One-Ton Diesel Pickups - 15% Grade w/10,500# Trailer:
After witnessing the dramatic results of the 15% contest among the gas trucks, we were even more interested to see what changes or stumbles might await the diesels on this steep hill.
Due to space and readability considerations, the bar graphs below only show the test results from 100-m to 250-m. See the table at the bottom for full test results from 50-m to 250-m.
All times and speeds measured are shown cumulative over the run.
In the graphs below: the lower a truck's bar is, relative to the other two trucks' bars, in the "Time Over 250-Meters" graph the better it performed. The higher a truck's bar is, relative to the other two trucks' bars, in the "Speed over 250-Meters" graph, the better it performed.
The shortest time required to cover the 250-meter distance determined the best performing truck.
One-Ton Diesel Pickups 15% Grade Assessment:
If we were directing marketing for GM, here's how we'd pitch its diesel motor. D for Duramax. D for dominant. Check out the how fast the Chevy finished the hill climb - more than 5-mph faster and over 2 seconds quicker than either the Ford or Dodge haulers! This time it even beat the Cummins in the first 50-meters.
Though the Power Stroke was still slower than the Cummins to the top, the gap between those two engines narrowed substantially on the 15% grade, in favor of the Ford. If the F-350 and Ram 3500 both had 3.73 rear gear ratios, instead of the Ram's 4.10, we think the F-350 almost certainly would have moved into the second spot.
But there was an incredibly interesting reason why the performance gap between the F-350 and Ram 3500 seemed to close so much. After two sprints up the hill, the F-350's diesel particulate filter (DPF) went into regeneration mode. The DPF is a new emissions control device required to reduce diesel particulate emissions 10-fold from 2006 model year levels. Regeneration is triggered when the truck senses too much back pressure from the DPF, because of a large amount of trapped particulates. To get rid of the soot, the engine injects metered amounts of diesel fuel into the exhaust stream to incinerate the soot in the filter. On the third run, after regenerating, the F-350 ran the hill up to a full 2-kilometers-per-hour (1.2-miles-per-hour) faster than its first two passes.
We were very surprised by the apparent power boost the Power Stroke received from regeneration. It illustrated just how much the new DPFs can restrict airflow through the exhaust system, and the challenge faced by all the OEMs to improve engine performance while complying with new emissions regulations.