Opposed piston engine

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An opposed piston engine is one in which the cylinders are double-ended, with a piston at each end and no cylinder-head. It should not be confused with the flat engine (often called a "Boxer") which has a single crankshaft with its two cylinder banks set 180° apart.

This type of configuration was used in the Junkers Jumo 205 diesel aircraft engine, with two crankshafts, one at either end of a single bank of cylinders. Even more remarkable were the Napier Deltic diesel engines, with three crankshafts serving three banks of double-ended cylinders arranged in an equilateral triangle, with the crankshafts at the corners. These were used in railway locomotives and to power fast patrol boats. Both types are now largely obsolete, although the Royal Navy still maintains some Deltic-powered Hunt Class Mine Countermeasure Vessels.

This configuration has also been used for marine auxiliary generators and for larger marine propulsion engines, notably Fairbanks-Morse diesel engines used in both conventional and nuclear US submarines. Fairbanks-Morse also used it in diesel locomotives, starting in 1944.

With the addition of a supercharger or turbocharger, opposed piston designs can make very efficient two-stroke cycle Diesel engines. Some attempts were made to build non-diesel 4-stroke engines, but as there is no cylinder head, the bad location of the valves and the spark plug makes them inefficient.

Both the Jumo and Deltic engines used one piston per cylinder to expose an intake port, and the other to expose an exhaust port. Each piston is referred to as either an intake piston or an exhaust piston depending on its function in this regard. This layout gives superior scavenging, as gas flow through the cylinder is axial rather than radial, and simplifies design of the piston crowns. In the Jumo 205 and its variants, the upper crankshaft serves the exhaust pistons, and the lower crankshaft the intake pistons. In designs using multiple cylinder banks, such as the Junkers Jumo 223 and the Deltic, each big end bearing serves one inlet and one exhaust piston, using a forked connecting rod for the exhaust piston.

Assembly and Function on an Example

Shown is the layout of an Otto cycle two-stroke engine similar to the one developed by engineer Kurt Bang at the Prüssing Office on the basis of the prewar DKW race engine. There existed two versions - one with an displacement of 250 cm³ and one with 350 cm³. The engine had two cylinders with four pistons, two crankshafts and a supercharger. The crankshafts were connected by gears. The fuel-air mixture was produced by a carburetor. This resulted in a high fuel consumption.

1. intake for the fuel-air mixture
2. supercharger (here: rotary vane pump; original: Centrix)
3. airbox to buffer and distribute the mixture
4. waste valve to limit the pressure level
5. outlet crank mechanism
6. inlet crank mechanism (runs app. 20° past the outlet to achieve an asymmetric control diagram)
7. cylinder with inlet and outlet slots
8. exhaust
9. water cooling jacket

The supercharger takes in the fuel-air mixture, compressing it and pushing it into the airbox. From here it reaches the crank housings. On the outlet side it cools the thermically high loaded piston. After ignition the pistons move outwards, performing the power stroke. At first, the outlet piston opens its slots in the cylinder. The remaining pressure accelerates the gas column towards the exhaust. Then the other piston opens the inlet slots. The pressurized fresh mixture pushes the remaining waste gas out. While the inlet is still opened, the outlet is closed. The supercharger presses additional gas into the cylinder until the inlet slots are closed by the piston. Now the compression stroke starts and the cycle repeats.