A six-stroke engine is one of several 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 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.
Engine types
Single-piston designs These designs use a single piston per cylinder, like a conventional two- or four-stroke engine. A secondary, nondetonating 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.
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"., linked photos and period diagrams 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 did not 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 , 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 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.
Dyer six-stroke engine Leonard Dyer invented a six-stroke, internal combustion, water-injection engine in 1915, very similar to Crower's design (see below). 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.
Bajulaz six-stroke engine The Bajulaz six-stroke engine is similar to a regular combustion engine in design, but modifications were made 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 (constant-volume) 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, multiple-fuel usage capability, and a dramatic reduction in pollution.
The Bajulaz six-stroke engine was invented in 1989 by Roger Bajulaz of the Bajulaz S.A. company, based in Geneva, Switzerland; it has and .
The Bajulaz six-stroke engine features claimed are: 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, fresh air is injected into the cylinder during the exhaust stroke, 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 power-to-weight ratio is slightly less than that of a four-stroke gasoline engine. The engine can run on a variety of fuels, ranging from gasoline and diesel fuel 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. The engine was developed in 2005 by a team of mechanical engineering students, U Krishnaraj, Boby Sebastian, Arun Nair, and Aaron Joseph George of the College of Engineering, Trivandrum.
NIYKADO six-stroke engine This engine was developed by Chanayil Cleetus Anil, of Cochin, India, who was granted an Indian patent for the design in 2012. The patent listing gives a filing date of 4 April 2005, a grant date of 25 May 2012, and records the patent's legal status as term expired. The name of the engine is taken from the name of his company, NIYKADO Motors. The engine underwent a preliminary round of full-throttle tests at the Automotive Research Association of India, Pune. The inventor claims this engine "is 23% more fuel efficient compared to a conventional four-stroke engine" and it is "very low on pollution".
Anil, a mechanic, developed the NIYKADO engine over the course of more than 15 years. The engine was first tested in 2004 and Anil applied for his patent in 2005. He claims that his design produces drastically less pollution and that use in the automotive industry could lead to "emission-less mobility". The patent description states that the engine specification includes both a two-valve system and a four-valve system; in the two-valve system, a buffer stroke and a retake stroke are added between the power and exhaust strokes.
Niykado Technologies Private Limited was later incorporated in Kerala in 2022. Kerala Startup Mission lists the company as a certified startup with Anil Cleetus Chanayil and Binesh E B as founders, and describes later products or applications including the NIYKADO Hybrid Six Stroke Engine, NIYKADO Power Generator and NIYKADO Pumps. The same profile claims that the six-stroke engine can reduce emissions to "very low or nearly negligible" levels and has "50 percent more fuel economy" compared to existing internal-combustion engine technology.
Engine functionality The patent specification describes two versions of the NIYKADO six-stroke engine: a two-valve system and a four-valve system. In the two-valve system, the additional strokes are described as a buffer stroke and a retake stroke between the power and exhaust strokes. In the four-valve system, the two additional strokes are an air intake stroke and an air final exhaust stroke, added after the normal exhaust stroke.
In the four-valve version, the six strokes are:
Intake stroke Compression stroke Power stroke Exhaust stroke Air intake stroke Air final exhaust stroke
The four-valve version uses four valves:
Air-fuel intake valve Air-only intake valve Combustion exhaust valve Air-only exhaust valve
Intake stroke The piston moves from top dead center (TDC) to bottom dead center (BDC). The intake valve opens and the air-fuel mixture enters the cylinder.
Compression stroke The piston moves from BDC to TDC, with the valves closed, compressing the air-fuel mixture.
Power stroke The spark plug ignites the compressed air-fuel mixture. The piston is forced from TDC to BDC, and the piston motion is converted into crankshaft rotation.
Exhaust stroke The piston moves from BDC to TDC while the combustion exhaust valve opens, forcing burned gases out of the cylinder.
Air intake stroke The air-only intake valve opens while the piston moves from TDC to BDC, drawing air through a separate air inlet port into the cylinder. The patent specification states that this increases the oxygen content in the cylinder and improves combustion in the next cycle. Overdrive described the prototype as using this extra stroke to introduce cold air into the combustion chamber after the exhaust stroke.
Air final exhaust stroke The air-only exhaust valve opens while the piston moves from BDC to TDC, forcing air out of the cylinder and completing the six-stroke cycle. Overdrive described the extra exhaust stroke as using the introduced air to clean leftover exhaust or unburnt gases from the cylinder before the next combustion cycle.
Overdrive reported that testing at the Automotive Research Association of India showed a 40 percent increase in economy and reduced NOx emissions, but also a 30 percent reduction in power. The article attributed the power reduction to the engine having only one power stroke in each six-stroke cycle.
Crower six-stroke engine In a six-stroke engine prototyped in the United States 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 requires an air or water cooling system to discharge in most engines 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 rotational speed. 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 the 75-year-old American inventor, who has applied for a patent on his design. That patent application was subsequently abandoned.
Porsche six-stroke engine
Opposed-piston designs These designs use two pistons per cylinder operating at different rates, with combustion occurring between the pistons.
Beare head