02-11-2012, 05:37 PM
A four-stroke engine
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A four-stroke engine, also known as four-cycle, is an internal combustion engine in which the piston completes four separate strokes—intake, compression, power, and exhaust—during two separate revolutions of the engine's crankshaft, and one single thermodynamic cycle.
There are two common types of four-stroke engines. They are closely related to each other, but have major differences in design and behavior. The earliest of these to be developed is the Otto cycle engine developed in 1876 by Nikolaus August Otto in Cologne, Germany,[1] after the operation principle described by Alphonse Beau de Rochas in 1861. This engine is most often referred to as a petrol engine or gasoline engine, after the fuel that powers it.[2] The second type of four-stroke engine is the Diesel engine developed in 1893 by Rudolph Diesel, also of Germany. Diesel created his engine to maximize efficiency, which the Otto engine lacked. There are several major differences between the Otto cycle engine and the four-cycle diesel engine. The diesel engine is made in both a two-cycle and a four-cycle version. Otto's company, Deutz AG, now primarily produces diesel engines.
The Otto cycle is named after the 1876 engine of Nikolaus A. Otto, who built a successful four-cycle engine based on the work of Jean Joseph Etienne Lenoir.[1] It was the third engine type that Otto developed. It used a sliding flame gateway for ignition of its fuel—a mixture of illuminating gas and air. After 1884, Otto also developed the magneto to create an electrical spark for ignition, which had been unreliable on the Lenoir engine.
Today, the internal combustion engine (ICE) is used in motorcycles, automobiles, boats, trucks, aircraft, ships, heavy duty machinery, and in its original intended use as stationary power both for kinetic and electrical power generation. Diesel engines are found in virtually all heavy duty applications such as trucks, ships, locomotives, power generation, and stationary power. Many of these diesel engine are two-cycle with power ratings up to 105,000 hp (78,000 kW).
The four cycles refer to intake, compression, combustion (power), and exhaust cycles that occur during two crankshaft rotations per power cycle of the four-cycle engines. The cycle begins at Top Dead Centre (TDC), when the piston is farthest away from the axis of the crankshaft. A cycle refers to the full travel of the piston from Top Dead Centre (TDC) to Bottom Dead Centre (BDC). (See Dead centre.)
1. INTAKE stroke: on the intake or induction stroke of the piston, the piston descends from the top of the cylinder to the bottom of the cylinder, reducing the pressure inside the cylinder. A mixture of fuel and air, or just air in a diesel engine, is forced by atmospheric (or greater) pressure into the cylinder through the intake port. The intake valve(s) then close. The volume of air/fuel mixture that is drawn into the cylinder, relative to the volume of the cylinder is called, the volumetric efficiency of the engine.
2. COMPRESSION stroke: with both intake and exhaust valves closed, the piston returns to the top of the cylinder compressing the air, or fuel-air mixture into the combustion chamber of the cylinder head.
3. POWER stroke: this is the start of the second revolution of the engine. While the piston is close to Top Dead Center, the compressed air–fuel mixture in a gasoline engine is ignited, usually by a spark plug, or fuel is injected into the diesel engine, which ignites due to the heat generated in the air during the compression stroke. The resulting massive pressure from the combustion of the compressed fuel-air mixture forces the piston back down toward bottom dead centre.
4. EXHAUST stroke: during the exhaust stroke, the piston once again returns to top dead center while the exhaust valve is open. This action evacuates the burnt products of combustion from the cylinder by expelling the spent fuel-air mixture out through the exhaust valve(s).
5. 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:
6. 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.[1] 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.
7. 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.
Types of engine 6 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".[2] 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.[3][4]
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.[5]
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.[6]