Mercedes’ new E-Class nudges gasoline efficiency toward diesel

  • 11-Mar-2013 05:15 EDT
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There are significant technology changes for Mercedes' new E-Class, but styling changes are modest; they include a choice of front ends: central star (illustrated) or traditional radiator.

Changes to Mercedes-Benz’ updated E-Class are many and varied, but particularly significant is a new 2.0-L, four-cylinder gasoline engine that combines stratified lean burn technology with turbocharging and external high pressure exhaust gas recirculation to help achieve (for its segment) very low fuel consumption and CO2 emissions. This technology integration is regarded by Mercedes as a “world first.”

Interviewed by AEI, Dr. Dieter Zetsche, Chairman of Daimler AG and Head of Mercedes-Benz cars, said some of this advanced technology was the result of work that included the DiesOtto research project, which merged several aspects of gasoline and diesel technology: “But it does not include variable compression ratios. For that we would need an entirely new engine block, which would only be achieved by a new-generation design.” Because of the use of turbocharging, the compression ratio of the new gasoline engine is only 10:1. The company's 2.1-L E250 diesel is at 16:1.

Zetsche neither confirmed nor denied whether such a radical design was likely to become a production reality, but he stressed that the new engine in the E250 makes it the only model in its segment to achieve “efficiency class A” within EU6 emission legislation. The E250 has a combined fuel consumption figure of 5.5 L/100 km, CO2 emissions of 135 g/km, and power output of 155 kW (208 hp). Performance figures for the E250 sedan with a 7G-Tronic Plus transmission include a 0-100 km/h (0-62 mph) time of 7.4 s and a top speed of 240 km/h (149 mph). Maximum torque of 350 N·m (258 lb·ft) is available from as little as 1200 rpm in a flat line on the graph to 4000 rpm.

Patrick Hawig, Powertrain Engineer for the E-Class, said the engine was a very significant part of Mercedes’ downsizing philosophy to provide V6 or even V8 performance using four-cylinder, fuel-efficient units. The E250 uses lean burn, a mode with significant excess air and precision multiple injection, at speeds up to 70 km/h (43 mph).

Hawig said the toughest challenge in the engine’s development concerned the use of lean burn in combination with turbocharging and the need for a NOx storage catalyst and a regeneration capability.

“By lowering the CO2 to 135 g/km, we are closing the gap between this engine and that of a similar diesel engine,” he explained. “The cost of the engine is less than that of a diesel despite all the high technology equipment we use.”

Third-generation spray-guided direct gasoline injection uses a system pressurized to 200 bar (2900 psi). Newly developed piezo injectors are fitted, facilitating up to five injections per power stroke. Rapid spark ignition provides a consecutive four spark discharge per millisecond. Timing of sparks and combustion period can be varied according to driving regime.

Low fuel consumption is also a major consideration for the latest version of the E300 BlueTEC Hybrid. The combined figure is 4.1 L/100 km with the engine driving through a 7G-Tronic Plus transmission.

The two-wheel-drive E63 AMG version of the E-Class, too, has an impressive official fuel consumption—9.8 L/100 km and 230 g/km CO2—for its engine size and power: The 5.5-L V8 produces 410 kW (550 hp), up from the previous model’s 386 kW (518 hp), with a maximum torque of 720 N·m (531 lb·ft). The 0-100 km/h sprint takes 4.2 s. The E63 AMG’s engine is also available in S form, adding a further 20 kW (27 hp) and taking maximum torque to 800 N·m (590 lb·ft) for even higher performance: 0-100 km/h in 3.6 s with 4Matic all-wheel drive. All versions use an AMG Speedshift MCT seven-speed sports transmission.

Dr. Jörg Gindele, Head of Powertrain Development Mercedes-AMG, said of the increased figures: “We did a lot of work on calibration to squeeze more power. We always look very deeply into the combustion system to learn more about optimization. We found we could use a higher boost pressure and saw that we could achieve better combustion regulation and better control of peak cylinder pressure.”

How are fresh power-output targets defined? “By the Board of Management; they say to the engine development guys: ‘What can you imagine?’ So we discuss this and come up with a figure. It doesn’t make sense to go for perhaps just 10 kW, so we say we’ll go for more.”

A challenge for the E63AMG was to achieve the required NVH in Comfort mode, said Gindele. “Reducing engine revs via higher gearing helps, but the engine always needs to sound ‘right.’ Sound and power output are essential for AMG buyers: 'It is an arbitrary sound—hard to define—but it also has to meet legislation. We worked with universities in analyzing the sound and have achieved it through the exhaust and engine calibration.'"

There is likely to be even more power to come from the AMG units because Mercedes wants to stay ahead of the competition (BMW M5 and Audi RS6): ”We look closely at the market, aware that if we don’t add more power we will lose customers. AMG cars are bought because they are the most powerful and emotional in their sector.”

While the power game continues and further ICE (internal-combustion engine) development such as the new E250 lean-burn turbo continues, alternative powertrains are very much on Zetsche’s agenda.

Daimler, Ford, and Nissan have signed an agreement for the joint development of a common fuel-cell system that will keep engineering, manufacturing, and technology costs down. The companies say they want to send signals to suppliers, policymakers, and industry in general to encourage the further development of hydrogen infrastructure globally.

“We can divide costs by three in this area; it is very difficult to get there on your own,” said Zetsche. “You also have to optimize the manufacturing processes. Nissan has their own fuel-cell technology and make stacks of less exotic materials.”

The aim is to cut fuel-cell unit cost to the level of a diesel hybrid by 2017. Zetsche said he then hopes to see production rates of “tens of thousands.”

The bosses of the three companies (Zetsche; Carlos Ghosn, Renault-Nissan Alliance; and Alan Mulally, Ford) are in regular contact about the development of fuel-cell collaboration. “That is through video conferencing and meetings at international motor shows. You need a personal relationship to make it work well,” stressed Zetsche.

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