A flat-4 or horizontally-opposed-4 is a flat engine with four cylinders arranged horizontally in two banks of two cylinders on each side of a central crankcase. 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 a boxing competitor punching his gloves together before a fight, which has led to it being referred to as a boxer engine.

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. 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. [cite book
last = Nunney
first = M J
title = Light and Heavy Vehicle Technology
publisher = Butterworth-Heinemann
date = 2007
pages = p. 13
isbn = 0-7506-8037-7

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 or Continental. Although they are somewhat superior to straight-4s in terms of vibrations, 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 interferes with the ability of the front wheels to steer. The latter problem has not stopped Subaru from using it in its four-wheel drive cars, where the difficulty of fitting the short 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 it, 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 machined into the external cylinder block walls to improve the engine cooling.

Balance and smoothness

Boxer engines are better balanced than other engine types in 4 cylinder configurations. The more common straight-4 configuration suffers from an engine balance problem, causing up-and-down vibrations, which grows worse with increased engine size and power. In general, straight-4s above 2.0 L usually have balance shafts and ones over 3.0 L are not seen in passenger cars. However, the flat-4 does have a less serious secondary imbalance 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 somewhat so the piston connecting rods can be on separate crank pins, which results in the forces being slightly off-centre. It does not usually require balance shafts, but unlike flat-6 and inline-6 engines, the flat-4 is not a fully balanced configuration.

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 strokes every 180 degrees of crankshaft rotation, and the crank throws 180 degrees apart, all the pistons must come to complete stop and reverse before the next one can start its power stroke. This results in a gap between power strokes and a pulsating delivery of power to the flywheel. 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 and lack of smoothness of the flat-4, most manufacturers now choose the straight-4 engine for economy models and have moved to straight-5 or V6 engines for ones requiring more power. These engines suffer from dynamic imbalance problems, but with modern computer-aided design techniques the problems can be overcome with a variety of complex crankshaft, balance shaft, and engine mounting designs.

Automobile use

Jowetts before the Second World War were best known for their flat twin engines, but they made a flat four for the Jason and 10hp models in the 1930s. Post war the 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.

VW used a water-cooled flat-4 Wasserboxer in the later third-generation Type 2 until 1991, and until 2005 in the Brazilian version, VW Kombi.

Citroën used an air-cooled flat-4 on the Ami Super, GS, GSA and Axel.

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.

Lancia used a water cooled flat-4 on the Lancia Flavia and high-end Lancia Gamma.

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. The engine, called the EJ series, is wide but very short, and is mounted ahead of the front axle with the transmission behind. With this layout, power can be taken off both ends of the transmission to drive both the front and back wheels. 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.

Motorcycle use

Honda introduced a liquid cooled flat-4 on a production motorcycle in 1975 on the Honda GL1000 Gold Wing.

Aircraft use

Lycoming manufactures a very successful series of flat-4 aircraft engines ranging up to 360 in.3, as used in many smaller Cessna and other general aviation aircraft. Similar engines are produced by Continental Motors, Franklin Aircraft Engines, 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-4 engines in 40 cm3 and 52 cm3 sizes for radio-controlled aircraft hobby use, with the 52 cm3 "FF-320" engine currently in production.


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