History and Applications
Flathead inline 4- and 6-cylinder engines were frequently used for automobiles, tractors, and other products from the beginning of the automotive industry until the 1960s, when overhead valve designs began to outnumber flathead designs in production numbers. Flathead designs are still commonly built new for applications such as lawnmowers, rotary tillers, two-wheel tractors, and other one- and two-cylinder equipment.
Some of the most famous automotive flathead engines were ones whose common nicknames reflected their flathead nature, such as the Ford flathead V8 engine and the Ford Sidevalve engine.
Due to cooling and efficiency problems, flathead engines fell out of favor in "high power" applications, such as aircraft engines, prior to World War I. However they lived on for some time in the automotive world and were used on the Jeep, for instance. Flatheads are no longer in common use for automobiles (except in some rodding and customizing circles), although they are still used for some small-engine applications like lawnmowers. Because of their design, the size of valves and the compression ratio are limited, which in turn reduces available power and economy.
A new arrival is the Belgian D-Motor LF26, a compact 2.5-litre flat-four aero-engine that has direct drive to a propeller, without any reduction gearbox. Dispensing with the complexity of an ohv valvetrain, the engine gives peak power of 80 bhp at only 2800 rpm. The resulting simplicity, lightness, compactness and reliability are ideal in an aero-engine. D-Motor is developing a 120 bhp 6-cylinder version.
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