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

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.