The six-stroke engine is a type of internal combustion engine based on the four-stroke engine, but with additional complexity intended to make it more efficient and reduce emissions. Two types of six-stroke engine have been developed since the 1990s:
In the first approach, the engine captures the heat lost from the four-stroke Otto cycle or Diesel cycle and uses it to power an additional power and exhaust stroke of the piston in the same cylinder. Designs use either steam or air as the working fluid for the additional power stroke. The pistons in this type of six-stroke engine go up and down three times for each injection of fuel. There are two power strokes: one with fuel, the other with steam or air. The currently notable designs in this class are the Crower six-stroke engine, invented by Bruce Crower of the U.S. ; the Bajulaz engine by the Bajulaz S.A. company of Switzerland; and the Velozeta Six-stroke engine built by the College of Engineering, at Trivandrum in India.
The second approach to the six-stroke engine uses a second opposed piston in each cylinder that moves at half the cyclical rate of the main piston, thus giving six piston movements per cycle. Functionally, the second piston replaces the valve mechanism of a conventional engine but also increases the compression ratio. The currently notable designs in this class include two designs developed independently: the Beare Head engine, invented by Australian Malcolm Beare, and the German Charge pump, invented by Helmut Kottmann.
 Engine types
 Griffin six-stroke engine
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 lbs sq in. 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.
 Bajulaz six-stroke engine
The Bajulaz six-stroke engine is similar to a regular combustion engine in design. There are however modifications to the cylinder head, with two supplementary fixed capacity chambers: a combustion chamber and an air preheating chamber above each cylinder. The combustion chamber receives a charge of heated air from the cylinder; the injection of fuel begins an isochoric burn which increases the thermal efficiency compared to a burn in the cylinder. The high pressure achieved is then released into the cylinder to work the power or expansion stroke. Meanwhile a second chamber which blankets the combustion chamber, has its air content heated to a high degree by heat passing through the cylinder wall. This heated and pressurized air is then used to power an additional stroke of the piston.
The claimed advantages of the engine include reduction in fuel consumption by at least 40%, two expansion strokes in six strokes, multi-fuel usage capability, and a dramatic reduction in pollution.
The Bajulaz six-stroke engine features:
- Reduction in fuel consumption by at least 40%
- Two expansion (work) strokes in six strokes
- Multifuel, including liquefied petroleum gas
- Dramatic reduction in air pollution
- Costs comparable to those of a four-stroke engine
 Velozeta six-stroke engine
In a Velozeta engine, during the exhaust stroke, fresh air is injected into the cylinder, which expands by heat and therefore forces the piston down for an additional stroke. The valve overlaps have been removed and the two additional strokes using air injection provide for better gas scavenging. The engine seems to show 40% reduction in fuel consumption and dramatic reduction in air pollution. Its specific power is not much less than that of a four-stroke Gasoline engine. The engine can run on a variety of fuels, ranging from Gasoline and diesel to LPG. An altered engine shows a 65% reduction in carbon monoxide pollution when compared with the four stroke engine from which it was developed.
 Crower six-stroke engine
In a six-stroke engine prototyped in the U.S. by Bruce Crower, water is injected into the cylinder after the exhaust stroke, and is instantly turned to steam, which expands and forces the piston down for an additional power stroke. Thus, waste heat that most engines require an air or water cooling system to discharge is captured and put to use driving the piston. Crower estimated that his design would reduce fuel consumption by 40% by generating the same power output at a lower RPM. The weight associated with a cooling system could be eliminated, but that would be balanced by a need for a water tank in addition to the normal fuel tank.
The Crower six-stroke engine was an experimental design that attracted media attention in 2006 because of an interview given by 75 year old American inventor has applied for a patent on his design. () That patent application was subsequently abandoned.  Leonard Dyer invented the first six-stroke internal combustion water injection engine in 1915, which is very similar to Crower's design. A dozen more similar patents have been issued since.  Dyer's six-stroke engine features:
- No cooling system required
- Improves a typical engine’s fuel consumption
- Requires a supply of pure water to act as the medium for the second power stroke.
- Extracts the additional power from the expansion of steam.
 Beare Head
The term "Six Stroke" was coined by the inventor of the Beare Head, Malcolm Beare. The technology combines a four stroke engine bottom end with an opposed piston in the cylinder head working at half the cyclical rate of the bottom piston. Functionally, the second piston replaces the valve mechanism of a conventional engine. first use of "sixstroke" 1994
The M4+2 engines have much in common with the Beare Head engines, combining two opposed pistons in the same cylinder. One piston working at half the cyclical rate of the other. But while the main function of the second piston in a Beare Head engine is to replace the valve mechanism of a conventional four stroke engine, the M4+2 take the principle one step further.
The working principle of the engine is explained in the Two- and four-stroke engines article.
 Piston charger engine
In this engine, similar in design to the Beare head, a "piston charger" replaces the valve system. The piston charger charges the main cylinder and simultaneously regulates the inlet and the outlet aperture leading to no loss of air and fuel in the exhaust. In the main cylinder, combustion takes place every turn as in a two-stroke engine and lubrication as in a four-stroke engine. Fuel injection can take place in the piston charger, in the gas transfer channel or in the combustion chamber. It is also possible to charge two working cylinders with one piston charger. The combination of compact design for the combustion chamber together with no loss of air and fuel is claimed to give the engine more torque, more power and better fuel consumption. The benefit of less moving parts and design is claimed to lead to lower manufacturing costs. Good for hybrid technology and stationary engines. The engine is claimed to be suited to alternative fuels since there is no corrosion or deposits left on valves. The six strokes are: aspiration, precompression, gas transfer, compression, ignition and ejection. This is an invention of Helmut Kottmann from Germany, working 25 years at MAHLE GmbH piston and cylinder construction.
 Related U.S. patents
- 1217788 Internal combustion and steam engine Feb 27, 1917. Hugo F. Liedtke seems to be one of the first to contemplate alternating between internal combustion and steam injection into the combustion chamber.
- 1339176 Internal combustion engine May 4, 1920. Leonard H. Dyer invented the first 6-stroke internal combustion/water-injection engine in 1915.
- 3964263 Six cycle combustion and fluid vaporization engine Jun 22, 1976
- 4143518 Internal combustion and steam engine Mar 13, 1979
- 4301655 Combination internal combustion and steam engine Nov 24, 1981
- 4433548 Combination internal combustion and steam engine Feb 28, 1984
- 4489558 Compound internal combustion engine and method for its use Dec 25, 1984
- 4489560 Compound internal combustion engine and method for its use Dec 25, 1984
- 4736715 Engine with a six-stroke cycle, variable compression ratio, and constant stroke Apr 12, 1988
- 4917054 Six-stroke internal combustion engine Apr 17, 1990
- 4924823 Six-stroke internal combustion engine May 15, 1990
- 6253745 Multiple stroke engine having fuel and vapor charges Jul 3, 2001
- 6311651 Computer-controlled six-stroke internal combustion engine and its method of operation Nov 6, 2001
- 6571749 Computer-controlled six-stroke cycle internal combustion engine and its method of operation Jun 3, 2003
- 7021272 Computer controlled multi-stroke cycle power generating assembly and method of operation Apr 4, 2006
- ↑ "Inside Bruce Crower’s Six-Stroke Engine". www.autoweek.com. 2006-12-26. http://www.autoweek.com/article/20060227/FREE/302270007. Retrieved 2010-05-20.
- ↑ "American Griffin Engine". Smokstak.com. Nov 2007. http://www.smokstak.com/forum/showthread.php?t=42526. , linked photos and period diagrams
- ↑ "Griffin Engineering Company of Bath". http://staff.bath.ac.uk/ensegb/griffin2.htm.
- ↑ Knight, Patrick. A to Z of British Stationary Engines. p. 83.
- ↑ "Only surviving Griffin engine returns home to Bath museum". April 15, 2007. http://www.culture24.org.uk/places+to+go/north+west/manchester/art45987.
- ↑ Yuen, W. W.; et al.. ""The Bajulaz Cycle: a Two-Chamber Internal Combustion Engine with Increased Thermal Efficiency"". SAE Technical Paper Series (Feb., 1986,): 1–10. No. 860534.
- ↑ 7.0 7.1 7.2 The StatesmanTemplate:Dead link
- ↑ 
- ↑ "Method and apparatus for operating an internal combustion engine". Google.com. http://www.google.com/patents/about?id=nNiYAAAAEBAJ. Retrieved 2011-12-06.
- ↑ "Internal-Combustion Engine". Google.com. http://www.google.com/patents?id=GMRnAAAAEBAJ. Retrieved 2011-12-06.
- ↑ A new Engine generation is born Kottmann-Motor-Team Six-Stroke-Engine. Accessed January 2008.
- Bajulaz Six-Stroke Engine Accessed June 2007
- Bajulaz Animation Accessed June 2007
- Lyons, Pete (February 23, 2006). "Inside Bruce Crower’s Six-Stroke Engine". AutoWeek. http://www.autoweek.com/apps/pbcs.dll/article?AID=/20060227/FREE/302270007/1023/THISWEEKSISSUE. Retrieved 2007-06-22.