Gliding in The Beast - I'm thinking its not worth it under 45mph.

[Jay2TheRescue]

Well, I tried neutral gliding on neighborhood streets on my way home last night, and this morning on my way to work. I'm beginning to think its not worth it at neighborhood speeds. This is what I noticed:

It glides nice and easy, and I guess because it weighs over 3 tons & gets a lot of momentum going, & at low speed wind resistance is not a large factor... It glides quite far. In a couple of places I was able to shift into N and glide up and down a few hills for a total distance of almost 1/2 a mile, and stay within +/- 5 mph of the speed limit. That's the good part.

Here's the bad part...

In most of the streets the speed limit is 35 mph. Traveling at 35 my tach reads about 1400 - 1500 rpm. Stopped, in N it idles at about 725 rpm. Logic would dictate that shifting into N would cut my rpm's roughly in half, making for considerable savings, right? Not exactly. At the same point if all I do is take my foot off the accelerator and let it coast in gear the tach reads 1000 - 1100 rpm. Shifting into N at that point only drops the rpm's to 900 - 1000. A savings at best of only 100 rpm, and at that point IMHO not worth the wear & tear on a 10 y/o 150,000 mile tranny to shift - even though the dealer looked at it this spring for the 150,000 mile checkup & scheduled maintenance and said they could not find anything wrong with the truck.

Then I really got to thinking. I was coming up on a red light, and I was traveling at 35 mph, 1350 rpm. I shifted into N and coasted to the light, watching my tach. Immediately upon shifting into N the RPM dropped to 950. The RPM slowly dropped as my speed decreased, and then quickly fell to 750 rpm when I hit 5 mph. I'm thinking obviously the computer is increasing the minimum idle speed for higher road speeds, and probably does not know and/or care that the vehicle is not in gear. Probably to keep the vehicle from loosing too much speed if you just take your foot off the accelerator. Its got to be an enhanced driveability thing, or is it just a truck thing? If the truck was fully loaded at 70 mph, and I took my foot off the accelerator, maybe it would start engine braking, and that's why the idle increases? To prevent the tranny from getting jerked around too much?

Anyway I'm thinking that under 45 mph I'm just going to take my foot off the pedal and coast in gear (just like I have been doing for years).

I welcome anyone else's comments/thoughts/suggestions on my findings, and would be curious if a passenger vehicle of the same vintage (1998) would behave in a similar fashion.

-Jay

[theholycow]

Well, I can tell you about my 2002 GMC full size truck. The line was updated for 1999, so it doesn't apply perfectly, but here goes.
- My truck normally idles very low, I think something like 550rpm.
- With GM's really nice torque converter, neutral is not going to make a big difference at speeds under 35mph, since it gets to drop to near idle speed anyway.
- It doesn't drop quickly; there's a bit of a delay, then it drops slowly.
- However, using N does make a slight difference, since my truck does not use DFCO easily, and yours may not use it at all; so a few more RPM, even with the throttle closed, still uses more gas than if you can lower the RPM at all.
- Also, using N when stopped does make a difference.
- I don't think you're putting any significant wear on the transmission by shifting between N and D.
- At higher speeds, it may be a good idea to rev-match when going from N to D. Just start to get lightly on the gas pedal at the moment that your shifter clicks into D; in my truck, this results in a perfect rev-match.
- The increased idle when at speed is probably to keep the transmission fluid moving fast enough to cool the transmission, since it is pumped by the input shaft. It's probably the same in most 1998 GM vehicles (except maybe Saturns, which I think have very different transmissions).

[ShadowWorks]

The difference is your engine is not under a load so even at 950rpm is will use less fuel compared to being in hear at 950rpm, the injectors duty cycle would be as small as possible with no load, compared to a labouring engine at 1400rpms it would tend to provide richer fuel, I maybe wrong?

[Jay2TheRescue]

The difference is your engine is not under a load so even at 950rpm is will use less fuel compared to being in hear at 950rpm, the injectors duty cycle would be as small as possible with no load, compared to a labouring engine at 1400rpms it would tend to provide richer fuel, I maybe wrong?
Sounds like more experiments are warranted once I get a scangauge... My birthday is coming up soon, maybe I can drop some hints...

-Jay

[theholycow]

The difference is your engine is not under a load so even at 950rpm is will use less fuel compared to being in hear at 950rpm, the injectors duty cycle would be as small as possible with no load, compared to a labouring engine at 1400rpms it would tend to provide richer fuel, I maybe wrong?

You are 100% correct, as I've observed using my fuel rate/fuel injector duty cycle meter (http://www.gassavers.org/showthread.php?t=7714). Duty cycle might be 1.5% at 550rpm in neutral, 2.7% at 950rpm in neutral, and 10% at 950rpm WOT. It's still more fuel than a lower idle but still far less fuel than under load.

[RoadWarrior]

Yeah, I'm trying to figure that out on Marvin, idle seems to float at 1100 while you're moving, it's only when stopped in N or P it goes to 800-850ish.

[COMP]

because you are turning the drive shaft,,,its turning the trans ,,shows you there isn't a complete neutral

[theholycow]

I still think that there is a complete neutral, and the reason for the increased RPM is to keep the transmission oil moving fast enough to cool it.

[COMP]

I still think that there is a complete neutral, and the reason for the increased RPM is to keep the transmission oil moving fast enough to cool it.

if the engine is running the pump is flowing/pumping fluid :thumbup:

[almightybmw]

sooo.....how is it at any speed I can put it in neutral and drop the pedal and watch the rpms rocket to rev limit? Thereby proving, there is a neutral.

Something to try, eliminate the accel to speed neutral drop. Start on a hill and push the vehicle from a dead stop, when idle is at its lowest. See if it increases. That may help reduce the variables as to why the rpms don't drop for a coast.

[theholycow]

I was thinking that the best way to figure out what's going on is with live sensor data.

When a modern engine is idling high, for example when it's cold, how does it do that? Does the throttle get opened, or does it run rich, or something else? I suspect high idle in neutral at speed would be done the same way. What sensor should I watch? I'll order my ELM327 tonight, I think...

[almightybmw]

I know on my protege I have about a dozen ecu "triggers" that tell the ecu to enrichen the mix, ie more fuel. This bumps the idle up slightly. Headlights, rear defog, rad and interior fan, wipers, etc are all triggers. I would assume most vehicles have some sort of trigger. At idle the Protege is nearly 15:1 for mix, but if I kick a few of these triggers on it increases to about 13.7:1. Small increase, more fuel, bigger bang, more air drawn in, idle increases to compensate against the alternators increased drag. Currently all these triggers are just bare wires, and I can increase my rpm manually without the alternator drag.

Okay, small sidetrack there. But yes, the ecu injects more fuel, leaves the injector open longer, wala, higher idle/rpms. You should be able to see it with your fuel rate monitor. You should see a higher value when you first fire it up in the morning.

[COMP]

sooo.....how is it at any speed I can put it in neutral and drop the pedal and watch the rpms rocket to rev limit? Thereby proving, there is a neutral.

Something to try, eliminate the accel to speed neutral drop. Start on a hill and push the vehicle from a dead stop, when idle is at its lowest. See if it increases. That may help reduce the variables as to why the rpms don't drop for a coast.

i'm mean there isn't 0 zero friction,,,,,have you ever been it a car sitting engine off,then start it in neutral and it slowly roll ??? i have tons off times

[almightybmw]

i'm mean there isn't 0 zero friction,,,,,have you ever been it a car sitting engine off,then start it in neutral and it slowly roll ??? i have tons off times
Yes I have actually. On top of a hill, with me holding the brakes pointed downhill.

You didn't read what I said did you?

[COMP]

Yes I have actually. On top of a hill, with me holding the brakes pointed downhill.

You didn't read what I said did you?

i re-read it ,,and yes i wouldn't be surprised it a rpm increase as it rolls down a hill ,,,,,,,,,my truck idles at 8-850,, at 50mph in N i see about 950-1000 rpm and its old school ,,77 chevy 350/350 ,,,and itstalls at 1200 rpm flashs to 1800

[theholycow]

You should be able to see it with your fuel rate monitor. You should see a higher value when you first fire it up in the morning.

I thought of that, but it won't tell me what's causing it. If neutral was still putting some power through to the input shaft (I can't imagine how, but let's say it is) then it would force the engine to turn a little faster and therefore use more fuel (yes, it does use more fuel for faster RPM even with my foot off the gas, unless it goes into DFCO). The fuel rate monitor won't tell me what the cause is, just the amount of fuel that's being used.

It should suffice to monitor O2 sensor readings via OBDII; I'd compare triggered fast idle and normal low idle to the neutral-at-speed fast idle.

[theholycow]

i'm mean there isn't 0 zero friction,,,,,have you ever been it a car sitting engine off,then start it in neutral and it slowly roll ??? i have tons off times

You know, you've got a point there.

Put a car up on jack stands and start it. In N the wheels will start turning. Damn. How does that work?

[COMP]

You know, you've got a point there.

Put a car up on jack stands and start it. In N the wheels will start turning. Damn. How does that work?

the internal friction,,thats the easest way i can explain,,,which i have a hard time at

[almightybmw]

input shaft spins, fluids move, primary and secondary shafts move. When loaded (ie on level ground) the car doesn't move, but in the air, the friction is close to nothing, therefore shafts spin. Does this mean there is no neutral? no, it just means the tranny is built solid and with tight tolerances. Neutral still exists. The engine has no direct mechanical connection with the tires.

[theholycow]

Ok, so the question is...is that fluid connection enough to increase engine RPM by 10% when the driveshaft is spinning at highway speed?

[Jay2TheRescue]

input shaft spins, fluids move, primary and secondary shafts move. When loaded (ie on level ground) the car doesn't move, but in the air, the friction is close to nothing, therefore shafts spin. Does this mean there is no neutral? no, it just means the tranny is built solid and with tight tolerances. Neutral still exists. The engine has no direct mechanical connection with the tires.
In an automatic the engine never has a direct connection with the wheels unless the TC is locked. The only exception that I know of is the old "Variomatic" transmissions from the late 50's. I forget who made them, but they utilized belts, not a fluid connection to the rear wheels.

-Jay

[almightybmw]

OKay, sounds like we all know the same stuff about how it works, and pretty much come to a conclusion that internal fluid friction is a pretty good deduction as to why--regardless of how you start or whether in D or N along the way--the rpms climb at speed if in Neutral. Can we prove it via testing?

Comp, you said you've seen it in your 77 Chevy? I guess I want to try it in my 02 GP then to see the results as well.

[Jay2TheRescue]

I'd like to try it in my 86 Chevy and my 81 Regal, but neither of those have a tach.

-Jay

[COMP]

Ok, so the question is...is that fluid connection enough to increase engine RPM by 10% when the driveshaft is spinning at highway speed?

looks like it ,,,this is a good reason sticks get better mikeage,,,,less HP loss

[ShadowWorks]

In most engine there is an idle control valve, which has a air by pass pipe in front of the butter fly plate, its a solenoid device and it allows a small amount of air to go around the plate to increase idle speed, its also connected to a speed sensor from the gearbox, the same one that gives the speedo reading on the dash, so it knows when the car is moving and will keep the idle above 1200rpms

It also helps keep idle higher when there is a heavy current draw from the alternator, modern cars have so many sensors and control systems in place, its amazing they don't break down more often.

[RoadWarrior]

I was cruisin' round the yard sales this morning, doing P+G, and I was noticing that at low speeds sub 30mph I do seem to get lower RPM coasting in gear than in neutral. Coasting down from 50ish, the engine only seems to drag more than Neutral down to about 40ish, after that I get no apparent huge difference in glide distance. Around 20mph-ish the motor seems to push just enough at idle speeds of around 850 to keep moving indefinitely, whereas if I shift to N, the rpm jumps to 1100ish... I am curious now as to whether it's using more fuel or not at that point. I'm figuring that it is probably using about the same in gear or in N below 50. I should be getting DFCO above 50 as well... so all in all it seems like there's a very narrow band, between 40 and 50 where N coast is of any real benefit. Although N coast at highway speed down hills and behind semis seems probably worthwhile. At a very rough figuring, knowing gas gauges are in no way accurate, and thinking my first quarter tank is above the full mark, and the rest of the scale is about 3/4 of the 15G tank, then I used around an 8th doing about 40 city miles this morning, which seems to indicate 28mpg-ish, probably too optimistic, but still around 22mpg if the gauge is "accurate" around the middle.

Edit: in case you're wondering about the economy of yard saling with gas so high, I did score a $200+ mechanics vice for $25 and a $100 carpet "runner" for $1, both things I needed, for an apparent cost of about $7 in gas. I'd have used the same amount of gas probably driving round stores "shopping" for them if I'd had the cash to buy new.

[malherbe]

coast as much as you can !!!!!!!! stop watching the mpg gauge, stop counting rpm's !!! coasting in gear = braking. You are always better off coasting.

[theholycow]

Around 20mph-ish the motor seems to push just enough at idle speeds of around 850 to keep moving indefinitely, whereas if I shift to N, the rpm jumps to 1100ish... I am curious now as to whether it's using more fuel or not at that point.

Almost definitely yes. You can certainly find out easily with a certain DIY fuel rate monitor that you know I really enjoy. :)

I'm figuring that it is probably using about the same in gear or in N below 50. I should be getting DFCO above 50 as well... so all in all it seems like there's a very narrow band, between 40 and 50 where N coast is of any real benefit.

Which of your vehicles is that for?

It's definitely not universally applicable. In my VW, coasting in neutral is far more efficient than DFCO, as determined by running whole tanks of gas one way or the other. If you think about it, using DFCO as a replacement for coasting can't possibly be more efficient. Consider the following:
- In neutral, you have aerodynamic drag, rolling resistance, and 1100rpm of engine friction and pumping.
- In DFCO, you have the same aerodynamic drag and RR, but now you have 1500 or 2000 (or whatever) rpm of engine friction and pumping, plus some additional drivetrain friction.
Which one uses more energy?

Edit: in case you're wondering about the economy of yard saling with gas so high

Even if it wasn't the most efficient option, it's probably something you enjoy. That's reason enough.