Diminutive Duramax Diesel: Big on Performance

GM gives its mid-size pickup customers what they’ve been clamoring for, a clean and quiet, high-torque, fuel-efficient diesel.

“The 2.8-liter Duramax engine originated with VM Motori during our joint venture days”—with said venture being with Fiat—“and is built in GM’s diesel engine plant in Rayong, Thailand,” says Chevy Colorado/GMC Canyon assistant chief engineer Scott Yackley. It is an evolution of VM Motori’s A 428 DOHC common rail diesel, and produces 181 hp at 3,400 rpm and 369 lb.-ft. of torque at 2,000 rpm. Designed to meet the ULEV 125, Bin 4 emission standards of 0.110-0.125 grams/mile of NMOG (non-Methane Organic Gasses) and NOx, the engine will be available in both California and the NESCAUM (Northeast States for Coordinated Air Use Management) states of Connecticut, Maine Massachusetts, New Hampshire, New Jersey, New York, Rhode Island and Vermont. And unlike certain other diesel engines in the news, Yackley claims the emissions levels have been tested both in the lab and on the road.

“We planned for the engine from the very beginning of the small truck program,” says Yackley, “and updated the engine we had been using overseas to meet the demands of our customers over here.” Many of these updates were made to help the engine meet the more restrictive U.S. emission standards, while many others were implemented to reduce NVH to levels expected of a premium small truck. And all of them had to be accomplished without degrading either fuel economy or towing ability.

“We spent about four years develop-ing the package for North America,” says Yackley. This engine was being put in a new application with a new transmission and with the addition, for the first time, of selectrive catalyst reduction (SCR). That regimen meant 35,000 hours on the dyno, running the powertrain in the vehicle, conducting NVH testing, and running many of the same validation tests used on the heavy-duty 6.6-liter Duramax V8. “All told,” claims Yackley, “that’s equivalent to about 2.5-million customer miles before the first North American-spec engine ever came down the assembly line.”

The 2.8-liter Duramax features a cast-iron block topped by an aluminum twin-cam head, forged steel crankshaft and connecting rods, and aluminum pistons. Bore and stroke is undersquare at 94 x 100 mm, and scrapers are used on the valve seats to keep every-thing clean. To keep noise and NOx emissions under control, the 2.8-liter four runs a 16.5:1 compression ratio. Each of the four cylinders is fed by a high-pressure common rail fuel system, and the engine uses three injections—pre, pilot and post—per cylinder. Oil jets cool the underside of the pistons, while the oiling circuit also features a dedicated feed for the Honeywell variable nozzle turbocharger that is designed to speed and improve lubrication of the turbine’s bearings. Maximum boost is 35-40 lb./in.2, and 90% of maximum torque is available from 1,400-2,800 rpm.

In addition to the multiple injections and slightly subdued compression ratio, NVH reduction required dedicated strategies. These include a pair of counter-rotating balance shafts mounted just below the crankshaft, a damped steel oil pan and timing cover, plus a damped panel located below the engine cover that runs along the top of the cam cover. In addition, there is a centrifugal pendulum vibration absorber located within the torque converter of the six-speed HydraMatic 6L50 automatic transmission to reduce both noise and vibration. 

Its secondary spring masses vibrate in a direction opposite to that followed by the engine’s torsional vibrations, and is the first use of this technology in a GM vehicle or in the mid-size truck segment. Not surprisingly, the intake manifold is plastic, which not helps to reduce noise, but provides smooth intake runners and integrates a number of parts into a single assembly. Ceramic glow plugs significantly reduce the time it takes for the cylinders to reach temperature, and run at higher temperatures with increased longevity.

The 2.8-liter Duramax uses a multistep emissions control system that begins with cooled EGR to lower combustion temperatures (and NOx emissions) by redirecting some of the exhaust gasses back into the intake manifold. Next up is an engine-mounted oxidation catalyst that reduces hydrocarbon formation, followed by an SCR system that adds urea to the exhaust gas stream before it is adsorbed onto a catalyst. The urea tank holds five gallons, and is topped off during the normal 7,500 mile service. Should it be necessary to fill the tank before the next service, the filler cap is located next to the fuel filler, and a warning light will illuminate 1,000 miles before a soft shut down is initiated. In addition, the engine has a diesel particulate filter with a dedicated diesel fuel injector. According to Yackley, “This allows us to follow an aggressive regeneration schedule without a reduction in engine power whether the vehicle is out on the road or idling through a ‘farm cycle’ in the fields.” 

One of the engine’s more inter-esting features has nothing to do with emissions, NVH or power output. A “Jake,” or exhaust, brake can be engaged to slow the vehicle and reduce brake wear. Unlike the version found on semi-trucks, this unit is quiet and has none of the loud reverberation that typifies an over-the-road system, though it works in the same way. As an unthrottled engine, a diesel always sends a full air charge into the cylinders where it is compressed and returns this energy to the crankshaft. By opening the exhaust valves on deceleration, the compression brake releases the pressure in the cylinders, and energy is absorbed. With cruise control engaged it can be used to maintain the desired speed in downhill runs without applying the brakes. When cruise control is not in use, it adds braking force to maintain vehicle control regardless of vehicle load or road grade. In either case, it works seamlessly with the truck’s standard integrated trailer brake controller.

When I spoke with Yackley, EPA mileage ratings hadn’t been released, but it seems as though 35 mpg for the two-wheel drive versions of the truck is not out of the question. The towing capacity is something that was established: Two-wheel drive trucks are capable of towing 7,700 lb or carrying 1,506 lb. of payload, while four-wheel drive versions can handle 7,600 and 1,470 lb., respectively.