How the F700 concept vehicle offers luxury-car performance, comfort, and econo-car fuel efficiency
Object of Desire: The F700 is a research car, but that didn’t stop some wealthy customers from making offers. Image: Steffen Jahn
Car buyers everywhere are spurning gas guzzlers, and some carmakers feel the pain more than others. Take Mercedes-Benz, long known for its luxury cars—its flagship S-Class is a full-size sedan that starts at around US $90 000. Without sacrificing performance or comfort, the company must meet the expectations of its increasingly environmentally conscious customers.
Mercedes, a division of Daimler, has ingeniously applied technology to come up with a stunning solution. It’s the F700 research car, which debuted last year. The F700 was created to give customers a preview of what they might expect from an S-Class sedan 10 years from now. The interior is hugely spacious, modern, and filled with electronics; the exterior is shiny and sleek, a dramatic evolution from Mercedes’s current upright look. The car is also fast: it goes from 0 to 100 kilometers per hour in 7.5 seconds.
And here’s the best part: the massive F700 sips no more fuel than a Toyota Prius.
The critical advance is the power plant. The F700 is a gasoline-electric hybrid, but it wrings most of its efficiencies out of good old-fashioned internal combustion. Mercedes is betting that such engines will be a mainstay of the industry for a very long time to come. But in the face of climate change and soaring global demand for oil, the company decided to show how much room for improvement there was in the 130-year-old internal combustion engine.
What makes this feat possible is a futuristic technology known as HCCI, or homo-geneous charge-compression ignition. HCCI combines the low emissions of gasoline engines and the fuel efficiency of diesel engines. Automakers have long been working to develop HCCI engines and the electronic controllers needed to tame their combustion. GM and Volkswagen, among others, have running prototypes. But Mercedes’s small power plant—a 1.8-liter four-cylinder twin-turbo HCCI engine that burns regular gasoline—shocked the experts.
“‘A lot more from a lot less’ is a fundamentally exciting vision for the future of an industry that’s under so many clouds right now,” says Nigel Griffiths, a managing director at the industry analysis firm Global Insight, in London.
Mercedes says the engine delivers the same performance as the 3.5-L V6 offered in European S-Class models while consuming just half the fuel.
“We had to demonstrate that good fuel consumption and premium luxury cars are not a contradiction,” says Herbert Kohler, who as head of group research and advanced engineering for vehicle and power train is responsible for all of Mercedes’s concept cars.
The company added the motor-alternator hybrid system to provide punch during acceleration. It also suspended the F700 with hydraulics that use infrared laser sensors to read and adapt to road conditions—not just in real time but ahead of time. In other words, the suspension isn’t active; it’s proactive.
Thanks to the compact engine, designers were able to extend the wheelbase, making the F700 no longer than an S-Class but far roomier. Finally, to convey the linked concepts of elegance and efficiency, Mercedes stylists found inspiration in…dolphins.
“The astounding thing about the F700 is that it showcases a vision of future technologies that doesn’t assume continued downsizing by consumers,” Griffiths says. “If they can really get those technologies working, we’re looking at a virtual quantum leap.”
Vehicle Visionary: Herbert Kohler mobilized Mercedes’s advanced engineering groups to create the F700.
Kohler, a measured and precise man, can pinpoint the exact steps that led to the conception of the F700. Sitting in his book-lined office in Daimler’s headquarters in Untertürkheim, on the outskirts of Stuttgart, Germany, he says the project began to take shape in the fall of 2005. That year, Mercedes unveiled a concept known as the Bionic Car, an exercise in structural and aesthetic design. It was natural to look next for improvements that could be made under the hood.
Kohler considered a conventional hybrid-electric drive but concluded that it wouldn’t give radical fuel reductions, especially during high-speed autobahn cruising. Could the car be fully electric? The problem was that even the best lithium-ion cells projected for 2018 would be too heavy to provide the 500 kilometers of range expected from luxury cars. Hydrogen fuel cells also appeared too far-fetched, as the necessary hydrogen fueling infrastructure was likely even further away.
Digging deeper into the company’s research, Kohler became convinced that the novel HCCI technology was ready to make the leap from lab bench to demonstration vehicle—in concert with a handful of additional engine modifications. HCCI could offer the advantages of gasoline and diesel engines while avoiding the disadvantages peculiar to each.
A gasoline engine achieves a well-controlled, efficient combustion with relatively low emissions by using a spark plug to ignite a vapor of fuel and air, compressed typically at a 10:1 ratio inside the cylinders. Diesel engines, on the other hand, have no spark plugs; combustion happens spontaneously when an injector sprays atomized diesel fuel into the cylinders, filled with extremely hot air compressed at ratios as high as 18:1. The higher compression ratio makes the engine more energy efficient but at a cost: higher temperatures generate nitrogen oxides, a major cause of air pollution.
The idea behind HCCI is to take advantage of both the cleaner burn of spark ignition and the higher efficiency of compression ignition. In a gasoline engine, the burn starts at the spark plug and propagates outward from there. In a diesel, combustion begins at the edges and moves inward. But in an HCCI engine, the fuel-air mixture is so thoroughly combined that it ignites at many points simultaneously. The benefits are twofold: the engine can use leaner fuel-air mixtures without suffering a loss in power, and the combustion temperatures are too low to form nitrogen oxides.
To take HCCI from the lab to the test track, Kohler turned to Gunther Ellenrieder, an old colleague who was director of group research and advanced engineering for vehicle concepts and human factors at the Mercedes-Benz Technology Center, at the company’s largest plant, in Sindelfingen, Germany. Ellenrieder, who is tall and prone to gestures to sketch out his explanations, nodded emphatically as he recalled their first conversation about HCCI. “The calculations looked pretty good,” he said, “but that didn’t mean it was mature enough”—meaning that getting HCCI working in a drivable car would be quite the challenge.
In early 2006, Ellenrieder mobilized a team of development engineers and gave them a tight deadline: September 2007, when Mercedes was to unveil the F700. To keep the fuel consumption close to the 50 percent mark, they settled on a four-cylinder HCCI engine of 1.8 L—the same size as those used in midsize European sedans. The team added two turbochargers, pumps driven by exhaust gases that pack additional air into each cylinder, allowing the injection of more fuel. As the engine starts to rev from idle, the pump with the smaller output nozzle boosts pressure immediately; the pump with higher output, from a larger nozzle, takes over at higher speeds.
The one thing the new engine couldn’t manage was the torque of a larger power plant. So the engineers added a 15-kilowatt electric motor, which provides an electric assist when maximum power is needed, and an integrated starter-generator, tucked inside the seven-speed automatic transmission, to both start the engine and recharge a battery pack on overrun.
Under regular loads, the engine acts conventionally, using spark ignition. Under heavy loads or for better acceleration—passing on a highway or going up a hill—the electric motor may add power as well. But under a partial load—level highway cruising, for instance—the engine switches over to work as an HCCI, cutting fuel consumption. And if there is no load at all, the engine simply switches off until it’s needed again.
Mercedes engine wizards hunkered down in the Sindelfingen lab for weeks to assemble the F700’s HCCI heart. Some parts they borrowed and adapted from production vehicles, including air-intake valves that open at different times and fuel injectors that squirt gasoline into the cylinders at different pressures.
But the engineers also perfected two systems not normally found in other engines. The first retained exhaust gas left over from the previous combustion cycle, using it to prewarm the gasoline-air mixture to reach ignition temperature. This required pricey pressure transducers inside the combustion chamber; they would feed data to the engine controller, which would determine in real time how much to open the exhaust and intake valves for each cylinder and when to inject the fuel.
The engineers next devised a mechanism to change the position of the piston at the top of its stroke. This complex piece of engineering raises the compression ratio to create the optimal environment for self-ignition under light loads and lowers the ratio for greater power in spark-ignition mode. It comes as no surprise that Mercedes won’t reveal the specifics, declining to answer questions about the range of compression ratios or the mechanical methods used.
The assembled engine block is a compact cube of metal and plastic. The company’s marketers christened it DiesOtto, combining the names of the two 19th-century German pioneers of diesel and gasoline combustion engines, Rudolph Diesel and Nikolaus Otto, respectively. Mercedes says that while several other makers are researching HCCI concepts, the DiesOtto engine is unique in its combination of spark-ignition and prewarmed HCCI modes, its variable compression ratio, and the application of turbocharging.
And then there’s the engine control unit. Conventional engine controllers keep combustion efficient and emissions low by constantly adjusting the amount of fuel injected and the timing of the ignition. They make adjustments based on data received from dozens of sensors measuring parameters as varied as engine rotation, atmospheric pressure, and tailpipe oxygen levels.
Designing such controllers is fiendishly complicated, even for standard cars. Designing the F700 controller—which had to manage such a complex range of combustion modes—was “an order of magnitude” worse, as Ellenrieder puts it. The switching between spark and compression-ignition modes proved especially hard. In the end, the engineers realized that the controller can figure out which mode to use from the data coming from the pressure transducers inside each cylinder. The controller then automatically adjusts the compression ratio, the amount of fuel injected, the valve timing, and other parameters. It’s the main brain behind the DiesOtto.
Suspension was next on Kohler’s list. Naturally, the car would have the most advanced electronic systems for safety and comfort used in Mercedes cars—traction control that varies power and braking on each wheel and adaptive cruise control that maintains a safe distance behind the car ahead at any road speed. But as a vision statement, the F700 needed more.
Once again, Kohler turned to Ellenrieder, who suggested what he called the “magic carpet.” Like today’s active suspension systems, it would take in data like road speed, acceleration rate, brake application, and force from each suspension component. But on top of that, it would monitor the conditions of the road just ahead of the vehicle.
If the system worked, the F700 would whiz over the asphalt almost as if floating in midair, already prepared to smooth out the bumps and irregularities it “knew” were coming—something no other vehicle could do. There were doubts inside Mercedes on whether the system—aptly named Pre-Scan—was far enough along to be exposed to the global glare of publicity. But Ellenrieder prevailed, after what he diplomatically termed a “push-pull process,” and the system went into full-on development.
The key innovation was the use of lidar, or light detection and ranging technology. Originally developed for military remote-sensing applications, lidar is now used by geoscientists and construction crews to map the elevations of a terrain. The device emits pulsed beams of infrared laser light, detects their reflections, and computes the distance to the target. The idea was to use lidar sensors in the headlights to scan the road so that the car’s suspension could firm up or soften its damping and direct the hydraulic shocks to absorb or counter the loads on each wheel.
Programming the suspension’s controller, fitted with multiple processors, brought its own challenges. Among other things, the controller had to keep track of the motion of the car body—where the sensors were mounted—relative to that of the wheels and suspension. And because the lidar sensors mapped the road so accurately—they could “measure” the thickness of the painted lines on the asphalt—the controller also had to cleverly smooth out the input data to make sense of different road surfaces. But the result paid off: sitting behind the wheel of the F700, a driver going over a pothole at high speed barely feels a jolt.
Finally, it was time to consider what the car would look like. Kohler discussed his early ideas with Peter Pfeiffer, head of the company’s design department, and they settled on rough dimensions. The length would be about 5 meters, the size of the current S-Class. With the small engine up front, the wheelbase could be a full 3.5 meters—about as long as that of ultraluxury vehicles like the Maybach 57 or a Rollsâ¿¿Royce Phantom—giving interior space that would be impossible in regular cars.
Kohler asked Pfeiffer what to do with this roominess. Pfeiffer told him that the company’s advanced design studio for interiors in Como, Italy, had experimented with repositioning seats to let passengers have more relaxing conversations or do business more effectively. One idea that emerged: if the rear seats rotated 180 degrees, passengers would be able to talk face to face, limousine style.
With those and other specs in hand, Kohler and Pfeiffer contacted the company’s design studios in Germany, Japan, and California and asked them for proposals. Above all, the design had to denote a sense of inherent fuel efficiency and environmental concerns—and, of course, still convey the instant emotional impression of a large, prestigious Mercedes-Benz.
In July 2006, Mercedes’s board of directors reviewed several proposals and chose a design submitted by the California studio, the Advanced Design North America division, in Irvine [see “Making it real”]. Its chief designer is Chris Rhoades, who, like several of his colleagues, is a surfer. ”Design is inspired by nature,” he says, “to give back to nature.” After Rhoades sketched, pondered, surfed, and sketched some more, what emerged in his flowing shapes and design elements was the image of a dolphin breaching the waves. Sleek, smart, efficient, it seemed to Rhoades the perfect metaphor for the F700. The board agreed, and the dolphin got the nod.
The result was a flowing roof and gently rounded shoulder lines, highest in the center, and a short snout up front. A dorsal fin down the car’s roof added aerodynamic stability—and undeniable marine overtones. Over the next five months, the design team worked frantically to build a full-size clay study, refine every detail and surface, and produce a final hard-foam model. From this model, the template for the actual F700 metal body finally emerged.
But challenges still loomed. As it turned out, the paint job almost ruined the car. The process was multilayered, with many cycles of painting, buffing, and sanding to leave fine metal flakes suspended in a deep iridescent silver surface. After a week in the paint shop, the car’s surface emerged flawlessly smooth—but the wrong shade! It wasn’t until a week later that the last coat of hardener finished curing in the sunlight and the slight green tint vanished. The paint shop foreman could only repeat, over and over, “It’s a miracle!”
The fully assembled F700 emerged for final review at the top-secret Mercedes-Benz design studio in the Sindelfingen complex. A circular building designed by world-renowned architect Renzo Piano, the studio has several roll-up garage doors that allow cars to be taken outside, where fences and shrubbery shield them from prying eyes. Here the F700 was positioned on a turntable, where it shone in the sunlight.
It looked just as good on the test track. The F700 has a rated power of 175 kW (238 horsepower) at maximum load. At cruising speeds, it consumes only 5.3 liters per 100 kilometers (44 miles per gallon), with CO2 emissions of 127 grams per kilometer—and this for a full-size vehicle that weighs 1700 kilograms. By comparison, the S350’s 3.5-L gasoline V6 produces 200 kW (268 hp) and takes 7.3 seconds to go from 0 to 100 km/h—just a bit better than the F700—but its fuel consumption is 10.3 L/100 km (23 mpg) and its CO2 emissions are 242 g/km, almost twice those of the concept car.
The F700 debuted at the Frankfurt Motor Show on 11 September 2007. It was like no Mercedes the world had ever seen, and the reception was rapturous. Some wealthy customers who were shown the car made offers on the spot or asked if Mercedes would build one for them. Several German commentators used the phrase aus einem Guss, meaning “from a single cast,” or seamless, all of a piece. The DiesOtto, in particular, received much acclaim, some even calling it a “green engine.” Industry analysts and critics praised the car’s other innovations as well—most of them, at least. The control interface of the navigation system was too far from the driver, some critics said, and talking to an avatar to input settings didn’t work very well. And then there was the dolphinlike body: some observers simply found it too radical for an S-Class.
But the question that customers, analysts, and other carmakers have all been asking is when the F700 features—especially the innovative DiesOtto engine—will appear in production models. Kohler is coy. He says Mercedes has no exact timetable, although he can point to dozens of features in today’s cars that first appeared in earlier concept cars.
Some industry analysts predict that HCCI engines will become practical for mass production no sooner than 2015. But with a new generation of S-Class models slated for 2012, some of the F700’s innovations may trickle into the showroom over the next few years. One possibility is a direct-injection four-cylinder engine, which would bring some of the fuel-saving advances of the F700 project to the S-Class models. For a line of sedans that Mercedes has always fitted with big engines, this alone would be a radical move.
Just don’t expect a dolphin-shaped body anytime soon.
About the Author
JOHN VOELCKER went to Daimler headquarters in Stuttgart, Germany, to report on the Mercedes-Benz F700, a prototype for future gas-sipping luxury cars. “They brought me into the top-secret design lab,” he says, “and, unexpectedly, rolled the full-size styling model of the F700 out into the sunlight and onto a turntable surrounded by 8-meter-tall hedges. I got exactly the same viewing that the top brass at Daimler did!”