Gas use while coasting

[DarcShadow]

I just read an article in Dec. Car & Driver and it stated that most vehicles built since 1990 will turn off the fuel when there is no throttle and the engin RPMs are higher then idel, ie when you're coasting to a stop the engine doesn't burn any fuel. The reason the article brought this up was to disprove the myth that putting your car in neutral will save you gas.

It was a very short article and didn't go into much detail but did mention once automatic should not be put into neutral. Does any one know if this applies to manuals and our Vipers? Is it better, in terms of gas usage, to leave the car in gear while coasting to the stop light or put it in neutral?

 

[GTSnake]

How can you not use any gas when in neutral??? If the engine is running you're using gas. The difference between decelerating in gear or putting in neutral to decelerate is extremely small. You may slow a little faster in gear because the drive train acts like a brake.

Personally I just nudge it into neutral as I'm slowing down just under 1k rpm. At that point the gears are pretty much in sync so I don't need to push on the clutch to get them to disengage. Saving a little wear on the clutch.

 

[Dom426h]

Considering my viper sounds like WWII with all the popping upon deaccel im pretty sure that some fuel is burning in there.

Ths topic was discussed a few months ago here, check er out:

http://forums.viperclub.org/sneaky-petes-place/617071-instant-gas-mileage-improvement-70s.html

 

[DarcShadow]

Don't think I made my question clear.

Clearly some gas is still burnt right after you take your foot off the gas, and the link you gave says to shift to neutral to save gas, but the article says that actually uses more gas.

When in neutral, obviously the car is using gas, the question is, when coasting, still in gear, does gas still get used?

The article indicated that automatics built since 1990 don't fire the injectors when you're coasting, but if you were to shift into neutral you would then have to burn gas in order to keep the engine spinning where as if you just left it in gear the coasting would keep the engine turning and the injectors wouldn't need to fire up till the RPMs dropped below a set level.

I have heard of this before, just didn't realize it was as common as the article made it seem. Does manual transmission cars work the same way though?

 

[ViperTony]

Coasting in a Viper??? Is that possible? It's always been idle or WOT for me...nothing much in between. I use my Vec3 to stop dumping fuel when I lift off the gas at a certain MAP reading so I guess its possible to save gas but based on the way I drive it does nothing for me in terms of fuel savings. Other than that...the Viper wasn't meant for gas economy, its meant to be driven...driven hard. F the EPA.

 

[Mopar Boy]

If the piston is moving, fuel is being injected regardless of if the vehicle is in gear or not. If you were to fill your tank at the top of a mountain and drive down with zero throttle application, then fill at the bottom, you would be down fuel. This even applies to a diesel motor - a motor design that injects fuel based on engine load!

As for idleing, I burnt threw 1/4 tank in about 1.5 hours straight idling. No I did not do it for fun! I was trying to leave a car show and it was only moving about 15 feet a minute!!

Robert

 

[JonB]

I just read an article in Dec. Car & Driver and it stated that most vehicles built since 1990 will turn off the fuel when there is no throttle and the engin RPMs are higher then idel, ie when you're coasting to a stop the engine doesn't burn any fuel. The reason the article brought this up was to disprove the myth that putting your car in neutral will save you gas.

It was a very short article and didn't go into much detail but did mention once automatic should not be put into neutral. Does any one know if this applies to manuals and our Vipers? Is it better, in terms of gas usage, to leave the car in gear while coasting to the stop light or put it in neutral?

This is specific to the PRIUS and some other gas-elec hybrids.

A gasoline engine uses less fuel idling / coasting than with the gearbox engaged. But not ZERO gas! The Dodge MDS motor fuels fewer cylinders when coasting idling...

But only an elec-gas hyprid uses ZERO gas while 'coasting' as it uses regenerative braking to build the batteries

 

[GR8_ASP]

If the piston is moving, fuel is being injected regardless of if the vehicle is in gear or not. If you were to fill your tank at the top of a mountain and drive down with zero throttle application, then fill at the bottom, you would be down fuel. This even applies to a diesel motor - a motor design that injects fuel based on engine load!

As for idleing, I burnt threw 1/4 tank in about 1.5 hours straight idling. No I did not do it for fun! I was trying to leave a car show and it was only moving about 15 feet a minute!!

Robert

Wrong. Vehicles equipped with decel fuel shut-off (most) use no fuel while decelerating at closed throttle down to somewhere near idle speed. Some even engage the torque converter clutch to maximize the time in the fuel shut-off mode.

 

[Mopar Boy]

Wrong. Vehicles equipped with decel fuel shut-off (most) use no fuel while decelerating at closed throttle down to somewhere near idle speed. Some even engage the torque converter clutch to maximize the time in the fuel shut-off mode.

Um. OK. I ****. I will try to be more informed before I speak in the future.

I hope I did not confuse anyone by my incorrect rambeling statement.

I willingly give up 10 HP bounus points for my mistake.

Robert

 

[mad0953]

According to my AEM wideband A/F meter when I let off of the gas and decelerate barely any gas(if any) is being used. The gauge acually goes blank (it's digital) off of the lean side, which is 20 to 1.

 

[DarcShadow]

Just curious, what does your meter say when you are sitting still ideling?

 

[ViperTony]

Just curious, what does your meter say when you are sitting still ideling?

Mine says 14.7 to 15.1 during pure idling.

 

[mad0953]

Same here, it flucuates between about 14.2/1 to 15.6/1.

 

[GTSnake]

Wrong. Vehicles equipped with decel fuel shut-off (most) use no fuel while decelerating at closed throttle down to somewhere near idle speed. Some even engage the torque converter clutch to maximize the time in the fuel shut-off mode.

How can an engine use "NO" fuel on decel? Wouldn't that mean that the engine quits? If it's at idle it's still using some fuel.

 

[chimazo]

How can an engine use "NO" fuel on decel? Wouldn't that mean that the engine quits? If it's at idle it's still using some fuel.

It's not at idle, it occurs while the car is in gear upon deceleration only. It is typically referred to as DFCO (Deceleration Fuel Cut Off). During off-throttle deceleration, the PCM cuts off the injectors. You can usually feel it when it occurs, and the engine note will change slightly (after about second into decelerating). It reduces emissions and helps fuel economy. The engine does NOT turn off.

 

[ViperTony]

How can an engine use "NO" fuel on decel? Wouldn't that mean that the engine quits? If it's at idle it's still using some fuel.

On decel, and in gear, when I get completely off the gas peddle my AFR hovers around 16,17 for a couple of seconds and jumps to 20 which is all air (I think). Yes, at idle it uses fuel indicated by the fact the engine is running as well as my AFR reading 14.7 ish (stoich) but there's obviously a difference in how the fuel delivery works between idle and decel. Maybe GR8_ASP can explain it in more detail.

 

[Dom426h]

Ive been reading about this DFCO (Deceleration Fuel Cut Off).
Fuel is cut while deaccel/coasting upon certain constraints some of which being engine temp, vehicle speed, rpm. Once up to temp the vehicle will cut the fuel after some ?sec delay after lifting off the throttle, then fuel will come back on once the engine gets down to some ?rpm above idle rmp or some speed above 0mph

So the question is, Do our vipers do this? and what are its constraints?
Judging by how my GTS sounds like it is blasting off cannons when i get to 1500rpms id say that it is definatly burning some fuel from 1500 on down.