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Fuel Savings Transmission Neutral vs In Gear at StopHelpful Member!(10) 

EurekaArchimedes1 (Electrical)
29 Jul 08 14:48
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

SonyAD (Computer)
29 Jul 08 14:58
You're probably putting more wear on the brake bands and clutch packs when you reengage gear at the green light.

It doesn't really matter because torque converter automatics all fundamentally suck anyway and deserve to die o thousand agonizing deaths.

Yes, I drive a stickshift.
Helpful Member!  BrianPetersen (Mechanical)
29 Jul 08 17:20
If the engine in question has a governed idle speed, it should save a little bit of fuel. I have heard that some newer GM automatics shift to neutral on their own when stopped as long as the brake pedal is being held.

If the engine in question has a plain ordinary throttle stop screw (example, old carbs) then this will save nothing, because the engine will just speed up from the reduced load.

I have no idea whether the saved fuel will offset possibly an earlier transmission overhaul.
carnage1 (Electrical)
29 Jul 08 17:28
Is it just me or is a big city taxi the perfect place for a hybrid. The engine shuts off every time you stop plus the regenerative breaking. Any idea if any of the automakers have considered this for their fleet vehicle sales?
SonyAD (Computer)
29 Jul 08 18:38
I've never liked the hybrid concept. The engine wear from the atrocious start/stop cycle is probably wearing the engine to death. Unless they used prepressurization pumps for oil and maybe oil preheaters as well.

On top of that, you're just piling on cumulative inefficiencies that, in the end, are maybe not as efficient as a whole as a well designed diesel engine could be for the same application.

The gas engine's thermal and mechanical efficiency (although it's probably run continuously at its most efficient setting), the generator's inefficiency, the batteries' inefficiency and losses, even the electric engine's minute inefficiency.

They all add up. I'm not sure it's any better than what you can get with a well designed diesel and stickshift or some other direct coupling transmission. I read somewhere that the 2007 generation TDI engines from VW have a peak thermal efficiency of over 40%.
JSteve2 (Automotive)
29 Jul 08 18:48
The Prius gasoline cycle is 38% thermally efficient. Add in the recovery from the hybrid side and the Prius compares favorably to a 42-3% thermally efficient diesel (which is state of the art). Best would be a diesel with a hybrid cycle but the potential gains are lower than with gasoline.

There are losses cycling thorough the generator and batteries, but that's energy that previously was simply lost. Plus, the dips in inefficiency having to work the engine outside the sweet spot (e.g. accelerating onto the interstate, jack rabbit starts, etc.) that can be generally avoided in the hybrid scenario more than make up for it (exception being long hills that outlast the battery charge). Remember that the hybrid also gives you a ready source of extra power (the electric side) that you don't have frictional losses for, where in other engines you have to build in extra "unused" displacement.
Helpful Member!  ivymike (Mechanical)
29 Jul 08 18:57
you forgot to mention regenerative braking...  

but the thread wasn't about that.  If you have an obd-II car, you should be able to hook up a scan tool at idle and see what the fuel rate is, then drop it in gear and check again.
 
IceStationZebra (Mechanical)
29 Jul 08 21:33
I am with JStev2. I have been following hybrid/electric vehicle development since the early 90's, and I've always said that the ideal marriage would be a full series diesel hybrid. The engine could be set to run at a fixed RPM with a correspondingly high efficiency. My second choice would be a parallel diesel.

As for the original question putting the car in neutral would save some energy, especially when you consider that they spend a fair portion of the day stopped at red lights. The higher rev argument is bogus unless the engine is not fuel injected. The bigger question is if the fuel savings is would offset the increased transmission wear? With $4/gal gas probably yes.

ISZ
SonyAD (Computer)
30 Jul 08 3:38
http://www.peugeot.com.au/PEUGEOT/AU/me.get?site.sectionshow&FFFF2391

75.15 mpg, US.

Who knows how much of the mpg is really the merit of a low drag aerodynamic shape and low vehicle weight, though?

I suspect doing without the turbocharger would further increase mileage.

The Loremo GT is purported to do 87 mpg US whilst being able to reach 60 mph in 9 seconds and a max. speed of 137 mph.

I think there are many vehicles on the road already which will beat the Prius in the myriad of less than ideal driving conditions.

The best of all worlds would probably be the latest generation VW PD TDi because of the high compression ratio and high injection pressures, mated to a chain drive (or anything else that doesn't rely only on static friction to handle the torque) CVT in a lightweight, low drag body.
globi5 (Mechanical)
30 Jul 08 5:50
If plug in hybrids become popular, than engine weight and engine costs will be more important than engine efficiency, since 90% of the driving time the engine is just useless weight.

In this case a lighter, cheaper gasoline engine will be a better option in a hybrid than a diesel engine. (Besides there's also no need for particle filter, NOx reduction with ammonia injection etc.)
ivymike (Mechanical)
30 Jul 08 7:29
The higher rev argument is bogus unless the engine is not fuel injected.

Why's that?  

If your vehicle has an 1100rpm idle speed in neutral, and drops to 900rpm in gear, you're moving 18% less air volume per unit time.  If your throttle position stayed the same, air density would be about the same, and that would translate to ~18% less fuel.  If your throttle opened slightly, density and fueling would go up, and who knows where you'd land.
 
Helpful Member!  mattsooty (Automotive)
30 Jul 08 8:07
If the taxis in question are fitted with EFI then slipping the transmission into neutral will, more than likely, cost you mpg.

Almost immediately after the pedal position goes to zero input Over Run Fuel Cut Off will be initiated and no fuel will pass through the engine until the engine speed reaches upper idle, whereby the idle speed conroller will take over.

If the transmission is put straight into neutral the engine will immediately go into idle speed control with no fuel cut.

MS
ivymike (Mechanical)
30 Jul 08 8:08
actually, the density for fixed throttle position would have to go up a bit, due to lower flow (and thus lower pressure drop).  perhaps there isn't much effect after all.
 
mattsooty (Automotive)
30 Jul 08 8:16
Just re-read my post, by 'upper idle' I mean the point where the ISC takes control of the engine speed on overrun and controls it down to steadystate idle speed NOT the same as high idle in a diesel.

MS
drwebb (Automotive)
30 Jul 08 8:40
I recall my 2001 V70 with JATCO transmission claims in the OM that it shifts to neutral at stop to save fuel.  Those Swedes- always thinking about the planet. Also has catalysts on the radiator that decompose ground-level ozone.   

NYC has for at least 2 years had hybrid taxis in the fleet.
EurekaArchimedes1 (Electrical)
30 Jul 08 22:20
Thanks for all the insightful comments, I'm going to work out this stuff in notation for variables & functions for kicks. Tried to scribble out a partial DE for air, fuel, throttle, idle, etc.. - then "may day"- the "batphone" rang sad!

The taxi's I've been in NYC were all gas engine naturally aspirated. Older, maybe late 90's/early 00's, no fancy stuff in the mechanicals - this was in the Bronx, Queens - probab Manhattan cabbies drive the nice stuff smile. Auto trans, all.

PS That German Low Resistance Mobile (LOREMO), I had not heard of. Would love to see a multiphyics simulation (i.e. COMSOL)of that GT. Great design from the quick glance I took around the web.

PPS <em>"If you have an obd-II car, you should be able to hook up a scan tool at idle and see what the fuel rate is, then drop it in gear and check again." </em>- cuts the "Gordian knot" quite well. They must have the obd-II - I don't think I rode in anything OLDER than a 96, for pete's sake smile(Then, maybe I did, there were some real beaters in the bunch, especially in parts of Queens). I might have a few laughs nxt time I'm in Gotham and ask the cabbies to let me hook up a scantool.

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

izzmus (Automotive)
3 Aug 08 10:43
ivymike, you also have to figure that the engine is under load at 900rpm in gear, and is not under load at 1100rpm free-running.

In practice, watching pulsewidths, it ends up being a wash, except on engines with idle controllers that don't allow the engine to rev up out of gear.

If the computer has a coastdown fuel cut, you may end up actually using more fuel by shifting to Neutral instead of coasting down in gear...
ivymike (Mechanical)
3 Aug 08 13:43
It's got a bit of trans load on it, but it has slightly lower friction and accessory loads.  Seems to me that if you took an engine running at idle (0% throttle), added a bit of load so that rpm dropped (still 0% throttle), you'd be burning the same amount of fuel - the engine rpm would have dropped until other loads decreased enough to equilibrate.
  
 
carnage1 (Electrical)
3 Aug 08 22:15
The unasked question is if they switch to neutral to save fuel or because they feel it will keep the trans cooler? maybe this practice has something to do with transmission longevity as the trans in the crown vic does not seem to be up to a heavy duty task. (I can say this, I replaced a lot of them as a transmission mechanic)
EurekaArchimedes1 (Electrical)
4 Aug 08 22:05
The unasked question is if they switch to neutral to save fuel or because they feel it will keep the trans cooler? - carnage1 (Electrical)

yes, but...

You're probably putting more wear on the brake bands and clutch packs when you reengage gear at the green light. - SonyAD (Computer)

Do you think the decrease in overall wear to the transmission by keeping it x degrees cooler overall via shifting to neutral at stop will pay off compared to the incremental increased wear to the trans brake bands and clutch packs, and other components from shifting in and out of gear at stop?  

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

Helpful Member!  Azmio (Automotive)
4 Aug 08 23:19
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.  
Helpful Member!  hemi (Automotive)
5 Aug 08 0:54
Before I answer that, define "load on the engine".
Azmio (Automotive)
5 Aug 08 2:16
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 (Mechanical)
5 Aug 08 3:47
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 (Automotive)
5 Aug 08 5:23
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 (Automotive)
5 Aug 08 7:03
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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

hemi (Automotive)
5 Aug 08 16:52
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.
Helpful Member!  Bribyk (Mechanical)
6 Aug 08 14:34
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 (Mechanical)
6 Aug 08 15:17
Edit: Sorry. it's actually "Menu", "Map/Guide" and "Cancel".
Helpful Member!  santorta (Automotive)
28 Aug 08 15:34
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 (Automotive)
7 Sep 08 12:08
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

Quote (Steve):

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 (Industrial)
8 Sep 08 9:28
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
www.ViscoAnalyser.com
 

GregLocock (Automotive)
9 Sep 08 0:01
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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

jmw (Industrial)
9 Sep 08 6:08
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
www.ViscoAnalyser.com
 

GregLocock (Automotive)
9 Sep 08 7:11
Sorry, I meant the recommendation I was given sounded a bit unlikely. The advice you were given sounds OK.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

ivymike (Mechanical)
9 Sep 08 7:43
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 (Electrical)
9 Sep 08 8:17
(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!).

--------
-----------
--------------

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 (Industrial)
9 Sep 08 12:28
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
www.ViscoAnalyser.com
 

jmw (Industrial)
9 Sep 08 12:37
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
www.ViscoAnalyser.com
 

Helpful Member!(3)  DigitalGT (Military)
7 Oct 08 14:07
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.
EurekaArchimedes1 (Electrical)
7 Oct 08 18:39
"Very occasionally the gauge drops to 0.2 GPH for a second or two. I've only noticed this on extremely hot days."

Puzzling.

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

btrueblood (Mechanical)
8 Oct 08 11:28
Good job, DigitalGT, and a star for that post.  Data (no matter how crude) beats theory any day.
dieselhellfire (Mechanical)
26 Oct 08 14:25
i have a scanguage installed in my truck, and it allows me to view anything the ECM "sees" (eng. load, rpm, mpg, MAP, IAT, Ign. adv., etc). I was curious about this myth as well, and from what i have seen, no significant difference in fuel consumption can be obtained by shifting into neutral. Besides, in neutral the engine has to spin the flywheel, clutch, input shaft and countershaft. When clutched all that spins is the flywheel and pressure plate. Yes, it does wear on the throwout bearing, but you press and release the clutch only once, as opposed to twice when shifted into neutral. Engine load stays at a steady 4% whether clutched or idling in neutral, although I have noticed a short spike when you let out on the clutch once in neutral (the engine has to get the input and countershafts spinning again). I highly recommend the Scangauge. If your vehicle has OBD2, buy it. It displays real-time sensor outputs, tracks fuel mileage, even calculates horsepower in real-time (uses engine load and RPM).  
BrianPetersen (Mechanical)
26 Oct 08 17:09
Dieselhellfire, the terminology that you are using suggests manual transmission. That is a different ball game. A manual in neutral is essentially no different from a manual with the clutch in; the only diff is a minor amount of inertia because all that stuff is just spinning in ball bearings. An automatic, which has been the main subject of discussion in this thread, is a WHOLE different deal. The torque converter puts a significant load on the engine when its output stage is stalled (i.e. trans in gear) compared to when its output stage is freewheeling (i.e. trans in neutral).
EurekaArchimedes1 (Electrical)
26 Oct 08 17:57
Many thanks BP. As you stated, manual vs auto trans are two completely different ball games, especially pertaining to this discussion. Thanks to DHF for the Scangauge recommendation. It looks like a reputable piece of measurement equipment from a cursory look on the web.

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

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