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.
These engines were produced by Hermann Michel of the Michel Engine Company of Kiel, Germany, in the 1920s and 1930s. A US patent application was filed in 1921 and granted in 1926.
This Michel engine should not be confused with the contemporary Michell engine, which was a swashplate engine.
Operation
The Michel engine was a two-stroke diesel engine, of piston-ported opposed-piston design. Its unusual feature was that rather than two pistons sharing a cylinder, the cylinders here were Y shaped and contained three pistons. The two upper pistons controlled the inlet ports, with the one lower piston controlling the exhaust ports. Having two inlet (scavenge air) pistons to one exhaust piston provided good scavenging and efficient combustion. The engine was water-cooled.
Variants
Early versions of the Michel engine were cam engines. These do not use a crankshaft in the conventional sense, but instead have a shaped cam. The pistons or their connecting rods have cam followers that slide over the surface of this cam. Although most cams use rotary motion to generate a linear motion, this 'crankshaft' use is also possible, where the linear motion of the pistons is used to drive the rotary motion of the cam and the engine's output shaft.
Cam engines have all been unsuccessful so far, although the idea enjoyed some popularity in the mid-20th century, amongst inventors if not engineers. A similar idea was the swashplate engine, an axial form of cam engine. Although the forces in a cam engine are often greater and the frictional losses are high, they can also allow the use of larger bearing surfaces than a conventional crankshaft. Before the development of high performance bearing materials, the cam and swashplate engines appeared to offer some advantages.
Cam engines are two-stroke, 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.
Rotary engine
The first version of this engine, as described in the initial patent, was a rotary engine: the 'crank' (in this case the cam ring) remained fixed and the cylinders revolved around it. This type of engine (although as spark-ignition petrol engines, not diesels) had developed before World War I as an aircraft engine, its advantage being improved cooling for the cylinders as they rotated through a stream of cooling air. A disadvantage of the rotary engine was the increased rotating mass, although in the Michel case, the cam track was itself of considerable mass and so the significance was less than it had been for the aircraft engine.
The rotary version of the engine had the fuel injection pump mounted on the rotating portion of the engine, along with the cylinders. Fuel, lubricating oil and cooling water were supplied through rotating couplings.
Little is known of the rotary engine version. It is described in two early German sources, but although mentioned in the 1928 NACA report, it appears to have been superseded by then.
Cam engine
A second version of the engine retained the cam drive from the pistons, but now had the cylinder block fixed in place with the cam ring rotating in the function of a crankshaft. This is illustrated in the NACA report, but not described in the text. The original roller followers for each piston were later replaced by plain bearings.
The engine was targeted at output powers between 120 bhp to 1,000 bhp. The cylinder diameter for these engines was 180mm. As for many radial engines, cylinder rows could be stacked for greater power. In most aircraft radial engines this is restricted to only one or two rows, by the need for air cooling of the rear rows. The Michel engine, being water-cooled, had no such restriction. A further advantage of the multiple rows was improved dynamic balance.