Fuel Savings Transmission Neutral vs In Gear at Stop
Fuel Savings Transmission Neutral vs In Gear at Stop
(OP)
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





RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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%.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.)
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
MS
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
NYC has for at least 2 years had hybrid taxis in the fleet.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
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
"Do not disturb my circles!" - 212 BCE, Siege of Syracuse
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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...
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
1) In drive, handbrake on.
2) In neutral, handbrake on.
Which lasts the longer on a full tank of fuel?
- Steve
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
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.
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
Puzzling.
"Do not disturb my circles!" - 212 BCE, Siege of Syracuse
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
RE: Fuel Savings Transmission Neutral vs In Gear at Stop
"Do not disturb my circles!" - 212 BCE, Siege of Syracuse