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 pistons move inwards and outwards at the same time.

The flat-twin design was patented by Karl Benz in 1896 and the first production flat-twin engine was used in the Lanchester 8 hp Phaeton car released in 1900. The flat-twin engine was used in several other cars since, however a more common usage is in motorcycles; early models oriented the cylinders in line with the frame, however later models switched to the cylinders being perpendicular to the frame to provide even cooling across both cylinders.

Flat-twin engines were also used in several aircraft up until the 1930s and in various stationary applications from the 1930s to the 1960s.

The Australian lawnmower manufacturer Victa also produced a flat-twin engine push mower from August 1975 to 1980 dubbed the ‘Twin 500’, and later the ‘Supreme’. These engines were manufactured in Canada. They are very sought after as only small numbers were produced, most likely due to ignition- and fuel-related problems in early models. In the Supreme (the later model) all these problems were fixed with a rear-domed piston, crankcase mixers and refined ignition system^{[citation needed]}.

Most flat-twin engines use a boxer configuration for the crankshaft and are therefore called "boxer-twin" engines. In a boxer-twin engine, the 180° crankshaft moves the pistons in phase with each other, therefore the forces generated by one piston are cancelled out by the other, resulting in excellent primary balance. The evenly spaced firing order also assists in reducing vibration. The equal and opposite forces in a boxer-twin engine do however generate a rocking couple, due to the offset distance between the pistons along the crankshaft.

A commonly used ignition system is wasted spark, which is a simple ignition system using a double-ended coil firing both spark plugs on each revolution (i.e. during both the compression and exhaust strokes). This system is distributorless and requires only a single contact breaker and coil for the engine.

The boxer-twin configuration can cause pressuring of the crankcase during each inward piston stroke and de-pressurisation during each outward piston stroke, since both pistons are moving inwards or outwards at the same time. This crankcase pumping effect (also found on single-cylinder engines and 360° parallel-twin engines) is usually addressed by means of a crankcase breather.

The Citroën 2CV boxer-twin engine took advantage of this pumping effect to maintain a partial vacuum inside the crankcase, in order to reduce oil leaks when an oil seal malfunctions. This was achieved by using a one-way valve (a leather or rubber flap over a hole in the crankcase), to let air escape the crankcase but not enter it.