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DiesOtto Aims To Be the Best of Both Worlds

DiesOtto Aims To Be the Best of Both Worlds

By Jeff Youngs, February 24, 2012
It sounds like just what everyone wants: all of the power and efficiency of a diesel engine mated to the low tailpipe emissions and lower production cost of a small gasoline engine. Several iterations of this engine are being worked on around the world, but the latest is the DiesOtto from Mercedes-Benz.

Mercedes showed the engine at the International Motor Show in Frankfurt in September 2007 and astounded everyone. The engine, which was showcased in the F700 concept car, combined hybrid technology with this version of a future internal combustion engine. The performance numbers were very impressive for a full-size luxury car: only 5.3 liters of gasoline per 100 kilometers (or 44.38 mpg) while still delivering acceptable acceleration.

Mercedes says this 4-cylinder engine is not far off in the future since its direct injection components are already in use in its current 3.5-liter gasoline engine. In the DiesOtto version, the engine has two-stage turbocharging so the 1.8-liter engine provides the same level of performance as the current 3.5-liter gasoline V-6 engine or the 3.0-liter turbodiesel V-6 engine currently offered in the S-Class.

The biggest change from current gasoline engines is the use of homogeneous charge combustion ignition (HCCI), which mimics the combustion process used in diesel engines. The small engine produces 238 hp from its DiesOtto engine (and a massive 400 Newton-meters or 295 lb.-ft. of torque). Mercedes says it will propel the large car to 62 mph in 7.5 seconds while still delivering 127 g/km of CO2 and meeting proposed EU 6 emissions regulations.

As Mercedes says, there is no reason to "reinvent the wheel" when it comes to internal combustion engines, but rethinking the principles of the 131-year-old gasoline engine is no small feat. "Our goal is to make gasoline engines as economical as diesels, and the new DiesOtto concept marks a major step in that direction, as it combines the best features of SI (gasoline spark ignition) and diesel engines," says Thomas Weber, the Daimler Board of Management member responsible for Group Research and Mercedes-Benz Cars Development.

The DiesOtto runs on normal gasoline, which means its modest fuel requirement can already be met at any filling station today. The fuel/air mixture in the new engine is ignited by a spark plug (the conventional SI method) during starting and full load operation, when the engine operates as a direct-injection unit. At low and medium speeds, however, the gasoline engine morphs into a self-igniting unit operating on the diesel principle. Since low and medium speeds are the most common operational modes of an engine, the fuel economy increases are substantial and with sophisticated turbocharging mated to the new combustion process, a smaller engine is possible. Because this engine emulates typical gasoline combustion, the need for aftertreatment to clean up exhaust is minimized.

The concept may seem relatively simple, but its execution is complicated, which is why it is not yet in production. The key is combination of state-of-the-art technology and cutting-edge engine management and controls.

The DiesOtto concept is modular, which means it can be used with different engine types and different-size vehicles. Further development is still required before the full combination of all the elements presented in the F700 can go into series production, according to Mercedes. In the meantime and even after HCCI engines go into production, both gasoline and diesel engines continue to have a right to exist side by side. Mercedes won't give a target date for DiesOtto engine's on-sale date, but hint that it's not far off.
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