Fuel Savings Transmission Neutral vs In Gear at Stop 10

[EurekaArchimedes1]

I notice this phenomenon when watching NYC cab drivers shift to neutral (auto trans) when approaching a stop. The technique is: when approaching a stop, immediately shift to neutral thus coasting to a stop and leave it in neutral while waiting at stop. Engine rev decrease while coasting and less load on engine while at stop is the purported benefit. I've read that the gas savings are close to nil, if any. One argument being that load on engine while in gear is offset by higher engine revs while in neutral. I've seen some equations bandied about but none have really nailed it for me.

"Do not disturb my circles!" - 212 BCE, Siege of Syracuse

 

[Azmio]

I've been having this habit of shifting to neutral everytime I idle my car.

Here's my argument, few years ago when I was working for a big japanese OEM, the Japanese automakers always compete against one and another for the lowest idling speed. They came up with lots of innovative devices to make this happen. Low idling speed saves fuel. Low idling speed can only be achieved when the engine load is very low.

Now the question that all of you should be asking, will the load on engine get lowered when the transmission is left at neutral? Torque converter and hydraulic fluid inside the AT transmission create friction when moving.

 

[Lou Scannon]

Before I answer that, define "load on the engine".

 

[Azmio]

During engine testing and development, we have many test in which we call no load test. The test rig is so simple in which the engine flywheel is not connected to anything.

During the test, the load that is connected at this moment is just its own load that come from engine accessories.

If the engine crank palm is connected to a torque converter instead of being disconnected, we simply assume that external load is connected to the engine.

When extra load comes in the form of torque converter, AC compressor is on, extra current drawn from alternator, ECU will tell the ISC to allow more air to get in. In case of drive-by-wire throttle, the throttle will allow more air to get in. Otherwise, the engine rpm will drop and it may stall.

Let's go from here, I'm interested to read your answer

 

[PLeon]

I shift to N when I see a red light. It saves wear on the transmission and gears because I skip the downshifts to 3rd, 2nd and 1st gear.

 

[SomptingGuy]

This whole debate could be settled by a simple test.

1) In drive, handbrake on.
2) In neutral, handbrake on.

Which lasts the longer on a full tank of fuel?

- Steve

 

[GregLocock]

A slightly less gruesome test would be for someone with a car that measures instantaneous fuel consumption in litres per hour to shift between neutral and drive when stationary.

That's a standard feature in EEC V.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Lou Scannon]

For such a test, the engine should be fully warmed up, as there may be some algorithms in play designed to light off the catalyst when the engine is below full operating temp, such as fast idle, spark retard and/or enrichment + air pumping, that may affect in-gear and neutral idle fuel consumption differently.

 

[Bribyk]

I tried Greg's test on my drive home yesterday at multiple stoplights in my 2005 Acura MDX. The FUP (I assume fuel-usage per a certain unit of time) went from flashing between 0.6ml and 0.8ml to flashing between 0.4 ml and 0.6 ml when I shifted to neutral at the lights. Obviously the resolution is poor as it only reads in .2ml increments but it did seem to indicate fuel savings when shifting to neutral.

PS - if you want to play with the cool stuff like your yaw rate reading and fuel consumption calculations in a Nav-equipped Acura, hold down the "Menu", "A/C Info" and "Cancel" buttons at the same time.

 

[Bribyk]

Edit: Sorry. it's actually "Menu", "Map/Guide" and "Cancel".

 

[santorta]

Lets take the two situations,

Scenario 1: When you are approaching a stop with the car in neutral: The Engine speed drops from 2500rpm (for eg) to the idle rpm, during this phase the deceleration fuel shut off kicks in and cuts the fuel. And as mentioned in previous post, the fueling restarts when the upper idle rpm is reached and ISC takes over.

Scenario 2: When you are approaching stop with the car in gear, since the load is attached to the engine, the car wheels will be driving the engine and so the engine rpm will drop slowly and reach a minimum rpm sustainable at that gear. So all along this journey of reaching the minimum speed, the fueling will happen all the way. So there wont be any cut in fueling.

Deceleration fuel shut off works only when there is significant drop in rpm, atleast a difference of 1000 rpm, and then re-initiates the fueling based on gear vs engine rpm lookup table.

This is the algorithm used to enhance the fuel economy. So the cab drivers are doing the right thing.

Santhosh Arasan

 

[crysta1c1ear]

Pulling up at traffic lights, the engine can be turned over using the kinetic energy of the vehicle instead of fuel. If the foot comes completely off the accelerator (and it should), then fuel can be cut off. In neutral, fuel is required to keep the engine at idle RPM.

I think that's half the question ... what to do while pulling up to stop. The other half is what to do when stopped, and that is covered by

This whole debate could be settled by a simple test.
1) In drive, handbrake on.
2) In neutral, handbrake on.
Which lasts the longer on a full tank of fuel?

Modern cars (with for example an integrated starter generator) should turn the engine off when stopped and in neutral.

So I see no problem with driving as taught: in gear when driving and in neutral when stopped.

 

[jmw]

Safety, not economy, might be the reason (which would suggest a better level of driver training than I'd expect of a cabbie, most of whom I think tend to have pretty bad driving habits, so I may be wrong).

Automatic or manual, my drill is the same when I stop; I shift to neutral an apply the handbrake.

If I get rear ended then the car isn't going to go any further forward than the rear end impact will shove it. However, with the car in gear/drive anything could happen including me suddenly accelerating into the oncoming traffic as my foot slips to the accelerator for some reason.

Unless you're a stopo driver or simply a boy racer, this bit of technique costs nothing but one day, just might prevent a bad day getting worse.
This bit of training came to me courtesy of our company who decided driver training would reduce their insurance bills and we all got a brief exposure to police style driving. Very informative.

I also switch off when held at railway crossings or in heavy stop go traffic.

JMW

http://www.ViscoAnalyser.com

 

[GregLocock]

Interestingly thae advice i was given was exactly the opposite- when stationary in a traffic queue, if you are the last in line, stop reasonably far back from the vehicle in front, stay in gear and watch your rear view mirror just in case the next vehicle along doesn't brake in time, if it looks like he's going to hit you drive around the car in front.

Sounds a bit unlikely to me.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jmw]

Our instructors were retired Yorkshire Police Senior Driving Instructors employed by Schlumberger.
I can only tell you what they told us and why.
I can also tell you they scared the living daylights out of us during our driving experience sessions.

JMW

http://www.ViscoAnalyser.com

 

[GregLocock]

Sorry, I meant the recommendation I was given sounded a bit unlikely. The advice you were given sounds OK.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ivymike]

just 2 weeks ago I pulled partway into an intersection to make room for a guy who wasn't stopping behind me... almost avoided being bumped. As it turned out, I got a little ding in my tailgate after he used up those extra 10ft.

 

[EurekaArchimedes1]

(re: ivymike's 09/09/08 7:43 post)

That's a bad situation all the way around. Either voluntarily edge into the intersection hoping to avoid or minimize the rear impact, or keep the pedal firmly depressed (maybe with the hand brake on) and take the hit. Either way you risk physical harm. I'd probably put it in park and apply the parking brake, then try to not tense up during impact (but I'm doubtful I'd actually be able to be relaxed!).

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Again, thanks to all the engineers who have taken the time to provide their unique perspectives from the point-of-view of their various engineering disciplines. The answers you've provided to my my original question have been very informative and insightful. I hope this topic continues to grow. It's been a welcome diversion from the world of electronics!

"Do not disturb my circles!" - 212 BCE, Siege of Syracuse

 

[jmw]

Ah, I understand you now Greg.

The instructors maintained, incidentally, that there is no such thing as an accident, (in the UK the motorists tend to refer to collisions saying "I had an accident". Perhaps a shade less judgemental is the US "I had a wreck") they maintain it is all driver error of some kind or other, you or the other guy or both.

JMW

http://www.ViscoAnalyser.com

 

[jmw]

Incidentally Greg, and Ivymike, I'd not say you are completely wrong. I had a similar experience to Ivy mike while sitting at traffic lights. I heard a screeching of tyres, glimpsed someone approaching fast in my rearview and somehow also checked the traffic flow through the intersection, dropped it into gear and humped the lights away from trouble....

Now it seems like an awful lot to be able to do when sitting at the lights with your mind in neutral but looking back at it I recognised that the driver training had instilled some useful behaviour patterns so that my mind wasn't switched off, I had my hand on the gear lever and it is a moment to engage gear and then drop the hand to the brake release and I was anyway assessing traffic flow through the intersection so when I glanced in the mirror I already knew what to expect from the noise and was already reviewing what I knew about the junction traffic and making decisions. In such situations the mind and body can be pretty damn quick and it is only afterwards that you can sort out the sequence of events and actions.

Oh, yes, once I had completed my escape, the next thing into my mind was to worry if the junction had cameras in which case I could expect a dangerous driving ticket for going through a red light.... it didn't happen.

One other useful tip that came was never to stop too close to the car in front and I'd guess that is what helped Ivymike; he had room to manoeuvre around whatever obstacle lay in front of him.

JMW

http://www.ViscoAnalyser.com

 

[DigitalGT]

Sorry to dig up the old.

My beater/commuter car is a '96 Toyota Avalon. They only came with a 3.0L mated to a 4 speed automatic. I have a ScanGauge II plugged into the ODB II port and I experimented for a while with bumping the trans lever in to neutral once completely stopped. Florida has some fairly long stop light cycles. Unfortunately the ScanGauge only has 1/10th of a gallon per hour resolution so the data is not exactly convincing but it is consistent. With air conditioning on and the transmission in drive at a complete stop, the engine draws an indicated 0.5 GPH. Turning off A/C drops that to 0.4 and pushing the transmission lever into neutral drops it further to 0.3 GPH. Very occasionally the gauge drops to 0.2 GPH for a second or two. I've only noticed this on extremely hot days.

It's not possible to do any accurate short term consumptions tests and I have not noticed a mileage increase by shifting to neutral while and standing.

FWIW though the gauge does indicate a change.