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 term "inline" is used somewhat differently for aircraft engines than automotive engines. For automotive engines, the term ‘inline’ refers only to straight engines. But for aircraft, ‘inline’ can also refer to engines which are not of the straight configuration, such as v, w, or horizontally opposed.
;Straight or Inline: Engines with a single bank of cylinders which can be arranged at any angle but typically upright or inverted,. ;V:Engines with two banks of cylinders with less than 180° between them driving a common crankshaft, typically arranged upright or inverted. engine ;O or Horizontally Opposed: Engines with two banks of cylinders arranged at 180° to each other driving a common crankshaft, almost universally mounted with banks horizontal for aircraft use, or with crankshaft vertical for helicopter use,. engine on display at the Royal Air Force Museum Hendon. ;W:Engines with three banks driving a common crankshaft, arranged so that first and last banks are 180°or less apart,. ;X: Multiple bank engines with four banks arranged around a common crankshaft, usually spaced evenly,. 2-stroke opposed pistondiesel engine ;Fan: Engines with more than three banks with 180° or less between first and last bank, akin to W engines. Note: Fan engines with single cylinder banks, typically from Anzani, are usually regarded as variants of the Radial engine. engine ;U:Engines with two banks of cylinders side by side driving separate crankshafts geared to a single output,. ;H:Engines with four banks of cylinders driving two crankshafts geared to a single output, in effect, two Opposed engines coupled together and mounted either horizontally or vertically,. ;Opposed piston:Two-stroke engines, typically compression ignition/Diesel, with a single bank of cylinders driving two crankshafts where the pistons travel towards each other forming single combustion chambers,. Note: There is no theoretical limit to the number banks in an opposed piston engine, limitations include cost, complexity and reliability. ;Multiple bank:Engines with more than two banks, arranged around a common axis and/or crankshaft with more than 180° between first and last banks. multibank aviation engine, four cylinders per bank. NoteThe BMW 803 is not only an inline radial engine but is also a coupled engine with two engines arranged back to back on a common axis driving separate co-axial propellers through a common gearbox.
Inverted engine
Some inline aircraft engines, including the de Havilland Gipsy Major used in the de Havilland Tiger Moth, and the Daimler-Benz DB 601 used in the Messerschmitt Bf 109, were designed to be installed in airframes inverted, such that the crankshaft was at the top of the engine and the cylinder heads were at the bottom. The advantages of inverted engines included improved visibility for the pilot, improved access to cylinder heads and manifolds for the ground crew, having the centre of mass of a multi-bank engine lower in the engine, and having the widest part of a multi-bank engine being closer to the midline of the fuselage, which is also generally wide.
