Straight engine

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4-cylinder straight engine scheme Straight engine.svg
4-cylinder straight engine scheme

The straight or inline engine is an internal combustion engine with all cylinders aligned in one row and having no offset. Usually found in four, six and eight cylinder configurations, they have been used in automobiles, locomotives and aircraft, although the term in-line has a broader meaning when applied to aircraft engines, see Inline engine (aviation).[ citation needed ]


A straight engine is considerably easier to build than an otherwise equivalent horizontally opposed or V engine, because both the cylinder bank and crankshaft can be milled from a single metal casting, and it requires fewer cylinder heads and camshafts. In-line engines are also smaller in overall physical dimensions than designs such as the radial, and can be mounted in any direction. Straight configurations are simpler than their V-shaped counterparts. Although six-cylinder engines are inherently balanced,[ vague ] the four-cylinder models are inherently off balance and rough, unlike 90-degree V fours[ citation needed ] and horizontally opposed 'boxer' four cylinders.

Automobile use

The inline-four engine is the most common four-cylinder configuration, whereas the straight-6 has largely given way to the V6 engine, which although not as naturally smooth-running is smaller in both length and height and easier to fit into the engine bay of smaller modern cars. Some manufacturers, including Acura, Audi, Ford, Mercedes-Benz, Volkswagen and Volvo, have also used straight-five configurations. The General Motors Atlas family includes straight-four, straight-five, and straight-six engines. Some small cars have inline three engines.

Once, the straight-eight was the prestige engine arrangement; it could be made more cheaply than a V-engine by luxury car makers, who would focus on other specifics than the geometric ones, and even built engines more powerful than any V8 engine. In the 1930s, Duesenberg used a cylinder block made from aluminium alloy, with four valves per cylinder and hemispherical heads to produce the most powerful engine on the market. It was thus a selling point for Pontiac to introduce the cheapest straight-eight in 1933. However, following World War II, the straight-eight was supplanted by the lighter and more compact V8 engine, which allowed shorter engine bays to be used in the design.

When a straight engine is mounted at an angle from the vertical it is called a slant engine. [1] Chrysler's Slant 6 was used in many models in the 1960s and 1970s. Honda also often mounts its straight-four and straight-five engines at a slant, as on the Honda S2000 and Acura Vigor. SAAB initially used the Triumph Slant-4 engine tilted at 45 degrees for the Saab 99, but later versions of the engine were less tilted.

Two main factors have led to the recent decline of the straight-six in automotive applications. First, Lanchester balance shafts, an old idea reintroduced by Mitsubishi in the 1980s to overcome the natural imbalance of the inline-four engine and rapidly adopted by many other manufacturers, have made both inline-four and V6 engines smoother-running; the greater smoothness of the straight-six layout is no longer as great an advantage. Second, fuel consumption became more important, as cars became smaller and more space-efficient. The engine bay of a modern small or medium car, typically designed for an inline-four, often does not have room for a straight-six, but can fit a V6 with only minor modifications.

Some manufacturers (originally Lancia, and more recently Volkswagen with the VR6 engine) have attempted to combine advantages of the straight and V configurations by producing a narrow-angle V; this is more compact than either configuration, but is less smooth (without balancing) than either.

Straight-6 engines are used in some models from BMW, Ford, Jeep, Chevrolet, GMC, Toyota, Suzuki and Volvo Cars.

Bus and rail use

Some buses and trains with straight engines have their engines mounted with the row of cylinders horizontal. This differs from a flat engine because it is essentially an inline engine laid on its side. Underfloor engines for buses and diesel multiple units (DMUs) commonly use this design. Such engines may be based on a conventional upright engine with alterations to make it suitable for horizontal mounting.

Aviation use

Mercedes D.IIIs on a WW1 AEG G.IV AEG G.IV, Canada Aviation Museum, Ottawa.jpg
Mercedes D.IIIs on a WW1 AEG G.IV
Inverted Gipsy Major in a DHA-3 Drover DHGipsyMajorengineDrover.JPG
Inverted Gipsy Major in a DHA-3 Drover

In aviation, the term "inline engine" is used more broadly, for any non-radial reciprocating engine, including V engines, W engines, H engines and horizontally opposed engines. [2]

Many straight engines, in the stricter sense, have been produced for aircraft, particularly from the early years of aviation and through the interwar period leading up to the Second World War. Straight engines were simpler and had low frontal area, reducing drag, and provided better cockpit visibility.

Straight sixes were especially popular in the First World War, and most German and Italian and some British aircraft used descendants of Daimler's pre-war inline six. Prominent examples include the German Mercedes D.III and BMW IIIa, Italian Isotta Fraschini V.4 and British Siddeley Puma.

The British de Havilland Gipsy family of engines and their descendants included straight-four and straight-six upright and inverted air-cooled engines which were used on a wide range of smaller aircraft around the world, including on the Tiger Moth biplane, and helped made the configuration popular for light aircraft. Menasco and Fairchild-Ranger in the United States, Renault in France, Walter in Czechoslovakia, and Hirth in Germany all built a similar range of engines which were popular in their respective markets.

Some straight aircraft engines have been inverted, with the crankshaft at the top of the engine, and the pistons hanging down rather than the reverse. Advantages of the inverted arrangement include a raised thrust line for improved clearance for the propeller, which either allows for the use of a larger, more efficient propeller, or for shorter undercarriage. Since the thrust line is higher, the engine can be mounted lower in the airframe, improving visibility forward, which is no longer blocked by the cylinder heads. It also allows for a simpler exhaust to keep gasses clear from the cockpit.

Motorcycle use

Straight-4 engine installed in line with the frame of an Indian Four motorcycle Indian 4 engine manifold side.jpg
Straight-4 engine installed in line with the frame of an Indian Four motorcycle

In motorcycling, the term "in-line" is sometimes used narrowly, for a straight engine mounted in line with the frame. A two-cylinder straight engine mounted across the frame is sometimes called a parallel twin. [3] Other times, motorcycling experts treat the terms parallel, straight, and inline as equivalent, and use them interchangeably. [4] [5]

Related Research Articles

Cylinder bank Line of cylinders in a piston engine arranged in two or more lines

Piston engines are usually arranged so that the cylinders are in lines parallel to the crankshaft. Where they are in a single line, this is referred to as a straight engine.

Aircraft engine Engine designed for use in powered aircraft

An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Most aircraft engines are either piston engines or gas turbines, although a few have been rocket powered and in recent years many small UAVs have used electric motors.

The engine configuration describes the fundamental operating principles by which internal combustion engines are categorized.

Flat engine Combustion engine using pistons facing to the sides on a common crankshaft

A flat engine, also known as a horizontally opposed engine, is a piston engine where the cylinders are located on either side of a central crankshaft. A flat engine should not be confused with the opposed-piston engine, in which each cylinder has two pistons sharing a central combustion chamber.

Flat-twin engine

A flat-twin engine is a two-cylinder internal combustion engine with the cylinders on opposite sides of the crankshaft. The most common type of flat-twin engine is the boxer-twin engine, where both cylinders move inwards and outwards at the same time.

Straight-twin engine Inline piston engine with two cylinders

A straight-twin engine, also known as an inline-twin, vertical-twin, or parallel-twin is a two-cylinder piston engine where two cylinders are arranged in a line along a common crankshaft.

Straight-eight engine Inline piston engine with eight cylinders

The straight-eight engine or inline-eight engine is an eight-cylinder internal combustion engine with all eight cylinders mounted in a straight line along the crankcase. The type has been produced in side-valve, IOE, overhead-valve, sleeve-valve, and overhead-cam configurations.

Straight-four engine Inline piston engine with four cylinders

A straight-four engine is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.

Straight-six engine Internal combustion engine

The straight-six engine is an internal combustion engine, with six cylinders mounted in a straight line along the crankcase with all the pistons driving a common crankshaft.

The straight-five engine or inline-five engine is an internal combustion engine with five cylinders aligned in one row or plane, sharing a single engine block and crankcase. The justification for a five cylinder engine is that it is almost as compact as an inline-four, and almost as smooth as a straight-six engine.

Straight-three engine Type of engine

A straight-three engine is a three-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.

Overhead camshaft engine Valvetrain configuration

An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion chamber in the engine block.

Flat-six engine

A flat-six engine, also known as a horizontally opposed-six, is a six-cylinder piston engine with three cylinders on each side of a central crankshaft. The most common type of flat-six engine is the boxer-six engine, where each pair of opposed cylinders moves inwards and outwards at the same time.

Multi-valve Type of car engine

In automotive engineering a multi-valve or multivalve engine is one where each cylinder has more than two valves. A multi-valve engine has better breathing and may be able to operate at higher revolutions per minute (RPM) than a two-valve engine, delivering more power.

Longitudinal engine Internal combustion engine mounted with the crankshaft lengthwise

In automotive engineering, a longitudinal engine is an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicle, front to back.

Motorcycle engine Engine that powers a motorcycle

A motorcycle engine is an engine that powers a motorcycle. Motorcycle engines are typically two-stroke or four-stroke internal combustion engines, but other engine types, such as Wankels and electric motors, have been used.

IOE engine Type of combustion engines

The intake/inlet over exhaust, or "IOE" engine, known in the US as F-head, is a four-stroke internal combustion engine whose valvetrain comprises OHV inlet valves within the cylinder head and exhaust side-valves within the engine block.

Inline engine (aeronautics) Reciprocating engine arranged with cylinders in banks aligned with the crankshaft

In aviation, an inline engine is a reciprocating engine with banks of cylinders, one behind another, rather than rows of cylinders, with each bank having any number of cylinders, although more than six is uncommon. The major reciprocating-engine alternative configuration is the radial engine, where the cylinders are placed in a circular or "star" arrangement.


The LFG Roland D.XVII was a single-seat, single-engine, parasol wing German fighter aircraft flown close to the end of World War I. Only one was built.


  1. Today's Technician: Automotive Engine Performance. Douglas Vidler. Cengage Learning, 1 Jul 2003
  2. Johnson, E. R. (2011-04-20). United States Naval Aviation, 1919-1941: Aircraft, Airships and Ships Between the Wars (illustrated ed.). McFarland. p. 326. ISBN   978-0786445509. INLINE ENGINEA type of reciprocating piston engine in which an even (4-6-8-12) number of cylinders are arranged either in a straight line or in a V-type configuration directly above (or below) the crankcase.
  3. Wilson, Hugo (1995). "Glossary". The Encyclopedia of the Motorcycle . London: Dorling Kindersley. pp.  309–310. ISBN   0-7513-0206-6. in-line Engine layout in which the cylinders are arranged in a row, and in-line with the wheels of the machine.
  4. Hunt, Phil; McKay, Malcolm; Wilson, Hugo; Robinson, James (2012), Duckworth, Mick (ed.), Motorcycle: The Definitive Visual History, DK Publishing, Penguin Group, pp. 126, 210, ISBN   9781465400888
  5. Tuttle, Mark Jr. (December 2005), "BMW F800S", Rider(General OneFile. Web. 29 June 2012.)|format= requires |url= (help), p. 15