The Rotary (Wankel) Engine

Dr. Yehia Eldrainy

HistoryIdeas have existed since the 16th centuryGerman scientist Felix Wankel was the first to put the idea into a working designFunded by the German Aviation Ministry during WWIIGermany believed that the rotary engine would propel their industry into eventual greatnessInvented in 1957 Wankel perfected his design and sold the rights for the design to several car companiesMazda produced its first rotary power car in 1961 and created their Rotary Engine Division in 1963

HistoryPopularity for the rotary powered vehicles increased rapidly until the gas crisis in the mid 70sRotary engines were not very fuel efficient compared to piston enginesStrict emissions standards could not be met with current rotary technologyThese two factors severely hurt the sale and development of rotary enginesMazda was the only car company that continued to produce cars with rotary engines through the 90s

Automotive Success with RotariesIn 1991, the Mazda 787B won the 24-hour Le Mans endurance raceRotary engines were then banned from the C2 circuitThe RX-8 is able to produce 238 hp from its 1.3L engine and with good gas mileage and favorable emissions

The Parts of a Rotary EngineA rotary engine has an ignition system and a fuel-delivery system that are similar to the ones on piston engines. RotorThe rotor has three convex faces, each of which acts like a piston. Each face of the rotor has a pocket in it, which increases the displacement of the engine, allowing more space for air/fuel mixture.At the apex of each face is a metal blade that forms a seal to the outside of the combustion chamber. There are also metal rings on each side of the rotor that seal to the sides of the combustion chamber.The rotor has a set of internal gear teeth cut into the center of one side. These teeth mate with a gear that is fixed to the housing. This gear mating determines the path and direction the rotor takes through the housing.

The Parts of a Rotary EngineRotor.

In the center of each rotor is a large internalgearthat rides around a smaller gear that is fixed to the housing of the engine. This is what determines the orbit of the rotor. The rotor also rides on the large circular lobe on the output shaft.

The Parts of a Rotary EngineHousingThe housing is epitrochoid in shape. The shape of the combustion chamber is designed so that the three tips of the rotor will always stay in contact with the wall of the chamber, forming three sealed volumes of gas.Each part of the housing is dedicated to one part of the combustion process. The intake and exhaust ports are located in the housing. There are no valves in these ports. The exhaust port connects directly to the exhaust, and the intake port connects directly to the throttle.

The Parts of a Rotary EngineHousing.

The Parts of a Rotary EngineOutput ShaftThe output shaft has round lobes mounted eccentrically, meaning that they are offset from the centerline of the shaft. Each rotor fits over one of these lobes. The lobe acts sort of like the crankshaft in a piston engine. As the rotor follows its path around the housing, it pushes on the lobes. Since the lobes are mounted eccentric to the output shaft, the force that the rotor applies to the lobes creates torque in the shaft, causing it to spin..

The Parts of a Rotary Engine

The Parts of a Rotary EngineRotor

Housing

Side Housing

Eccentric Shaft

Stationary Gear

Operation of the engine

Operation of the engineWankels original design had an outer-toothed gear with 20 teeth, while the larger inner-toothed gear had 30 teeth.Due to this gear ratio, the rate of turning speed between the rotor and the shaft is defined as 1:3. This means that while the smaller gear does a single revolution, the larger inner-toothed gear rotates three times. Because theeccentric shaft, which is analogous to a crankshaft in a piston engine, is connected to the smaller toothed-gear it means that with the engine running at 3,000 rpm, the rotor will run at only 1,000 rpm. This not only means the rotary combustion engine runs smoother, but it also allows achieving a higher redline.

Rotary Engine IgnitionSince the working chambers of each rotor fire in the same geographic location in the engine, only one set of spark plugs are needed per rotor housing.

Due to the complexities of combusting a long chamber, two spark plugs are used in each housing.

The lower one is called the "leading" spark plug, while the top one is called the "trailing" spark plug.

As the working chamber approaches Top Dead Center (TDC), the leading plug fires first, starting the ignition of the air-fuel mixture and contributing most to the generation of power.

The trailing plug typically fires 10 to 15 degrees later and effectively completes the combustion of the remaining air-fuel mixture.

Engine Seals

Lubrication System

Cooling System

Engine displacement

P-V diagram

IntakeBegins when apex passes intake portIncrease in chamber volumeCreates low pressure zonePulls in Fuel/Air mixtureCompletes when next apex passes intake port

CompressionBegins after intakeVolume of chamber decreasesFuel/Air mixture compressedChamber compresses to its minimum size

CombustionSpark plugs ignite mixtureTwo spark plugs to maximize amount of fuel ignitedCauses rapid chamber expansionTurns rotor which produces work output on shaftPower stroke continues until apex passes exhaust port.

ExhaustChamber decreases in sizeForces combustion bi-products out the exhaust portContinues until next apex passes exhaust port.Entire cycle repeats

The CycleRotor mounted eccentrically on shaftOne rotation of rotor provides three rotations of shaftSpark plugs fire 3 times per rotor revolutionOne rotation of shaft for each firing of spark plugs

Wankel or Rotary engineNo Piston.Rotor that spins in an oval chamber (shaped like a flat during burning fuel).Rotor has three lobes.Rotor rotates in an eccentric pattern. The lobes remain in contact with the oval housing, creating a tight seal.

AdvantagesVibrationNo unbalanced reciprocating massesPower/WeightFor similar displacements, rotaries are generally 30% lighter and produce twice as much powerSimplicityContain half as many moving partsHave no connecting rods, crankshaft, or valve trainsNo Seizure at high temperature.Smooth power flow and high RPM.

DisadvantagesFuel Efficiency and EmissionThe shape of the combustion chamber, which is long instead of small and concentrated, makes the combustion travel longer than a piston engineDue to the longer combustion chamber, the amount of unburned fuel (HC) is higher which is released into the environmentSealing, leaks from the apex seals greatly reduces efficiency of the engineCostThe lack of infrastructure and development for the rotary engine has caused their production and maintenance costs generally to be moreUnsuitability for use as a diesel engine.high local temperatures and unequal thermal expansion.

ApplicationsMazda's first Wankel engined car was the 1967 Mazda Cosmo

Mazda used the Wankel in their RX-7 sports car until August of 2002In 2003, Mazda introduced the RENESIS engine with the new RX-8

The Soviet aircraft engine design bureau, is known to have produced Wankel engines for aircraft and helicopters

The People's Republic of China is also known to have experimented with Wankel engines

Rotor Vs Piston13B 1.3 Liters (146hp @ 6500rpm and 138ftlbf )KA24DE 2.4 Liters (155 hp @ 5600 rpm and 160 ftlbf)Mazda Rx-7 (1989-1992)Nissan 240sx (1991-1994)

Mazda RxMazda has been a pioneer in developing production cars that use rotary engines. The RX-7, which went on sale in 1978, was probably the most successful rotary-engine-powered car. But it was preceded by a series of rotary-engine cars, trucks and even buses, starting with the1967 Cosmo Sport. The last year the RX-7 was sold in the United States was 1995, but the rotary engine is set to make a comeback in the near future.The Mazda RX-8 , a new car from Mazda, has a new, award winning rotary engine called theRENESIS. Named International Engine of the Year 2003, this naturally aspirated two-rotor engine will produce about 250horsepower.

Mazda Rx-8 (2011)

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