From Autocar Magazine, Week ending 8th November 1973
A look at Wankel engine history and the latest unit to appear, the Comotor Bi-rotor announced for the Citroen GS
ROTARY ENGINES go back in history a long way before Dr Felix Wankel's obsession with them before and after the last war. If you admit to the basic similarity between engines and pumps (the action of many engines can be reversed to form pumps), then an Italian engineer called Ramelli was the first in 1588, with his invention of the vane-type pump. Pappenheim, a German, followed this 48 years later with the gear-type pump, which did away with Ramelli’s slide valves but suffered from leaky seals, water or gas tightness being the weak point of all rotary designs.
In 1799 a colleague of James Watt adapted Pappenheim's gear pump into a rotary steam engine. Despite wooden scrapers in the tips of the teeth, there was a lot of leakage and a poor efficiency. Sixty years later an Englishman called Jones simplified the gear pump down to two double-lobed intermeshing rotors, this configuration later being adopted in the famous Rootes supercharger.
One of the first rotary engine patents was filed by an American called Cooley in 1901. His invention featured an epicycloid with internal meshing- gears and an enveloping casing with three fixed seals. In 1908 Umpleby, an Englishman, converted Cooley’s rotary steam engine into an internal combustion engine, but the gas sealing problems prevented the design from being successful.
Inventors throughout modern history have repeatedly been fired by enthusiasm for rotary engines, the attraction generally being smoothness of operation from the elimination of reciprocating parts and a high specific output for the weight and bulk, cylinders being arranged in a much more compact layout than the conventional side-by-side cylinder configuration of a reciprocating engine. Apart from the often impossibility of actually making some of the parts, in the main it has been gas sealing that has caused the designs to fail. Even when combustion was sustained, power outputs were very low due to poor combustion chamber shapes and the thermal problems of putting the cylinders so close together.
Born in the Black Forest in 1903, Felix Wankel acquired a keen interest in rotary engines during the five years he spent working for a scientific publishing house in Heidelberg. At the age of 21 he set up his own workshops and made models of rotary piston engines. Very soon he established that the weakness of all designs was the gas sealing and he set about beating this problem with a programme of intensive development. With the co-operation of BMW, he designed a practical unit in 1934 and in 1936 he received support from the German Air Ministry to set up a research organization. In 1942 the laboratories were expanded to undertake work on rotary-valve aircraft engines, but in 1945 the Allied Forces occupying Germany destroyed Wankel's whole operation.
Nothing daunted, Wankel immediately started work again in his Black Forest home and in 1951 his first contract with NSU was signed. In 1954 the discovery was made that a four-stroke cycle could be performed by the rotation of a three-lobed rotor in an epitrochoidal casing and suddenly the door to success was open. Two years later, an NSU motorcycle with a rotary-piston supercharger wiped the board in competition and took several world speed records on the Great Salt Lake.
In 1957 the first Wankel engine ran. This early type DKM was extremely complicated with both rotor and housing rotating. From a capacity equivalent to only 250 cc, 29 bhp was developed at 17,000 rpm. The next step was to make the rotor trace an orbital path and keep the casing stationary and the engine in this form (known as the KKM) successfully ran in 1958. Endurance trials the following year proved the reliability and in January 1960 at a German engineering convention in Munich, Wankel told the world about his revolutionary new engine.
Already the Curtiss—Wright Corporation in the USA had a licence to develop the Wankel engine, and enquiries poured in from all over the world. By the beginning of 1961, Toyo Kogyo and Yanmar Diesel in Japan had taken out licensing agreements, to be followed by Daimler-Benz, Alfa Romeo, Rolls-Royce, Porsche, Nissan, General Motors, Toyota, Ford of Cologne, BSA, Yamaha, Kawasaki and American Motors, to name only the automotive companies in the total list of 25 licensees. In 1967 Citroen set up a joint company with NSU, named Comotor, to manufacture and market rotary-piston engines, and in 1971 the Anglo-Rhodesian mining company, Lonrho Ltd, acquired Wankel's interest in the original design and thus became the recipient of a share of all licence fees.
The first Wankel engine to go into series production was a 300 cc unit developing 18 bhp at 6,000 rpm. lt was used as the power unit for a small water scooter controlled by a ski-mounted driver. From October 1962 onwards a total of 3,000 were manufactured.
ln 1959 the Wankel engine made history by powering a car for the first time. Two NSU Prinz minicars were fitted with a 500 cc unit developing 44 bhp at 9,000 rpm and sent out on long-term endurance testing. These cars helped convince the NSU management that the engine had potential, but it was still greeted with a lot of scepticism, all previous rotary engines having been considerable white elephants.
A year later and an engine specifically designed for fitting to a car was ready, six Prinz Sport coupés being built as prototypes. There followed a long period of development and an increasing undercurrent of unrest amongst NSU shareholders about the considerable financial commitment. This precipitated the premature launch of a production car, the NSU Spider in September 1964, powered by a single—rotor Wankel engine of 996 cc developing 50 bhp at 6,000 rpm. Like the early experimental engines, this power unit suffered from very poor bottom-end torque and a useful range of only 4,000 to 6,000 rpm, top end performance being limited by the seal life at that time. For this reason and the fact that there had to be a premium price on a low volume output, the installation chosen was a convertible version of the Sport Prinz, the idea being that sports car drivers expected engines with these characteristics. No more than 15 per day were produced, the total built until production ceased in 1968 being only 3,200.
A lot of valuable experience was gained in service, however, often at the expense and inconvenience of the customer, although NSU have always been very quick to replace Wankel engines free of charge. We ran one of these Spiders as a staff car and suffered excessive seal wear and a cracked rotor casing.
From single rotor engines work progressed to double rotors and more, Toyo Kogyo and Mercedes eventually getting as far as four rotors experimentally. NSU's great landmark was the launch of the twin—rotor Ro80 in 1967. For the first time in Europe there was a new car designed around the Wankel engine, the compact size of the power unit allowing a low aerodynamic nose and front-wheel drive. To compensate for the still-evident deficiency in bottom—end torque, a semi—automatic transmission was employed, with manual shifting and a torque convertor. The Ro80 engine had exactly the same internal dimensions as the Spider unit, two rotors being ganged together in series.
As early as 1964, Citroen, who have never been afraid of complication or advanced engineering, established an association with NSU, the aims being to develop a joint rotary-engined car. There can be little doubt that some of the Citroen thoughts on passenger car layout went into the Ro80.
In 1967 Comotor was established in Luxembourg as the Compagnie Europeenne de Construction Automobile, with objectives more specifically towards the manufacturing and marketing of rotary engines. Two years later a large factory site was acquired in the Saar and in June this year the first phase was opened with an initial work force of 200 and an output of 30 engines per day. Eventually the plant will be four times its present size and capable of producing between 5,000 and 6,000 engines per day.
Since the original formation of Comotor, NSU have been taken over by Volkswagen, so there is now a potential of Wankel—powered cars from NSU, Citroen, Audi and Volkswagen. Of these companies, Citroen are most in need of a new engine range, especially for their big D—range of saloons. To gain experience of the Wankel engine in production, they first embarked on a massive proving trial with single rotor units using components from the Ro80 engine. Originally it was planned that 500 experimental cars based on the Ami 8 and called the M.35 would be placed in the hands of typical high—mileage French drivers for endurance testing. Later the number was reduced to 350 and in fact between 1971 and 1972 only 260 were delivered. The M.35 was a fastback coupe using mainly Ami 8 body panels but also fitted with prototype hydropneumatic suspension which was later introduced with the GS saloon.
Logically speaking the M.35 should have led directly to a twin—rotor saloon, but Citroen were nervous about the Wankel and wanted more time to prove its reliability. Consequently the GS was launched with a flat—four reciprocating piston engine while the M.35 prototypes continued to build up a total mileage of more than 18 million miles between them.
At Last, the Bi-rotor
Development wheels turn slowly when new factories and new designs are involved, but at this year's Paris Salon the twin—rotor Citroen saloon was launched officially, even though the model will not go on sale until next March. As expected, the engine is a double version of the M.35 unit, which brings it back to being very like the one in the Ro80. Equivalent swept volume is 1,990 c.c. and from a compression ratio of 9 to 1 107 bhp (DIN) at 6,500 rpm is produced. This is 8 bhp less than the Ro80 (115 at 5,500) but the shape of the torque curve has been improved dramatically. As the graph shows, the Citroen engine's torque rises rapidly to reach 90 |b.ft. by about 1,600 rpm and is then flat (apart from a slight peak of 100 lb.ft. at just over 3,000 rpm) all the way to 5,000 before it tails slowly off. On the Ro80 engine the torque rises slowly to a peak of 117 lb.ft. at 4,500 rpm and then falls off sharply.
To increase the life of the vital rotor tip seals, Citroen use sintered iron rubbing against a nickel—silicon liner to the alloy trochoid casing. Both the rotor and its side seals are made from cast steel. To minimize the danger of over revving (rubbing speed is the critical factor for seal life), a warning buzzer is fitted which is triggered at 6,800 rpm.
The installation of the Comotor engine in the GS saloon is so neat that it must have been envisaged before the reciprocating piston engined version was conceived. Mounted transversely, the engine drives a torque converter and manual three-speed transmission in line with the rotor shafts, before the final drive which is ahead of and slightly below the gearbox. A unique arrangement is a cross-shaft with bearing on the opposite side to permit equal length drive shafts. To make the installation even more compact, the air injection pump for the anti—pol|ution equipment is mounted in front of the gearbox and driven by a thin shaft connected to the back of a jockey pulley at the other end of the engine. This jockey is driven by twin vee-belts off the rotor shaft nose, as is the water pump. A single vee-belt drives the alternator off a pulley on the air pump.
As well as air injection into a reactor, emissions are controlled by electronic ignition timing which is varied according to gear lever position, oil temperature, engine speed, air intake temperature and induction vacuum.
According to the terms of their licence, Comotor can develop Wankel engines with power outputs between 40 and 200 bhp for land vehicles. This more than covers all the Citroen needs, even the Maserati engine in the SM developing only 178 bhp. In its present stage of development, the Comotor engine would be eminently suitable for most of the D-range of cars, with the exception of the DS 23. By the time Citroen are ready with a replacement large car (1975 is my calculated guess), Comotor could easily have developed a three—rotor unit with around 150 bhp. The attraction of Wankel engineering, apart from the simplicity and small size, is that the ganging up of modules is feasible and relatively easy. For this reason I think Comotor will stick to the 67mm rotor width that has been a feature of NSU Wankels since the Spider unit designed in 1962. All the combustion development has been based on this rotor width with the radial dimension to its tip of 100mm and an eccentricity of 14mm.— However, the terms of Wankel licences stipulate an exchange of technical information with licencees and that other producer of Wankel car engines, Toyo Kogyo, have wider and bigger rotors in their Mazda RX range, so Comotor could get to 130 bhp another way.
Whatever turns up in 1975, Citroen seem to have committed themselves to a big stake in the Wankel engine and l doubt if we shall see them design another four, or even a six cylinder reciprocating engine. From what we know, the GS Bi—rotor will sell for a premium price in France next year, partly to throttle the demand and partly to allow for the possibility of trouble in service. Despite the complicated manufacturing process, however, Comotor have the advantage of starting manufacture from scratch in a new factory, so eventually the unit cost per engine must come down to the level of conventional engines. By then, Citroen believe, the few remaining snags will have been eliminated.