Mercedes Diesel Sets New Standard

The Ward's 10 Best Engines competition has been recognizing outstanding powertrain development for 14 years. In this installment of our 2008 series, Mercedes reveals the technical challenges associated with the deliciously powerful, yet eco-friendly, 3.0L V-6 turbodiesel.

HOW MANY AMERICANS HAVE DRIVEN A STATE-of-the art diesel? Very few, we'll bet.

Most still operate on old perceptions: rough, noisy, smoky, smelly and slow. And if they've filled up a diesel at the self-serve pump, they remember smelly fuel on their hands.

That is, unless they have traveled to Europe lately. There, legions of fast, quiet and soot-free modern turbodiesels are more popular than gas burners due to their higher fuel economy, invigorating performance and (until recently) lower fuel prices.

An awful lot has changed since diesel's bad old days in the U.S. The fuel in Europe, and now over here, is a lot less smelly due to dramatically reduced sulfur content.

But those Americans who have not forgotten everything they used to know about diesels should do so at once.

With its 210 hp and prodigious 400 lb.-ft. (542 Nm) of torque, the thoroughly emissions-scrubbed 3.0L Mercedes-Benz Bluetec V-6 turbodiesel delivers a rare combination of V-8-like performance (zero to 60 mph in 6.6 seconds), 4-cyl. fuel-economy and up to 700 miles (1,127 km) of range.

It is better on all counts than a similar-size gasoline V-6, while showing little overt evidence of dieselness, except for its impressive 23 city/32 highway (10-7 L/100 km) U.S. Environmental Protection Agency fuel-economy rating.

What Mercedes markets as Bluetec is a modular diesel emissions-cleansing process that marries several technologies to minimize emissions generated by the engine and effectively scrub the exhaust.

It includes an oxidation catalyst and a diesel particulate filter (DPF) in addition to new techniques for reducing oxides of nitrogen emissions, the last regulated element still higher in diesel exhaust compared with gasoline.

The multi-staged package consists of optimized engine and combustion processes — including dual overhead cams with four valves per cylinder, electronic engine control, third-generation common-rail high-pressure piezo direct injection, variable-geometry turbocharger and exhaust gas recirculation — to minimize engine-out emissions.

The package also includes oxidation catalytic converters to minimize emissions of carbon monoxide and unburned hydrocarbons; a DPF that cuts particulate emissions by up to 98%, which is standard on all Mercedes diesel passenger cars in many countries since mid-2005; and a refined NOx storage catalytic converter to lower emissions.

All this earned the '08 E320 Bluetec turbodiesel the EPA's Bin 8 emissions category, clean enough for 42 states in 2008 but not for California and seven others (Connecticut, Massachusetts, Maine, New York, Rhode Island, Pennsylvania and Vermont) that insist on emulating California's standards.

For 2009, New Jersey, Oregon and Washington also have adopted the tougher Bin 5 California standard, which becomes a federal requirement in 2010.

The next step needed to satisfy all 50 states is an additional module with liquid urea (commercial name: AdBlue) injection in a selective catalytic reduction (SCR) process that reduces smog-producing NOx up to 80%.

That ultra-clean engine has arrived for 2009 in Mercedes' U.S.-market ML320, GL320 and R320 Bluetec cross/utility vehicles.

The Bluetec V-6, known internally as OM 642, was developed by Mercedes engineers in Stuttgart, Germany and offering up to 40% lower fuel consumption than a gasoline engine.

Work on the program began in 2001, and the engine debuted in the European C-Class in 2005. It has been used in a variety of Mercedes passenger cars and commercial vehicles since reaching the U.S. in October 2006 in the E320 Bluetec sedan.

Joachim Schommers, head of passenger car diesel engine development, says the primary objective of the Bluetec diesel program was to deliver a lightweight, powerful and fun-to-drive powertrain that also offers excellent fuel economy.

“The additive-free particulate filter enabled it to meet the most stringent particulate emissions, and SCR now achieves the lowest global NOx limits. Bluetec thus enables a worldwide usage of diesel engines in Mercedes-Benz cars.”

Schommers points out a number of difficult challenges were faced in development of the Bluetec V-6 and its complex but highly effective emissions-cleansing system, including:

To meet 2009 11-state and 2010 federal emissions standards, which require diesels to be as clean as gasoline engines, the earlier version's NOx Storage Catalyst (NSC) system was replaced by the SCR technology. The operating principle of a NSC is based on two successive steps, Schommers says.

First, NOx is oxidized to nitrogen dioxide (NO2) through precious-metal catalytic coating. The NO2 is stored in the catalyst as nitrates, then cleaned up by the exhaust gas. But as the amount of nitrates in the catalyst increases, its storage capacity decreases. So, before a critical loading is reached, the engine controller starts NOx-regeneration by causing the engine to generate rich exhaust gas that reduces the stored nitrates to nitrogen.

To fulfill the ultra-stringent Bin 5 demands, engineers developed the SCR technology, using the AdBlue urea solution. The system consists of an oxidizing catalytic converter, a particulate filter, a feeding valve behind the particulate filter, the SCR catalyst and the AdBlue tank under the cargo area. The tank is large enough to last through two routine service intervals.

“When AdBlue is injected into the hot exhaust,” Schommers says, “it is broken down into water and urea. The urea is then converted into ammonia, the real agent of this process, at temperatures of around 200°C (392°F). The ammonia is stored in the catalytic converter and when exhaust gases pass through it, it helps remove the NOx.”

Schommers adds that the injectors, the mixing area between nozzles and the catalytic converter and the inflow of the converter had to be configured to ensure uniform distribution of ammonia to the SCR catalytic converter.

And while the diesel engine and complex Bluetec emissions-cleansing system is more costly than a gasoline engine, Mercedes is charging only a $1,000 premium for the E320 over the otherwise comparable $51,200 E350 V-6 gas-engine sedan.

Given the diesel's torque and fuel-economy advantages over the gasoline engine, that's a world-class value proposition.

ENGINE SPECS

Daimler AG

3.0L DOHC V-6 TURBODIESEL

Displacement (cc): 2,987

Block/head material: aluminum/aluminum

Bore × stroke (mm): 83 × 92

Horsepower (SAE net): 210 @ 3,800 rpm

Torque: 400 lb.-ft. (542 Nm) @ 1,600-2,400 rpm

Specific output: 69 hp/L

Compression ratio: 18.0:1

Assembly site: Stuttgart, Germany

Application tested: Mercedes E320 Bluetec

EPA fuel economy, city/highway (mpg): 23/32

Ward's 10 Best Engines 2008
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