On the topic of "Hyper-Mileing"...

[byez]

I drive an auto and I occasionally drop it into neutral and coast to a stop when safe to do so as in there being no other cars around.

When stopped, is there a fuel economy difference between neutral and drive? Is there some inner workings within the transmission that will cause fuel consumption in drive and not in neutral. Remember I am asking this question as it relates to a stopped vehicle.

Thanks

 

[dwendt44]

Yes, you are using more fuel in Drive.
There are pumps and stuff ( a technical term) turning while in drive that aren't moving in neutral.

 

[Simplysimple]

Originally Posted By: dwendt44
Yes, you are using more fuel in Drive.
There are pumps and stuff ( a technical term) turning while in drive that aren't moving in neutral.


So the extra power it takes to overcome the friction caused by being in drive as opposed to neutral 'costs' more in fuel than the extra fuel used at the higher r.p.m. of being in neutral? Without actually measuring it, I just don't know. I'm only thinking of the vehicle being at a stop.

On another hyper-mileing note, on a fuel injected vehicle with an auto transmission, does it take less fuel to coast over a given distance in neutral or in drive? Someone told me that the injectors shut off while coasting in drive, but not in neutral, so you'd use less fuel coasting in drive. I doubted that, but just don't know. That seems like something I should have asked long ago, or should have known for a long time.
Of course, you could coast longer in neutral, I think.

[Edited to add- will someone please post links to some good 'hyper-mileage' sites?]

 

[Shannow]

Also, when on over-run in an auto, the spragged vanes in the torque converter will be freewheeling on over-run.

 

[Benzadmiral]

But your idle speed increases in neutral, sitting at a light, so I'd think you'd use more fuel.

 

[Shaman]

Originally Posted By: Simplysimple
Originally Posted By: dwendt44
Yes, you are using more fuel in Drive.
There are pumps and stuff ( a technical term) turning while in drive that aren't moving in neutral.


On another hyper-mileing note, on a fuel injected vehicle with an auto transmission, does it take less fuel to coast over a given distance in neutral or in drive?


Yes, many cars will shut off fuel injectors until you get close to idle RPMs. IF you have a stick shift, you cuold ride the stick down, using the compression braking, without using any gas to stop.

check out
http://www.gassavers.org/index.php

 

[JHZR2]

you have to think ahead, as it is all about maintenance of momentum.

Take a stoplight ahead as a good example. If you are far enough away from it, the best best is to put the car into neutral and coast it until traffic picks up and goes. The key is that being far ahead, you can maintain momentum better in neutral, and then not have to re-accelerate as much.

If you know that you will have to shed significant speed, or come to a stop, then your best bet is to leave it in gear until about 1200 RPM, after which the injectors will turn back on, so your best best is neutral to minimize parasitics. This way youre saving gas from the injector turnoff, and saving brake wear.

The key is anticipation and maintenance of momentum where applicable. different situations have different applications of techniques, and there isnt a one fits all.

JMH

 

[ekpolk]

Originally Posted By: Benzadmiral

But your idle speed increases in neutral, sitting at a light, so I'd think you'd use more fuel.


But with no (or very little) load, the fuel flow may be lower than when the ECU is programming in some gas to overcome the drag of the trans.

 

[The_Jeff]

^ yup, RPM is not the end all to be all in fuel consumption.

Like when someone goes from 4.11 gears to 2.73, they can't expect 150% of the previous mileage.

 

[PandaBear]

My personal experiment:

getting off the freeway via coasting in N and in D makes my engine sound louder in D and the car slow down faster in D. Since my car has a butterfly and cable controlled throttle, and O2 sensor feedback controlled injector and ignition, I'd say coasting down in N is more efficient, and in D is more like engine braking.

How much more efficiency? I'd expect no more than an extra 10 miles per tank (270-300 miles). It takes some effort and I only do it if I know for sure I have to do a complete stop and wouldn't need to push it back in D when my engine is 800rpm and my car is going 50mph.

 

[meep]

alot depends on ECU.

I DO NOT KNOW which method is more prevalent:
1. I recall reading from an older shop manual a few years back that an engine would open the throttle just a bit (IAC) during coasting/deceleration to raise combustion pressures in order to maintain thorough mixture burn for emissions. This was in a manual, I believe it.
2. Someone on this board stated as a "fact" that during coast/decel newer cars cut fuel mix to the cylinders completely until... something undefined... happens (speed? rpm?)

In our 93 jeep, the idle is forced high (1100rpm) until the vehicle drops below ~7mph during decel/coast.

If method #2 exists, that may affect your experiment.

M

 

[glennc]

My car, which is an '89, does cut fuel at coast. I can see it on the instantaneous fuel economy gauge (which goes to the stop on infinity) and hear it in the exhaust note when it cuts the fuel. It does seem to depend on rpm and engine temp. Sometimes on a long downhill it will begin to inject some fuel, and I see the gauge leave the stop and indicate some very high mpg level. I think it does this to bring engine temps back up.