the limits of diesel

wintersun wrote:
To produce more power from an engine you need to reduce pumping loses (cooling system, oil pump, fuel pump, etc.) and burn the fuel more efficiently. High compression diesel engines are already more efficient in terms of the percent of the available energy in a gallon of fuel than the lower compression gasoline engines.

Sometimes to meet new emissions levels requirements the manufacturers need to improve the efficiency of their engines so that more of the fuel is burned inside the cylinders and not sent out the tailpipe. Cummins is doing this with its next generation of engines for medium to heavy trucks and this technology may find its way into the small engines used in pickup trucks in time.

What is unfortunate is that the public wants more powerful cars and trucks and will buy a new truck or truck to get an extra 50HP when their current vehicle is not in need of replacement. They will buy the new vehicle to get an extra 10-15 percent gain in horsepower but are less eager to buy a new vehicle to gain an extra 10-15% in fuel economy. Otherwise there would be no Hummers on the road and the automakers would be focusing on hybrids.

The gating factor with today's pickups is the amount of torque that the transmissions and drivetrain can handle and auto transmissions can handle more power than a manual transmission. Drag racers switched to automatic transmissions starting in the 1970's for this reason. There is also a limit as to how much of that power can be directed to the rear tires before they start to spin.

GM provided a lot of information for the 2011 trucks with the LML engine that produced a great deal more horsepower and torque but needed stronger bearing journals, improved oil flow, and stronger drivetrain components to insure that overall reliability was not reduced.

The MPG figures are from EPA tests of vehicles that are indoors and not subject to air drag or changes in elevation. They are not close to real world performance in terms of fuel usage and this is particularly true with highway MPG. The manufacturers continue to use very tall gearing and this helps with higway mileage figures but these same gears work against the truck that is being used to haul or tow a heavy load.

You bring up some really good points. I noticed that the EPA mileage estimates on the gasoline engines seem to be inflated in some cases. I have always been lucky to get near what they claim. I have owned two VW TDI cars and both diesels got much more mileage than the EPA rating. My current Passat with an automatic transmission gets around 38+ mpg in town, with most of the driving being done with the AC on. On the road it gets around 45 mpg. Those numbers are significantly higher than the EPA estimate and my TDI Jetta was the same way. It didn't get quite as good of mileage with a manual transmission, as the Passat does, but it is close. My 1994 Dodge dually got 14 mpg pulling my 34' Avion. My 2000 Ford 7.3 Powerstroke gets about 11 mpg pulling that same trailer. That trailer has been pulled by gasoline trucks but the mileage was so bad that I wouldn't even consider it again. Maybe the new gasoline trucks will do much better, but the older ones didn't.