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Fairbanks-Morse opposed piston diesel engines on the submarine USS Pampanito.

An opposed piston engine is one in which the cylinders are double-ended, with a piston at each end and no cylinder head. Some variations of the Opposed Piston or OP designs can use a single crankshaft like the Doxford ship engines [1] and the Commer OP truck engines [2] They should not be confused with a flat engine, which is referred to as horizontally opposed, or sometimes as a "Boxer" engine.

Junkers Jumo 205

A more common layout uses 2 crankshafts or even 3 Crankshafts like the Napier Deltic diesel engines. These engines use 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.

The first opposed-piston diesel engines were developed in the beginning of 20th century. In 1907, Raymond Koreyvo, the engineer of Kolomna Works, patented and built opposed-piston two-stroke diesel with two crankshafts, connected by gearing. Although Koreyvo patented his diesel in France in November, 1907, the direction would not go on to manufacture opposed-piston engines.

The first Junkers engines had one crankshaft, the upper pistons having long connecting rods outside the cylinder. These engine were the forerunner of the Doxford marine engine. There is currently a resurgence of this design in a boxer configuration [3] Later Junkers engines like the Junkers Jumo 205 diesel aircraft engine, use two crankshafts, one at either end of a single bank of cylinders. There is also an effort to reintroduce the OP diesel aircraft engine http://www.dair.co.uk/

An April, 1950 print advertisement for Fairbanks-Morse opposed piston engines.

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. 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.

Koreyvo, 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.

The Doxford Engine Works of the UK designed and built very large opposed-piston engines for marine use. These engines differ in design from Jumo and Fairbanks-Morse engines by having external connecting rods outside the cylinder linking the upper and lower pistons, thus requiring only a single crankshaft. The first engine of this type was developed by Karl Otto Keller in 1912. Doxford obtained a sole UK license from Oechelhauser and Junkers to build this design of engine. After World War I these engines were produced in a number of models, such as the P and J series, with outputs as high as 20000 hp. Certain models were license-built in the US. Production of Doxford engines in the UK ceased in 1980. [4] [5] [6]

An example of an opposed piston engine.
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
10 sparkplug
For an animation see here (587 kB), for a view in perspective here.

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 a displacement of 250 cm³, and one with 350 cm³ displacement. 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.

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 forces additional gas into the cylinder until the inlet slots are closed by the piston. Then the compression stroke starts and the cycle repeats. This type of two cycle system is similar to the famous Grey Marine Diesel, later to be known as the GM Diesel (Detroit Diesel). In 1998 the production of that brand was halted as well due to the lower cost of available four cycle diesels.

The U.S. and British Militaries still purchase remanufactured engines if needed due to high demand.

An interesting variation on the opposed-piston engine is the free-piston engine which was patented in 1934 by Raúl Pateras de Pescara. It has no crankshaft and the pistons are returned after each firing stroke by compression and expansion of air in a separate cylinder. Early applications were for use as an air compressor or as a gas generator for a gas turbine. There is now renewed interest in it for powering vehicles by using it to drive a linear alternator.

"Amazine New Lightweight Turbine Engine" was the cover story in the February 1969 issue of Mechanix Illustrated magazine. It was actually a free-piston engine, not a turbine, and was used to power a go-cart.

• Gas turbine locomotive

• Fairbanks-Morse 38D8 Diesel Engine