A fixed part, the stator, with an interior shape that resembles a bean. On its inner edge, a piece, the rotor, rotates around an axis, in the shape of an equilateral triangle. Its three edges must maintain permanent contact with the limit of the stator. We are talking, in a very simple explanation, about a rotary engine. Its first advantage over a conventional piston engine is to eliminate irregularities in the operating cycle as much as possible since the number of rotating parts is reduced to a minimum. This was the approach, in 1929, of its creator, the German engineer Felix Wankel which, in 1951, convinced NSU of this. In 1963, the public discovered the first rotary engine from this German company at the Frankfurt Motor Show. Several brands are interested in this type of engine, including Japanese ones like Mazda. And the French Citroën.
Citroën GS Birotor engine interior
P.F.
In 1964, NSU and Citroën created Comobil, a joint project that was born to develop the Wankel rotary engine. That same year, NSU presented its Spider Wankel at the Paris Motor Show, a two-seater convertible equipped with a 1000 cc and 50 HP DIN engine, with a maximum speed of 150 km/h. It will be on sale until 1968.
In April 1967, NSU and Citroën, as a continuation of Comobil, launched Comotor in Luxembourg, a company dedicated to the manufacture of this type of engines, not only for their own use but also for other brands. And a year later, NSU presents its RO 80 saloon.
The French on the Quai de Javel liked the idea. But its technicians were also aware of the problems that arose. The tightness of the rotor segments was, without a doubt, its Achilles heel. But there were also problems with the ignition and the spark plugs had a very short life: between 500 and 2500 kilometers. Oil consumption was high and up to six liters had to be refilled every 1,000 kilometers. Furthermore, the engine aged poorly and, as the kilometers passed, gasoline and oil consumption increased while power decreased.
The M35, a prototype for customers
But Citroën continues working and after many kilometers covered with crutches based on the Ami 6, to better understand and try to resolve the weaknesses of the Wankel engine, in November 1969, it unveiled the M35.
It is a coupe with a rotary engine with a body closely derived from the Ami 8, but with two doors. However, it does not complement this double chevron model. In reality, it is not even a series model, but rather a prototype or rather a special series with a planned production of five hundred units. It is a small number to merit its own bodywork that requires specific manufacturing machinery and, therefore, entails a high cost, which is why many elements of the Ami 8 are used.
The M 35 used a coupe body derived from the Ami 8
P.F.
Under its front hood beats a Comotor mechanic, it had a single rotor and a displacement of 995 cc. It transmitted 49 hp at 5,500 rpm and a torque of 7 m/kg at 2,745 rpm to its front wheels. With a total curb weight of only 815 kilos (happy times!) it reached a top speed of 144 km/h.
The increase in performance and weight compared to the air-cooled twin forced the Ami 8’s original suspension to be changed (a direct inheritance from that of the 2 CV) for a hydropneumatic one similar to that used then by the DS.
The objective of this model is to launch an operation with a chosen group of clients who test it, in a period of time. For a few years, Citroën has been working to perfect this type of engine, and carried out many kilometers of tests. But, logically, it was a job carried out by professionals, with a mechanical “education”, and above all a driving experience that allowed them to demand the maximum from the engine, but without the “bad” habits of a street driver (over speeds, useless accelerations…). With conventional piston engines, manufacturers knew perfectly well the correction coefficient to apply to a tester’s mileage in relation to a customer, but this was not the case with the rotary engine. The first objective of Citroën’s operation was to know this correction coefficient. But also, it was about carrying out a dissemination operation for their project and seeing the market’s reaction to this novelty.
The initial idea was to build, with a daily production rate of two M35s, a total of five hundred cars, in the facilities of the coachbuilder Heuliez, between 1969 and 1971, although this figure would never be reached: they would remain at around two hundred and sixty-seven M35s.
The clientele was selected from users of the brand who traveled approximately 30,000 kilometers annually. In the event of the slightest incident, these collaborating clients received immediate special assistance and, in addition, a replacement car was provided in case of immobilization. The parts and labor warranty was total for two years for the engine and for one year for the rest of the parts, with no mileage limitation: remember that we are talking about 1970. The maintenance costs, of course, were borne by the customer. The car had a price of over 14,000 francs at the time. And Citroën also planned to deliver some units to its rental company Citer, so that other motorists could try this mechanics. Citroën will buy back the M35s and, after studying the wear of the engine components, will proceed to scrap them, a destruction from which few units will be saved. But the project was the prologue to a new Citroën.
The perfect storm against the GS Birotor
On Thursday, October 4, 1973, at the Porte de Versailles, the president of the French Republic Georges Pompidou (a great automobile fan), inaugurated the Paris Motor Show. The new Citroën GS Birotor is displayed at the Citroën stand, and its commercialization for 1974 is announced.
Two days after this act, the Arab coalition led by Egypt and Syria launches an offensive on the territories conquered by Israel in the Six-Day War of 1967.
But let’s return to our protagonist. Under the already known body of the GS (it had been presented in 1970) a Wankel-type engine with two rotors is discovered, with a displacement of 1990 cc associated with a three-speed torque converter automatic transmission, all located in a transverse front position. . The manufacturer announces 107 DIN HP and, with a curb weight of 1,140 kilos, it reaches a maximum speed of 175 km/h, which places it between the 188 km/h of the DS 23 Injection Electronique and the 151 km/h of the GS 1220.
With the smooth and silent two-rotor Wankel engine, Citroën wanted to disseminate the advantages of this mechanism and at the same time show that the GS had qualities to compete with the best travel models of the time. And, apart from small changes in suspension, steering and brakes, and a front subframe, the Birotor differed little from its brothers. Of course, its interior had a high-end finish.
The GS Birotor was distinguished externally by widened wheel arches and wider rims with chrome hubcaps.
P.F.
The tests of the time showed excellent road qualities. It was a car with a high level of driving pleasure, comfortable, quiet, without vibrations at high speeds (at low speeds it was another matter), fast, with excellent stability and powerful braking. Among the criticisms was a somewhat harsh steering in parking maneuvers, a gearbox poorly adapted to the engine and, above all, the high consumption: at 130 km/h it used 14.2 liters per 100 kilometers compared to 12.1 for the DS. 23 Injection Electronique and the 10.9 of a GS 1220. But at 50 km/h its consumption was already 10.4 liters, while those of the DS 23 were at 8.5 liters and those of the GS 1220 at 5 .4 liters. And as soon as it was driven at a good pace, it would reach 18 liters at 100 km/h on average.
If we put ourselves in the circumstances of the moment, we are talking about 1974, with restrictions and increases in oil prices by OPEC as a consequence of the Yom Kippur War, we found the perfect storm for that adventure of the GS Birotor to end. after only 847 units manufactured. Unsold examples will be destroyed and the brand, as with the M 35, will try to recover the rest for scrapping.
Citroën no longer wanted to know anything about his expensive adventure. But the existence of the rotary engine does not end there. But that, you know, is another and, by the way, beautiful story, written from Japan by Mazda.