HOME
        TheInfoList






A flat-four engine, also known as a horizontally opposed-four 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 cylinders 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 an inline-four engine. 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.

Design

Boxer-four animation

Most flat-four engines are designed so that each pair of opposing pistons moves inwards and outwards at the same time, which is known as a "boxer" configuration (somewhat like boxing competitors punching their gloves together before a fight). Therefore, the terms "flat-four" and "boxer-four" are often used synonymously.

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. The layout also lends itself to efficient air cooling with the airflow being evenly distributed across the four cylinders. In aircraft, this avoids the need to carry heavy water cooling systems.

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.[1] Due to these factors, inline-four engines are more commonly used than flat-four engines, and V6 engines are often used where larger displacements are required.

Engine balance

The equal and opposing forces generated in a boxer-four engine results 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 practice, each cylinder in a boxer engine is slightly offset from its opposing pair due to the distance between the crankpins along the crankshaft. This offset distance means that the equal and opposite forces from each cylinder pair produces a rocking couple.[2](p27) The resulting vibration is not usually high enough to require balance shafts.

As per all four-stroke engines with four cylinder or less, the lack of overlap in the power strokes results in a pulsating delivery of torque to the flywheel, causin

A boxer-four engine has perfect primary and secondary balance, however the two cylinder heads means the design is more expensive to produce than an inline-four engine. 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.

Most flat-four engines are designed so that each pair of opposing pistons moves inwards and outwards at the same time, which is known as a "boxer" configuration (somewhat like boxing competitors punching their gloves together before a fight). Therefore, the terms "flat-four" and "boxer-four" are often used synonymously.

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. The layout also lends itself to efficient air cooling with the airflow being evenly distributed across the four cylinders. In aircraft, this avoids the need to carry heavy water cooling systems.

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.[1] Due to these factors, inline-four engines are more commonly used than flat-four engines, and V6 engines are often used where larger displacements are required.

Engine balance

The equal and opposing forces generated in a boxer-four engine results 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 practice, each cylinder in a boxer engine is slightly offset from its opposing pair due to the distance between the crankpins along the crankshaft. This offset distance means that the equal and opposite forces from each cylinder pair produces a rocking couple.[2](p27) The resulting vibration is not usually high enough to require balance shafts.

As per all four-stroke engines with four cylinder or less, 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.centre of gravity, and a short engine length. The layout also lends itself to efficient air cooling with the airflow being evenly distributed across the four cylinders. In aircraft, this avoids the need to carry heavy water cooling systems.

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.[1] Due to these factors, inline-four engines are more commonly used than 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 results 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 practice, each cylinder in a boxer engine is slightly offset from its opposing pair due to the distance between the crankpins along the crankshaft. This offset distance means that the equal and opposite forces from each cylinder pair produces a crankpins along the crankshaft. This offset distance means that the equal and opposite forces from each cylinder pair produces a rocking couple.[2](p27) The resulting vibration is not usually high enough to require balance shafts.

As per all four-stroke engines with four cylinder or less, 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.[3] If necessary, this vibration can 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.

The other common exhaust configuration (such as used by Subaru since the mid-2000s) is to pair the cylinders with a firing interval offset of 360 degrees, in order to optimise the exhaust pulses.exhaust pulses.[4][5][6] This configuration requires long exhaust manifolds, in order to pair the cylinders on opposite banks, and results in a less distinctive exhaust sound.

In 1900, the first flat-four engine was produced by Benz & Cie, based on Benz's 1897 "contra" flat-twin engine.[citation needed] This engine was used in Benz racing cars, produced 20 hp (15 kW), had a displacement of 5.4 L (330 cu in) and was designed by Georg Diehl.

London company Wilson-Pilcher released its first car in 1901, which was powered by a flat-four engine. This engine was mounted longitudinally in the chassis, water-cooled, produced 9 hp (7 kW) and had a displacement of 2.4 L (146 cu in). Unusually for its day, the bore and stroke were equal, with each being 95 mm (3.7 in).

In 1902 the Buffum automobile was equipped with opposed four cylinder engines that were rated at 16 horsepower. Herbert H. Buffum produced an American Automobile called the Buffum in Abington, Massachusetts from 1903 to 1907.[7]

Having previously produced flat-twin engines, the 1926 Tatra 30 was the Czech company's first model powered by a flat-four engine. Tatra produced various flat-four engined model through the 1920s and 1930s.

1936-1999