A Pickup Truck Project

[DieselFan]

I think you are right about the testing having been done under full load.
Anyway, as you said before let's build on tested ideas.
My rear end is a 3.73 but so is the front... its a 4wd remember? All Dana heavy duty stuff. My transmission 6th gear is 0.73 ratio. I had investigated swapping to a different gear but doing both differentials was costly and low ratio gears is not a good idea for pulling. No problem for the engine but not good for the differentials and tranny. The gear vendor additional 6 gears sounds like a better engineered solution to retain pulling power but lower rpm matched to the weight.
One thing I find surprising in Gear Vendor instructions is that oil needs to be changed every 5000 miles! Doesn't make any sense to me. Transmissions can go 100,000 miles without renewing fluids although I do mine every 20,000. How often to you change oil in your gear vendor?
If the only change I make from my current set up is the GV overdrive, so I keep my tranny 0.73 gear and 3.73 diff ratio, can I expect something like 10% highway mileage improvement at the same 60 mph?
Thanks for all the help.

[Diesel_Dave]

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.

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.

[Big Dave]

Optimizing the gearing to lowest BSFC "island" is a fine idea for generators, boats, or even airplanes (Junkers Jumo 205) but cars and trucks operate under a fairly wide speed range. Keeping the vehicle on the optimum island is a piupe dream.

Secondly, turbo diesels have flat torque curves (at the low end of the operating range) and their BSFC "islands" at wode toward lower RPMs.

I have a hard time believing a Cummins at 1950 RPM will get better MPG than my International at 1350 RPM.

[oil pan 4]

All the guys that clam there engine is most efficient around 2000RPMs, live by that code and believe its the best engine speed for there truck to run at are the ones that will never see over 20mpg.

After I install the intercooled turbo set up with water injection I may swap to 2.73 rear end gears. Could end up just shy of 35mpg with just using mostly 1980s and 1990s technology.

One problem I see is you could get the engine chugging along at a low enough RPM to where the alternator does not produce enough power to run the electrical system.
Another would be if you had an auto transmission, the pump pressure out put could be so low that you are sliping your torque converter lock up when chuging along. I can adjust the pressure on my TH700R4 so it wont be a problem for me and the 6.2L came with just about the lowest stall converter ever produced.

[slowmover]

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

Click to BSFC Thread posts #170-173 on Cummins.

A chart/graph might show 1,800-rpm as optimal, but it may also NOT account for winds. Speed, as the final variant, in other words.

[Diesel_Dave]

Originally Posted by slowmover

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

Click to BSFC Thread posts #170-173 on Cummins.

A chart/graph might show 1,800-rpm as optimal, but it may also NOT account for winds. Speed, as the final variant, in other words.

Slowmover, that chart in post #170 was exactly what I was referring to (to not use). That's BSFC vs engine speed at full load. So 1800 rpm would be optimal if you're running up at 100% load--but that's almost never the case, so that particular graph is not really useful for our discussion here.

I did a little digging and I'm pretty sure now that Cummins doesn't publicly release the full BSFC maps. But having worked in diesel R&D, I can tell you that the basic characteristics are the same as that TDI map that I referred to earlier (post #37 in that other thread). Tho only exception to that is what Big Dave pointed out--many of the torque curves (including the Cummins ISB engine) have a flat top--just like you see in post #170. FYI, that's usually done not because the engine cannot produce more torque, but because the transmission can't take the higher torque level, so the engine limits the fueling such that that maximum torque is not exceeded.

Oil pan, I don't think you should have any trouble with the alternator. I've never sized alternators myself, but my understanding is that they are sized such that they can keep up with all the electrical loads with the engine at idle speed. If they weren't then your battery would get drained down if you were sitting idling.

[slowmover]

Slowmover, that chart in post #170 was exactly what I was referring to (to not use). That's BSFC vs engine speed at full load. So 1800 rpm would be optimal if you're running up at 100% load--but that's almost never the case, so that particular graph is not really useful for our discussion here.


Okay, I'm all for clarification. I wondered what you were posting. It's still the only thing I've found. And those of us with trucks have a tendency to hook up trailers, carry gravel, etc. Not disputing your point. (Therein enters the question of gear choice, road speed and EGT when heavily loaded: optimum rpm on grades, etc).

I can tell you that the basic characteristics are the same as that TDI map that I referred to earlier (post #37 in that other thread).

Then how do we use it? I haven't read up on how others interpret these maps since we didn't have one. I much appreciate your pointing out this map.

[Big Dave]

Real BSFC maps are expensive and are generally closely held propriatary information.

[oil pan 4]

The battery in my truck was getting drained at idle.
Even city driving was draining them, I would charge them up, drive around shut the truck off and they would be a 12.4v.
I have caused a few electrical problems by switching to an electric coolant pump, electric fans and electric fuel pumps on the truck. I also plan to add a gast compressor (I tested it and it draws 50 amps at no load) and a 2,500w power inverter.
The alternator that the 6.2L came with was sized for an all mechanicaly controlled and driven diesel engine.
There is a rebuilt 95 amp ACdelco unit ready to replace what ever is in there now and I picked up some free solar panels that should barely cover "off" power draw and hopefully be some what good for battery float charging.

[Big Dave]

A larger drive pulley solves the "accessories at low idle" problem.