[TestDrive]

Originally Posted by Geonerd

It all depends....

On aero drag, vehicle mass, rolling resistance, gearing, and the shape/size of your BSFC sweet spot.

Even then, you'd need some heavy math and/or computer modeling to obtain optimal pulse/glide target speeds.

Thanks for the reply. Sorry I took so long getting back to you, but I was thinking. (This is after all a thought experiment. )

For the most part you have all good points, but it's not quite what I'm looking for (throttling technique - see below). This is no doubt mostly due to a poorly designed and poorly worded thought experiment.

The perfectly flat, level, deserted stretch of highway; zero wind, single even moderate temperature were all included as parameters in order to eliminate them as variables. The 400 mile distance to be completed in 24 hours or less makes average speed a factor, but not necessarily a critical one. The 75 mph maximum travel rate was an attempt to give the problem some sort of real world grounding. The automobile was probably overly specified, so let's revise.

Automobile: A typical EFI, ICE, 5sp MT.

Though most typical cars don't have readily found BSFC Maps, let's assume this one -


I certainly agree about the heavy math and/or (probably and) computer modeling in order to determine optimal target speeds. What I wish to discuss here however is optimal rate(s) of accelerations from target low = x mph to target high = y mph. Let's assume you know: vehicle mass including driver, fuel and cargo = 2,000 lbs., cd = 3.0 , frontal area = 25 sq. ft. , (Rolling Resistance coefficient) RRc = 0.008 and gearing is --- (your choice). Since it is a perfectly flat, level road lets assume you also know the throttle position required to maintain a constant x mph = TPx as well as the throttle position required to maintain a constant y mph = TPy.

There are many different ways you could handle accelerating from x mph to y mph - some being.

1. When speed drops to x (restart the engine & put in gear),
   then depress pedal to floor (100% TP) and hold until you attain y mph.
2. When speed drops to x, depress pedal to TPy and hold until you attain y mph.
3. When speed drops to x, depress pedal to TPx + a.
   As mph increases, increase the throttle position at a constant rate until you attain y mph at TPy + a.
   ...(If this is the method selected as best, what can be said about the optimal value for a?)
4. When speed drops to x, depress pedal to TPy + b and hold until you attain y mph.
   ...(If this is the method selected as best, what can be said about the optimal value for b?)
5. When speed drops to x, depress pedal to TPx + a and as vehicle speed increases, increase pedal depression at a fixed rate such that you attain y mph at TPy + b.
   ...(If this is the method selected as best, what can be said about the optimal values for a and b?)
6. When speed drops to x, depress pedal to TPx + a and as vehicle speed increases, depress pedal at a uniformly accelerating rate until you attain y mph at TPy + b.
7. ...

That a pretty broad range of strategies. Most if not all of them must be incorrect. Some must be wildly incorrect. Given the facts assumed, what does anyone think the best strategy is? Which would be the first one you'd try? Why? Or do you have a different strategy that you think would work better? If so, what is it?

Originally Posted by Geonerd

I think the best quick-and-dirty attempt would involve picking speeds that keep the engine in or near the low SFC island for as much time as possible.

I doubt you'd spend much time in or near that SFC island unless you're at or near WOT. And that seems unlikely to be an effective strategy.

Remember SFC is a ratio of fuel consumed / work accomplished. Literally - the amount of fuel consumed, divided by the power being produced. - grams fuel/kilowatt hour (gm/kWH) or pounds fuel/horsepower (lb/hp)

Suppose for a moment that the driver of our imaginary (2,000 lbs gross weight including driver) vehicle weighs 200 lbs. Let's also assume that experimentation has determined the that the most FE fixed rate of speed for the vehicle under previously described conditions is 45 mph (in 5th gear at x rpm). Now suppose we swap out our 200 lb driver with a different driver and 3 passengers each one of whom weighs 300 lbs. The gross weight of the vehicle now rises to 3,000 lbs., a 50% increase. If we now operate the vehicle at 45 mph (in 5th gear at x rpm - MT so no slippage = same rpm), our mpg will drop, but SFC will decrease as much more work is being accomplished with a comparatively moderate increase in fuel consumed. We've use more fuel, but we've used that fuel more efficiently.

If you're doing a high percentage of P&G driving, you spend most of your time accelerating. I suspect that how you accelerate may be at least as important as target speeds and I'd love to see other peoples thoughts and observations regarding this.