Wankel engine

First Wankel Engine NSU KKM 57P Autovision und Forum, Germany

Wankel Engine in Deutsches Museum Munich, Germany

The Wankel rotary engine is a type of internal combustion engine, invented by German engineer Felix Wankel, which uses a rotor instead of reciprocating pistons. This design promises smooth high-rpm power from a compact, lightweight engine; however Wankel engines are criticized for poor fuel efficiency and exhaust emissions.

Naming

Since its introduction in the NSU Motorenwerke AG (NSU) and Mazda cars of the 1960s, the engine has been commonly referred to as the rotary engine, a name which has also been applied to several completely different engine designs.

Although many manufacturers licensed the design, and Mercedes-Benz used it for their C111 concept car, only Mazda has produced Wankel engines in large numbers. As of 2005, the engine is only available in the Mazda RX-8.

How it works

The Wankel cycle. The "A" marks one of the three apexes of the rotor. The "B" marks the eccentric shaft, turning three times for every revolution of the rotor.

In the Wankel engine, the four strokes of a typical Otto cycle engine are arranged sequentially around an oval, unlike the reciprocating motion of a piston engine. In the basic single rotor Wankel engine, a single oval (technically an epitrochoid) housing surrounds a three-sided rotor (similar to a Reuleaux triangle, but with the middle of each side a bit more flattened) which turns and moves within the housing. The sides of the rotor seal against the sides of the housing, and the corners of the rotor seal against the inner periphery of the housing, dividing it into three combustion chambers.

As the rotor turns, its motion and the shape of the housing cause each side of the rotor to get closer and farther from the wall of the housing, compressing and expanding the combustion chamber similarly to the "strokes" in a reciprocating engine. However, whereas a normal four stroke cycle engine produces one combustion stroke per cylinder for every two revolutions (that is, one half power stroke per revolution per cylinder) each combustion chamber of each rotor in the Wankel generates one combustion 'stroke' per revolution (that is, three power strokes per rotor revolution). Since the Wankel output shaft is geared to spin at three times the rotor speed, this becomes one combustion 'stroke' per output shaft revolution per rotor, twice as many as the four-stroke piston engine, and similar to the output of a two stroke cycle engine. 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. This design also allows the Wankel engine to have a much higher redline as there is less friction working against the internals of the engine.

National agencies which 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 regulatory agencies view it as offering so pronounced an advantage that they ban it altogether.

Advantages

Wankel engines have several major advantages over reciprocating piston designs, in addition to having higher output for similar displacement and physical size. Wankel engines are considerably simpler and contain far fewer moving parts. For instance, because valving is accomplished by simple ports cut into the walls of the rotor housing, they have no valves or complex valve trains; in addition, since the rotor is geared directly to the output shaft, there is no need for connecting rods, a conventional crankshaft, crankshaft balance weights, etc. The elimination of these parts not only makes a Wankel engine much lighter (typically half that of a conventional engine with equivalent power), but it also completely eliminates the reciprocating mass of a piston engine with its internal strain and inherent vibration due to repetitious acceleration and deceleration, producing not only a smoother flow of power but also the ability to produce more power by running at higher rpm.

In addition to the enhanced reliability due to the elimination of this reciprocating strain on internal parts, the construction of the engine, with an iron rotor within a housing made of aluminum which has greater thermal expansion, ensures that even if severely overheated the Wankel engine can not seize, as an overheated piston engine is likely to do; this is a substantial safety benefit in aircraft use.

A further advantage of the Wankel engine for use in aircraft, is the fact a Wankel engine can have a smaller frontal area than a piston engine of equivalent power.

The simplicity of design and smaller size of the Wankel engine also allow for a savings in construction costs, compared to piston engines of comparable power output.

As another advantage, the shape of the Wankel combustion chamber and the turbulence induced by the moving rotor prevent localized hot spots from forming, thereby allowing the use of fuel of very low octane number without preignition or detonation, a particular advantage for Hydrogen cars. Mazda has recently placed a hydrogen-burning rotary engine in one version of its RX-8 sports car. This feature also led to a great deal of interest in the Soviet Union, where high octane gasoline was rare.

Disadvantages

The design of the Wankel engine requires numerous sliding seals and a housing that is typically built as a sandwich of cast iron and aluminum pieces that expand and contract by different degrees when exposed to heating and cooling cycles in use. These elements led to a very high incidence of loss of sealing, both between the rotor and the housing and also between the various pieces making up the housing. Further engineering work by Mazda brought these problems under control, but the company was then confronted with a sudden global concern over both hydrocarbon emission and a rise in the cost of gasoline, the two most serious drawbacks of the Wankel engine.

Just as the shape of the Wankel combustion chamber prevents preignition, it also leads to incomplete combustion of the air-fuel charge, with the remaining unburned hydrocarbons released into the exhaust. At first, while manufacturers of piston-engine cars were turning to expensive catalytic converters to completely oxidize the unburned hydrocarbons, Mazda was able to avoid this cost by enriching the air/fuel mixture enough to produce an exhaust stream which was rich enough in hydrocarbons to actually support complete combustion in a 'thermal reactor' (just an enlarged open chamber in the exhaust manifold) without the need for a catalytic converter, thereby producing a clean exhaust at the cost of some extra fuel consumption.

Unfortunately for Mazda, their switch to this solution was immediately followed by a sharp rise in the cost of gasoline worldwide, so that not only the added fuel cost of their 'thermal reactor' design, but even the basically lower fuel economy of the Wankel engine caused sales to drop alarmingly.

A related cause for unexpectedly poor fuel economy involves an inherent weakness of the Wankel rotor design when used with conventional fuels. Some studies have indicated that at high speeds, the rate at which the volume of the combustion chamber increases in the moments after ignition actually outpaces the expansion of the burning fuel. The result is that, at high speeds, less useful energy is extracted from the same volume of fuel, as the exhaust has to expend time and energy "catching up" to the rotor before it can accomplish any work.

In engines having more than two rotors, or two rotor race engines intended for high-rpm use, a multi-piece eccentric shaft maybe used, allowing additional bearings between rotors. While this approach does increase the complexity of the eccentric shaft design, it has been used successfully in the Mazda's production three-rotor 20B-REW engine, as well as many low volume production race engines. (The C-111-2 4 Rotor Mercedes Benz eccentric shaft for the KE Serie 70, Typ DB M950 KE409 is made in one piece! Mercedes Benz used split bearings.)

Unlike a piston engine, where the cylinder is cooled by the incoming charge after being heated by combustion, Wankel rotor housings are constantly heated on one side and cooled at the other, leading to very high local temperatures and unequal thermal expansion. This places high demands on the materials used.

Many disadvantages of the Wankel engine have been solved in the Renesis engine of the RX-8. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber. This approach allowed Mazda to eliminate overlap between intake and exhaust port openings, while simultaneously increasing exhaust port area. Fuel consumption is now within normal limits while passing California State emissions requirements.

Proponents of Wankel engines would argue that the claims about the Wankel's emissions and fuel consumption problems are unfair. A rotary engine, it is said, cannot be compared to a piston engine of the same displacement. An average 1.3 liter piston engine might develop something like 60 to 70 bhp. A rotary engine of the same displacement can develop in excess of 230 bhp. This is over three times the power; a figure that might well be expected from an engine twice the size. Therefore, any comparison should be made between engines of similar power output, rather than engines of identical displacement.

History

Wankel first conceived his rotary engine in 1954 (DKM 54) and the KKM 57 (the Wankel rotary engine) in the year 1957. Considerable effort went into designing rotary engines in the 1950s and 1960s. They were of particular interest because they were smooth and very quiet running, and because of the reliability resulting from their simplicity.

In Britain, Norton Motorcycles developed a Wankel rotary engine for motorcycles, which was included in their Commander and F1; Suzuki also produced a production motorcycle with a Wankel engine, the RE-5. Arctic Cat produced snowmobiles powered by 303cc Wankel rotary engines in 1971 and 1972. John Deere Inc, in the US, had a major research effort in rotary engines and designed a version which was capable of using a variety of fuels without changing the engine. The design was proposed as the power source for several US Marine combat vehicles in the late 1980s.

After occasional use in automobiles, for instance by NSU with their Ro 80 model, Citroën with the M35 and GS Birotor using engines produced by Comotor, and abortive attempts by General Motors and Mercedes Benz to design Wankel-engine automobiles, the most extensive automotive use of the Wankel engine has been by the Japanese company Mazda.

NSU Wankel Spider the first serien car with a wankel

3-Rotor Eunos Cosmo engine

After years of development, Mazda's first Wankel engined car was the 1967 Mazda Cosmo. The company followed with a number of Wankel ("rotary" in the company's terminology) vehicles, including a bus and a pickup truck. Customers generally loved them, notably the smoothness. However they had the very bad luck of being released in the middle of efforts to decrease emissions and increase fuel economy. Mazda later abandoned the Wankel in most of their automotive designs, but continued using it in their RX-7 sports car until August of 2002 (although RX-7 importation for North America ceased with the 1995 model year). The company normally used two-rotor designs, but received considerable attention with their 1991 Eunos Cosmo, which used a twin-turbo three-rotor engine. In 2003, Mazda introduced the RENESIS engine with the new RX-8. The RENESIS engine relocated the ports for exhaust and intake from the periphery of the rotary housing to the sides, allowing for larger overall ports, better airflow, and further power gains. The RENESIS is capable of delivering 238 hp from its minute 1.3 L displacement at better fuel economy, reliability, and environmental friendliness than any other Mazda rotary engine in history.

The Malibu Grand Prix chain, similar in concept to commercial recreational kart racing tracks, operates several venues in the United States where a customer can purchase several laps around a track in a vehicle very similar to open wheel racing vehicles, but powered by a small Curtiss-Wright rotary engine.

Although VAZ, the Soviet automobile manufacturer, is known to have produced Wankel-engine automobiles, and Aviadvigatel, the Soviet aircraft engine design bureau, is known to have produced Wankel engines for aircraft and helicopters, little specific information has surfaced in the outside world; what has been seen indicates a general similarity to Wankel designs by NSU, Comotor, and Mazda, therefore it is likely that many Western patents were infringed upon by these designs, the probable reason for their being hidden.

The People's Republic of China is also known to have experimented with Wankel engines, but even less is known in the West about the work done there, other than one paper, #880628, delivered to the SAE in 1988 by Chen Teluan of the South China Institute of Technology at Guangzhou.

Automobile racing

In the racing world, Mazda has had substantial success with two-rotor, three-rotor, and four-rotor cars, and private racers have also had considerable success with stock and modified Mazda Wankel-engine cars.

The Sigma MC74 powered by a Mazda 12A engine was the first engine and team from outside Western Europe or the United States to finish the entire 24 hours of the 24 Hours of Le Mans race, in 1974. Mazda is the only team from outside Western Europe or the United States to have won Le Mans outright and the only non-piston engine ever to win Le Mans, which the company accomplished in 1991 with their four-rotor 787B (2622 cc actual displacement, rated by FIA formula at 4708 cc). The following year, rules were changed at Le Mans which made the Mazda 787 ineligible to race. Mazda is also the most reliable finisher at Le Mans (with the exception of Honda, who have entered only three cars in only one year), with 67% of entries finishing.

The Mazda RX-7 has won more IMSA races in its class than any other model of automobile, with its one hundredth victory on September 2, 1990. Following that, the RX-7 won its class in the IMSA 24 hours of Daytona race ten years in a row, starting in 1982. The RX7 won the IMSA Grand Touring Under Two Liter (GTU) championship each year from 1980 through 1987, inclusive.

Formula Mazda Racing features open-wheel race cars with Mazda Wankel engines, adaptable to both oval tracks and road courses, on several levels of competition. Since 1991, the professionally organized Star Mazda Series has been the most popular format for sponsors, spectators, and upward bound drivers. The engines are all built by one engine builder, certified to produce the prescribed power, and sealed to discourage tampering. They are in a relatively mild state of racing tune, so that they are extremely reliable and can go years between motor rebuilds.[1]

Aircraft engines

The Wankel's superb power-to-weight ratio, reliability, and small frontal area make it particularly well suited to aircraft engine use. There was intense interest in them in this role in the 1950s when the design was first becoming well known, but it was at this same time that almost the entire industry was moving to the jet engine, which many believed would be the only engine in use within a decade. The Wankel suffered from a lack of interest, and when it later became clear that the jet engine was far too expensive for all roles, the general aviation world had already shrunk so much that there was little money for new engine designs. Nevertheless, interest in them for small aircraft has continued.

The first rotary-engine aircraft was the experimental Lockheed Q-Star civilian version of the U.S. Army's reconnaissance QT-2, basically a powered Schweizer sailplane, in 1968 or 1969. It was powered by a 185 hp (138 kW) Curtiss-Wright RC2-60 Wankel rotary engine.

Aircraft Wankels have made something of a comeback in recent years. None of their advantages have been lost in comparison to other engines, and the introduction of better materials has helped the tip-seal (Apex-seal) problem. They are being found increasingly in roles where their compact size and quiet running is important, notably in drones, or UAVs. Many companies and hobbyists adapt Mazda rotary engines to aircraft use; others, including Wankel GmbH itself, manufacture Wankel rotary engines dedicated for the purpose.

Since Wankels are high revving and have little torque compared to a similarly powered conventional aircraft piston engine, which are low revving and have a high torque, they are less suited to directly driving conventional propellers, and more to driving ducted fans. The Moller M400 skycar has harnessed the power of the ducted fan and uses 2 redundant Wankel engines to drive each fan. Despite this, there are many experimental aircraft flying with Wankel engines via the use of a PSRU ("Propeller Speed Reduction Unit") which allows the high revving rotary to drive a slower un-ducted propeller.

Other uses

Small Wankel engines are being found increasingly in other roles, such as go-karts, personal water craft and auxiliary power units for aircraft. The Graupner/O.S. 49-PI is a 1.27 hp (947 W) 5 cc Wankel engine for model airplane use which has been in production essentially unchanged since 1970; even with a large muffler, the entire package weighs only 13.4 ounces (380 grams).

The simplicity of the Wankel makes it ideal for mini, micro, and micro-mini engine designs. The MicroElectroMechanical Systems (MEMS) Rotary Engine Lab at the University of California, Berkeley has been developing Wankel engines of down to 1 mm in diameter with displacements less than 0.1 cc. Materials include silicon and motive power includes compressed air. The goal is to eventually develop an internal combustion engine that will deliver 100 milliwatts of electrical power; the engine itself will serve as the rotor of the generator, with magnets built into the engine rotor itself.

The largest Wankel engine was built by Ingersoll-Rand; available in 550 hp (410 kW) one rotor and 1100 hp (820 kW) two rotor versions, displacing 41 liters per rotor with a rotor approximately one meter in diameter, it was available between 1975 and 1985. It was derived from a previous, unsuccessful, Curtiss-Wright design, which failed because of a well-known problem with all internal combustion engines; the fixed speed at which the flame front travels limits the distance combustion can travel from the point of ignition in a given time, and thereby the maximum size of the cylinder or rotor chamber which can be used. This problem was solved by limiting the engine speed to only 1200 rpm and use of natural gas as fuel; this was particularly well chosen, as one of the major uses of the engine was to drive pumps on natural gas pipelines.

From 1974 to 1977 Hercules produced a limited number of motorcycles powered by Wankel engines. The tooling was later used by Norton to produce the Norton Commander model in the early 1980s.

Aside from being used for internal combustion engines, the basic Wankel design has also been utilized for air compressors, and superchargers for internal combustion engines, but in these cases, although the design still offers advantages in reliability, the basic advantages of the Wankel in size and weight over the four-stroke internal combustion engine are irrelevant. In a design using a Wankel supercharger on a Wankel engine, the supercharger is twice the size of the engine!

Perhaps the most exotic use of the Wankel design is in the seat belt pretensioner system of the Volkswagen New Beetle. In this car, when deceleration sensors sense a potential crash, small explosive cartridges are triggered electrically and the resulting pressurized gas feeds into tiny Wankel engines which rotate to take up the slack in the seat belt systems, anchoring the driver and passengers firmly in the seat before any collision.

Trivia

Wankel Rotary Engine is a phrase included in a Monty Python audio sketch, titled "Are You Embarrassed Easily?" on Monty Python's Previous Record (1972). It is here considered to sound rude due to the word "Wankel," which sounds dirty but is actually a name of a person.

Wankel engine: Encyclopedia BETA

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