Cam engine

A cam engine is a reciprocating engine where, instead of the conventional crankshaft, the pistons deliver their force to a cam that is then caused to rotate. The output work of the engine is driven by this cam.

Cam engines have been a success. The first engine to get an airworthiness certificate from the United States government was, in fact, a radial cam engine. A variation of the cam engine, the swashplate engine (also the closely related wobble-plate engine), was briefly popular.

These are generally thought of as internal combustion engines, although they have also been used as hydraulic- and pneumatic motors. Hydraulic motors, particularly the swashplate form, are widely and successfully used. Internal combustion engines, though, remain almost unknown.

Some cam engines are two-stroke engines, rather than four-stroke. Two modern example are the KamTech and Earthstar, both radial-cam engines. In a two-stroke engine, the forces on the piston act uniformly downwards, throughout the cycle. In a four-stroke engine, these forces reverse cyclically: In the induction phase, the piston is forced upwards, against the reduced induction depression. The simple cam mechanism only works with a force in one direction. In the first Michel engines, the cam had two surfaces, a main surface on which the pistons worked when running and another ring inside this that gave a desmodromic action to constrain the piston position during engine startup.

Usually, only one cam is required, even for multiple cylinders. Most cam engines were thus opposed twin or radial engines. An early version of the Michel engine was a rotary engine, a form of radial engine where the cylinders rotate around a fixed crank.

The short dwell time that a crank produces does not provide a more-or-less constant volume for the combustion event to take place in. A crank system reaches significant mechanical advantage at 6° before TDC; it then reaches maximum advantage at 45° to 50°. This limits the burn time to less than 60°. Also, the quickly descending piston lowers the pressure ahead of the flame front, reducing the burn time. This means less time to burn under lower pressure. This dynamic is why in all crank engines a significant amount of the fuel is burned not above the piston, where its power can be extracted, but in the catalytic converter, which only produces heat.

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Wikipedia