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Opposed piston engine 1

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

An opposed-piston engine is a reciprocating internal combustion engine in which each cylinder has a piston at both ends, and no cylinder head.

ConfigurationsEdit

Jumo205 cutview 02

Junkers Jumo 205 aircraft engine

Some variations of the Opposed Piston or OP designs use a single crankshaft, such as the Doxford ship engines[1] and the Commer OP truck engines.[2] They should not be confused with flat engines. Though flat engines are sometimes referred to as horizontally opposed, they are very different mechanically.

A more common layout uses two crankshafts, with the crankshafts geared together, or even three geared crankshafts in the Napier Deltic diesel engines. The Deltic uses 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 the 20th century. In 1907, Raymond Koreyvo, the engineer of Kolomna Works, built an opposed-piston two-stroke diesel with two crankshafts connected by gearing. Although Koreyvo patented his engine in France in November 1907, the management 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 engines were the forerunner of the Doxford marine engine, and this layout was also used for two- and three-cylinder car engines from around 1900-1922 by Gobron-Brillié.[3] There is currently a resurgence of this design in a boxer configuration as a small aircraft Diesel engine, and for other applications, called the "OPOC"[4] engine by Advanced Propulsion Technologies, Inc. of California.[5] Later Junkers engines, such as the Junkers Jumo 205 diesel aircraft engine, use two crankshafts, one at either end of a single bank of cylinders. There are efforts to reintroduce the opposed-piston diesel aircraft engine with twin geared crankshafts for General aviation applications, by both Dair and PowerPlant Developments in the UK.[6]

This configuration has also been used for marine auxiliary generators and for larger marine propulsion engines, notably Fairbanks-Morse 38 8-1/8 diesel engines used in both conventional and U.S. nuclear 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 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 linking the upper and lower pistons, thus requiring 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 20,000 horsepower (15,000 kW). Certain models were license-built in the US. Production of Doxford engines in the UK ceased in 1980.[1][7][8][dead link]

Assembly and functionEdit

Opposite piston engine

An example of an opposed-piston engine. 1 intake for 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 (runs app. 20° past outlet to achieve an asymmetric control diagram)
6 inlet crank mechanism
7 cylinder with inlet and outlet slots
8 exhaust
9 water cooling jacket
10 sparkplug Other views: animation (587 kB), or perspective.

Shown at right is the layout of a 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 were two versions: one with a displacement of 250 cm³ (15 cu in), and one with 350 cm³ (21 cu in) displacement. The engine had two cylinders with four pistons, two crankshafts and a supercharger. The crankshafts were connected by gears.

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.

Production ceased in 1998, but the U.S. and British Militaries still purchase remanufactured engines on occasion.[citation needed]

Free-piston engineEdit

Main article: Free-piston engine

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, such as the Pratt & Whitney PT1 design. There is now renewed interest in it for powering vehicles by using it to drive a linear alternator.

See also Edit

ReferencesEdit

  1. 1.0 1.1 "Marine Engines – Doxford - Page 1". OldEngine.org.
  2. "Rootes-Lister – TS3 Horizontally Opposed Piston Engine Page 1". OldEngine.org. Archived from the original on February 25, 2008.
  3. L.J.K. Setright. Some Unusual Engines. 
  4. "The Patented opoc Engine".
  5. Peter Hofbauer. "Stroke of genius – OPOC takes two". Engine Technology International.
  6. "The 100hp Liquid Cooled Diesel Aircraft Engine". Diesel Air Limited.
  7. "Doxford Engines 1878–1980". Doxford Engine Friends Association.
  8. "Junkers Ship Engines". Horst Zoeller. Archived from the original on 2009-10-25.

External linksEdit

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