A cam engine is a piston 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.
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.
Cam engines are two-stroke engines, rather than four-stroke. 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.
Even if the cam engine had worked as designed, it offered only a few potential advantages.
One advantage is that the bearing surface area can be larger than for a crankshaft. In the early days of bearing material development, the reduced bearing pressure this allowed could give better reliability. A relatively successful swashplate cam engine was developed by the bearing expert George Michell, who also developed the slipper-pad thrust block.
Unlike a crankshaft, a cam may easily have more than one throw per rotation. This allows more than one piston stroke per revolution. For aircraft use, this was an alternative to using a propeller reduction gear: high engine speed for an improved power to weight ratio, combined with a slower propeller speed for an efficient propeller. In practice, the cam engine design weighed more than the combination of a conventional engine and gearbox.
The only internal combustion cam engines that have been remotely successful were the swashplate engines. These were almost all axial engines, where the cylinders are arranged parallel to the engine axis, in one or two rings. The purpose of such engines was usually to achieve this axial or 'barrel' layout, making an engine with a very compact frontal area. There were plans at one point to use barrel engines as aircraft engines, with their reduced frontal area allowing a smaller fuselage and lower drag.
A similar engine to the swashplate engine was the wobble plate engine. This used a bearing that purely nutated, rather than also rotating as for the swashplate. The wobble plate was separated from the output shaft by a rotary bearing. Wobble plate engines are thus not cam engines.
Pistonless rotary engines
Some engines use cams but are not 'cam engines' in the sense described here. These are a form of pistonless rotary engine. Since the time of James Watt, inventors have sought a rotary engine that relied on purely rotating movement, without the reciprocating movement and balance problems of the piston engine. These engines don't work either.[note 1]
Most pistonless engines relying on cams, such as the Rand cam engine, use the cam mechanism to control the motion of sealing vanes. Combustion pressure against these vanes causes a vane carrier, separate from the cam, to rotate. In the Rand engine, the camshaft moves the vanes so that they have a varying length exposed and so enclose a combustion chamber of varying volume as the engine rotates. The work done in rotating the engine to cause this expansion is the thermodynamic work done by the engine and what causes the engine to rotate.
- "Cam engines". Douglas Self.
- "Axial Internal-Combustion Engines". Douglas Self.
- "Comments on Crankless Engine Types". NACA Technical Memorandum (462). Washington DC: NACA. May 1928. p. 5.
- "Comments on Crankless Engine Types". NACA Technical Memorandum (462). Washington DC: NACA. May 1928. pp. 2–4.
- NACA 462, pp. 5–7, 15
- US 1603969, Hermann Michel, "Two-stroke-cycle internal combustion engine", issued 19 October 1926
- "Rotary Principle". Reg Technologies Inc.