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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. [1]
The first engine to get an airworthiness certificate from the United States government was a radial cam engine. A variation of the cam engine, the swashplate engine (also the closely related wobble-plate engine), was briefly popular. [2]
Cam engines 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. 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.
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.
A modern cam can be manufactured with computer numerical control (CNC) technology so as to have a delayed mechanical advantage.
Other advantages of modern cam engines include:
To suggest that cam engines were or are a failure when robustness is concerned is in error. After extensive testing by the United States government, the Fairchild Model 447-C 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 30 to 50 hours, the Model 447-C was far more robust than any other aircraft engine then in production. [5] Sadly, in this pre-CNC age it had a very poor cam profile, which meant it shook too severely for the wood propellers and the wood, wire, and cloth airframes of the time.
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] [6]
The Michel engine (no relation) began with roller cam followers, but switched during development to plain bearing followers. [7] [8]
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 speed reduction unit: 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. [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 time 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 is the wobble plate engine, also known as nutator or Z-crank drive. This uses a bearing that purely nutates, rather than also rotating as for the swashplate. The wobble plate is separated from the output shaft by a rotary bearing. [2] Wobble plate engines are thus not cam engines.
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. [9] 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.
A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting rods.
A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common features of all types. The main types are: the internal combustion engine, used extensively in motor vehicles; the steam engine, the mainstay of the Industrial Revolution; and the Stirling engine for niche applications. Internal combustion engines are further classified in two ways: either a spark-ignition (SI) engine, where the spark plug initiates the combustion; or a compression-ignition (CI) engine, where the air within the cylinder is compressed, thus heating it, so that the heated air ignites fuel that is injected then or earlier.
The rotary engine is an early type of internal combustion engine, usually designed with an odd number of cylinders per row in a radial configuration. The engine's crankshaft remained stationary in operation, while the entire crankcase and its attached cylinders rotated around it as a unit. Its main application was in aviation, although it also saw use in a few early motorcycles and automobiles.
A two-strokeengine is a type of internal combustion engine that completes a power cycle with two strokes of the piston in one revolution of the crankshaft. A four-stroke engine requires four strokes of the piston to complete a power cycle in two crankshaft revolutions. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust functions occurring at the same time.
A starter is a device used to rotate (crank) an internal-combustion engine so as to initiate the engine's operation under its own power. Starters can be electric, pneumatic, or hydraulic. The starter can also be another internal-combustion engine in the case, for instance, of very large engines, or diesel engines in agricultural or excavation applications.
The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders "radiate" outward from a central crankcase like the spokes of a wheel. It resembles a stylized star when viewed from the front, and is called a "star engine" in some other languages.
The engine configuration describes the fundamental operating principles by which internal combustion engines are categorized.
A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor.
A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crankshaft. The connecting rod is required to transmit the compressive and tensile forces from the piston. In its most common form, in an internal combustion engine, it allows pivoting on the piston end and rotation on the shaft end.
The Sarich orbital engine is a type of internal combustion engine, invented in 1972 by Ralph Sarich, an engineer from Perth, Australia, which features orbital rather than reciprocating motion of its central piston. It differs from the conceptually similar Wankel engine by using a generally prismatic shaped piston that orbits the axis of the engine, without rotation, rather than the rotating trilobular rotor of the Wankel.
Controlled combustion engine (CCE) is a term used by TechViki, an engine design company, to identify a type of experimental internal combustion engine (ICE) designed by Brad Howell-Smith. It uses two counter-rotating cams instead of a crankshaft. Pairs of cylinders oppose each other in a boxer flat engine or X engine arrangement.
A rotary engine is a type of internal combustion piston engine used in some early aircraft, motorcycles, and cars. Almost the entire engine rotates about a fixed crankshaft.
Reciprocating motion, also called reciprocation, is a repetitive up-and-down or back-and-forth linear motion. It is found in a wide range of mechanisms, including reciprocating engines and pumps. The two opposite motions that comprise a single reciprocation cycle are called strokes.
An axial engine is a type of reciprocating engine with pistons arranged around an output shaft with their axes parallel to the shaft. Barrel refers to the cylindrical shape of the cylinder group whilst the Z-crank alludes to the shape of the crankshaft.
An axial piston pump is a positive displacement pump that has a number of pistons in a circular array within a cylinder block.
A swing-piston engine is a type of internal combustion engine in which the pistons move in a circular motion inside a ring-shaped "cylinder", moving closer and further from each other to provide compression and expansion. Generally two sets of pistons are used, geared to move in a fixed relationship as they rotate around the cylinder. In some versions the pistons oscillate around a fixed center, as opposed to rotating around the entire engine. The design has also been referred to as a oscillating piston engine, vibratory engine when the pistons oscillate instead of rotate, or toroidal engine based on the shape of the "cylinder".
A swashplate, also known as slant disk, is a mechanical engineering device used to translate the motion of a rotating shaft into reciprocating motion, or vice versa. The working principle is similar to crankshaft, Scotch yoke, or wobble/nutator/Z-crank drives, in engine designs. It was originally invented to replace a crankshaft, and is one of the most popular concepts used in crankless engines. It was invented by Anthony Michell in 1917.
A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement (rotation). The hydraulic motor is the rotary counterpart of the hydraulic cylinder as a linear actuator. Most broadly, the category of devices called hydraulic motors has sometimes included those that run on hydropower but in today's terminology the name usually refers more specifically to motors that use hydraulic fluid as part of closed hydraulic circuits in modern hydraulic machinery.
The Michel engine was an unusual form of opposed-piston engine. It was unique in that its cylinders, instead of being open-ended cylinders containing two pistons, were instead joined in a Y-shape and had three pistons working within them.
An internal combustion engine is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is typically applied to pistons, turbine blades, a rotor, or a nozzle. This force moves the component over a distance, transforming chemical energy into kinetic energy which is used to propel, move or power whatever the engine is attached to.