Flat twin

A flat-twin is a two-cylinder internal combustion engine with the cylinders on opposite sides of the crankshaft. It is a flat engine with two cylinders. Used in motorcycles for more than a century, flat-twins have also been used in automobiles, light aircraft, stationary powerplants, and household appliances.

Early flat-twin motorcycles' engines were mounted with the cylinders in line with the frame. This caused uneven cooling of the cylinders and required the motorcycle to have a long wheelbase. Later flat-twin motorcycles' engines were mounted with their cylinders across the frame for better air cooling and a shorter wheelbase. Disadvantages of this layout include torque reaction in the motorcycle in turns and the potential to damage cylinder heads.

The traditional crank configuration used with flat-twin engines is the boxer twin whose 180° crankshaft moves the pistons in phase with each other. In this configuration, the primary force generated by one piston counteracts that generated by the other at all times, resulting in excellent primary balance. Since the pistons are not in line with each other, the equal and opposing forces generate a rocking couple on the crankshaft. Unlike the ignition of a V-twin or a 180° parallel twin, a boxer engine has equally spaced firing intervals, adding to its smoothness.

The flat-twin engine in the 1897 Lanchester 8 hp phaeton had two counter-rotating crankshafts. Each piston was attached to one crankshaft by a thick connecting rod and to the other crankshaft by two thinner connecting rods, one on either side of the other piston's thick connecting rod. This allowed both cylinders to have the same axis. It also had the torque reaction of one crankshaft cancel the torque reaction of the other, cancelling torque reaction in the engine. The pistons in the Lanchester engine moved so that one piston was on top while the other was at bottom dead center, creating an uneven firing cycle. Lanchester used this engine design until 1904.

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