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Wankel Rotary Engine 


Wankel was born in 1903 in Lahr in the Schwarzwald in Germany. 

He was employed in the sales department of a scientific publishing house in Heidelburg from 1921 to 1926. 

In 1924, he set up a workshop in Heidelburg where he made his first models of a rotary piston engine.  Realising that the principal problem with such designs was gas tightness, he spent considerable time attempting to resolve it.  By 1927, the problems were largely solved. 

During World War II, he worked for the German Luftfahrtministerium (Air Ministry). 

In 1951, Wankel and NSU signed a contract establishing a partnership to develop the rotary piston engine.

On 13 April 1954, the first Wankel rotary engine was built by NSU - Vier Takte in einer Maschine, das sind vier Erfindungen in einer einzigen (Four cycles in a single engine are no less than four inventions in one).

In 1956, an NSU prototype motorcycle won all the trials in its category and broke several world records on the Great Salt Lake in the USA - its engine was fed by a Wankel supercharger.

In 1958, NSU started tests on the Wankel engine.

In 1960 the Wankel engine was first discussed in public at the congress of the German Engineers’ Association.

In 1963, NSU launched the Wankel powered Spider at the Frankfurt Motor Show.

In 1964, the COMOBIL company was set up in Geneva - a joint venture between NSU and CitroŽn - to develop a rotary engined car.

In 1967, the COMOTOR company, another NSU/CitroŽn joint venture was set up in Luxembourg for the purpose of manufacturing and marketing the Wankel engine.  At the Frankfurt Motor Show, NSU unveiled the Ro80 which went on sale in 1968.

In 1969, COMOTOR purchased 850 000m3 of land in the Saar to build a factory where the Wankel engine would be produced.

In 1970, CitroŽn produced the M35 prototype

Single rotor Wankel
993 cm3 swept volume
6 CV fiscal rating
9:1 compression ratio
49 bhp @ 5 500 rpm
7 mkg (50,6 lb ft) torque @ 2 745 rpm
Water cooled

Hydropneumatic self levelling on all four wheels

Front discs, rear drums

Modified Ami 8 two door coupť
Wheelbase 2,4m,
Front track 1,26m
Rear track 1,22m
Length 4,05m
Width 1,554m
Height 1,35m
Weight 815kg

Max speed 144 kph
0 - 100 kph 19 seconds
9,68 l/100km

Between 1971 and 1972, 267 M35s were provided to selected clients under the supervision of CitroŽn who maintained the vehicles.  30 000 000 km were covered by these mobile test beds and the information gathered allowed the design to be improved - particularly with regard to gas sealing, the trochoid’s wall lining and cooling.

In parallel, a new twin rotor engine, based on the M35 unit, was developed and in 1972, the first such engines were delivered from the COMOTOR plant at Altforweiler in the Saar.

How the COMOTOR engine works
The engine operates on the four stroke principle, viz:
Stage 1. The air/petrol mixture moves in through the inlet port. 
Stage 2. The rotor then shuts off this port and compresses the mixture.
Stage 3  The fuel mixture is ignited by the spark plugs at the point of highest compression. The burning mixture expands causing the rotor to rotate thereby providing the motive power.
Stage 4  The rotor clears the exhaust port allowing evacuation of the exhaust gases.

The rotary piston, also known as the rotor is shaped like an equilateral triangle with curved side and moves within a stator or trochoid in a motion known as epitrochoid which allows for volume changes within the “combustion chamber”.  The gearing between the rotor and output shaft is such that for a rotor speed of 1 000 rpm, the output shaft revolves at 3 000 rpm.

The COMOTOR 624 engine was fitted in the short lived GZ Birotor
Number of rotors 2
Swept volume 1 990 cm3
11 CV fiscal rating
9:1 compression ratio
107 bhp @ 6 500rpm
14 mkg (101,3 lb.ft) @ 3 000 rpm 

Problems with gas tightness, exhaust emissions and high fuel consumption which also coincided with both the Middle East oil crisis and the Peugeot take over led to the abandonment of the Wankel project by CitroŽn.  The Ro80 soldiered on for a few more years and nowadays, only Mazda builds Wankel powered cars, having solved the rotor tip wear problems.  Mercedes Benz, Rolls Royce and GM have all investigated the Wankel engine but the reality sadly did not meet expectations. 

In theory, a rotary engine should be turbine smooth and much more efficient than a conventional engine.  Furthermore, the number of moving components is greatly reduced in comparison to a conventional engine while the total number of components is approximately ten per cent of those in a normal engine.  This results in a considerable reduction in weight, an improvement in efficiency and a reduction in manufacturing costs. 

Certainly power outputs were considerably greater than an equivalent capacity reciprocating engine - the swept volume figures quoted here are actually double those of the actual cubic capacity; thus the M35 engine was really 497 cm3 and the Birotor was 995 cm3 .  The engines were indeed much smoother than conventional engines and were willing to rev quite freely - indeed it was felt necessary to fit both a warning buzzer and an ignition cut out to prevent over revving.  The prime shortcomings were an almost linear torque curve (which meant that both the Ro80 and Birotor had to be fitted with semi automatic gearboxes equipped with a torque converter to allow for torque multiplication at low engine speeds) and excessive rotor tip wear which led to high oil and fuel consumption, loss of power and high emissions.  With modern technology and electronic controls, the engine can be made to work reliably and is extremely attractive but high fuel consumption remains a bugbear.  In the seventies, the technology was simply not up to the task.  Had the engine been made to work efficiently, it is likely that it would have provided the power source for the CX; in the event, the CX was initially fitted with the engines from the DS. 

NSU was absorbed within the Volkswagen/Audi group and CitroŽn within the Peugeot group.

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Click for large image below - comparison between Wankel rotary engine and a conventional 4 cylinder engine

Click for large image

Above the CitroŽn helicopter project initiated in the late 1960s was fitted with a twin-rotor Wankel rotary engine developing 180 bhp, the helicopter made several hundred flights and had a top speed of over 200 km/h but when production of the Wankel ended in 1979, the project came to an end.

Left and above - electric-powered cutaway of Wankel engine on display at the Musťe Auto Moto Vťlo in Ch‚tellerault

Above NSU Spyder and M35 - two Wankel-engined cars
Below M35 prototype No. 442 is on display at the Musťe Auto Moto Vťlo in Ch‚tellerault

Above Comotor power train
Below another view of prototype No. 442 on display at the Musťe Auto Moto Vťlo in Ch‚tellerault

 How the Wankel rotary engine works

The four pictures above showing the two CitroŽn production Wankel-engined cars, the M35 and Birotor are © Klaus Nahr
© 1999 Julian Marsh/CitroŽnŽt