DRAG-REDUCTION vs BSFC

[Daox]

Well, its really not too hard to correlate MPH and gear ratios with RPM. But, yes, it doesn't have anything to do with aero drag. It is simply an engine efficiency chart.

[mobilerik]

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

Is there any kung fu that can make an engine think it's smaller?

I'm wondering about WAI and that sort of sorcery that I'm totally n00bish about...

[MetroMPG]

Kung fu -

WAI, in theory... Increased EGR (to a point), in theory... Cylinder deactivation (displacement on demand)... Atkinsonized cam...

[aerohead]

Originally Posted by TestDrive

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 http://ecomodder.com/forum/showthrea...-got-1466.html


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


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 !!!

The premise of Sovran's report addresses any vehicle operating on SAE cycles.He addresses urban,hwy,and mixed composite cycles.From his charts,the theoretical relationships between drag-reduction and MPG are laid out,however his caveat,and this is what he wants to stress the most,is that without proper gear-matching,the realized improvements from drag reduction may be lower than what is achievable.------------ You'll notice on your BSFC maps,that the island of highest BSFC occurs between either a range of mean piston speed,or engine rpm at a given range of load.The rpm or piston speed equates to a given cruise speed in top gear.At this speed,if the engine does not see this much load,it will operate at a less efficient level.The only way to correct this is to put taller gears in the car,or as MetroMPG has offered,move to a smaller displacement powerplant.

[some_other_dave]

I don't think we were disagreeing on the underlying facts, just on the terminology.

I contend that the BSFC map itself does not change--that it depends solely on the engine and ancillary systems. However, reducing your drag means that you need to make less power to cruise at a given speed, so you change the "load" coordinate on the BSFC graph for that cruising speed.

In most cases, you will put the engine into a regime where it has worse BSFC--but the overall fuel consumption will also decrease, because the BSFC talks about fuel used per horsepower generated, and you need to generate less horsepower overall.

-soD

[aerohead]

Right! I think akaschic thought the actual map might morph around a bit(and go real psychedelic) and I didn't know how to represent it, without it straight in front of me.Hucho actually shows the shift in the load line,Sovran goes deeper in trying to explain it. --------- But yeah,with the lower load,mpg rises,and for any of us that can massage the gearing,we have the best chance to see top mpg.

[Whoops]

Conflating, kung foo! So, what I think this suggests is that I can aero my car and I should see some improvement in mileage. However, to maximize the return, in turn I need to know what rpm my particular engine is optimum, for the reduced load of the aero. Additionally, realistically I only have some alternative fixed gear ratio's, but I should be looking to find the closest to optimum.

Are there BSFC maps for different engine/camshaft combinations available, somewhere?

[TestDrive]

Originally Posted by some_other_dave

I don't think we were disagreeing on the underlying facts, just on the terminology.

Yes!

Originally Posted by Whoops

Are there BSFC maps for different engine/camshaft combinations available, somewhere?

BSFC maps aren't easy to find, but you can start with http://ecomodder.com/forum/showthrea...-got-1466.html

[aerohead]

The loose rule for BSFC is that it will top out at the rpm where maximum torgue occurs for the engine.By streamlining, and cutting drag,the engine has more mechanical advantage between the flywheel and drive wheels and "see's" less load as it's "easier".------------

By putting "taller" gears between the flywheel and drivewheel(s),the engine has to strain as much as it used to,before you did the drag reduction.This moves the engine back into it's "sweet-spot" on the BSFC map--------------------------

For the gearing,you would calculate the new theoretical top speed for the vehicle,and then choose a tranny,or differential,or combination of the two,which would give the same engine rpm the vehicle had at it's original top speed,at the new top speed.-------------------------

If your engine has a very flat torque-curve,all this may be unnecessary,as the island of top BSFC may occur over a very broad rpm range.---------------------

The hitch to just changing the ratio,is that you can kill your acceleration and hill-climbing ability.Adding an "extra" gear may be necessary to maintain all the other performance abilities.--------------------

My T-100 is geared for 156-mph @ redline.The horsepower peaks @ 138-mph,which is where my road load and horsepower peaks(minus drivetrain losses),so 138-mph is probably a reasonable top speed for the truck.-----------

With no gear-matching,I've been able to get a 37% increase in mpg.

[Christ]

Let's not forget that tire sizing is a factor in overall drive gearing, and might be a cheaper alternative than actually swapping gears in your vehicle, especially as a "test" setup, where the result might not be optimal.