Diesels are smelly, smoky and loud, right? Not if my advance trial of Cummins’ new 6.7-liter QSB common-rail diesel is any indication. This marine power plant offers clean, Tier 3 emissions as rated by the Environmental Protection Agency along with quiet operation and laudable fuel economy. The new in-line six-cylinder series debuts this summer in 380, 425 and 480 hp offerings, mirroring the current horsepower ratings of the 5.9-liter engine it replaces.
New federal EPA Tier 3 emissions standards require 55 percent fewer exhaust particulates and 17 percent less NOx — the industry designator that includes both nitric oxide and nitrogen dioxide. That translates to nearly unnoticeable exhaust. Cummins and other engine manufacturers reach these goals with common-rail fuel delivery, which supplies all injectors with full fuel pressure at all times. Older systems — even those with electronic governors — relied upon the position of the rotating camshaft to create fuel pressure only at the exact moment an injector fired. With a common rail, the injector becomes an electronic valve with limitless control over the time, shape and duration of each spray of fuel.
Common rail delivers a tiny charge of fuel after the main power injection to clean up exhaust, and a bit of fuel introduced before the main power injection to smooth out the preignition “rattle” heard on many diesels. When I was at the Cummins plant last fall to preview this new engine, engineers turned this “pilot injection” off, and then back on, so I could hear the difference — they claim an 83 percent reduction of noise and vibration at idle. I’ve supplied an audio clip of the engine running with and without pilot injection. You can hear it in the video below.