Issue: Sep 2007


MERCEDES GASOLINE ENGINE CUTS FUEL USE 29% BELOW DIESEL



by Bob Brooks

Mercedes data on its advanced technology DiesOtto gasoline engine reveals the fuel consumption impact of breakthrough technology that combines diesel and gasoline(Otto) engine features.. Gasoline use per mile is cut 42% vs the.conventional gasoline engine in the same vehicle and 29% less vs. diesel power . Expressed in US mpg terms, DiesOtto improves mpg by 72% vs. the conventional gasoline engine and 41% vs. conventional diesel.

Data received from Mercedes in Germany is for three otherwise equal advanced S-class cars; one fitted with a conventional 3.6L gasoline engine, one with a 3.0L diesel engine and the third with the new 1.8L DiesOtto engine. Fuel consumption data expressed in liters/100 km along with KW power are given as follows:

- 3.6L gasoline engine 10.3 liters/100 km 200 KW
- 3.0L diesel engine 8.5 liters/100 km 173 KW
- 1.8L DiesOtto gasoline engine (less than)6.0 liters /100 km 175+15(ISG) KW
source: Group R&D Mercedes Car, DaimlerChrysler AG, Stuttgart

note: The term ISG refers to Integrated Starter Generator hybrid system

The DiesOtto name is derived from the respective engine inventors’ names:, Rudolf Diesel and Nicholas Otto whose basic technologies have been combined by Mercedes in a dramatic example of engine downsizing plus a number of added systems. Engine displacement is cut virtually in half to gain inherent higher efficiency possible at moderate load conditions . Twin turbocharging in series is employed to achieve high power when needed. Mercedes reports that premium grade gasoline is required but conventional gasoline exhaust aftertreatment systems are used rather than diesel emissions controls..

To this is added variable valve actuation, piezo type direct fuel injection and as yet not fully explained variable compression ratio along with “controlled auto ignition” (often called HCCI - homogenous charge compression ignition); a key aspect of the engine’s exceptionally high fuel efficiency. .

HCCI will surely be identified as the single most important development within the package of enhancements which collectively result in the remarkable gain in fuel efficiency Mercedes has achieved. It will also be the topic of intense discussion at engine engineering meetings since it has long been recognized as a very difficult combustion system to manage...

To understand HCCI is to realize that in the spark ignited (Otto) gasoline engine, a charge of fuel and air is compressed as the piston moves up to near its top dead center position at which point it depends on a timed spark to ignite the mixture thereby increasing the pressure of the charge to drive the piston down releasing energy to turn the crankshaft. The objective is to “light off” the charge at a precise time.

In the diesel system engine, air only is compressed by the upward travel of the piston to the point near top dead center at which fuel injection adds combustion pressure to drive the piston down and also turn the crankshaft.

HCCI combustion employs a mixture of fuel and air that is compressed but relies on compression heat to “light off” the charge at the desired precise time. This has been very difficult to achive as it requires a new level of sensors, systems and complex computer program engine engineers have strugged with for many years. The pay of, however, is simultaneous combustion everywhere in the cylinder resulting in lower emissions and total energy use.

HCCI will result in packed meeting rooms at engineering society events where engine engineers will gather to learn how HCCI has been made to work. It will also drive recently graduated engine engineering students back for refresher courses now that the Diesel and Otto systems have been joined by yet another basic system.



The fuel economy improvement also reflects the addition of an automatic stop/start system( employing integrated starter generator) that shuts the engine down when the vehicle is stopped in urban conditions. Mercedes says this accounts for 10% of the efficiency gain and adds 15% to power...This and the various other enhancements in the DiesOtto system are to be phased into production vehicles over time.


Adding to the importance of Mercedes’ achievement is the fact that gasoline contains about 11% less energy per gallon than diesel fuel, hence, on a thermal efficiency basis, DiesOtto performance gain is particularly significant.

Mercedes has indicated it will release more detail about the engine’s operational characteristics and perhaps give an indication of the degree to which fuel consumption is even less than 6 liters/100 km at the coming Frankfurt auto show in September. The firm also indicates the elements of the DiesOtto system will be phased in over time; the schedule for which is also expected at the Frankfurt show.

For now, the degree to which DiesOtto gasoline engine technology has exceeded the energy efficiency of both conventional gasoline and diesel systems, has all the
earmarks of a major milestone. And since DaimlerChrysler is a member of the HEDGE consortium of engine and vehicle companies, it will be asked to what degree HEDGE technology may be employed in the DiesOtto system..

A central question will be how DiesOtto cost will compare with current technology and to what extend its overall economics will permit use in both smaller cars and heavier commercial vehicles. For vehicles that are used mostly at highway speeds, the added cost of the DiesOtto system will surely be the better choice compared with hybrids. The high efficiency of DiesOtto technology combined with stop/start technology may compete as well with traditional hybrids in city conditions.

Historians will note that the DiesOtto development comes from Mercedes which traces its origin back to the beginning of the internal combustion engine powered automobiles of Gotleib Daimler, Wilhelm Maybach and Karl Benz...


About the author: Bob Brooks is a member of the Society of Automotive Engineers and long time automotive technology journalist specializing in powertrains and fuels.


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