Volume 3 Issue 2 - Diesel Articles

The 2011 Duramax LML replaces the LMM with More Horsepower and Torque While Adding More Than Ten Percent Fuel Efficiency

If you were looking to buy a new diesel truck and I told you that the Duramax brought additional horsepower to the table over last year's engine you might stop to consider it a worthy contender. If I told you that the table was shattered with 765 foot pounds of torque (up from 660 in last year's model) I'd get your attention. You might ask, extra horsepower and torque are great, but how much am I gonna pay for it at the fuel pump? As Joel Paynton pointed out before the new Duramax's release this Summer, the engine actually produces all that additional power while improving fuel efficiency and emissions. Now we learn from GM that the improved fuel economy may be as high as 11 percent. They also managed to make the EPA happy, or maybe just a little less unhappy, by reducing NOx emissions by at least 63 percent over the 2010 LMM.

How did GM pull it off: more power on less energy? GM's press release boasting of the 2011 Duramax offers some clues. In this article, we will investigate some of the mechanical wizardry that made it possible.

Looking at GM's press release on enhancements for the 2011 Duramax, we discover:

• Main bearings’ profiles changed to enhance oil film thickness
• Oil pump flow increased for increased pressure at low speeds
• The turbocharger’s oil circuit is changed to provide increased pressure at the turbo and faster oil delivery
• The connecting rods’ pin ends are modified to provide increased piston support
• New, higher-strength piston design that eliminates bushings to provide lower reciprocating weight

Let's look at these one-by-one to discover why they make the LML stronger and more fuel efficient at the same time.

Top view of the new Duramax LML Engine

Main Bearings’ Profiles Changed to Enhance Oil Film Thickness

The main bearings connect to the front and the rear of the crankshaft. By slightly enlarging the main bearings, the thickness of the oil film increases. This increase in film strength allows the crankshaft to sit further away from the bearings. The result? Each turn of the crankshaft produces less drag. Drag occurs between the crankshaft and the bearings because of the extreme pressure that occurs between them. Oil, under normal circumstances, possesses liquid properties; however, under extreme pressure, like that which occurs in the crankshaft, the oil film thins and begins to act more like a solid. Instead of allowing the crankshaft to roll freely as it should when surrounded by lubricating oil, friction (drag) increases as if the crankshaft were rubbing against a solid (even though a very thin oil film is still, present). When a thicker oil film is present, as a result of the heavier duty, bigger main bearings like those in the LML, the oil film thickness increases and the oil tends to maintain it liquid characteristics, meaning less drag.

For Example

At 1,500 revolutions in one minute, the decreased drag with every turn of the crankshaft means improved efficiency 25 times every second. At 3,000 RPM, the reduced drag burns less fuel 50 times a second. Of course reduced drag means being able to produce the same amount of power with less fuel: it also means that if you use the same amount of fuel as in the LMM you will get more power, a fact reflected in the LML's higher power ratings.

While the beefier fittings positively affect fuel efficiency and power output, the only negative impacts should be the additional cost to manufacture the bearings and a little bit of space since they would take up more room in the engine. Perhaps the increased cost involved in the bearings led the GM engineers to only replace the main bearings and not the eight connector rod bearings but it also makes sense from a mechanical perspective. While the main bearings are constantly engaged in the transfer of power between it and the transmission, the connector rod bearings only transfer power to the crankshaft during the power stroke; that is, improving the film strength for those bearings through the expense of making them bigger would only yield improved fuel efficiency or power following combustion in one of the eight cylinders with no net improvement during the intake, compression and exhaust strokes. Another reason to not use bigger bearings for the connector rods? Right. Increasing the space inside the bearings increases the potential damage caused by the rods as they hammer down on the bearings during each power stroke. Any significant play means deformed bearings sooner or later. At the very least, using bigger rod bearings would necessitate using a heavy viscosity (lower fuel efficiency) oil to insure proper film strength in those bearings. These issues, on the other hand, are not present with the main bearings.