Pulse and Glide with the engine running?

[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.

[DRW]

Here's another good post to illustrate peak vs poor efficiency. http://www.gassavers.org/showpost.ph...8&postcount=46
Be sure to click on the map and notice how quickly efficiency drops off as load gets lighter approaching idle load. Notice how efficiency is best around 2k to 3k rpm and 50% to 75% load. This is why traditional P+G works so well.
Granted, a vehicle with lean burn capability has different efficiency islands, as leaner A/F ratios are used at light load, which helps reduce the inefficiency of running at light load.

[groar]

In my cars, I can coast in neutral for longer than 600m from 90 to 60km/h (0.4 miles from 56 to 37mph).

Aerodynamic deceleration is stronger at 90km/h than at 80, so I will coast longer from 80 to 50km/h (49 to 31mph).

I use coasting in neutral as much as possible and DFCO when I didn't anticipated correctly.

Out of city, DFCO decelerates around 3 times faster than neutral.
In city, DFCO decelerates much more than neutral. Tires pumped up at near max sidewall and coasting in neutral are certainly what permitted me my best improvement but I have only manual transmissions.

About idle and lean burn, does lean burn is possible at idle or is it only at steady speeds ?

Denis.

[DracoFelis]

Originally Posted by theholycow

That's called DFCO - Deceleration Fuel Cut Off. It's great if you already intend to slow down.

However, it doesn't save any gas at all if you intended to keep going at speed.

That's clearly a YMMV thing. Yes, you don't save as much fuel as with EOC, but in at least some cars P&G using DFCO does save fuel. I've noticed in my CRX, for example, that my best fuel tanks are often when I'm in very light traffic and can therefore do P&G DFCO.

OTOH it really depends upon your level of "engine braking", and that will depend a LOT on both what car you have, and what you have done to allow the car to move easier, as there are a lot of factors that control how far you coast with DFCO (in high gear). Here are just a few that I've noticed helped with my CRX:

1) Since the engine is connected during DFCO (but not during a neutral coast), extra good engine lubrication (including quality synthetic oil) seemed to noticeably extend/help my DFCO "coasting" range (presumably due to less engine friction).

2) And the transmission is also connected during DFCO, so lowing losses in the transmission (for example, good synthetic manual transmission fluid) also helps lower the "braking" you get with DFCO.

3) Lubing up the wheel barrings (with good synthetic grease) made a big difference for all coasting (DFCO and otherwise), as the wheels spun much more freely. And don't think just because your barrings are supposedly "factory sealed" that you can't do this with a little effort. For example, it's amazing where you can inject grease with a grease gun hooked up to a hypodermic needle...

4) Since the alternator is connected during DFCO, saving electricity (LED lights, turning off accessories when not needed, etc) helps lower the alternator drag you experience with DFCO (again, lowing the slowdown hit of DFCO relative to a neutral coast).

5) Many cars suffer from their brakes dragging a bit. And fixing this problem will allow you to coast further (DFCO or otherwise), by lowing the slowdown that this minor brake drag can cause.

6) And simply adding air to the tires helps with both DFCO and neutral (or even "engine off") coasting.


NOTE: For safety reasons, I generally like DFCO when I'm in traffic, over EOC. And while I'm OK with neutral coasting, I've found that in my car I get a surprisingly long "coast" using DFCO. In fact, after working on those issues listed above, I now get roughly twice the range with DFCO that I used to get with a neutral (or engine off) coast. Granted, my neutral coasts have gotten longer as well, so I still go further in neutral than DFCO. But after doing those improvements to the CRX, DFCO in 5th gear (in my CRX) is surprisingly useful, with a "coast" long enough to make P&G actually useful (even if/when I stay in 5th gear the whole time).

[theholycow]

Originally Posted by DracoFelis

That's clearly a YMMV thing. Yes, you don't save as much fuel as with EOC

I wasn't comparing to EOC, though.

but in at least some cars P&G using DFCO does save fuel.

I suppose it's possible for some cars to do better with P&DFCO than with P&G (engine on, trans in neutral), but I suspect it would be rare. The losses with P&DFCO would have to be very abnormally small, and the losses with idling would have to be abnormally high.

Don't forget: Any work you do while in neutral, you do while in DFCO; but there's additional work that needs to be done in DFCO -- extra engine revolutions.

I've noticed in my CRX, for example, that my best fuel tanks are often when I'm in very light traffic and can therefore do P&G DFCO.

I suspect that you would get the same or better FE with engine-on P&G under that light traffic condition than with P&DFCO.

A few line items...

OTOH it really depends upon your level of "engine braking"

Definitely. This is affected by engine size, gearing, and friction losses.

1) Since the engine is connected during DFCO (but not during a neutral coast), extra good engine lubrication [...]
2) And the transmission is also connected during DFCO, so lowing losses in the transmission

That would help with those friction losses.

4) Since the alternator is connected during DFCO, saving electricity (LED lights, turning off accessories when not needed, etc) helps lower the alternator drag you experience with DFCO (again, lowing the slowdown hit of DFCO relative to a neutral coast).

The electricity is not free. It's the same amount of electricity, just made at different times from different sources depending on which technique you use. Therefore, saving electricity makes approximately the same improvement whether you prefer to P&DFCO, P&G, or P&EOC. If you don't run the alternator now you'll have to run it later.

NOTE: For safety reasons, I generally like DFCO when I'm in traffic, over EOC.

Safety first. If you feel that you might be sacrificing safety to EOC or neutral coast, then it doesn't matter how much gas you're saving.

[DracoFelis]

Originally Posted by theholycow

Don't forget: Any work you do while in neutral, you do while in DFCO; but there's additional work that needs to be done in DFCO -- extra engine revolutions.

True. However, as I mentioned there are things you can do to lesson those losses. And the other side of the coin is that DFCO saves more fuel than neutral coasting does.

So even though there are greater DFCO losses (and therefore less total "coast" distance), that somewhat is made up for by the fact that DFCO uses zero fuel, whereas neutral coasting uses fuel to maintain idle.

Originally Posted by theholycow

I suspect that you would get the same or better FE with engine-on P&G under that light traffic condition than with P&DFCO.

In my experience it's about equal on my CRX. i.e. I get a little longer coast in neutral, but I also use a little more fuel. So I seem to get approximately the same mileage either way. And since I don't really gain much (if anything) by constantly putting the car in/out of neutral (wearing out the clutch faster, and having slightly less "control" if/when the traffic is heavy), I've started using DFCO more and more. I still use neutral coasting some (especially at speeds too slow for DFCO to work in 5th gear), but I find I generally prefer just staying in (5th) gear (and use DFCO) when I'm at highway speeds.

Originally Posted by theholycow

The electricity is not free. It's the same amount of electricity, just made at different times from different sources depending on which technique you use. Therefore, saving electricity makes approximately the same improvement whether you prefer to P&DFCO, P&G, or P&EOC. If you don't run the alternator now you'll have to run it later.

Good point. So I guess saving electricity should be in the "it helps period" category (just like say greasing up wheel barrings is). However, while it might just change where/when the savings occurs, I can tell you that saving electricity can make a very noticably difference in DFCO "coasting distance" (at least on my small CRX).

i.e. The drag from the alternator is more noticeable under DFCO, than under many of the other conditions that you drive under.

[DRW]

Originally Posted by groar

About idle and lean burn, is lean burn possible at idle or is it only at steady speeds?

My car has lean burn at idle, but only when the car is moving fast enough (over 45mph) to trigger lean burn mode. This is a quirk of the programming, which was done by a few DSM enthusiasts and myself.