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Old 09-07-2004, 07:17 AM
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Mercedes-Benz C111 Research Car 1969-1979

C 111 – power, elegance and speed

Stuttgart, Aug 14, 2004
The compact wedge in bright orange, a shade internally called “weissherbst”, expressed power, elegance and speed. C 111 was the designation of the futuristic study displayed by Mercedes-Benz in September 1969 at the Frankfurt International Motor Show (IAA). The car broke new ground in terms of both engineering and design. Motor show visitors crowded around the sports car, marveling at its intriguing – as well as polarizing – design. Was this the worthy successor to the famous 300 SL Gullwing? The car’s style, dynamic lines and classic gullwing doors promised just that to lovers of refined cars with the three-pointed star on the hood. This happened 35 years ago, at the C 111’s presentation in Frankfurt. In the spring of 1970, an even more elegantly clad C 111-II made its appearance at the Geneva Motor Show, prompting interested parties to send blank checks to Stuttgart to secure one of these cars for themselves.
However, it had never been planned to produce the new Gullwing in series, and the C 111 was not to appear in showrooms. The coupe may have looked like the systematic further development of the Sport Light models from the 1950s, but it was not a design study for a new SL: it was to serve as an experimental car. Laboratory machines as beautiful as this Mercedes-Benz, designed among other things to test glass-fiber-reinforced plastics as bodywork material, were – and still are – few and far between, however. The coupe’s lightweight skin, opening up new possibilities in the aerodynamic design of sports cars, was bonded to a steel frame/floor unit.

Powered by a Wankel engine

The second revolutionary feature of the C 111 was hidden under its skin. The first experimental car of 1969 was powered not by a reciprocating-piston engine but by a Wankel – or rotary-piston - engine. At the time, many manufacturers were interested in Felix Wankel’s unconventional propulsion system. Mercedes-Benz, too, had been experimenting with Wankel engines – KP through to KC series – since 1962. However, the Wankel engine was to be extensively road-tested before being fitted in production cars. The last Mercedes with a rotary-piston engine from this series was the four-rotor DB M950 KE409 of the C 111-II in 1970.

The performance of the C 111 even with the three-rotor engine was convincing right from the start. In 1969, the Wankel engine developed 280 hp from 600 cubic centimeters of chamber volume per rotary piston and gave the car a top speed of 260 km/h; with this engine, the car accelerated from standstill to 100 km/h in five seconds. The C 111-II of 1970 was powered by a large four-rotor Wankel engine which developed 350 hp and gave the car a top speed of 300 km/h. The second C 111 accelerated from standstill to 100 km/h in highly respectable 4.8 seconds. While some of the engines in the C 111-I cars had still featured dual ignition which was difficult to adjust, the four-rotor engine was equipped with single ignition exclusively. Both engines were direct-injection units.

The development department of Mercedes-Benz eventually succeeded in solving the engineering-design problems involved in the rotary-piston principle, especially in engine mechanics, but the problem of the Wankel engine’s poor degree of efficiency, due to the elongated, variable combustion chambers of the rotary-piston principle, was not to be overcome with technical modifications. This problem was simply inherent in the design: in a Wankel engine, the fuel burns within the space between the convex side of the rotary piston and the concave wall of the piston housing rather than the cylindrical combustion chamber of a reciprocating-piston engine. The variable, anything but compact combustion chambers of the Wankel engine were responsible for poor thermodynamic fuel economy as compared to a reciprocating-piston engine, resulting in significantly higher fuel consumption for the same output. The engines of the first two C 111 versions were straightforward gas-guzzlers. And since the pollutant content in the exhaust gas of the Wankel engines was also too high, Mercedes-Benz discontinued work on this type of engine in 1971, in spite of its impressively smooth running characteristics and compact size.

In retrospect, Dr. Kurt Obländer, head of engine testing in the C 111 project, described the Wankel engine as follows: “Our four-rotor engine with gasoline injection represented the optimum of what could be reached with this engine concept. The multi-rotor design called for peripheral ports for the intake-air and exhaust-gas ducts. We were able to solve the difficult problems in engine cooling and engine mechanics by technical means. But the main problem of the concept, its low thermodynamic degree of efficiency, remained. Due to the elongated, not exactly compact combustion chambers, fuel economy was poor, resulting in high fuel consumption and unacceptably high pollutant emissions. These drawbacks were inherent in the design principle.”

Testing of the diesel engine

Then, in the fall of 1973, a boycott of the oil-producing countries brought about the so-called oil crisis and crude oil, hitherto an inexpensive commodity, became a precious resource. Developers were requested to come up with new engines which, more than anything else, used the expensive fuel sparingly. The most obvious proposition was the low-consumption diesel engine but the compression-ignition unit was still thought to be sluggish and noisy. There had certainly been examples of either vice in automotive history but the diesel engine had long since been developed into a refined power unit, perfectly capable of driving sporty passenger cars.

In 1976, Mercedes-Benz decided to disprove the old prejudice – and what could have been better suited to providing counter-evidence than a C 111 with diesel engine? The engineers installed a three-liter naturally-aspirated compression-ignition engine with five cylinders in the C 111-II for the first tests. In the car, now called C 111-IID, the OM 617 LA engine developed as much as 190 hp, thanks to turbocharging and intercooling, as opposed to the 80 hp output of the production engine which powered the Mercedes-Benz 240 D 3.0 (W 115, Stroke Eight) and, at a later stage, other models. In June 1976, the C 111-IID reached spectacular speeds on the test track at Nardo near Lecce in Italy. In the course of 60 hours, four drivers established a total of 16 world records – thirteen of these applying to diesel-engined cars and three to cars in general, irrespective of their type of engine. During the tests, an average speed of 252 km/h was recorded, and Mercedes-Benz proved impressively that diesels also have sprinter qualities.

Second career as a record-breaking car

The success of the sparsely modified C 111-II in Nardo spurred the developers on to new heights. This time, they did not create a design study for a road-going sports car but a thoroughbred racing car for the sole purpose of establishing speed records: the C 111-III. The new car was built in 1977; it was narrower than the first C 111, had a longer wheelbase and perfect aerodynamic properties, thanks to complete streamlining and rear airfoils. In 1978, the C 111-III lined up at the start in Nardo. Once again, a diesel engine growled under the silver-painted plastic bodywork. While this engine had been derived from a production unit, it had been tuned to develop 230 hp and gave the streamlined car a top speed well over 300 km/h. With this Silver Arrow, Mercedes-Benz established nine absolute world records in the late 1970s.

However, the C 111 was still to take the final evolutionary step towards becoming an all-out racing machine. The last version of the sports car, the C 111-IV presented in 1979, broke the track record by reaching a speed of 403.978 km/h. This time, it was no longer a diesel engine working under the plastic skin but a V8 gasoline engine with a displacement of 4.5 liters and an output of 500 hp. The shape of the bodywork was equally a far cry from the first version. Ten years on, and the bodywork crafted with esprit and courage in 1969 had become a slim, elongated rocket with two airfoils and massive spoilers in a silvery livery.

Setting the standards for the design of modern sports cars

Even the first C 111 hadn’t deserved its nickname, “test lab on wheels”. As well as incorporating highly innovative parts and assemblies, the coupe set the standards for the design of modern sports cars.

The gullwing doors, retained for all four series of the C 111, identify this charismatic experimental car as a member of the legendary Mercedes-Benz sports car family. These doors, hinged at the roof, developed into hallmarks of Mercedes-Benz sports cars. They first featured on the filigree 300 SL (W 194) which was entered in racing in 1952; from this car, a road-going sports car (W 198/I) was derived of which just about 1,400 units were built between 1954 and 1957. The coupe with its breathtaking metal skin on a spaceframe was powered by a modified version of the six-cylinder in-line engine from the 300 “Adenauer Mercedes”. Rudolf Uhlenhaut, head of the passenger car testing department at Mercedes-Benz and creator of the 300 SL, topped the development of the dynamic sports car line with a 300 SLR in 1955. The engine of the “Uhlenhaut Coupe” was based on that of the open-top 300 SLR racing sports car in which Stirling Moss and Denis Jenkinson won the Mille Miglia in 1955: the sports car was powered by the eight-cylinder in-line engine from the company’s racing cars at the time but the plans to put the SLR coupe to the acid test in the Carrera Panamericana long-distance race in Central America were thwarted.
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