Pulse and Glide with the engine running?

[Geonerd]

I think the original idea might be made to work, although I doubt the FE gains would be impressive. Instead of a full DFCO throttle chop, apply just enough gas to roughly offset the engine drag. Your car will slow at a rate close to it's coast down deceleration. During this glide phase, you'll be using a lot more gas than at idle, but less than you would at steady-state.

Overall efficiency would depend on the battle between the peak BSFC pulse, and the low efficiency (but low power) semi-powered coast period. I'm guessing that FE at highway speeds would be slightly improved over steady-state.

At slower speeds, where you can gear the engine to run at low revs, this technique should work pretty well. Engine drag is vastly reduced and you're probably not using a whole lot more gas than at idle.

(A slipper clutch would sure be nice, eh?)

-Moo

[DRW]

Originally Posted by Geonerd

I think the original idea might be made to work, although I doubt the FE gains would be impressive. Instead of a full DFCO throttle chop, apply just enough gas to roughly offset the engine drag. Your car will slow at a rate close to it's coast down deceleration. During this glide phase, you'll be using a lot more gas than at idle, but less than you would at steady-state.

Overall efficiency would depend on the battle between the peak BSFC pulse, and the low efficiency (but low power) semi-powered coast period. I'm guessing that FE at highway speeds would be slightly improved over steady-state.

Bad guess.
PaleMelanesian already gave the correct answer in post #5

Originally Posted by PaleMelanesian

The momentum lost due to engine braking will outweigh the fuel not used due to DFCO. You'll do better by maintaining a steady speed than trying to pulse-and-dfco.

Best: P&G, engine off glide
Good: P&G, engine on, neutral glide
Ok: steady speed
Bad: Pulse-and-dfco

With the VX, though, you have the option of running in lean-burn at moderate speed, which should be at least as good, or possibly better. That might move steady speed up into 2nd place on the list above.

To add a 5th option to the above list, 'worse than bad' would be to decel using very light throttle. In fact, that would be worse than normal driving since you're using gas to keep the engine turning against it's own drag. If you're not using gas to propel the vehicle, then you're wasting gas.

[Geonerd]

Originally Posted by DRW

Bad guess.
PaleMelanesian already gave the correct answer in post #5

Brave (and rather rude) talk, but do you have the data to prove it?
If your car has a small engine and tall gears,the penalty for 'keeping the engine running' will not be that great. At the same time, the large benefits of a peak efficiency pulse are well established.

If you're not using gas to propel the vehicle, then you're wasting gas.

No kidding, Sherlock. I never said it was an ideal solution.

[theholycow]

Originally Posted by Geonerd

Brave (and rather rude) talk, but do you have the data to prove it?
If your car has a small engine and tall gears,the penalty for 'keeping the engine running' will not be that great. At the same time, the large benefits of a peak efficiency pulse are well established.

My car doesn't quite meet your specifications (2.5l on a 3000 pound car & short gears -- 3000rpm @ 70mph), but going from P&DFCO to cruise control made my FE jump up. I tried P&DFCO for 4 tanks. See my gaslog for Effram.

P&DFCO was a highly unpleasant way to drive. Even if it saved gas I wouldn't continue doing it.

[DRW]

Originally Posted by Geonerd

Brave (and rather rude) talk, but do you have the data to prove it?

Sorry to step on your toes, but I wanted to point out in no uncertain terms that the technique you were recomending was worse for FE than simply driving at a steady speed.
See www.gassavers.org/showthread.php?t=3140 read the first post, although the whole thread has good info, and specifically this post http://www.gassavers.org/showpost.ph...4&postcount=23

Originally Posted by Geonerd

If your car has a small engine and tall gears,the penalty for 'keeping the engine running' will not be that great. At the same time, the large benefits of a peak efficiency pulse are well established.

Then why recomend keeping the engine running at higher speed with very light throttle? That would be at the lowest efficiency. The idea behind P+G is to maximize peak efficiency use, and minimize use at poor efficiency.

Originally Posted by Geonerd

I never said it was an ideal solution.