|Subject: Rotary engine Sat Oct 17, 2009 9:30 am|| |
The [i][b]rotary engine[/b][/i] was an early type of internal-combustion engine, usually designed with an odd number of cylinders per row in a radial configuration, in which the crankshaft remained stationary and the entire cylinder block rotated around it. The design was used mostly in the years shortly before and during World War I to power aircraft, and also saw use in a few early motorcycles and cars.
By the early 1920s the rotary aircraft engine was becoming obsolete, mainly because of an upper ceiling to its possible output torque, which was a fundamental consequence of the way the engine worked. It was also limited by its inherent restriction on breathing capacity due to the need for the fuel/air mixture to be aspirated through the hollow crankshaft and crankcase, which directly affected its volumetric efficiency. However, at the time it was a very efficient solution to the problems of power output, weight, and reliability
|Subject: Re: Rotary engine Sat Oct 17, 2009 9:32 am|| |
A rotary engine is essentially a standard Otto cycle engine, but instead of having a fixed cylinder block with rotating crankshaft as with a conventional radial engine, the crankshaft remains stationary and the entire cylinder block rotates around it. In the most common form, the crankshaft was fixed solidly to an aircraft frame, and the propeller simply bolted onto the front of the crankcase.
Three key factors contributed to the rotary engines success at the time: -
-[b]Smooth running[/b]: Rotaries delivered power very smoothly because (relative to the engine mounting point) there are no reciprocating parts, and the relatively large rotating mass of the cylinders acted as a flywheel.
-[b]Weight advantage[/b]: many conventional engines had to have heavy flywheels added to smooth out power impulses and reduce vibration. Rotary engines gained a substantial power-to-weight ratio advantage by obviating the need for an added flywheel.
-[b]Improved cooling[/b]: when the engine was running the rotating cylinder block created its own fast-moving cooling airflow, even with the aircraft at rest.
Most rotary engines were arranged with the cylinders pointing outwards from a single crankshaft, in the same general form as a radial, but there were also rotary boxer engines and even one-cylinder rotaries.
Like radial engines, rotaries were generally built with an odd number of cylinders, so that a consistent every-other-piston firing order could be maintained, to provide smooth running. Rotary engines with an even number of cylinders were mostly of the "two row" type.
|Subject: Re: Rotary engine Sat Oct 17, 2009 9:33 am|| |
[b]Rotary engine control[/b]
It is often asserted that rotary engines had no carburettor and hence power could only be reduced by intermittently cutting the ignition using a "blip" switch, which grounded the magneto when pressed, shutting off power to the spark plugs and stopping ignition. However, rotaries did have a simple carburettor which combined a gasoline jet and a flap valve for throtting the air supply. Unlike modern carburettors, it could not keep the fuel/air ratio constant over a range of throttle openings; in use, a pilot would set the throttle to the desired setting (usually full open) then adjust the fuel/air mixture to suit using a separate "fine adjustment" lever that controlled the fuel valve.
Due to the rotary engine's large inertia, it was possible to adjust the appropriate fuel/air mixture by trial and error without stalling it. After starting the engine with a known setting that allowed it to idle, the air valve was opened until maximum engine speed was obtained. Since the reverse process was more difficult, "throttling", especially when landing, was often accomplished by temporarily cutting the ignition using the blip switch.
By the middle stages of World War I some throttling capability was found necessary to allow pilots to fly in formation, and the improved carburettors which entered use allowed a power reduction of up to 25%. The pilot would close off the air valve to the required position, then re-adjust the fuel/air mixture to suit. Experienced pilots would gently back off the fuel lever at frequent intervals to make sure that the mixture was not too rich: a too-lean mixture was preferable, since power recovery would be instant when the fuel supply was increased, whereas a too-rich mixture could take up to 7 seconds to recover and could also cause fouling of spark plugs and the cylinders to cut out.
The Gnôme Monosoupape was an exception to this, since most of its air supply was taken in through the exhaust valve, and so could not be controlled via the crankcase intake. Monosoupapes therefore had a single petrol regulating control used for a limited degree of speed regulation. Early models also featured variable valve timing to give greater control, but this caused the valves to burn and therefore it was abandoned.
Later rotaries still used blipping the ignition for landing, and some engines were equipped with a switch that cut out only some rather than all of the cylinders to ensure that the engine kept running and did not oil up. A few 9 cylinder rotaries had this capability, typically allowing 1, 3, or 6 cylinders to be kept running. Some 9 cylinder Monosoupapes had a selector switch which allowed the pilot to cut out six cylinders so that each cylinder fired only once per three engine revolutions but the engine remained in perfect balance. Some documentation regarding the Fokker Eindecker shows a rotary selector switch to cut out a selected number of cylinders suggesting that German rotaries did as well.
By 1918 a Clerget handbook advised that all necessary control was to be effected using the throttle, and the engine was to be stopped and started by turning the fuel on and off. Pilots were advised to avoid use of the cut out switch as it would eventually damage the engine.
The blip switch is, however, still recommended for use during landing rotary-engined aircraft in modern times as it allows pilots a more reliable, quick source of power that lends itself to modern airfields.
The landing procedure using a blip switch involved shutting off the fuel using the fuel lever, while leaving the blip switch on. The windmilling propeller allowed the engine to continue to spin without delivering any power as the aircraft descended. It was important to leave the blip switch on while the fuel was shut off to allow the spark plugs to continue to spark and keep them from oiling up, while the engine could easily be restarted simply by re-opening the fuel valve. If a pilot shut the engine off by holding the blip switch down without cutting off the fuel, fuel would continue to pass through the engine without combusting and raw fuel/air mix would collect in the cowling. This could cause a serious fire when the switch was released, or alternatively could cause the spark plugs to oil up and prevent the engine restarting
|Subject: Re: Rotary engine Sat Oct 17, 2009 9:38 am|| |
The Wankel engine is a type of internal combustion engine which uses a rotary design to convert pressure into a rotating motion instead of using reciprocating pistons. Its four-stroke cycle takes place in a space between the inside of an oval-like epitrochoid-shaped housing and a rotor that is similar in shape to a Reuleaux triangle. This design delivers smooth high-rpm power, from a compact size. Since its introduction the engine has been commonly referred to as the rotary engine, though this name is also applied to several completely different designs.
The engine was invented by German engineer Felix Wankel. He began its development in the early 1950s at NSU Motorenwerke AG (NSU) before completing a working, running prototype in 1957. NSU then licensed the concept to companies around the world, who have continued to improve the design.
Because of their compact design, Wankel rotary engines have been installed in a variety of vehicles and devices such as automobiles including racing cars, along with aircraft, go-karts, personal water craft, chain saws, and auxiliary power units. The most extensive automotive use of the Wankel engine has been by the Japanese company Mazda.
|Subject: Re: Rotary engine Sat Oct 17, 2009 9:48 am|| |
In the Wankel engine, the four strokes of a typical Otto cycle occur in the space between a three-sided symmetric rotor and the inside of a housing. In the basic single-rotor Wankel engine, the oval-like epitrochoid-shaped housing surrounds a rotor which is triangular with bow-shaped flanks (often confused with a Reuleaux triangle), a three-pointed curve of constant width, but with the bulge in the middle of each side a bit more flattened. From a theoretical perspective, the chosen shape of the rotor between the fixed apexes is basically the result of a minimization of the volume of the geometric combustion chamber and a maximization of the compression ratio, respectively. Thus, the symmetric curve connecting two arbitrary apexes of the rotor is maximized in the direction of the inner housing shape with the constraint not to touch the housing at any angle of rotation (an arc is not a solution of this optimization problem).
The central drive shaft, called the eccentric shaft or E-shaft, passes through the center of the rotor and is supported by fixed bearings. The rotors ride on eccentrics (analogous to cranks) integral with the eccentric shaft (analogous to a crankshaft). The rotors both rotate around the eccentrics and make orbital revolutions around the eccentric shaft. Seals at the corners of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers. The turning of each rotor is caused and controlled by a pair of synchronizing gears. A fixed gear mounted on one side of the rotor housing engages a ring gear attached to the rotor and ensures the rotor moves exactly 1/3 turn for each turn of the eccentric shaft. The power output of the engine is not transmitted through the synchronizing gears. The force of gas pressure on the rotor is (to a first approximation) directed to the center of the eccentric, on the output shaft.
The best way to visualize the action of the engine in the animation at left is to look not at the rotor itself, but the cavity created between it and the housing. The Wankel engine is actually a variable-volume progressing-cavity system. Thus there are 3 cavities per housing, all repeating the same cycle. Note as well that points A and B on the rotor and e-shaft turn at different speed, point B moves 3 times faster than point A, so that one full orbit of the rotor equates to 3 turns of the e-shaft.
As the rotor rotates and orbitally revolves, each side of the rotor gets closer and farther from the wall of the housing, compressing and expanding the combustion chamber similarly to the strokes of a piston in a reciprocating engine. The power vector of the combustion stage goes through the center of the offset lobe.
While a four-stroke piston engine makes one combustion stroke per cylinder for every two rotations of the crankshaft (that is, one half power stroke per crankshaft rotation per cylinder), each combustion chamber in the Wankel generates one combustion stroke per each driveshaft rotation, i.e. one power stroke per rotor orbital revolution and three power strokes per rotor rotation. Thus, power output of a Wankel engine is generally higher than that of a four-stroke piston engine of similar engine displacement in a similar state of tune; and higher than that of a four-stroke piston engine of similar physical dimensions and weight.
Wankel engines also generally have a much higher redline than a reciprocating engine of similar power output, in part because the smoothness inherent in circular motion, but especially because they do not have highly stressed parts such as a crankshaft or connecting rods. Eccentric shafts do not have the stress-raising internal corners of crankshafts. The redline of a rotary engine is limited by wear of the synchronizing gears. Hardened steel gears are used for extended operation above 7000 or 8000 rpm. Mazda Wankel engines in auto racing are operated above 10,000 rpm. In aircraft they are used conservatively, up to 6500 or 7500 rpm. However, as gas pressure participates in seal efficiency, running a Wankel engine at high r.p.m. under no load conditions can result in the engine destruction.
National agencies that tax automobiles according to displacement and regulatory bodies in automobile racing variously consider the Wankel engine to be equivalent to a four-stroke engine of 1.5 to 2 times the displacement; some racing sanctioning bodies ban it altogether.
The Wankel cycle. The "A" marks one of the three apexes of the rotor. The "B" marks the eccentric shaft and the white portion is the lobe of the eccentric shaft. The shaft turns three times for each rotation of the rotor around the lobe and once for each orbital revolution around the eccentric shaft.
|Subject: Re: Rotary engine || |