Shifting to neutral when going downhill

[TheBDB]

Is there anything wrong with putting an automatic transmission in neutral when going downhill to save gas?

 

[Fenixgoon]

there is no reason to do this in an automatic. it may even hurt the transmission (i've accidentally gone D-->N--->D and it didn't make a pretty sound)

 

[Jumpem]

It will actually use more gas.

 

[woodie1]

If your engine is fuel injected you save gas by leaving the car in gear when going down a hill.

 

[thomsbrain]

Yes, there is something wrong with this.

You'll burn gas in neutral (car has to burn gas to idle), but if you leave it in gear, a modern fuel-injected car will shut off the fuel completely. According to my Scangauge, at 55 MPH, fuel cutoff is the difference between ~80 MPG in neutral and infinity MPG in gear. So leave it in gear, save yourself trouble, save wear and tear on the transmission, and save yourself gas. Triple win!

The one exception to this rule is if the car coasts to speed that is low enough that the engine gets near idle RPM and therefore re-engages idle fuel flow. In that case, there isn't much difference, because either way the engine uses the same amount of gas, and since the engine is near idle and using the normal idle fuel flow, it doesn't provide any significant thrust or engine braking. Again, no reason to use neutral.

 

[alfa147x]

i do that because the ladies that drive by can see me driving a manual

 

[zerocool84]

Originally posted by: alfa147x
i do that because the ladies that drive by can see me driving a manual

:laugh:

 

[TheBDB]

So you are saying when I am going 70mph downhill there is no combustion going on in the engine, it is still rotating at 3000 rpm because it is still connected to the wheels and they are turning?

 

[SparkyJJO]

Originally posted by: TheBDB
So you are saying when I am going 70mph downhill there is no combustion going on in the engine, it is still rotating at 3000 rpm because it is still connected to the wheels and they are turning?

Yes, or very little combustion, depending on how steep the hill is.
3000RPM at 70MPH, huh? Oh wait, 4 cylinder probably....

 

[alphatarget1]

But then if the hill isn't steep enough you might lose speed because the gravity isn't enough to run the engine, in which case if you have to maintain speed you still need to burn gas.

I have no good explanations for this.

 

[LTC8K6]

It works uphill, too. Momentum plays a big role. Gravity isn't necessary.

 

[Q]

I'm confused.

So what's the answer?

 

[fleabag]

Originally posted by: TheBDB
Is there anything wrong with putting an automatic transmission in neutral when going downhill to save gas?

It depends. If the grade is enough to warrant the use of engine braking or one that by using engine braking, you'll be able to maintain speed while not needing to or lightly using the brakes, then it's more beneficial to keep it in drive and let the engine do engine braking. The rule I read was that anything 15% above idle speed when engine braking will cut off the fuel injectors but I don't think that's true for all cars. One example of a car is the ES350 which seems to not turn off the fuel injectors until the engine reaches 2000rpm (when engine braking).

Otherwise if you want to coast a long time, put it into neutral however I don't suggest this with an automatic because of the issue with shifting back into gear can make some nasty sounds.

So the simple rule is this: If you can utilize engine braking at a decent engine speed and not lose speed (if you don't want to) then do it. Otherwise using neutral would reduce the drag the engine causes on the drivetrain while you're coasting, therefore slowing you down. But again, I have to emphasize that on an automatic, I believe it's a bad idea to be shifting into and out of drive. I have no anecdotes to prove this is a bad idea so be my guest in being a guinea pig and let world know whether this has adverse effects or not.

 

[LTC8K6]

The answer is that coasting in neutral generally uses more fuel than coasting in gear in modern cars.

Coasting in neutral used to be illegal in some states and may still be because you have less control of your car in neutral when rolling downhill. Coming down a mountain being the most obvious example.

So I would say that overall, it's probably better to be in gear when coasting. It saves fuel, keeps your speed under some control, and if there's a stop light ahead, you can drop another gear and get some preparatory deceleration too.

Personally, I have a Ford transaxle, and it's fragile enough already...

 

[shabby]

Originally posted by: woodie1
If your engine is fuel injected you save gas by leaving the car in gear when going down a hill.

I dont think that works with an automatic transmission, only manual.

 

[AMCRambler]

How can the fuel injectors be shut off? Your car would shut off wouldn't it? In which case then you'll get some real engine braking!

I'm just trying to figure out what's going on in the engine when you're going downhill. You're off the acclerator pedal, and the weight of the car is pushing it down the hill rather than the engine doing the pushing, so I can see that a lot less fuel would be needed. However you'd still need some fuel entering the cylinders or the engine would stop firing and then you'd basically be rolling down the hill with the wheels of the car turning the engine.
So no, I don't think the fuel injectors get shut off completely. I think the car would use less fuel then it would at idle since the wheels would be helping to turn the engine so not as much fuel would be needed to turn the engine. The EFI component of the car's computer should adjust the fuel flow down, if it is indeed set up to recognize this situation.

 

[LTC8K6]

The same thing that is going on when you push start a car. The forward motion of the car is turning the engine. No fuel is necessary to turn the engine when the car is in motion and in gear.

As long as a downshift causes the engine to rev up, then the forward motion is turning the engine. A downshift causes the engine to rev up in both auto and manual transmissions.

The engine computer recognizes this "over-revving" situation, since the throttle is at idle, and it turns off the fuel.

Imagine you are a giant who can play with real cars. If a car is in gear and you push it along, the engine will turn.

For an auto, you just need to push faster than with a manual, iirc. This is what makes it impractical to push start an auto.

As another example, you can turn the engine backwards and damage it if you push the car backwards while it's in a forward gear. This sometimes happens when a race car spins and goes backwards at high speed such as in NASCAR.

 

[overst33r]

Originally posted by: AMCRambler
How can the fuel injectors be shut off? Your car would shut off wouldn't it? In which case then you'll get some real engine braking!

I'm just trying to figure out what's going on in the engine when you're going downhill. You're off the acclerator pedal, and the weight of the car is pushing it down the hill rather than the engine doing the pushing, so I can see that a lot less fuel would be needed. However you'd still need some fuel entering the cylinders or the engine would stop firing and then you'd basically be rolling down the hill with the wheels of the car turning the engine.
So no, I don't think the fuel injectors get shut off completely. I think the car would use less fuel then it would at idle since the wheels would be helping to turn the engine so not as much fuel would be needed to turn the engine. The EFI component of the car's computer should adjust the fuel flow down, if it is indeed set up to recognize this situation.

Think of it this way. Under engine braking, the engine is still going through its strokes, but there is no combustion going on. As I understand it, it is just intaking and compressing air at this point. There is no fuel being ignited.

 

[Toastedlightly]

Recall, the person may be dealing w/ a torque converter, so I have no clue on this.

 

[LTC8K6]

BTW, this is the type of over-rev that the rev-limiter can't save you from...

 

[Zenmervolt]

Originally posted by: LTC8K6
As another example, you can turn the engine backwards and damage it if you push the car backwards while it's in a forward gear. This sometimes happens when a race car spins and goes backwards at high speed such as in NASCAR.

The problem there is the sudden transition from forward to backward. An engine really doesn't care which way it spins from a purely mechanical viewpoint, but a sudden change in which way it is spinning puts a lot of stress on reciprocating components.

ZV

 

[DivideBYZero]

:facepalm:

 

[exdeath]

Can't believe some people still think the engine absolutely after numerous threads has to be consuming fuel if it's 3000+ RPM and coasting in gear.

http://members.cox.net/exdeath/coast.jpg

At the peak on the yellow is vehicle speed, you can see how it peaks out then drops sharply under the drag of engine braking. The red vertical line is where I pressed the clutch in coasting in neutral with the engine RPM rapidly dropping back to idle and vehicle speed no longer dropping sharply with no engine braking.

Unfortunately I didn't get injector pulse width in this log, and unlike injectors, the fuel pump never shuts off completely even when coasting (rail pressure is constant even with the engine off and key on). Never the less you can see the log of the fuel pump in red and how it's lowest point is while coasting in gear (the point between peak speed in yellow and where I push the clutch in at the vertical red line) and between shifts where the throttle is completely lifted, and how it comes up a little bit reflecting the idle speed fuel demand when I press the clutch in and coast in neutral.

Case in point, coasting in gear will use less fuel, unless of course you coast in gear excessively to the point that you keep dropping below your desired speed and have to constantly re-accelerate again. If you are coasting with the throttle closed, there is no air entering the engine (ignoring idle bypass insufficient for the RPM anyway) due to a closed intake tract; you are pumping complete vacuum, and no air = no fuel.

If this isn't sufficient to end this debate once and for all I'll try to bum a laptop of someone again and get a similar log showing injector pulse width going static zero while coasting in gear.

 

[mooseracing]

Originally posted by: exdeath
Can't believe some people still think the engine absolutely after numerous threads has to be consuming fuel if it's 3000+ RPM and coasting in gear.

So are you trying to say the engine runs with no gas while coasting?

Please tell me no.

 

[exdeath]

Originally posted by: mooseracing

Originally posted by: exdeath
Can't believe some people still think the engine absolutely after numerous threads has to be consuming fuel if it's 3000+ RPM and coasting in gear.

So are you trying to say the engine runs with no gas while coasting?

Please tell me no.

I'll put it simple. Coast in gear with the throttle closed with the momentum of the car turning the engine. If the throttle is closed there is no air in the engine, how are you burning fuel if you did inject it during this time? If throttle is closed and RPM is above the idle threshold, injector pulse width is zero. Once the RPM falls via friction, ECU starts to open the idle air control and resumes fuel injection to stabilize idle, assuming you aren't still in gear and about to mechanically stall it at this point.

If you have a manual you experience fuel cut every time you lift the throttle to shift.

I think people over estimate the friction in an engine and under estimate the amount of kinetic energy in a rolling vehicle.