Neutral at stop lights, save gas??

[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.

[omgwtfbyobbq]

Awesome info, I've been meaning to ask about this. Imo it means that no matter what, coasting in N should result in better mileage. I think. If you have the time, could you check the fuel flow when coasting in N compared to decelerating in gear at a kinda slow speed, like ~40mph? If the idle consumption when decelerating is greater than coasting in N, then coasting must be a win win no matter what.

[LxMike]

i have found with my car that in gear coastng shows higher milage reading on my scanguage than neutral coasting.

[omgwtfbyobbq]

Really? That's very interesting. Do you know if fuel is being injected or not? For example, on the Camry, depending on coolant temps, when I coast (in gear) and drop below ~1200-1500rpm fuel starts being injected, and it pretty much idles down the road in gear. But if I'm coasting (in gear) above those rpm in top gear (greater than 50mph), no fuel will be injected. I'm wondering if the fuel flow for in gear idling is the same as N idling...

[LxMike]

I'll have to check gph readings and see.

[rh77]

i have found with my car that in gear coastng shows higher milage reading on my scanguage than neutral coasting.

My SG does the same thing, but what it doesn't know is that the injectors actually cut fuel while coasting in D (so it should read 9999).

Several vehicles benefit from Neutral idling -- the GPH arguement is quite valid. For maximum efficiency, long idling situations should be in engine-off mode (like at traditionally long lights, etc).

BUT...

The TSX, for example, has variable cam timing (i-VTEC) that cuts the cam timing dramatically at idle and uses very little fuel. It actually uses more fuel to shut-down and re-start if you're not sitting for at least 2-minutes (according to the on-board FE recorder). I don't use neutral idle anymore in the 'Teg either, as I'm worried about the longivity of the transmission going from N to D at a stop (a bit of a hard engagement). Coasting in engine-off N, starting up, and back to D with a rev-match seems to work well, tho...

RH77

[Peakster]

i have found with my car that in gear coastng shows higher milage reading on my scanguage than neutral coasting.

Same with my dad's Chrysler. It depends on the car whether its sensitive to being in gear or not.

[budomove]

According to Ben (SVOboy) the fuel injectors on hondas turn off while coasting in gear, so the same goes for us.

[repete86]

Do you know if this applies to all Hondas? I generally drop into neutral for short coasts and turn the engine off for longer coasts because when I eave it in gear it slows down considerably faster and I know that when I'm stopped that going into N will save gas. I would just experiment, but I don't drive very often except when I'm on a job, so for me to go from tank to tank would be difficult, and I don't have a SuperMID.

[Bruce]

Using liters instead of gallons on the SGII will give better resolution of the idle fuel consumption. I found that my idle fuel consumption at a stop went down about .1l/h in neutral. One gallon = 3.785l. Watch for any transitional changes that may eat up any gains over a short time out of gear.

If you do a lot of ICE-off coasting, changing the SGII engine type to "hybrid" and keeping the key in the "run" position as much as possible should give the trip mileage better accuracy.

GPH Fuel consumption in my car is about the same coasting in neutral or drive, but above 30 MPH it slows down faster in drive because of engine braking. MPH/GPH = MPG, so the faster you're going during ICE-on coasting, the better the gas mileage. ICE-on coastdowns in neutral at high speeds will give better gas mileage in my car because it remains at high speed longer.

[CO ZX2]

My SG does the same thing, but what it doesn't know is that the injectors actually cut fuel while coasting in D (so it should read 9999).

Several vehicles benefit from Neutral idling -- the GPH arguement is quite valid. For maximum efficiency, long idling situations should be in engine-off mode (like at traditionally long lights, etc).

BUT...

The TSX, for example, has variable cam timing (i-VTEC) that cuts the cam timing dramatically at idle and uses very little fuel. It actually uses more fuel to shut-down and re-start if you're not sitting for at least 2-minutes (according to the on-board FE recorder). I don't use neutral idle anymore in the 'Teg either, as I'm worried about the longivity of the transmission going from N to D at a stop (a bit of a hard engagement). Coasting in engine-off N, starting up, and back to D with a rev-match seems to work well, tho...

RH77

It doesn't appear to me that you guys are differentiating well between coasting with engine on and engine off. When you are coasting with engine off NO fuel is being consumed. So how can your fuel consumption be less than that? Coasting in gear only slows your car down faster, how can that be good? If there was total fuel cut at any time with any car, the engine would die. Engine Off Coasting is a sure fire way to substantially raise your FE. It worked for me and has worked for many others.

[Ted Hart]

According to Ben (SVOboy) the fuel injectors on hondas turn off while coasting in gear, so the same goes for us.

Turn off the fuel injectors, and the engine will die! Will it not? Does it restart itself, too? I'm confused....:confused:

[omgwtfbyobbq]

All modern FI cars should have the injectors turn off (or maybe pulse depending on the post cat O2 reading) above a certain rpm with no throttle, depending on coolant temps (maybe other things too?), when the vehicle is above a certain rpm in any gear. Once the engine drops below a certain rpm, the car starts "idling" again, in or out of gear. I suppose manufacturers could tie the transmission switch into the vehicle idle, but the problem with this is if the switch goes, the car can stall at stops.

[ELF]

Turn off the fuel injectors, and the engine will die! Will it not? Does it restart itself, too? I'm confused....:confused:


If your speed is high enough the most auto trannys will spin the motor if you shut it off, So at higher speeds cutting fuel would not kill the engine.But once you get to a lower speed the tranny will stop turning the motor and engine will kill.

[CO ZX2]

If your speed is high enough the most auto trannys will spin the motor if you shut it off, So at higher speeds cutting fuel would not kill the engine.But once you get to a lower speed the tranny will stop turning the motor and engine will kill.

ELF. You are wrong on this one. If the fuel is shut off the engine is killed, not running under it's own power. Even though the engine is turning, it is being powered through the transmission, transmission being powered by the wheels. :mad: This scenario results in reverse FE. You are using energy you have already generated to turn over a dead engine. Not that hard to understand. Does this make any sense to you??

[SVOboy]

ELF. You are wrong on this one. If the fuel is shut off the engine is killed, not running under it's own power. Even though the engine is turning, it is being powered through the transmission, transmission being powered by the wheels. :mad: This scenario results in reverse FE. You are using energy you have already generated to turn over a dead engine. Not that hard to understand. Does this make any sense to you??
Coasting in gear doesn't have to equal reverse FE. You can coast to stops or in locations where it would be advantageous to have the resistance to keep you from going 90 down the hill and dying.

Ted, the fuel is cut and the engine becomes an air pump as it is still connected to the wheels and rotated through that physical connection.

[Matt Timion]

According to Ben (SVOboy) the fuel injectors on hondas turn off while coasting in gear, so the same goes for us.

I've found that to be true in my old honda (89) but not true in my new honda (2007)

[ELF]

ELF. You are wrong on this one. If the fuel is shut off the engine is killed, not running under it's own power. Even though the engine is turning, it is being powered through the transmission, transmission being powered by the wheels. :mad: This scenario results in reverse FE. You are using energy you have already generated to turn over a dead engine. Not that hard to understand. Does this make any sense to you??


I was only trying to explain to Ted that the engine would still be turning over at speed and not completely stopped, so it wouldn't have to restart. I didn't mean to imply the FE would be better.

[ELF]

[QUOTE=omgwtfbyobb If you have the time, could you check the fuel flow when coasting in N compared to decelerating in gear at a kinda slow speed, like ~40mph? If the idle consumption when decelerating is greater than coasting in N, then coasting must be a win win no matter what.[/QUOTE]

I checked today, hitting neutral at 40 mph....GPH was at .07 no throttle in drive.
switching to N at 40mph.. GPH went to 0.9 for about 2 seconds and then went back to .07

Seems to be no help going to neutral for coasting , at least in my car.

[repete86]

But at the same time though you're losing inertia energy because of engine braking. If you drop it into neutral, you'll coast longer, or at least that's the case with the cars I've owned.

[omgwtfbyobbq]

Exactly. In my case, I loose energy twice as fast with the car in gear, so since the idle rate is pretty much the same in both cases, coasting in N must always be a win-win situation.

[ELF]

But at the same time though you're losing inertia energy because of engine braking. If you drop it into neutral, you'll coast longer, or at least that's the case with the cars I've owned.

Your right, I wasn't thinking about engine braking just the SG readout. I did some neutral coasting today, my tranny shifts back in gear real smooth and easy even at speed so I'm going to see if this helps with overall mpg.

[Ryland]

It doesn't appear to me that you guys are differentiating well between coasting with engine on and engine off. When you are coasting with engine off NO fuel is being consumed. So how can your fuel consumption be less than that? Coasting in gear only slows your car down faster, how can that be good? If there was total fuel cut at any time with any car, the engine would die. Engine Off Coasting is a sure fire way to substantially raise your FE. It worked for me and has worked for many others.

If your engine runs extreamly rich on start-up, then to restart your engine can use more fuel, then if you had let it sit there idleing if it would only have been idleing for a few seconds.

[BeeUU]

I have noticed that on my newer car (2002 Mazda Mpv with auto trans) when I lift the transmission disengages and the engine falls to idle. When I hit the throttle the trans reengages. The MPV is a ULEV and to keep the converters at temperature the exhaust gas has to be hot. So no fuel cut while coasting. This is my guess......

So FE is sacrificed for emissions in this case. Pllus it makes for some nasty bumps from the driveline while driving in traffic, not so good. :(

[cfg83]

Hello -

Idling at neutral at speed X gives me roughly a 3/1 ratio of MPG/MPH :

http://home.earthlink.net/~cfg83/gassavers/sw2_mph_mpg.jpg

When I am in gear at these speeds, the increased RPM decreases to roughly a 2/1 ratio. This tells me I am still injecting fuel, even when my foot is off the accelerator pedal.

CarloSW2

[skewbe]

Hello -

...
When I am in gear at these speeds, the increased RPM decreases to roughly a 2/1 ratio. This tells me I am still injecting fuel, even when my foot is off the accelerator pedal.

CarloSW2

Don't believe your scanguage GPH (or mpg) reading in an overrun condition.

[JanGeo]

Strange as it seems - I see fuel being injected at .3gph going down First Beach hill at 25-30mph with foot off gas in gear but I also do NOT see the engine temp rise - if I put it in neutral at the bottom of the hill to coast a little I see the same fuel burn rate and the temp increases . . . indicating that the injectors are being driven but no fuel is getting into the engine when going down the hill (engine braking). This may also skew the fuel use in the SGII at fillup showing more fuel used than actual if engine braking is used a lot for the tank of gas.

[LxMike]

I think i got it figured why i sometimes see a high mpg reading when coasting in gear. out of gear coasting my engine idles a bit higher than when stopped and when in gear it drops and i think thats why i sometimes see higher readings.

I have started doing alot more Engine Off Coasting and it's showing on the SG, now just gotta see what it shows at the pump.

[cfg83]

LxMike -

I think i got it figured why i sometimes see a high mpg reading when coasting in gear. out of gear coasting my engine idles a bit higher than when stopped and when in gear it drops and i think thats why i sometimes see higher readings.

I have started doing alot more Engine Off Coasting and it's showing on the SG, now just gotta see what it shows at the pump.

The SG is way off for me in that condition. It is always a 10/1 factor. If I am coasting with engine off at 39 MPH, the instantaneous display shows 390 MPG on the SG. Since this is an "infinity condition", aka divide by zero (something akin to X MPH / 0 RPMs), I think the SG is just pasting in a safety number to keep from going bonkers.

What is interesting is that for my car, this might mean that I am getting better MPG for "run X" than the SG is reporting. If it is averaging in "10/1" MPG for the period that I am coasting with the engine off, then it is lowering my actual MPG figures.

CarloSW2

[cfg83]

skewbe -

Don't believe your scanguage GPH (or mpg) reading in an overrun condition.

Do you really think so? I was hoping to *infer* gas consumption from SG behavior.

From reading saturnfans.com, this behavior is consistent to what other people have written. What they say is that fuel injenction does decrease, but does not get very close to zero until you almost reach the car's idle RPM. However, at the idle RPM, the fuel injection will be maintained in order to keep the engine turning over.

The following is *NOT* the post I got this info from, but it does talk about ECU/PCM behavior in Saturns :
http://www.saturnfans.com/forums/showpost.php?p=964640&postcount=25

That's what drives me nuts about the SG. I don't know how it is programmed. How does it actually derive fuel consumption? Since it is a jack of all trades, it must be using lowest common denominator inputs across all OBD II implementations.

I mean, why does it need to know my engine size? For real accuracy, it should only be looking at the actual amount of fuel being injected into the engine. If however, it is extrapolating the fuel consumption by monitoring other ECU/PCM input data, then the engine info makes more sense to me.

I love the SG, but it really just makes me hunger for one of the high end engine data monitors that are in the $500+ range. :(

CarloSW2

[skewbe]

skewbe -

Do you really think so? I was hoping to *infer* gas consumption from SG behavior.


2 reasons why I think so (I've been meaning to hook up an LED to my injector leads to prove that it ISN'T getting voltage when the scangauge says it is delivering fuel, but haven't gotten to it)

1. in my car, when it is cold and I'm in overrun, I can feel the effects of turning the injector kill switch on and off, but not when it is warmed up. The scan gauge still will read .2 gph in both cases.

2. The scangauge will read .2gph with the injector kill switch held in, though this is understandable as a pulse is still being created, just intercepted on the way to the injector.

It could be that the metro ECU is a little stupid about it? It could be the scangauge is. Or maybe it's an obdII limitation? Or I'm hallucinating?

I don't worry about it too much since I don't normally have any downhills that require engine braking and I try and drive in a style that doesn't require ANY kind of braking.

[JanGeo]

The engine size relates to the injector size - bigger engines use bigger injectors that inject more fuel per msec of pulse so by setting the engine size they can calculate the amount of fuel injected.
I have seen 500-600 mpg on my scangaugeII when I get the idle down to .1gph and am coasting at 50-60 mph.
Keep in mind that all the data that the SG uses is from the ECU an dwe don't know exactly what that info contains.
The Liters setting sounds interesting I may try that.
ALSO DON'T play with the SG with the engine off - seems that can make it crash and lock up - happened to me again this week had to unplug but didn't loose the data this time at least. My remote door locks activated the SG and I didn't turn the ignition key on and it locked up.

[ELF]

My scan gauge lights up when I hit the remote door locks too.

[rh77]

My scan gauge lights up when I hit the remote door locks too.

I've had some wierd SG behavior, but it gets around (plugging it into all sorts of rental vehicles). On the '06 Kia Spectra, the car's ECU or the SG malfunctions and loses the ability to calculate MPG and the TPS position. It worked on a new one, so I'm thinking the car didn't like it.

It still is a great, simple, instant-feedback device for OBD-II'ers. The engine displacement is a good question, but seems to be accurate. I've driven everything from a 6.0L V-8 on the SG, down to a 1.6L -- and the pump calcs aren't that far off (as accurate as pumps calcs can be).

I also have a data logging chip for sensing and diagnosis a variety of variables -- to spot trends, etc. I bought it before I heard about the SG -- it uses the same OBD-II port, so I should've waited. I'd like a direct link to a laptop, but I can't see getting much use out of it. With the logger, I can come back and crunch the raw data if I want to.

RH77

[skewbe]

The engine size relates to the injector size - bigger engines use bigger injectors that inject more fuel per msec of pulse so by setting the engine size they can calculate the amount of fuel injected...

I think that's right. Then they multiply the calculated amount by however far off the tank was when compared with a fillup, to account for variances i.e. different injectors, different fuel pressures.

[cfg83]

rh77 -

I've had some wierd SG behavior, but it gets around (plugging it into all sorts of rental vehicles). On the '06 Kia Spectra, the car's ECU or the SG malfunctions and loses the ability to calculate MPG and the TPS position. It worked on a new one, so I'm thinking the car didn't like it.

It still is a great, simple, instant-feedback device for OBD-II'ers. The engine displacement is a good question, but seems to be accurate. I've driven everything from a 6.0L V-8 on the SG, down to a 1.6L -- and the pump calcs aren't that far off (as accurate as pumps calcs can be).

I also have a data logging chip for sensing and diagnosis a variety of variables -- to spot trends, etc. I bought it before I heard about the SG -- it uses the same OBD-II port, so I should've waited. I'd like a direct link to a laptop, but I can't see getting much use out of it. With the logger, I can come back and crunch the raw data if I want to.

RH77

Does the logger help you to monitor the Air/Fuel ratio over time? That way you could see if your Hot Air Intake was working (right?).

CarloSW2

[occupant]

Speaking of Hondas...I worked at the Civic factory in 1996 and 1997, in the VQD (vehicle quality department or something to that effect), and 2 of my 5 stations required me to drive the car from one line to the next, either out of the water pressure booth and onto final inspection, or from final inspection to the exit line which applied a black wax coating to the underside. We were instructed to start the car, drive it to the next line, and shut it off as fast as possible. This was an effort which allowed two benefits...1) saves fuel, and those guys running around the plant in the older '93 Civic sedan HATED to go get gas for us...2) keeps exhaust temperatures down to avoid setting off the heat alarm because the black wax is flammable until it sets. The automatic cars benefitted more from this procedure than the manuals, according to data collected by me when letting the car idle for x seconds and seeing what readout the temperature gauge on the exit line gave me.

Without question, the Saudi/UAE/Kuwaiti EX coupes with manual transmission could idle the longest time before exceeding the maximum allowable safe temperature. So I would assume the least fuel was being burned in those if the temperatures were lower. That or maybe they didn't have cats.

[brucepick]

All modern FI cars should have the injectors turn off (or maybe pulse depending on the post cat O2 reading) above a certain rpm with no throttle, depending on coolant temps (maybe other things too?), when the vehicle is above a certain rpm in any gear...

Cool. Do you know when they started doing that? My car is an '89 Volvo 240 auto trans; can you guess whether the injectors shut off?

[rh77]

rh77 - Does the logger help you to monitor the Air/Fuel ratio over time? That way you could see if your Hot Air Intake was working (right?). CarloSW2

It doesn't show A/F ratios but does show long and short-term fuel trim. During inital tests, it showed that higher IA temps improved the trim into a leaner condition, but would tend to enrich the mix at very hot IATs. It's a great device that I rarely use anymore because it plugs-in where the SG does :rolleyes: Lots of parameters to choose from. Davis Carchip E/X. (http://www.davisnet.com/drive/products/drive_product.asp?pnum=08221)

(Wondering) Hmmm...is there an OBD-II splitter out there? like a 1 into 2 deal?

RH77

[omgwtfbyobbq]

Cool. Do you know when they started doing that? My car is an '89 Volvo 240 auto trans; can you guess whether the injectors shut off?

I think they started around OBD1, and that pre OBD systems like VW's digi2 didn't, but don't quote me on that. You could hook up an led to an injector circuit pathway and see if it's blinking behavior changes. Or better yet, I've noticed on downhills that are steep enough to coast down in gear at 55mph, the engine temps drop a lot because there's no fuel being injected, but if I'm coasting in N, they stay pretty consistent. Find a big enough hill and watch what happens to the coolant temperatures.

[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]

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]

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]

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.