The Rotary-Powered 1969 Mercedes-Benz C111 Is The Real Reason Daimler Went With Diesels

Patented by inventor Felix Wankel in the 1920s and developed for the modern era at German outfit NSU at the end of the 1950s, the Wankel rotary was a sought-after innovation thanks to its promised simplicity, high-revving potential and excellent power-to-weight ratio. It was also stupidly maintenance-intensive in those first years, consuming oil and combustion seals in equal measure. The high failure rates of early Wankels doomed NSU's own carmaking efforts—it was bought by Volkswagen in 1969 and merged with Auto Union to form the modern incarnation of Audi.

Meanwhile, the various licensing deals it had made with automakers like General Motors, Toyota and Mercedes to produce their own rotaries reached R&D dead ends. Only Mazda would figure it out. Mostly.

[Editor's note: It's easy to dismiss concept cars as marketing gimmicks and dead-end design exercises. But every once in a while, a company gives away the secret to its future without anyone noticing. So with ever-grander promises about electrification, autonomy and material advances being made by today's concepts, I thought it'd be useful to take a look through the archives to see how and when the major engineering and design trends that define the present were actually seeded. This is The Most Important Concept Cars You Forgot All About, and its aim is to give you the tools to understand what's really coming next. -- KC]

As viewed through the Wankel-powered C111, the story of Mercedes' flirtation with rotary power is no different at first. The aero-wedge, gullwing-door concept and its completely unorthodox powertrain hits the modern eye as another empty promise of its time, a mere design exercise marrying SL heritage and aggressive Giugiaro styling.

But look a little closer, and the C111's multi-stage development story shows how serious the automaker was about exploring the limits of the rotary as its engine of the future. Mercedes really wanted this to work. Ultimately, though, the fruitless experience it had with the C111 in the late '60s and early '70s pushed the automaker to embrace what would become a gigantic part of its global image for decades after: the diesel engine.

A surprised public might've been convinced that the C111's big splash at the 1969 Frankfurt Auto Show was an indication that the company was making overtures to the supercar market. The reality was far more focused on engineering than eye candy. Daimler engineers had stuffed the car full of new chassis and drivetrain ideas it was eager to test out in a real-world setting, and viewed the C111 as more of a research platform than any particular clue as to the future of its sheet metal.

That's why, unlike the way the 2001 Dodge Super8 in our last installment previewed designs for cars that are still on sale today, there are no real-world uber-Benzes from the Seventies borrowing its low front end or buttressed rear. That, and Mercedes' pathological avoidance of high-performance sports cars in the wake of the 1955 Le Mans Disaster.

Speaking of sheet metal, the C111 didn't have any. At least not on the outside. Its skin was created out of a fiberglass and plastic material that marked the first time Mercedes experimented with this type of body construction. It was particularly useful when working with new aerodynamic concepts, although the technique was later abandoned when the plastic material couldn't live up to passive safety requirements.

Underneath all that composite was another fresh idea: a multi-link independent rear suspension that would go on to underpin many of the brand's sedans and coupes in the years that followed.

Everything else about the C111 was secondary to its most intriguing piece of kit: a mid-mounted, three-rotor M950F Wankel engine capable of producing 280 horsepower. Mercedes had been working on the rotary question for five years at that point, having built a series of engines for testing purposes as it attempted to deal with the reliability, efficiency, and cooling concerns that had plagued early implementations of the odd design.

Picture a typical engine cylinder. You have a piston moving up and down, compressing the fuel and air mixture at the top so it can be ignited, expanding the chamber as combustion occurs and pushing up once again to expel the exhaust gases.

The Wankel rotary applies the same principles to a rotor instead of a pumping piston, a fat, rounded triangle spinning around a central shaft in a roughly circular chamber carrying the fuel-air mixture around the perimeter as it goes. This .gif will do a better job than words explaining how the compression, ignition, and exhaust stages follow:

By 1969, Daimler engineers figured it had the threat of a wonky Wankel pretty well licked and moved on to the much more difficult issue facing rotaries. While powerful, the characteristics of a Wankel—with its large, variable combustion chambers that form between the spinning rotor and the housing wall—made it extremely difficult to match the thermodynamic efficiency of a traditional piston engine.

Fuel economy remained poor. It turned out Wankel hadn't quite cracked the code for compact efficiency. Still, the promise was too compelling to ignore.

A second version of the car—the imaginatively-named C111-II—drew even more attention in 1970 at the Geneva show with its upgraded four-rotor, 350-horsepower Wankel and polished bodywork. Capable of hitting 60 mph in less than five seconds and a top speed of over 180 mph, the C111-II's fuel-injected rotary drivetrain elevated it to true supercar status for its time.

The thing looked ready for production, and there are tales of fervent Benz fans mailing in blank checks to the company's Stuttgart HQ with C111 on the memo line.

Unfortunately, Mercedes was no closer to quenching the extreme thirst of the C111-II's Wankel despite its fresh four-rotor design, nor was it able to do anything about the over-the-top emissions even after building more than a dozen test models. Convinced that it could neither pass a government sniff test from the newly emergent EPA nor lure in the general public beyond an excitable few, Mercedes shelved the rotary for good in 1971 and parked the C111 in storage.

It's here that fate intervened—not just in the C111's story, but also in the future of the automaker itself. The 1973 oil crisis hit not long after Mercedes mothballed the C111-II, bringing global repercussions for car companies who were suddenly struggling to prioritize fuel efficiency above all else. And the rotary, while compact compared to the huge V8s of the era, just couldn't deliver on the economy front. 

To chart a path forward, Mercedes approached a very old idea from a new perspective. What if diesel engines—around forever and relatively thrifty, but panned for their noisy, clattering sloth—could be refined to the point where they could satisfy both the performance demands of its well-heeled clientele and the needs of average drivers seeking a break at the pump?

It was decided that the C111 would be pulled out of storage and repurposed to prove to the world that diesel had a future outside of low-buck economy cars and heavy trucks. Mercedes pulled its OM617 five-cylinder diesel from the pedestrian 240D sedan and added a turbocharger and intercooler to more than double its output to a respectable 190 horsepower. It then stuffed the entire setup into the C111's midships engine bay and decided it was going to break a whole bunch of speed records.

The attempts were made at the Nardo Ring in Italy in 1976, where the C111-IID would set 13 diesel-related records alongside a trio of general speed records, with the average pace being listed at just under 160 mph. It was the same spot that Mercedes would later return to in 1978 with a more powerful 230 horsepower diesel engine and a host of dramatic aero improvements in the form of the 202-mph C111-III to add another nine records to its trophy case.

In 1974, when the decision was made to swap the C111 from rotary to diesel power, Mercedes sold just a single diesel model in the United States if you don't count the Unimog: the pokey W115 200D and 220D sedans, in all their 50-odd horsepower glory. By the middle of the 1980s, you could get a turbodiesel stateside in everything from the economical W201 compact to the big S-Class.

There's no question that the Mercedes-Benz C111 closed one drivetrain door and opened another during its decade of existence as an exotic sports car testbed. If the German automaker had been able to dial down the rotary's troublesome fuel consumption and emissions issues to its own satisfaction, and had the 1973 oil crisis not forced the issue, it's likely the Wankel power plant would have lived on as more than just a novel aspect of Mazda's niche RX line of sports cars. 

With a global automaker throwing its might behind the technology, who knows how far it could've gone?

If you doubt Mercedes' ability to move the market, one only needs to consider the impact that the C111-IID's turbodiesel star turn had on transforming the technology in the minds of motorists and company executives. The company released its first turbodiesel model in its executive S-Class sedan 1978—and three years later the next-generation of its oil-burning flagship would show up on U.S. shores to begin the slow and steady process of persuading American drivers of diesel's potential (just as domestic manufacturers were doing the exact opposite, I should note).

In the space of just 36 months, the C111 had convinced Mercedes' product planners of the Wankel rotary's intractability. In nearly the same length of time, the C111-IID would perform the same trick in reverse by engineering away the diesel engine's historically dull and doughty reputation at Mercedes-Benz, within Europe, and eventually, for the rest of the world.

If nothing else, it's proof that the path of progress isn't often a straight line. The auto industry's big current push toward electrification is being helped along by the diesel emissions cheating scandal. Sometimes you set out to solve one problem and end up solving another instead.

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