Most folks will never know what's humming under the hood of the Mercedes-Benz ML320 Bluetec. The characteristic diesel rattle, smoke, and smells are gone. There's no starting hesitation in frigid weather, and the tailpipe residue is gray ash instead of black soot. From idle to the 4500-rpm redline, the Bluetec V-6 sounds more like an electric motor than an internal-combustion engine.
A seven-speed automatic restricts the engine to its torque-rich 2000-to-3000-rpm band with rare excursions beyond 4000 rpm. The shift paddles are fun to play with but are unable to stop the transmission from upshifting well before the redline.
The Mercedes diesel V-6's core components are an aluminum block with iron cylinder liners, aluminum heads supporting dual overhead camshafts and four valves per cylinder, and a forged-steel crankshaft. To achieve even firing and low vibration with this engine's 72-degree V-angle, a split-pin crankshaft and a balance shaft are necessary. The electronically controlled common-rail injection system delivers two to five squirts of fuel per combustion cycle at 22,720 psi through piezo-electric injectors. Multiple injections quell the rattle while the ultrahigh delivery pressure fosters fast fuel vaporization and more complete combustion. Half of the intake ports are throttled at times to swirl the air entering the combustion chamber. A variable-nozzle device gets the turbo spinning and boost rising at low rpm. External emissions controls include an oxidation catalyst, a particulate filter, a valve to meter the AdBlue aqueous urea, and a selective catalytic reduction stage. To burn accumulated soot from the particulate filter, the exhaust temperature is periodically raised to 1100 degrees Fahrenheit by injecting extra fuel. The seven-gallon supply of AdBlue fluid is replenished every 10,000 miles during normal oil-change service.
Don Sherman, Technical Editor ...next page >>