Opposed piston engine
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 1 crankshaft like the Doxfordship engines  and the Comer OP truck engines 
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 Junkers engines had but one crankshaft the upper pistons having long connecting rods outside the cylinder. These engine were the forrunner of the Doxford marine engine. There is currently a resurgence of this design in a boxer configuration  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/
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.
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 1, 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.   
Assembly and function
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 presses additional gas into the cylinder until the inlet slots are closed by the piston. Now the compression stroke starts and the cycle repeats. This type of two cycle system was a similar version of the famous Grey Marine Diesel later to be known as the GM Diesel (Detroit Diesel). In 1998 the seize of production of that brand was halted as well due to the cost of four cycle diesel's being more readily available cheaper. The U.S. and British Militarys still receive remanifacted engines if needed due to high demand.
|Piston engine configurations|
|Straight||Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14|
|V||2, 4, 5, 6, 8, 10, 12, 16, 20, 24|
|Flat||2, 4, 6, 8, 10, 12, 16, H|
|W||8, 9, 12, 16, 18|
|Other inline||H, VR, Opposed, U (Square), X|
|Other||Hemi, Radial, Rotary, Pistonless, Deltic, (Wankel)|
|Stroke cycles |
One • Two • Four • Six •
|Engine types |
Gas turbine • Piston • Jet • Rocket engine • Steam engine • Stirling engine • Tschudi• Twingle
Rotary • Wankel • Free-piston • Britalus • Coomber • Swing-piston • Orbital • Quasiturbine
Cylinder head porting • D slide • Four-stroke • Manifold • Multi • Piston • Poppet • Sleeve
|Piston layouts |
Single cylinder • Straight • Opposed • Flat • V • W • H • Deltic • Radial • Rocket engine nozzle • Rotary • Stelzer • Controlled Combustion • Bourke
|Motion mechanisms |
Cam • Connecting rod • Coomber rotary • Crank • Crank substitute • Crankshaft • Linkages (Evans • Peaucellier-Lipkin • Sector straight-line • Watt) • Double acting/differential cylinder