I've worked for a manufacturer of engines, compressors (reciprocating and centrifugal), gas turbines, turbochargers and related control systems for 31 years, the last 26 in technical management. Our experience in the R&D lab, the factory test floor and in the field is that best light-to-moderate load BSFC will vary according to type, size and application of the engine since accessory (including pump drives - oil, jacket water, aftercooler water and hydraulically driven cooling fans) and friction losses are a function of RPM. It really depends how these parasitic losses (which increase linearally and/or exponentially with RPM) balance out against the RPM at which the combustion process is at its optimum. If the "bare engine theoretical" BSFC curve as measured on the flywheel dynomometer without accessories tends to be very flat, then the optimum light load RPM insofar as BSFC is concerned could be lower than the peak torque (peak BMEP) RPM since the reduction in parasitic loads could more than offset the slight degradation in "bare engine theoretical" BSFC.

Rusty

 

I figured as such Rusty.

I guess all I have to go with are the manufacturer's suggested BSFC curves which certainly don' t account for the parasitic losses of the accessories.

What I was shooting for is what you said and thats the combustion process optimum. If you can shoot for the RPM band where the engine combustion is at the peak effieciency, then the account for parasitic losses my not be required as we are not talking full load in this case.

If the combustion process is optimized, then maintaining that RPM for the large percentage of the engines operation should yield lower fuel consumption rates.

J-eh

 

Not to , but my point was that at lighter loads, parasitic losses become more significant in terms of percentage of total horsepower output, so the lighter the load, the more the best BSFC RPM might vary from (likely be lower than) the peak torque/BMEP RPM.

Does that make more sense? (I fear that I'm not communicating very well...)

Rusty

 

That helps.. I think it would probably be easier to discuss over a coffee..


I found an article that Joe Donnelly wrote in Issue 29 (Fall 2000) of the TDR magazine. IT has alot of good stuff on just what we are discussing here, that with some factory BSFC curves for the 12V and "new" 24V at that time.

It has alot in it that we have discussed here. I wish it was online available, or I could scan it.

 

For a CTD, the HP and TQ curves should never cross. Curves only cross when engine RPM exceeds 5252. Above this hp>tq. Below this hp<tq-- ALWAYS.

JLH

 

Sorry, didn't mean to submit a vague post. I would have pasted the engine efficiency map I referred to directly in my post if I could figure out how to do it (it's incorporated [page 2] in a .pdf file).

Based on this map, peak thermal efficiency (>43%) is reached at about 1800 rpm and at about 12 BMEP, which is at a "heavy" load, but not at "full" load. This of course is only for the EPA test engine, but it would in all likelihood apply to diesel engines in general, I would think. Efficiency drops off to less than 30% at extremely light loads though the entire rpm range.

 

12 BMEP??? You must mean 12 bar BMEP, not 12 PSIG BMEP since BMEP at peak torque for the 245/505 2001 and 2002 HO is around 200 PSIG (I'll run the formula if anyone wants an exact value). 12 PSIG BMEP is certainly not a "heavy" load.

Rusty

 

Yes, 12 BAR BMEP.

Can anyone advise on how I can get the map from a .pdf file into my post?

 

If you have the Adobe writer software you might be able to select an image in the document and save it as a Jpg... Or you could revert it to Word and edit it that way too.

I haven't tried either of these as I usually only write to adobe.

 

Think I'll just go 65 or less. There are way too many acronyms flying around for me. I'm hunkering down in my fox hole.

- JyRO