Mercedes has made it official: Diesel power will be offered in its 2007 SUVs, giving the Stuttgart three-pointed star bragging rights for the first diesel seven-seat 'ute on the market following the cancellation of the Excursion.This is just one aspect of Mercedes' approach to consumption reduction, as its hybrids, fuel cells, and direct-gasoline-injection vehicles continue to evolve. The company doesn't see diesel as the only answer, but with fuel savings of 25 to 40 percent and minimal to no loss in performance, it's a viable option.

The 320 CDI badge will find its way onto the ML- and R-Class this fall and the GL by early 2007, starting with 45-state sales and eventually expanding to 50-state applications. As it did with its 450 and AMG 55 and 63 labels, Mercedes took some license; the 320 CDI is a 3.0-liter and the Euro-only (for now) 420 CDI is a 4.0-liter.

Undersquare dimensions (bore is less than stroke) are typical in diesels and are 3.27 x 3.62 inches for this all-aluminum V-6. However, the V angle is 72 degrees, crank pins are offset, and a balance shaft was fitted, all to ensure smoothness. Dual chain-driven camshafts impel four valves per cylinder around central piezo injectors, with maximum injection pressure of 23,500 psi, an amount that would quickly erode the top of a traditional-material piston; all we could find out about the piston content was that it's "proprietary."

The injection system is the most modern version of common-rail and runs five events per cycle, two pre, one main, and two post injections, through eight-hole injectors. The variable-nozzle turbocharger generates close to 29-psi maximum boost pressure.

As fitted to the 'utes, the OM 642 engine will be rated 224 horsepower at 3800 rpm and 376 pound-feet of torque from 1600 to 2800 rpm. It starts making usable boost at 1400 rpm and is on full steam shortly thereafter, falling off from 2800 rpm. However, there's more than 200 horsepower available from 2800 to 4200 and still 260 pound-feet on tap as fuel flow cuts back approaching the governed no-load engine speed at around 4600 rpm. It'll be teamed exclusively with the 7G-Tronic seven-speed automatic.

For the new Sprinter coming to the U.S., the engine will be derated to 180 horsepower and 325 pound-feet maximum torque in deference to gearbox, cooling, and commercial-use requirements. A mainstay of Mercedes propulsion, the OM 642 is offered in everything from the Viano van to the flagship S-Class. In most of Europe, the ML diesel accounts for 80 percent of sales, over 95 percent in Austria. If the diesels meet with anything approaching similar success, it's possible the 4.0-liter twin-turbo 420 CDI might come stateside, where its 306 horsepower and 516 pound-feet would paint a comparison-test target on the Touareg TDI's doors.

Mercedes Powertrain senior manager Dr. Frank Duvinage notes that Mercedes-Benz's goal is to make gasoline engines as efficient as diesels and diesels as clean as gasoline. With the Blue-tec system applied on the E-Class, NOx is the only category in which the diesel's emissions remain higher than a gas engine's. But CO2 is considerably lower in the diesels.

In the future, the AdBlue system of urea injection will come to the utility family, pending certification as of press time. It won't be unproven technology, as it's already been used on 20,000 trucks, and refilling will be done at regular maintenance interval services.

Over 400 miles of highway passing, start/stop driving, cold starts, and 'wheeling, we never saw a wisp of smoke nor smelled diesel fuel. Good fuel and particulate filters go a long way to cleaner exhaust, but to be fair there was always a breeze during our stops.

The 320 CDI drives like a GL450 with less lunge when you hit the accelerator; boost feeds in progressively and smartly, urging you forward in a subdued rush of torque. At 65 mph, the engine turns around 1650 rpm, right in the sweet spot for towing torque and low consumption. Unless idling, it's as quiet as a current gas engine, and after many miles cruising at 50-65 mph, our co-driver rightly observed there's nothing to suggest it isn't gasoline powered, except for the exceptional 500-mile range.

European economy figures put a GL320 CDI at an average 9.8 liters/100 km, or roughly 24 mpg, about 30 percent better than the 4.6-liter gas engine with 110 more horsepower and 40 pound-feet less torque. The 4.6 will get to 60 almost two seconds earlier, yet on the road a 320 CDI, apart from losing the race across the crosswalk, has no trouble keeping up or towing a load.

Over fast, winding roads interspersed with long idle periods and low-gear trail driving, we managed to return 18.5 mpg, a four mpg gain over what a GL450 delivered in similar conditions and overall average speed. With reduced heavy-throttle application, no idling, and no trail use, we ran 20.5 in the diesel V-8, so a real-world number in the low to mid-20s sounds reasonable for a 320 CDI. With less weight and better aero respectively, the ML and R might improve slightly on that figure. In other applications, the diesel-price premium in a Mercedes-Benz runs about $1000. But since the GL comes standard with a V-8, it may be only the ML and R that get a price bump. In any event, Mercedes believes the time is right for a diesel-powered SUV lineup. Who are we to argue?

Mercedes-Benz GL/ML/R320 CDI
Engine type TD V-6, alum block/heads
Bore x stroke, in 3.27 x 3.62
Displacement, ci/L 182/3.0
Compression ratio 17.7:1
SAE horsepower, hp @ rpm 204 @ 3800
SAE torque, lb-ft @ rpm 376 @ 1600-2800
Transmission type 7-speed automatic
1st 4.38:1
2nd 2.86:1
3rd 1.92:1
4th 1.37:1
5th 1.00:1
6th 0.82:1
7th 0.73:1
Reverse 3.42:1
Axle ratio 3.45:1
Final-drive ratio 2.52:1
Base curb weight, lb 5225
Max payload capacity, lb 1705
Max GVWR, lb 7150
Max towing capacity, lb 7700
Fuel capacity, gal 26.4
EU fuel econ (comb, converted to mpg) 24.0


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