Usually found in 4- and 6-cylinder configurations, the straight engine (often designated inline engines) is an internal-combustion engine with all cylinders aligned in one row, with no or only minimal offset.

A straight engine is considerably easier to build than an otherwise equivalent horizontally opposed or V engine because the cylinder bank can be milled from a single metal casting and it requires fewer cylinder heads and camshafts. In-line engines are also much smaller in volume than designs like 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, the four-cylinder models are inherently off balance and rough, unlike 90 degree V fours and horizontally opposed 'boxer' 4 cylinders.

Automobile use

The straight-4 is by far the most common 4-cylinder configuration, whereas the straight-6 has largely given way to the V6, 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, notably Acura, Audi, Mercedes-Benz, Toyota, VW and Volvo, have also used straight-5 configurations. The General Motors Atlas family includes straight-4, straight-5, and straight-6 engines.

Once, the straight-8 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 V-8. In the 1930s, Duesenberg used an aluminium block 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-8 in 1933. But following the Second World War, even the simple technology of overhead valves was not applied to the straight-8 blocks but only to V-engines, which were comparatively lighter.

Many manufacturers mount straight engines at an angle from the vertical, referring to them as slant engines. Chrysler's famous Slant 6 was used in many models in the 1960s and 1970s. Honda also often mounts their straight-4 and straight-5 engines at a slant, as on the Honda S2000 and Acura Vigor. SAAB first used an inline-4 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-6 in automotive applications. Lanchester balance shafts, an old idea reintroduced by Mitsubishi in the 1980s to overcome the natural harshness of the straight-4 engine and rapidly adopted by many other manufacturers, have made both straight-4 and V-6 engines much more refined than used to be the case. The inherent smoothness of the straight-6 is no longer as great an advantage as it used to be. Secondly, at around the same time, fuel consumption became a much more important factor. Cars became smaller and much more space-efficient. The engine bay of a modern small or medium car, typically designed for a straight-4, often does not have room for a straight-6, but can fit a V-6 with only minor modifications. Conversely, if the car is designed to have enough room for a straight-6, the 4-cylinder version of the same car will have wasted space and, all else being equal, cost a little more to manufacture and use more fuel.

Some manufacturers (originally Lancia, and more recently Volkswagen with the VR6) 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 can still be found in passenger vehicles like the Australian Ford Falcon, some entry-level Lexus models, the Toyota Land Cruiser, and BMW 6-cylinder models.

Aviation, bus and rail use

Renault produced an inverted air-cooled Straight-6 for airplanes, this was used on the Stampe. A similar design was the de Havilland Gipsy series of engines, used on the Tiger Moth and other aircraft. Advantages include improved visibility for the pilot in single engined craft, and lower center of gravity.

Engines of this type in some buses and trains have been built in a horizontal form. This differs from a flat engine because it is essentially an inline engine laid on its side. Underfloor engines for buses and DMUs are commonly seen in this design. Such engines may be based on a conventional upright engine with alterations to make it suitable for horizontal mounting.

Automotive, aircraft and marine use

• Straight-twin
• Straight-3
• Straight-4
• Straight-5
• Straight-6
• Straight-8
• Straight-9
• Straight-10
• Straight-12
• Straight-14
• Straight-24

See also

Piston engine configurations  •  • [ e]
Straight	Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14
Flat	2, 4, 6, 8, 10, 12, 16
V	2, 4, 5, 6, 8, 10, 12, 16, 20, 24
W	8, 12, 16, 18
Other inline	H, U, Square, VR, Opposed, X
Other	Radial, Rotary, Pistonless (Wankel)

Heat engine types/configurations using thermodynamic cycles (Power cycles)
Stroke number and stroke parting
Crower six stroke Four-stroke cycle Scuderi Split Cycle Engine One-stroke cycle Six stroke engine Two-stroke cycle
Different work volume types (incl. Pistonless rotary engine)
Britalus Rotary Engine Combustion chamber Controlled Combustion Engine Jet engine Orbital engine Piston engine Quasiturbine Rocket engine Swing-piston engine Toroidal engine Trochilic engine Tschudi engine Twingle engine Wankel engine
Different work volume ports and main forms of
Cylinder head porting D slide valve Four-stroke cycle engine valves Manifold Multi-valve Piston valve Poppet valve Rocket engine nozzles Sleeve valve
Different pistons layouts
Bourke engine Delta engine Double acting/differential cylinder Opposed piston engine Radial engine Rotary engine Single cylinder engine Stelzer engine Straight engine
Different main rotational motion mechanisms or arc to (or even almost) pistons back-and-forth.
Sector straight-line linkage [1] Connecting rod Coomber Rotary Engine [2] Crank Substitute Engine [3] Crankshaft Evans linkage [4] Cam Parallel motion Peaucellier-Lipkin linkage Piston rod QRMC Stirling/HydraLink [5] Revolving Cylinder Engine [6] Rhombic drive Scotch yoke Swashplate Swashplate engine Watt's linkage
One-way stop-and-go like rotational motion mechanisms to rotation.
Toroidal engine Trochilic engine