DRAG-REDUCTION vs BSFC

[aerohead]

Hello all.Geebee had posted a thread which got cfg83,akashic,PaleMelanesian,some_other_dave,and myself off into BSFC,as related to drag.I was reluctant to get into it without my references,and I also thought this was of such general interest,that it deserved it's own "thread" in the aero "sticky".

First,let me apologize to anyone who went off on a wild goose chase in Hucho's book looking for the nuts and bolts on this topic.I found the material I was thinking of,and while Hucho touches on it,the following fleshes it out better than anything I've run across.

The reference is:SAE Paper#830304 "Tractive-Energy-Based Formulae for the Impact of Aerodynamics on Fuel Economy Over the EPA Driving Schedules",by Gino Sovran,General Motors Research Laboratories,presented,International Congress & Exposition,Detroit,Michigan,February 28-March 4,1983.


The paper revolves around changes in fuel economy" resulting from specific changes in one of the overall vehicle characteristics.",specifically,"resulting from a prescribed change in tractive energy"--- "The only direct change considered is -----resulting from a change in aerodynamic drag."---------------- "If the tractive energy is reduced,bsfc will usually increase because engine operation occurs at smaller fractions of full load.Since bsfc is the dominant multiplier in the fuel consumption equation the possibility that it might change must be accounted for."------------------------- "Fuel economy studies -----show an essentially linear relationship between g (fuel economy[my italics] and CdA for any given vehicle----."------------------"Without appropriate drivetrain changes to accomodate the reduction in CdA the impact of fuel consumption is only about 60% of what is available and attainable."------------------- "(engine speed / vehicle speed ratios desirable for low-drag vehicles may not always be available-------------------- particularly for vehicles with spark-ignition engines."

Too actually ascertain all the data to intelligently analyze and arrive at the "best-fit" gearing for highway mpg may prove daunting for the mere mortal.I furnish the information for those who intuit that they should be achieving better mpg because of their mods.There appears to be a bonafide scientific explanation for how that could be the case.

And the long answer to the question stands on the evidence,yes,aerodynamics directly impacts BSFC.

[aerohead]

I thought I was in the aero sticky when I began typing this in.Sorry if it gets lost over time.Everyone needs to know about this one!

[MazdaMatt]

So in effect... reducing drag means reduced HP requirement. In order to cruise at the same speed with the different HP you have a lower throttle position. Lower TP at the same RPM puts your engine in a less efficient point on the BSFC chart?

[aerohead]

Originally Posted by MazdaMatt

So in effect... reducing drag means reduced HP requirement. In order to cruise at the same speed with the different HP you have a lower throttle position. Lower TP at the same RPM puts your engine in a less efficient point on the BSFC chart?

Yep,that's pretty much it.

[MetroMPG]

I just want to throw in this point, since it sometimes gets confused when people are talking about moving away from the "ideal" BSFC zone: operating at a less efficient point on the BSFC map as a result of aero mods doesn't imply increased fuel consumption.

Fuel consumption will in almost all normal situations still be improved, but overall efficiency (the engine's ability to produce a unit of power for a given amount of fuel) is reduced.

Not saying anyone in this thread misunderstands the point. Just putting it out there because I've seen it happen.

[i_am_socket]

Huh. So to get better BSFC efficiency on the same vehicle with reduced drag, you'd have to ... drive faster?

[Daox]

You'd have to gear lower.

[MetroMPG]

Driving faster would probably move the engine's operation closer to the BSCF ideal zone, but it would also increase overall fuel consumption.

The technical solution is to change gearing... or use a smaller engine.

[aerohead]

Originally Posted by i_am_socket

Huh. So to get better BSFC efficiency on the same vehicle with reduced drag, you'd have to ... drive faster?

At the higher velocity,the road load would increase,and that WOULD push the engine closer to it's best BSFC,but the overall increase in fuel for the added load would provide you a net loss in mpg.Sovran recommends that gear-matching be used to wring-out every last drop of fuel,however as MetroMPG points out,you're going to see an overall improvement in mpg,just maybe not as much as if you had infinite gear ratios to throw at your car".Speed costs money."E.C.Burnett

[TestDrive]

Originally Posted by aerohead

And the long answer to the question stands on the evidence,yes,aerodynamics directly impacts BSFC.

Isn't this conflating BSFC (of the engine) with the BSFE and most Optimally FE Cruise Speed (of the vehicle).

Referring to the first chart in the The BSFC chart thread (post 'em if you got 'em)


I see FUEL CONSUMPTION mapped against engine RPM and TORQUE - nothing involving GEAR RATIOS or MPH.

Originally Posted by MazdaMatt

So in effect... reducing drag means reduced HP requirement. In order to cruise at the same speed with the different HP you have a lower throttle position. Lower TP at the same RPM puts your engine in a less efficient point on the BSFC chart?

Assuming the cruise speed in question was already optimally FE prior to the reduction in drag, I think that sums it up nicely.

Of course assuming that the initial cruise speed was already optimally FE can hardly be considered a given. For both my '94 Escort 5sp MT and my '99 Escort 4-door 5sp MT, the most optimally FE cruise speed is about 37 MPH !!!