Quote:
Originally Posted by DieselFan
I looked up a Cummins report showing lb of fuel per hour at various rpm and it had the lowest consumption at 1950 rpm. I don't know how much load was put on that engine during the test though.
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I think I know the report that you are referring to (if you have a link, please provide). If it's what I think it is, it's a chart that shows BSFC vs speed
at the torque curve (100% load). The purpose of this chart is primarily to determine the max fuel flow at any given speed. You should NOT use this type of chart to determine what cruising rpm will give you the best FE.
The correct type of chart to use is a BSFC map (BSFC vs speed AND torque). I'm not sure it Cummins publically releases it's BSFC maps, but most all modern turbodiesel BSFC maps have more or less the same general shape. For an example see here (post 37):
http://ecomodder.com/forum/showthrea...ot-1466-4.html
The thing to remember is that when you shift gears your engine speed changes but the power does not, therefore the torque changes (HP=speed[rpm]xtorque[lb-ft]/5252). So let's say your cruising along at 2000 rpm, 100 lb-ft of torque. That means you need 38 hp. Now let's say you shift to a higher gear and now you're at 1500 rpm. To get the same power, you'll be at 135 lb-ft. I think your 5.9 torque peak is about 650 lb-ft of torque so you've gone from 15% load to 21% load. Notice from that BSFC map how bad your BSFC gets when you get to low load%. So the big benefit of the lower rpm isn't so much the lower rpm itself, but the higher load%.
Generally speaking on a modern turbodiesel, until you get up near the torque curve 80-90% load, a lower rpm will almost always give better FE. The thing to keep in mind with trucks like ours is that when you're just cruising down the highway with little or no payload, the engine is WAY oversized (you're lucky if your over 30% load). It pays to shift to a higher gear to get that load% as high as possible.
I personally keep under 55mph in 6th pretty much all the time. This means I almost never go over 1600 rpm.