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

 

[SonyAD]

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.

 

[BrianPetersen]

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]

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]

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]

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.

 

[ivymike]

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]

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]

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]

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]

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.

 

[mattsooty]

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]

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]

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]

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]

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 :-(!

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 . 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 (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]

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]

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]

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]

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