Cam engine

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.^[1]

Cam engines have been a success The first engine to get an airworthiness certificate from the US government was in fact a radial cam engine. A variety of the cam engine, the swashplate engine (also the closely related wobble-plate engine), was briefly popular.^[2]

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.

Operation

Operating cycle

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.^[3]

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.

Advantages

(a) Perfect balance, a crank system is impossible to dynamically balance, because you cannot attenuate a reciprocal force or action with a rotary reaction or force. The modern KamTech cam engine uses another piston to attenuate the reciprocal forces. It runs as smooth as an electric motor.

(b) A more ideal combustion dynamic, a look at a PV diagram of the “Ideal IC engine” and you will find that the combustion event ideally should be a more or less “constant volume event“. ^[4] 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 degrees before TDC, it then reaches maximum advantage at 45 to 50 degrees. This limits the burn time to less than 60 degrees. Also the quickly descending piston lowers the pressure ahead of the flame front slowing 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. A modern cam can be manufactured with CNC technology so as to have a delayed mechanical advantage. The KamTech cam for example reaches significant advantage at 20 degrees permitting the ignition to start sooner in the rotation and maximum advantage is moved to 90 degrees permitting a longer burn time before the exhaust is vented. This means the burn under high pressure takes place during 110 degrees with a cam rather than 60 degrees as happen when a crank is used. The result, the KamTech Engine at any speed and under any load never has fire coming out of the exhaust, because there is time for full and complete combustion to take place under high pressure above the piston.^[5]

A few other advantages of modern Cam Engines

• Ideal piston dynamics,
• Lower internal friction,
• Cleaner exhaust,
• Less fuel consumption, Longer life,
• More power per kilogram
• Modern compact modular design permits better vehicle design,
• and because it has less parts it costs less to make.

To suggest that Cam engines were or are a failure as far as being robust is in error. After extensive testing by the US Government the Fairchild Model 447-C which was a radial Cam engine had the distinction of receiving the very first Department of Commerce Approved Type Certificate. At a time when aircraft crank engine had a life of from 30 to 50 hours the Model 447-C was far more robust than any other aircraft engine then in production.^[6] Sadly in this pre CNC ^[7]age it had a very poor cam profile which ment it shook very hard. Too hard in fact for the wood props and the wood, wire and cloth airframes of the time.

Bearing area

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.^[2][8]

The Michel engine (no relation) began with roller cam followers, but switched during development to plain bearing followers.^[9][10]

Effective gearing

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.

Swashplate engines

The only internal combustion cam engines that have been remotely successful were the swashplate engines.^[2] 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.^[2] 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.^[11] 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.

Notes

References