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The pistons are usually mounted on the crankshaft such 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 automobile use such engines and it is a common configuration for smaller aircraft engines such as made by Lycoming, Continental and Rotax. Although they are superior to straight-4s in terms of secondary vibration, they have largely fallen out of favor because they have two cylinder banks thus requiring twice as many camshafts as a straight-4 (if an OHC rather than OHV or F-head configuration is used) 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 4-cylinder configurations. The more common inline-4 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, so inline-4s larger than 2.0 L usually have balance shafts and ones over 3.0 L are seldom used in passenger cars. In contrast, the flat-4 has much less secondary imbalance at the expense of larger rocking vibration, that causes 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 exhaust ports located on opposing banks in order to achieve evenly spaced exhaust pulse. In a four stroke engine, each cylinder creates 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.
In addition, four-stroke cycle flat-4s 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-4 compared to the inline-4, most manufacturers now choose the inline-4 engine for economy models and have moved to inline-5 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-6, flat-6, or V8 configurations because these designs are much smoother than the flat-4, particularly at larger displacements.
Tatra introduced an air-cooled flat four engine in the 1926 Tatra 30, followed by the T52 of 1930, T54 of 1931, T57 in 1931, and T75 in 1933, all with air-cooled flat fours of varying displacement. The 1936 T97 model pioneered the rear-engined, air-cooled flat-four, backbone chassis layout, later copied by 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.
Volkswagen used air-cooled flat-4s extensively in their early days, in the VW Beetle and most early VW designs. Porsche also used the VW engine in the early Porsche 356. This engine was replaced by a Porsche designed flat-4 in the late 356s and the 912. The 914 that replaced the 912 was built in partnership with VW using a VW engine.
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.
Water-cooled Alfa Romeo flat-4 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-4 engine marketed as H-4, by which they mean Horizontal rather than the H cross-section normally meant by 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-4, it allows Subaru to build an all-wheel drive vehicle at little extra cost over two-wheel drive.
- Honda introduced a liquid-cooled shaft drive flat-4 on a production motorcycle in 1975 on the Honda GL1000 Gold Wing.
- In 1955 in Britain Wooler produced a small number of 500 cc flat opposed fours with shaft drive.
- The BFG-Citroën (French) of 1982 was powered by a flat-4 1,299 cc Citroën automobile engine and shaft drive. About 450 of them were built in 1981 and 1982. One-quarter of them were purchased by the French police.
Lycoming manufactures a very successful series of flat-4 aircraft engines ranging up to 360 cu in (5.9 L), as used in many smaller Cessna 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.
Homebuilt aircraft have been built with Volkswagen and Subaru flat-4 power.
- Nunney, M J (2007). Light and Heavy Vehicle Technology. Butterworth-Heinemann. p. 13. ISBN 0-7506-8037-7.
- 'Ordinary' means left-right-left-right or right-left-right-left crank throws. A left-right-right-left (or, right-left-left-right) crankshaft is theoretically possible, but leaves a gap in between two cylinders on the left (or, right) bank.
- Subaru developped such "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 Legacy in 2003, Forester in 2005 and Impreza in 2007. As a result, Subaru flat-four engines no longer have the "flat-four burble".
- Announcement on Legacy exhaust in Japanese: "Fuji Heavy Industries, Press Information". 2003-05-23. Retrieved 2013-12-16.
- Announcement on Forester exhaust in Japanese: "Fuji Heavy Industries, News Release". 2005-01-27. Retrieved 2013-12-16.
- Announcement on Impreza exhaust in Japanese: "Fuji Heavy Industries, News Release". 2007-06-05. Retrieved 2013-12-16.
- Stepler, Richard (09 1994). "Preview Drive: Three Ways to Get a Grip". Popular Science 245 (3): 44. Retrieved 2011-10-23.
- Julian Marsh (2000-06-10). "Citroënët - BFG and MF motorcycles". Citroenet.org.uk. Retrieved 2011-12-06.
- Julian Marsh. "Citroënët - GS & GSA boxer engine". Citroenet.org.uk. Retrieved 2011-12-06.
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