Configuration
Flat engines offer a low centre of gravity and thereby may offer a drive configuration with better stability and control. Flat engines lend themselves well to aircraft engines.
Front-mounted air-cooled flat-twin engines were used in Tatra 11 and Tatra 30, by Citroën in their model 2CV and its derivatives. The GS and GSA and the Oltcit Club used a flat-four and a flat-six was proposed for the Citroën DS but rejected. BMW has used air- or air/oil-cooled flat-twin engines in its motorcycles from 1923 until the present day. Cars such as the four-cylinder Volkswagen Beetle and the six-cylinder Porsche 911 use a flat-engine at the rear of the car, where its extra width does not interfere with the steering of the front wheels and there is a weight-saving since no prop-shaft is required.
All versions of the Subaru Impreza, Forester, Tribeca, Legacy, Outback, Baja and SVX use either a flat-four or flat-six engine.
True boxers have each crankpin controlling only one piston/cylinder while 180° engines, which superficially appear very similar, share crankpins.
- The boxer engine has corresponding pistons reaching top dead centre (TDC) simultaneously.
Boxer engines must not be confused with opposed-piston engines, which are essentially the inverse, with two pistons compressing a single combustion space. These can be used in vehicles such as tanks.
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