Six-stroke engine

The term six-stroke engine has been applied to a number of alternative internal combustion engine designs that attempt to improve on traditional two-stroke and four-stroke engines. Claimed advantages may include increased fuel-efficiency, reduced mechanical complexity and/or reduced emissions. These engines can be divided into two groups based on the number of pistons that contribute to the six strokes.

In the single-piston designs, the engine captures the heat lost from the four-stroke Otto cycle or Diesel cycle and uses it to drive an additional power and exhaust stroke of the piston in the same cylinder in an attempt to improve fuel-efficiency and/or assist with engine cooling. The pistons in this type of six-stroke engine go up and down three times for each injection of fuel. These designs use either steam or air as the working fluid for the additional power stroke.

The designs in which the six strokes are determined by the interactions between two pistons are more diverse. The pistons may be opposed in a single cylinder or may reside in separate cylinders. Usually one cylinder makes two strokes while the other makes four strokes giving six piston movements per cycle. The second piston may be used to replace the valve mechanism of a conventional engine, which may reduce mechanical complexity and enable an increased compression ratio by eliminating hotspots that would otherwise limit compression. The second piston may also be used to increase the expansion ratio, decoupling it from the compression ratio. Increasing the expansion ratio in this way can increase thermodynamic efficiency in a similar manner to the Miller or Atkinson cycle.

These designs use a single piston per cylinder, like a conventional two- or four-stroke engine. A secondary, non-detonating fluid is injected into the chamber, and the leftover heat from combustion causes it to expand for a second power stroke followed by a second exhaust stroke.

In 1883, the Bath-based engineer Samuel Griffin was an established maker of steam and gas engines. He wished to produce an internal combustion engine, but without paying the licensing costs of the Otto patents. His solution was to develop a "patent slide valve" and a single-acting six-stroke engine using it. By 1886, Scottish steam locomotive maker Dick, Kerr & Co. saw a future in large oil engines and licensed the Griffin patents. These were double-acting, tandem engines and sold under the name "Kilmarnock". A major market for the Griffin engine was in electricity generation, where they developed a reputation for happily running light for long periods, then suddenly being able to take up a large demand for power. Their large heavy construction didn't suit them to mobile use, but they were capable of burning heavier and cheaper grades of oil. The key principle of the "Griffin Simplex" was a heated exhaust-jacketed external vapouriser, into which the fuel was sprayed. The temperature was held around 550 °F (288 °C), sufficient to physically vapourise the oil but not to break it down chemically. This fractional distillation supported the use of heavy oil fuels, the unusable tars and asphalts separating out in the vapouriser. Hot-bulb ignition was used, which Griffin termed the "catathermic igniter", a small isolated cavity connected to the combustion chamber. The spray injector had an adjustable inner nozzle for the air supply, surrounded by an annular casing for the oil, both oil and air entering at 20 psi (140 kPa) pressure, and being regulated by a governor. Griffin went out of business in 1923. Only two known examples of a Griffin six-stroke engine survive. One is in the Anson Engine Museum. The other was built in 1885 and for some years was in the Birmingham Museum of Science and Technology, but in 2007 it returned to Bath and the Museum of Bath at Work.

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