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The pistons are usually mounted on the crankshaft so that opposing pistons move back and forth in opposite directions at the same time, somewhat like boxing competitors punching their gloves together before a fight, which has led to it being referred to as a "boxer" engine. The design is rarely seen with shared crank throws (See Coventry Climax FWMW for such a non-boxer flat engine), so "flat-four" and "boxer-four" are usually used synonymously.
The configuration results in inherently good balance of the reciprocating parts, a low centre of gravity, and a very short engine length. The layout also lends itself to efficient air cooling with excellent thermal balance. However, it is an expensive design to manufacture, and somewhat too wide for compact automobile engine compartments, which makes it more suitable for cruising motorcycles and aircraft than ordinary passenger cars.
This is no longer a common configuration, but some brands of automobiles use such engines and it is a common configuration for smaller aircraft engines such as those made by Lycoming, Continental and Rotax. Although they are superior to straight fours in terms of secondary vibration, they have largely fallen out of favour for overhead cam design engines, for which an opposed-four cylinder layout would require twice as many camshafts as a straight-four while the crankshaft is as complex to manufacture. The low centre of gravity of the engine is an advantage. The shape of the engine suits it better for mid engine or rear engine designs. With a rear engine layout, it allows a low-tail body while in front engine designs the width of the engine often interferes with the maximum front wheel steering angle. The latter problem has not stopped Subaru from using it in its all-wheel drive cars, where the difficulty of fitting the wide engine between the front wheels ahead of the front axle is compensated for by the ease of locating the transmission and four-wheel drive mechanisms behind the short length, between the front and rear axles.
The open and exposed design of the engine allows air cooling as well as water cooling, and in air-cooled applications fins are often cast into the external cylinder block walls to improve the engine cooling.
Balance and smoothness
Boxer engines tend to be better balanced than other four-cylinder configurations. The more common inline-four configuration suffers from a secondary balance problem caused by the fact that the pistons travel faster on the top half of the crank rotation than the bottom half, which causes the engine to vibrate up and down twice per crank rotation for a total of four times per crankshaft revolution for ordinary up-down-down-up crank throws. This problem becomes worse with increased piston speed and weight, effectively limiting the capacity of these engines. Inline-fours larger than 2.0 L usually have balance shafts whilst engines over 3.0 L are seldom used in passenger cars. In contrast, the flat-four has much less secondary imbalance at the expense of larger rocking vibrations, that cause it to rotate back and forth around a vertical axis. This is because the cylinders cannot be directly opposed, but must be offset so the connecting rods can be on separate crank pins, which results in the forces being slightly off-centre. The rocking vibration is usually not serious enough to require balance shafts.
Another disadvantage of flat-four boxer configuration is its requirement for long exhaust pipes merging the exhaust from those exhaust ports located on opposing banks in order to achieve evenly spaced exhaust pulse. In a four stroke engine, each cylinder creates an exhaust pulse every 720 degrees of crank rotation, and they need to be paired with another pulse that is 360 crank degrees offset in a four-cylinder configuration to achieve an even scavenging of exhaust gas in the cylinders, which is needed for uniform filling of intake and uniform combustion.
As the firing order on an ordinary flat-four boxer engine  on 'L'eft and 'R'ight banks are LLRR or RRLL with each ignition (and thus exhaust) being 180 degrees apart. By counting two characters (2 x 180 =360) to the right of each 'L' or 'R', the cylinders that fire with 360 degree crankshaft rotational angle offset are shown to be located on opposite banks. If the exhaust manifold is designed to merge two exhaust ports on a bank into one exit as was common in the designs in the past, the pulse spacing becomes irregular, which causes uneven filling of intake into cylinders and the characteristic 'burble' of exhaust note on older flat-four engines. Subaru developed an "equal lengths, even pulse spacing" exhaust system with exhaust pipes connecting the left and the right banks for WRC competition in the 1990s, and incorporated this design into production Legacys in 2003, Foresters in 2005 and Imprezas in 2007. As a result, most Subaru flat-four engines no longer have the "flat-four burble". The Impreza WRX and WRX STI still have unequal length exhaust pipes to feed the turbo sitting in the corner of the engine bay, and still have the characteristic burble. This was changed for the 2015 WRX which feeds a centrally mounted turbo, but the STI retained unequal length headers. They are, however, a somewhat popular aftermarket modification.
In addition, four-stroke cycle flat-fours have a problem common to all four-cylinder engines: the power strokes do not overlap. With a piston starting its power stroke every 180 degrees of crankshaft rotation, and the crank throws 180 degrees apart, all the pistons finish their power stroke before the next piston starts its power stroke. This results in gaps between power strokes and a pulsating delivery of torque to the flywheel, causing a rotational vibration on the entire engine along the crankshaft axis. By contrast, in engines with more cylinders the power strokes overlap; the next piston starts its power stroke before the previous one has finished, and the delivery of power is much smoother.
As a result of the relatively high manufacturing costs of the flat-four compared to the inline-four, most manufacturers now choose the inline-four engine for economy models and have moved to inline-five or V6 engines for models requiring more power. These engines are not without balancing imperfections, but with modern computer-aided design techniques, the problems can be overcome with a variety of complex crankshaft, balance shaft, and engine mounting designs. Luxury performance car manufacturers prefer to use the inline-six, flat-six, or V8 configurations because these designs are much smoother than the flat-four, particularly at larger displacements.
Use in automobiles
In 1897 Karl Benz developed the boxer engine. This drive system, in which two horizontally opposed cylinders turned a single crankshaft, was given the name “contra engine”. The unit was used from 1899 onwards, principally in passenger and racing cars. The final evolutionary stage of the Benz racing car equipped with a contra engine was the 20-hp Benz vehicle introduced in 1900. Whereas the two previous models both had a two-cylinder boxer unit, this vehicle – developed by Georg Diehl – featured a four-cylinder boxer with a 5,440-cc displacement
Wilson-Pilcher introduced their car in 1900, fitted initially with a flat-four engine and, shortly after, a flat-six version was also available. The engine was conventionally mounted lying between the frames of the chassis, water cooled, and having an attached four speed helical gearbox. Unusually for its day it had equal bore and stroke of 95 mm. The car was made in London until Armstrong Whitworth took over production in 1904, and then was made in Newcastle until c1907. One example still exists.
Tatra introduced an air-cooled flat-four engine in the 1926 Tatra 30, followed by the T52 in 1930, T54 in 1931, T57 in 1931, and T75 in 1933, all with air-cooled flat-fours of varying displacements. The 1936 T97 model pioneered the rear-engined, air-cooled flat-four, backbone chassis layout, later copied in the Volkswagen KdF-Wagen.
Jowetts before the Second World War were best known for their flat twin engines, but they made a flat-four for the Jason and 10 hp models in the 1930s. Post-war Gerald Palmer designed Javelin saloon and Jupiter sports models used a totally different design of flat-four. Alec Issigonis originally designed the Morris Minor for a flat-four, but cost constraints meant it was never used.
Porsche designed Volkswagen air-cooled flat-fours for VW Beetles and most early VW and Porsche cars such as the Volkswagen Type 2 (T1), Porsche 356, 550, and the 912. The Porsche 914 that replaced the 912 used an enlarged version of a VW engine originally designed for Volkswagen Type 4.
The Goliath 1100 appeared at the Geneva Motor Show in March 1957, with a water-cooled 1100 cc flat-four driving the front wheels. In 1958, the name was changed to the Hansa 1100, and this car was produced through 1961.
A water-cooled Alfa Romeo flat-four was introduced in 1971 on the Alfa Romeo Alfasud. That engine was later used on the Alfa Romeo Arna, the Alfa Romeo 33, the Alfa Romeo Sprint and the Alfa Romeo 145/146.
Subaru produces a water-cooled, front-mounted flat-four engine marketed as "H-4", meaning "horizontal" rather than the "H" cross-section normally indicated by an "H engine". Subaru has created a number of engines, starting with the EA series introduced in 1966, progressing towards the currently used EJ series, which is wide but very short and light, and is mounted ahead of the front axle with the transmission behind. With this layout, the gearbox can remain very similar to common transmissions in both design and weight without the need for a bulky and inefficient transfer case. Although it is more expensive than a straight-four, it allows Subaru to build an all-wheel drive vehicle at little extra cost from two-wheel drive.
In a joint venture between Toyota and Subaru a 1,998 cc flat-four engine with 200 PS (147 kW) and GDI was developed. Labelled as the Subaru FA20 and Toyota 4U-GSE, it is used in the two-door coupe Toyota 86 (Scion FR-S in the United States) and Subaru BRZ.
Porsche announced in 2012 that they were developing a new flat-four, their first flat-four since the 1970s Porsche 914 and 912E. This engine in 2.0 and 2.5 litre sizes is now used in Porsche 718 models, and produces up to 350 hp (261 kW). 
Use in motorcycles
- Honda introduced a liquid-cooled, shaft drive flat-four on a production motorcycle in 1975 on the Honda GL1000 Gold Wing
- From 1938 to 1939 Zündapp produced the shaft drive K800 opposed-four in Germany
- In 1955, in Britain, Wooler produced a small number of 500 cc flat-fours with shaft drive
- The BFG-Citroën (French) of 1982 was powered by a flat-four, 1,299 cc Citroën automobile engine and shaft drive. About 450 of them were built in 1981 and 1982. One-quarter of these were purchased by the French police.
Use in aircraft
Lycoming manufactures a popular series of flat-four aircraft engines ranging up to 360 cu in (5.9 L), as used in many smaller Cessnas and other general aviation aircraft. Similar engines are produced by Continental Motors, Franklin Engine Company, and others. Retired aircraft engines power many shallow draft boats in the Florida Everglades.
The OS Engines company in Japan has made miniature, air-cooled flat-four engines in 40 cc and 52 cc sizes for radio-controlled aircraft hobby use, with the 52 cc "FF-320" engine currently in production.
Homebuilt aircraft builders often favour flat four engines because of their smoothness and suitability for air-cooling. For some years modified Volkswagen and Subaru flat-fours were popular. In recent times, flat-fours manufactured specifically for kit planes have been successful; such motors include the Rotax 912, the Jabiru 2200 and the D-Motor LF26.
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- 'Ordinary' means left-right-right-left or right-left-left-right crank throws.
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