Pulse and Glide with the engine running?

[neverendz]

I'm talking about the VX here but other cars may apply.

If you were to pulse to 70mph, leave the engine running, car in fifth gear and glide down to 50mph, clutch engaged, the injectors will shut off during the glide with engine braking. Which would still be a fuel savings. The thing is that the car will slow down to 50 mph faster. But how much faster? Or how much of a distance difference?

I'm asking because I live in a congested area and pulse and glide is not an option.

I was wondering if someone may test this and let me know of the results?

Just curious. Seems like a safer, easier pulse and glide method.

Thanks.

, Bill

[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 because all the same work is done - the car is moved through the air, the drivetrain is turned the entire time, and the engine is turned the same quantity of revolutions.

Normal engine-on neutral-coasting P&G saves gas because the engine idles at low RPM while the car continues to coast without slowing much. Of course you should never exceed your comfort level, do anything you think is dangerous, or do anything that is illegal; but if it's not illegal or uncomfortable, you should be able to P&G safely with the engine on.

Turning the engine off during a P&G cycle's coasting segment, a strategy that I call P&EOC (a term which I hope will catch on), additionally saves gas for the obvious reason that you're not idling the engine during that time. As with

The 70 to 50 mph cycles you're talking about must be very unpleasant. I tried the same strategy, which I called P&DFCO, for my car on the highway due to P&G being difficult for me to do at those speeds when I first started driving it -- I had difficulty rev-matching the idle->3000rpm shift. Indeed, it was unpleasant, jumping on and off the gas, accelerating and then engine braking repeatedly. However, it did not save gas; in fact, it cost me a few MPG.

Tangent note: I use the term P&EOC to differentiate it from P&G which usually means leaving the engine on, and to be more specific than just EOC which isn't necessarily part of a P&G cycle -- one could EOC when descending without ever having pulsed and without intending to pulse afterwards.

[neverendz]

Thanks for the info!

The 70 to 50 mph cycles you're talking about must be very unpleasant.

What would be an optimal speed range?

, Bill

[aalb1]

Originally Posted by neverendz

Thanks for the info!



What would be an optimal speed range?

, Bill

You got a VX... I got a 1990 Civic with DPFI. P&G'ing, I found out that 50-60 is better than 50-65 or 50-70. Now I'm trying to figure out the correct amount of throttle to give it on the pulse. Initially I tried using my aftermarket cruise control unit that gently accelerates. But now I'm trying to give it heavier throttle to pulse up to 60 to see what works better since some other Honda owners have seen success with that.

Whether you got some mileage instrumentation or not it takes a lot of experimentation. Good luck.

[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.

[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.

[DRW]

Yes, Lean burn will complicate things.
My car has lean burn, although it works differently than the lean burn mode in a VX.

I noticed no benefit to P+G on flat ground, regardless if the engine was on or off during the glide. This is because holding a steady/light throttle lets the ecu use very lean A/F ratios. The 'pulse' portion of P+G uses heavier throttle and the ecu uses a richer A/F ratio at this time.

[1cheap1]

Originally Posted by DRW

Yes, Lean burn will complicate things.
My car has lean burn, although it works differently than the lean burn mode in a VX.

I noticed no benefit to P+G on flat ground, regardless if the engine was on or off during the glide. This is because holding a steady/light throttle lets the ecu use very lean A/F ratios. The 'pulse' portion of P+G uses heavier throttle and the ecu uses a richer A/F ratio at this time.

When i have no draft targets insight i just eoc from 62 to 52 mph or if lazy just use the cruise control at 57 mph. Using eoc yields slightly better results for my car but takes a lot of effort. If there are no cars even close i will slow to 45 mph and pulse to 55. The freeways here have a lot of humps for the overpasses so depending how i feel that day i use the above methods.