Neutral at stop lights, save gas??

[Snax]

Going back to ELFs original observation of saving gas while stopped in neutral, it's reasonable to expect that it would for the removal of the parasitic load of the torque converter. In gear, it is always working to transfer torque whether the vehicle is moving or not. In neutral it is allowed to freewheel and injector duty cycle is reduced to maintain the same idle rpm.

I've noticed what I would guess to be about a 15-20% increase in economy at stop lights by shifting into neutral, and that jibes with injector programming that I have done on other motors for other parasitic load compensations.

[ffvben]

Originally Posted by ELF

I don't know if this has been discussed before but, I learned something today.
Until recently I never paid much attention to my GPH on the scan gauge.
So today I had it on and while at a stop light I checked to see how much fuel I was using, 0.3 GPH in gear. I put the tranny in N and the GPH changed to 0.2

So at least in my car I can save some gas by using neutral at stop lights. Don't think I'll be saving a lot but every little bit helps.

geting back to the thread..neutral or in gear at stops... on my own truck idle is around 700rpm neutral/auto and about 550rpm drive/auto. i would think drive gear would be saving fuel at a stop just from the lower rpm. I don't have a sg to check this my self but I might look into geting one just to see. I think the best way to check this is check it in drive at a stop. then next stop check it in neutral.

[SVOboy]

Neutral uses less than drive because when it drive and stopped, the power is just going to the brakes, but in neutral, there is not power being tranfered to the wheels. This has been tested a bit by people with scanguages.

[Silveredwings]

Originally Posted by ffvben

geting back to the thread..neutral or in gear at stops... on my own truck idle is around 700rpm neutral/auto and about 550rpm drive/auto. i would think drive gear would be saving fuel at a stop just from the lower rpm. I don't have a sg to check this my self but I might look into geting one just to see. I think the best way to check this is check it in drive at a stop. then next stop check it in neutral.

Actually I would think it's the reverse. When in N, the engine needs a minimum amount of fuel flow to maintain idle rpm. In D, the engine is being loaded down by the torque converter so the rpm drops but not because of a drop in fuel flow. If anything, in D, the fuel flow may be a little higher in order to keep rpm high enough not to stall. YMMV.

[zpiloto]

Originally Posted by Silveredwings

Actually I would think it's the reverse. When in N, the engine needs a minimum amount of fuel flow to maintain idle rpm. In D, the engine is being loaded down by the torque converter so the rpm drops but not because of a drop in fuel flow. If anything, in D, the fuel flow may be a little higher in order to keep rpm high enough not to stall. YMMV.

That's what I see with my SG. In D the load drops the RPM and increases FF. On the Daewoo-woo idling in N the ECU will actually compensate after awhile and it will drop about 150 rpms once it adjust in N. I can't set the idle on my car the ECU does it. YMMV

[Snax]

The engine mapping engineers have to increase injector pulse width to compensate for even the smallest things at idle. Even turning the parking lights on is enough to affect idle in small displacement motors without a measureable increase in fuel to hold it steady. In a well mapped car, the effect of this seems relatively minor as the system quickly adapts, but anybody starting from scratch quickly learns that idle speed injector pulses can vary wildly depending on parasitic loads - and an active torque converter is definately parasitic.

I would argue that the reverse is actually true with manual transmissions however, as the load of a disengaged clutch against the throw-out bearing is less than the drag induced by spinning the main shaft inside the gearbox amongst the oil while in neutral. You may not see the difference on a Scanguage with the GPH display, as it is significantly smaller than the difference with an automatic, but the difference is definately there and would show up by examining injector pulse width. I think the 'engine load rate' display might provide similar resolution and insight, but I've not checked that out yet myself.

As an aside to that, one could probably more accurately deduce how much extra fuel is saved or consumed from one transmission oil to the next by paying attention this. Perhaps I am assuming too much about the resolution of the Scanguage engine load rate display however.

[Snax]

Ok, I just went out and checked this the load rate display against my theory here and it appears to be correct. After fully warming up the motor and allowing the idle to settle at 650 rpm, depressing the clutch pedal changed both the ignition timing and load rate data initially, with the load rate eventually settling to one point lower after about 30 seconds with timing settled back down. Releasing the pedal creates the same fluctuation until it settles down after 20-30 seconds or so, 1 point higher on the engine load.

The initial instability after engaging or disengaging is likely due to the variance in drivetrain mass as the main shaft is accelerated or decoupled from power.

It's also worth noting that turning parking lights on was worth a full point on engine load, low beams another 2, and high beams 3 more on top of that. Turning the heat on full blast with the high beams on took load from the baseline of 23 to a whopping 37. That's more than a 50% increase at idle.

Watch your amps!

[Bill in Houston]

Interesting stuff. I have noticed the idle speed in small cars going up and down in unison as the blinker blinks.

I think that for the sake of the longevity of the throwout bearing, it is better to be in neutral with your foot off the clutch. But it does cost a little more fuel...

[Snax]

Pay now or pay later I suppose.

As far as manuals are concerned, aside from the engine load figure, I think the difference would be difficult to nail down statistically without carefully measured tanks just left to idle through in both conditions. Clearly this is more significant with automatics.