crankshaft axis. Such vibration, if excessive, may be minimised using a harmonic damper.
The typical firing order for a boxer-four engine is for the left bank of cylinders to ignite one after another, followed by the right bank of cylinders (or vice versa), with the firing interval evenly spaced at 180 degrees. Traditionally, the exhausts from the two cylinders on each bank were merged, with the resulting uneven exhaust pulses causing a characteristic "flat-four burble" exhaust sound.
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