Flat-4
Encyclopedia
Flat engine
A flat engine is an internal combustion engine with multiple pistons that move in a horizontal plane. Typically, the layout has cylinders arranged in two banks on either side of a single crankshaft and is sometimes known as the boxer, or horizontally opposed engine. The concept was patented in 1896...
with four cylinders
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
arranged horizontally in two banks of two cylinders on each side of a central crankcase
Crankcase
In an internal combustion engine of the reciprocating type, the crankcase is the housing for the crankshaft. The enclosure forms the largest cavity in the engine and is located below the cylinder, which in a multicylinder engine are usually integrated into one or several cylinder blocks...
. The piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
s are usually mounted on the crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
such that opposing pistons move back and forth in opposite directions at the same time, somewhat like boxing
Boxing
Boxing, also called pugilism, is a combat sport in which two people fight each other using their fists. Boxing is supervised by a referee over a series of between one to three minute intervals called rounds...
competitors punching their 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.
This is no longer a common configuration, but some brands of automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
use such engines and it is a common configuration for smaller aircraft engine
Aircraft engine
An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. Aircraft engines are almost always either lightweight piston engines or gas turbines...
s such as made by Lycoming
Lycoming Engines
Lycoming Engines is a U.S. aircraft engine company, known primarily for its general aviation engines. For most of its history Lycoming has been part of the AVCO group as AVCO Lycoming. In 1987 AVCO was purchased by Textron to become Textron Lycoming...
or Continental. Although they are somewhat superior to straight-4
Straight-4
The inline-four engine or straight-four engine is an internal combustion engine with all four cylinders mounted in a straight line, or plane along the crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a common crankshaft....
s in terms of vibrations, they have largely fallen out of favor because they have two cylinder banks thus requiring twice as many camshaft
Camshaft
A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...
s 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
Subaru
; is the automobile manufacturing division of Japanese transportation conglomerate Fuji Heavy Industries .Subaru is internationally known for their use of the boxer engine layout popularized in cars by the Volkswagen Beetle and Porsche 911, in most of their vehicles above 1500 cc as well as...
from using it in its all-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 cast into the external cylinder block walls to improve the engine cooling.
Balance and smoothness
Boxer engines are better balancedEngine balance
Engine balance is the design, construction and tuning of an engine to run smoothly. Improving engine balance reduces vibration and other stresses and can improve the overall performance, efficiency, cost of ownership and reliability of the engine, as well as reducing the stress on other machinery...
than other engine types in 4-cylinder configurations. The more common inline-4 configuration suffers from an engine balance
Engine balance
Engine balance is the design, construction and tuning of an engine to run smoothly. Improving engine balance reduces vibration and other stresses and can improve the overall performance, efficiency, cost of ownership and reliability of the engine, as well as reducing the stress on other machinery...
problem caused by the fact that the pistons travel faster on the top half of the crankshaft rotation than the bottom half, which causes the engine to vibrate up and down twice per crankshaft revolution. This problem becomes worse with increased engine size and power so inline-4s larger than 2.0 L usually have balance shaft
Balance shaft
In piston engine engineering, a balance shaft is an eccentric weighted shaft which offsets vibrations in engine designs that are not inherently balanced...
s and ones over 3.0 L are seldom used 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 twice per crankshaft revolution. This is because the cylinders
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
cannot be directly opposed, but must be offset somewhat so the piston connecting rod
Connecting rod
In a reciprocating piston engine, the connecting rod or conrod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts linear motion into rotating motion....
s can be on separate crank pin
Crank pin
In a reciprocating engine, the crankpins, also known as crank journals are the journals of the big end bearings, at the ends of the connecting rods opposite to the pistons....
s, which results in the forces being slightly off-centre. The vibration is usually not serious enough to require balance shafts.
In addition, four-stroke cycle
Four-stroke cycle
A four-stroke engine, also known as four-cycle, is an internal combustion engine in which the piston completes four separate strokes—intake, compression, power, and exhaust—during two separate revolutions of the engine's crankshaft, and one single thermodynamic cycle.There are two...
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
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
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
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
. 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 engine
V6 engine
A V6 engine is a V engine with six cylinders mounted on the crankcase in two banks of three cylinders, usually set at either a right angle or an acute angle to each other, with all six pistons driving a common crankshaft...
s for models requiring more power. These engines also suffer from dynamic imbalance problems, but with modern computer-aided design
Computer-aided design
Computer-aided design , also known as computer-aided design and drafting , is the use of computer technology for the process of design and design-documentation. Computer Aided Drafting describes the process of drafting with a computer...
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
Flat-6
A flat-6 or horizontally opposed-6 is a flat engine with six cylinders arranged horizontally in two banks of three cylinders on each side of a central crankcase...
, or V8 configurations because these designs are in intrinsic primary and secondary balance and thus are much smoother than the flat-4, particularly at high power outputs.
Automobile use
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.Jowett
Jowett
Jowett was a manufacturer of light cars and light commercial vehicles in Bradford, West Yorkshire, England from 1906 to 1954.-Early history:Jowett was founded in 1901 by brothers Benjamin and William Jowett with Arthur V Lamb. They started in the cycle business and went on to make V-twin engines...
s 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
Jowett Javelin
The Jowett Javelin is an award-winning British car that was produced from 1947 to 1953 by Jowett Cars Ltd of Idle, near Bradford. The model went through five variants labelled PA to PE, each having a standard and "de luxe" option. The car was designed by Gerald Palmer during World War II and was...
saloon and Jupiter
Jowett Jupiter
The Jowett Jupiter was a British car made by Jowett Cars Ltd of Idle, near Bradford from 1950 to 1954. Following the launch of the all new Jowett Javelin and its successes in competition Jowett decided to use its power train in a sports car for export in the hope of increasing their inadequate...
sports models used a totally different design of flat four. Alec Issigonis
Alec Issigonis
Sir Alexander Arnold Constantine Issigonis, CBE, FRS was a Greek-British designer of cars, now remembered chiefly for the groundbreaking and influential development of the Mini, launched by the British Motor Corporation in 1959.- Early life:Issigonis was born into the Greek community of Smyrna ...
originally designed the Morris Minor
Morris Minor
The Morris Minor was a British economy car that debuted at the Earls Court Motor Show, London, on 20 September 1948. Designed under the leadership of Alec Issigonis, more than 1.3 million were manufactured between 1948 and 1971...
for a flat four, but cost constraints meant it was never used.
Volkswagen
Volkswagen is a German automobile manufacturer and is the original and biggest-selling marque of the Volkswagen Group, which now also owns the Audi, Bentley, Bugatti, Lamborghini, SEAT, and Škoda marques and the truck manufacturer Scania.Volkswagen means "people's car" in German, where it is...
used air-cooled flat-4s extensively in their early days, in the VW Beetle and most early VW designs. Porsche
Porsche
Porsche Automobil Holding SE, usually shortened to Porsche SE a Societas Europaea or European Public Company, is a German based holding company with investments in the automotive industry....
also used the VW engine in the early Porsche 356
Porsche 356
The Porsche 356 was the company's first production automobile. It was a lightweight and nimble handling rear-engine rear-wheel-drive 2 door sports car available in hardtop coupe and open configurations. Design innovations continued during the years of manufacture, contributing to its motorsports...
. This engine was replaced by a Porsche designed flat-4 in the late 356s and the 912
Porsche 912
The Porsche 912 is a sports car that was manufactured by Porsche of Germany between 1965 and 1969 as their entry-level model. The 912 is a nimble-handling compact performance four-seat vehicle, delivering 90 SAE horsepower at 5800 rpm. It is capable of up to fuel economy. This combination is...
. The 914
Porsche 914
The Porsche 914 or VW-Porsche 914 is a mid-engined, targa-topped two-seat roadster designed, manufactured and marketed collaboratively by Volkswagen and Porsche from 1969 to 1976.- Development :...
that replaced the 912 was built in partnership with VW using a VW engine.
The Goliath 1100
Goliath 1100
The Goliath 1100 is a small two-door saloon that was manufactured by the Bremen based Borgward subsidiary Goliath-Werke Borgward & Co from 1957-1961. A three-door kombi version was also available from launch and a two-door coupe was offered a year later...
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.
VW used a water-cooled flat-4 Wasserboxer
Wasserboxer
The Volkswagen wasserboxer is a four cylinder horizontally opposed pushrod overhead-valve petrol engine developed by Volkswagen. The engine is water-cooled, and takes its name from the ; which when translated into English, means "water-boxer" - or water-cooled boxer - with 'boxer' being an...
in the later third-generation Type 2
Volkswagen Type 2 (T3)
The Volkswagen Type 2 was the third generation of the Volkswagen Transporter and was marketed under various nameplates worldwide – including as the Transporter or Caravelle in Europe, Microbus in South Africa and as the Vanagon in North and South America.Larger and heavier than its...
until 1991.
Citroën
Citroën
Citroën is a major French automobile manufacturer, part of the PSA Peugeot Citroën group.Founded in 1919 by French industrialist André-Gustave Citroën , Citroën was the first mass-production car company outside the USA and pioneered the modern concept of creating a sales and services network that...
used an air-cooled flat-4 on the Ami Super
Citroën Ami
The Citroën Ami is a supermini produced by the French automaker Citroën from 1961 to 1978. The Ami and stablemate Citroën Dyane were replaced by the Citroën Visa and Citroën Axel . The Ami was for some years the best-selling car model in France...
, GS
Citroën GS
The Citroën GS and Citroën GSA are small family cars produced by the French automaker Citroën. The GS was voted European Car of the Year for 1971, and was probably the most technologically advanced car in its class when launched, with class leading comfort, safety and aerodynamics.-Market...
, GSA
Citroën GS
The Citroën GS and Citroën GSA are small family cars produced by the French automaker Citroën. The GS was voted European Car of the Year for 1971, and was probably the most technologically advanced car in its class when launched, with class leading comfort, safety and aerodynamics.-Market...
and Axel
Citroën Axel
The Citroën Axel was a supermini automobile produced between 1984 and 1990 and developed in co-operation by Citroën of France and Automobile Craiova, a joint venture company with the Romanian government....
.
Water-cooled Alfa Romeo flat-4
Alfa Romeo Flat-4 engine
Alfa Romeo developed a water-cooled flat-4 engine for their new Alfasud, introduced in 1971 at the Turin Motor Show.-1200:The original engine displaced with an 80 mm bore and 59 mm stroke and produced between .Applications:...
was introduced in 1971 on the Alfa Romeo Alfasud
Alfa Romeo Alfasud
The Alfa Romeo Alfasud is a compact car made by Alfa Romeo of Italy from 1971 to 1989. It was considered one of Alfa Romeo's most successful models, sold 893,719 examples from 1972 to 1983 plus 121,434 Sprint versions from 1976 to 1989. A common nickname for the car is ’Sud...
. That engine was later used on the Alfa Romeo Arna
Alfa Romeo Arna
The Alfa Romeo Arna is a subcompact automobile produced by the Italian manufacturer Alfa Romeo between 1983 and 1987.Launched at the 1983 Frankfurt Motor Show, the Arna was a product of a short-lived partnership between Alfa Romeo and Japanese manufacturer Nissan; the car's name was an acronym...
, the Alfa Romeo 33, the Alfa Romeo Sprint
Alfa Romeo Sprint
The Alfa Romeo Sprint is a coupé version of the Alfa Romeo Alfasud, produced from 1976 to 1989 by Alfa Romeo. 116,552 examples of the Alfasud Sprint and Alfa Romeo Sprint were built in total. The Sprint was sold in Europe, South Africa, Australia and New Zealand.-History:Until 1983, the car's name...
and the Alfa Romeo 145/146
Alfa Romeo 145
The Alfa Romeo 145 and 146 are small family cars produced by the Italian automaker Alfa Romeo between 1994 and 2001. They were launched at the 1994 Turin Motor Show...
.
Lancia
Lancia
Lancia Automobiles S.p.A. is an Italian automobile manufacturer founded in 1906 by Vincenzo Lancia and which became part of the Fiat Group in 1969. The company has a long history of producing distinctive cars and also has a strong rally heritage. Some modern Lancias are seen as presenting a more...
used a water-cooled flat-4
Lancia Flat-4 engine
Lancia, known for their advanced engineering, surprised the auto world by designing a new aluminum flat-4 engine for their 1961 Flavia. Though it was a pushrod engine, it was advanced for the time. The pushrod version of the Lancia boxer was only ever used in the Flavia...
on the Lancia Flavia
Lancia Flavia
The Lancia Flavia is a medium sized luxury saloon, launched with a 1500 cc engine at the 1960 Turin Motor Show by Lancia and introduced in major European markets during the next twelve months. Coupe and cabriolet versions developed by Pininfarina quickly followed, together with one or two low...
and high-end Lancia Gamma
Lancia Gamma
The Lancia Gamma is an executive car from Italian Fiat Auto's up-scale Lancia marque. Presented in 1976 at the Geneva Motor Show as Lancia's new flagship, it filled the void in Lancia's lineup left by the demise of the Flavia...
.
Subaru
Subaru
; is the automobile manufacturing division of Japanese transportation conglomerate Fuji Heavy Industries .Subaru is internationally known for their use of the boxer engine layout popularized in cars by the Volkswagen Beetle and Porsche 911, in most of their vehicles above 1500 cc as well as...
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
H engine
An H engine is an engine configuration in which the cylinders are aligned so that if viewed from the front, they appear to be in a vertical or horizontal letter H....
. Subaru has created a number of engines, starting with the EA series
Subaru EA engine
The Subaru EA engine is a series of automobile internal combustion engines manufactured by Subaru, a division of Fuji Heavy Industries. All EA series engines are of a flat-4 design....
introduced in 1966, progressing towards the currently used EJ series
Subaru EJ engine
The Subaru EJ engine is a series of automotive engines manufactured by Subaru and introduced in 1989, intended to succeed the previous Subaru EA engine. The EJ series is the mainstay of Subaru's engine line, with all engines of this series being 16-valve flat-4 horizontal, with configurations...
, 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.
Motorcycle use
- HondaHondais a Japanese public multinational corporation primarily known as a manufacturer of automobiles and motorcycles.Honda has been the world's largest motorcycle manufacturer since 1959, as well as the world's largest manufacturer of internal combustion engines measured by volume, producing more than...
introduced a liquid-cooled shaft drive flat-4 on a production motorcycle in 1975 on the Honda GL1000 Gold Wing.
- From 1938 to 1939 ZündappZündappZündapp was a major German motorcycle manufacturer. The company was founded in 1917 in Nuremberg by Fritz Neumeyer, together with the Friedrich Krupp AG and the machine tool manufacturer Thiel under the name "Zünder- und Apparatebau G.m.b.H." as a producer of detonators...
produced the shaft drive K800 opposed four in GermanyGermanyGermany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
.
- In 1955 in Britain WoolerWooler (motorcycles)Wooler was a British manufacturer of motorcycles and other vehicles, founded by engineer John Wooler in 1911 based in Alperton, Middlesex. The company became known for its unconventional designs which included several fore-and-aft twins, a vertical camshaft single cylinder machine, a...
produced a small number of 500 cc flat opposed fours with shaft drive.
- The BFG-Citroën of 1982 was powered by a flat-4 1,299 cc CitroënCitroënCitroën is a major French automobile manufacturer, part of the PSA Peugeot Citroën group.Founded in 1919 by French industrialist André-Gustave Citroën , Citroën was the first mass-production car company outside the USA and pioneered the modern concept of creating a sales and services network that...
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.
Aircraft use
LycomingLycoming Engines
Lycoming Engines is a U.S. aircraft engine company, known primarily for its general aviation engines. For most of its history Lycoming has been part of the AVCO group as AVCO Lycoming. In 1987 AVCO was purchased by Textron to become Textron Lycoming...
manufactures a very successful series of flat-4 aircraft engine
Aircraft engine
An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. Aircraft engines are almost always either lightweight piston engines or gas turbines...
s ranging up to 360 CID, as used in many smaller Cessna
Cessna
The Cessna Aircraft Company is an airplane manufacturing corporation headquartered in Wichita, Kansas, USA. Their main products are general aviation aircraft. Although they are the most well known for their small, piston-powered aircraft, they also produce business jets. The company is a subsidiary...
and other general aviation
General aviation
General aviation is one of the two categories of civil aviation. It refers to all flights other than military and scheduled airline and regular cargo flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights...
aircraft. Similar engines are produced by Continental Motors, Franklin Engine Company
Franklin Engine Company
The Franklin Engine Company was a manufacturer of aircraft engines, formed as the H. H. Franklin Co. in 1902, located in Syracuse, New York, United States. Barely surviving bankruptcy in 1933, the company was purchased by a group of ex-employees and renamed Aircooled Motors in 1937...
, 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 cc and 52 cc sizes for radio-controlled aircraft
Radio-controlled aircraft
A radio-controlled aircraft is controlled remotely by a hand-held transmitter and a receiver within the craft...
hobby use, with the 52 cc "FF-320" engine currently in production.
Homebuilt aircraft
Homebuilt aircraft
Also known as amateur-built aircraft or kit planes, homebuilt aircraft are constructed by persons for whom this is not a professional activity. These aircraft may be constructed from "scratch," from plans, or from assembly kits.-Overview:...
have been built with Volkswagen and Subaru flat-4 power.