The parallel twin, opposed pistons and the Atkinson cycle

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The ongoing search to improve engine efficiency has revived some very old engine layouts, such as the opposed piston engine with extended equivalent stroke that can operate in ‘Atkinson’ mode. There is precedent of an engine builder having ‘folded over’ an opposed piston engine and operating it as a modified parallel twin cylinder engine on a motorcycle. One cylinder featured inlet ports while the companion featured exhaust ports. While that engine is now part of motorcycle history, there may actually be scope to revive the layout in this modern era, to improve engine efficiency.

The traditional 4-stroke parallel twin engine powered many motorcycles (Triumph, BSA, Norton, Suzuki, Benelli and many other makes) as well as several motorcars such as the Fiat 500, the Honda N600 and numerous modern Japanese ‘micro-cars’. Modern parallel twin engines reduce vibration courtesy of weighted counter-shafts or weights attached to connecting rods set at 90-degrees from the pistons. While the parallel twin is still widely used in motorcycles and micro-cars, there may be scope to introduce a more efficient version of the engine to small motorcars in Asia and in some European markets.

Some modern American built V-8 engines are designed to ‘unload’ cylinders and operate on 4 or 6-cylinders when running under reduced load. The same ‘unloading’ technology can be adapted for use in a parallel twin cylinder engine, that is, it operates on both cylinders when power is needed and runs on a single cylinder when cruising at port load. Except that the parallel twin-cylinder design invites engineers to include a precedent from an earlier parallel twin engine. The challenge is to design an interconnecting pipe between the cylinders that will open when the engine runs under part load.

When running under part load, compression, injection and firing will occur in a single cylinder. Then the interconnection will open to allow expansion to occur across 2-cylinders, a variation of the Atkinson cycle. The use of ceramic coatings on top of the pistons, under the valves and inside the interconnecting pipe will enhance part-load engine efficiency. An engine of 750-cm3-displacement would operate as an engine of 375-cm3 with one cylinder unloaded and as an engine of 450 to 500-cm3 with the second cylinder assisting to expand combustion gas in Atkinson cycle mode.

Engines of 450-cm3 provide power in several modern Japanese micro-cars and these cars could attract customers across Asia and Europe. An engine of 750-cm3 could provide additional performance when power is needed and operate economically on a modified Atkinson cycle when running under part load. The combination of modern valve technology and a blower could allow the engine to operate either as a 4-stroke engine or as a head-breathing 2-stroke engine when additional power is required. It could also combine single cylinder 2-stroke operation with the 2nd cylinder providing the ‘extended’ stroke for Atkinson cycle operation.

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