Which uses more fuel and how much?

[Edited on November 29, 2005 at 12:54 PM. Reason : eh, neutral, couldn't hit the stop button quick enough]

11/29/2005 12:54:15 PM

NEGLIGIBLE. A point not worth arguing. If it's an automatic, just leave it in gear.

[/thread]

11/29/2005 12:56:27 PM

coasting uses idling gas.

engine braking uses more gas

[Edited on November 29, 2005 at 12:57 PM. Reason : manual or auto?]

11/29/2005 12:57:07 PM

But how negligible? My coworker posed this initially when we were coming back from lunch, and my first impression was that yea it was negligible but he was trying to claim that in gear would be better, which just didn't make sense to me.

11/29/2005 12:58:25 PM

Engine braking doesn't necessarily use more fuel. Some vehicles it actually conserves fuel to leave it in gear. In some vehicles, a torque sensor lets the ECU figure out that the tires are applying torque to the engine rather than the other way around, and actually switches off the injectors untill the accelerator is depressed or you start going uphill, lose momentum, etc. Like zxappeal said, still negligable, but an interesting fact not many people know.

This is especially true of most new parallel hybrid systems like the Prius and all the vehicles that "charge" themselves. When coasting/braking, engine/electric-motor torque is not required and the torque created by the tires (gravity) turns electric motor which is used to generate power to charge the battery pack.

[Edited on November 29, 2005 at 1:10 PM. Reason : -]

11/29/2005 1:06:23 PM

might save a qurter of an ounce of gas on a long hill around here, it'd be different in the mountains, but then you would burn up your brakes without engine braking

11/29/2005 1:07:22 PM

I would say that it'd be difficult to say it's negligible in an automatic car and here's why. The gas savings may be negligible, but the damage the average person will do shifting the car in and out of gear every time they coast will eventually take it's toll on the transmission.

As for gas savings, yeah, it's negligible.

Mike

11/29/2005 1:17:42 PM

Back in September I was driving in the mtns around Hendersonville, and coasted probably 3-4 miles while in neutral. I took the car out of gear going around 75mph and topped out around 96mph.

It was errie to be going that fast with no engine sounds, lol. I was going more than 90 for the majority of the time, i was impressed.

back OT: if you take this argument at face value, its negligible for the reasons stated. However if you put the car in neutral in order to coast until you have to hit the gas again, then yes coasting in neutral helps conserve some fuel. The car will come to a stop much quicker if left in gear than if in neutral, due to the engine braking. Therefore you can travel further without reapplying the gas.

[Edited on November 29, 2005 at 1:56 PM. Reason : c]

11/29/2005 1:54:43 PM

since you are going down hill
if you leave it in gear
it converts the potential energy from your lost height back into fuel

11/29/2005 2:07:13 PM

In a typical modern four cylinder engine, coasting at 60mph in 5th is going to get you around 250mpg. Coasting at 60mph in neutral is going to get around 400mpg. This is what I was told by a mercedes technician of 20 years when I asked the same question.

Even though the difference seems large, it doesnt matter in the long run because the amount of gas you use while coasting in gear is already negligible.

11/29/2005 2:15:10 PM

Question,

For a non hybrid automatic vehicle coasting down hill, are the injectors completely closed? How does the engine stay "lit".

11/29/2005 2:35:33 PM

No need to stay lit. once the vehicle's speed and/or the engine speed drops below a certain range and/or the throttle position sensor comes off of idle, the ECM begins firing the injectors...the engine still windmills the whole time, even in an automatic.

11/29/2005 2:40:14 PM

Yea but I would expect of feel a sort of dieseling feeling from coming back from coasting to foot on the gas if this were the case. Maybe it is there and I just don't feel it.

11/29/2005 2:47:40 PM

why the hell would you think that?... zxappeal is right...and I've seen the deceleration fuel tables for an LS1 engine, 0 fuel when coasting most of the time

11/29/2005 2:50:05 PM

I dunno. Poor assumption I suppose.

11/29/2005 2:51:04 PM

Jeez, nobody believes me unless zxappeal says something to the affect.

[Edited on November 29, 2005 at 3:04 PM. Reason : -]

11/29/2005 3:01:42 PM

I tell you what...if you drive a stick, let off the gas while coasting down a hill. Now turn the ignition switch off. Can you feel any difference? I doubt you will.

11/29/2005 3:03:13 PM

other than loss of power steering and boosted brakes, of course

11/29/2005 3:06:02 PM

Not with a stick...remember that the engine is windmilling. Even on a lot of newer automatics, as the lockup converter stays locked up. If it doesn't have a lockup converter, then the ECM HAS to supply fuel to the engine to keep 'er ticking.

Actually, if you cut the engine on a newer automatic, it'll drop out of gear and unlock as well, so you really can't try this trick...

[Edited on November 29, 2005 at 3:10 PM. Reason : more to it...]

11/29/2005 3:09:19 PM

Quote :

"other than loss of power steering and boosted brakes, of course "

wow, just wow

11/29/2005 3:11:18 PM

negligible

adj 1: so small as to be meaningless; insignificant; "the effect was negligible" 2: not worth considering; "he considered the prize too paltry for the lives it must cost"; "piffling efforts"; "a trifling matter" [syn: paltry, trifling]

11/29/2005 3:21:06 PM

Quote :

"Not with a stick...remember that the engine is windmilling. Even on a lot of newer automatics, as the lockup converter stays locked up. If it doesn't have a lockup converter, then the ECM HAS to supply fuel to the engine to keep 'er ticking."

So are agreeing with my original statement that the injectors will in fact cut off on most modern vehicles, or disagreeing? I'm just curious, sorry to split hairs, I'm honestly not trying to be a smart ass.

11/29/2005 3:25:52 PM

Quote :

"wow, just wow"

I'm assuming you're calling me a dumbass? Why? Haven't you ever tried to push/roll/coast a vehicle without the engine running? You didn't notice a drastic change in steering and stopping performance?

11/29/2005 3:28:43 PM

Given than most, if not all, vehicles with electronic fuel injection have lockup converters, I think it's a given that the fuel map goes to 0 when coasting.

11/29/2005 3:38:16 PM

In gear coasting uses absolutely no fuel what so ever in some cars under certain conditions, these include and are not limited to many Porsches, BMWs, VWs from the early 80s to present day. Do a search on it, or read an engine management book.
Ahmet

11/29/2005 3:41:28 PM

Been there, done that. You're right.

Damn near all cars today (even my '79 Datsun 280ZX) cut fuel when coasting. Emissions reasons.

[Edited on November 29, 2005 at 3:48 PM. Reason : yepper.]

11/29/2005 3:47:44 PM

Was that you I got in an argument with, can't remember for sure... Reading over your posts on this page, I'm not in disagreement.
Ahmet

11/29/2005 3:50:25 PM

and its illegal regardless.

11/29/2005 3:58:01 PM

illegal to coast downhill in neutral?

11/29/2005 4:00:17 PM

It definitely is for trucks. I remember getting that drilled into my head when I was getting my class B license when I was 18.

11/29/2005 4:02:32 PM

its illegal to coast uphill, downhill, or on a flat with your car not in gear.

11/29/2005 4:05:37 PM

houston won that round, an automatic is designed to bein gear, not in neutral so leave it there.

11/29/2005 4:13:35 PM

have ya'll read your owners' manuals?

11/29/2005 4:26:19 PM

Also, most older automatics do have have enough pressure to provide adequate bearing lubrication at low rpms, so while you are turning 800 rpms with your engine, your car is rolling 70 mph, the back end of your transmission is turning 3k rpm, good bye bearings.

11/29/2005 4:46:02 PM

Quote :

"I'm assuming you're calling me a dumbass? Why? Haven't you ever tried to push/roll/coast a vehicle without the engine running? You didn't notice a drastic change in steering and stopping performance"

Yes, I'm calling you a dumbass. The brakes work on engine vacum--at idle there is plenty, with the engine stopped there is none. The power stearing system uses hydraulic pressure from a pump--it makes plenty of pressure at idle and with the engine stopped it makes none. Both stearing and braking systems work fine at idle but basically not at all with the engine stopped, so you really can't use that as a comparison since they aren't the same at all..

11/29/2005 5:44:12 PM

Unless the engine is windmilling...then both will still work.

The engine still draws vacuum when it's spinning, even if it's not firing.

11/29/2005 5:52:16 PM

this topic was discussed at length a while back.

Quote :

"...VWs from the early 80s to present day..."

that would have to be mid-late 80s for the vws when they went to electronic fuel injection (as opposed to CIS mechanical fuel injection.

[Edited on November 29, 2005 at 6:28 PM. Reason : and i think it was me that argued with you awhile back, Ahmet.]

[Edited on November 29, 2005 at 6:29 PM. Reason : and i eventually admitted i was wrong.]

11/29/2005 6:24:19 PM

this is a fucking retarded argument, am i the first to realize this?

12/1/2005 11:21:30 AM

Quote :

"Was that you I got in an argument with, can't remember for sure... Reading over your posts on this page, I'm not in disagreement. "

ahmet the discussion you are referring to was with myself and Poe87 i believe.

12/1/2005 8:42:25 PM

Tis' all good w/me, I remember us all being happy at the end.
Ahmet

12/2/2005 12:19:24 AM

engine braking does not use gas as the injectors cut at, as the engine slows down the slowly kick back in around 1500 rpm to slow the engine down enough to idle.

12/2/2005 2:00:04 AM

Quote :

"stearing"

12/2/2005 3:50:13 AM

yeah... and I'm the dumbass I keep forgetting this is TWW and ppl don't understand sarcasm or light hearted jokes.

12/2/2005 12:02:59 PM