Coast in-neutral vs in-gear: I've been challenged to show the numbers

[reb01501]

Originally Posted by reb01501

Actually, this is the one I was trying to refute:
Coasting in Neutral or Gear to Save Gas - Coasting and Fuel Economy - Popular Mechanics

Oh! And the reason I think the author reaches the incorrect conclusion is that he fails to consider the lost momentum that needs to be regained at the end of the coast. Coasting in neutral loses less momentum so less fuel needs to be used to regain it.

[ecomodded]

The gained/maintained momentum is defiantly saving the fuel.
Which is why i suggested a coast down test.

[Whitey]

OK. So I just read that article and got a good chuckle out of it.

The author states:

The algorithm the trip computer uses is not based on how much fuel is actually consumed, but on some calculated value based on airflow past the mass airflow sensor, manifold vacuum and engine rpm. And it's not accurate under these coasting conditions. That's why when we report fuel economy here at PM, we never just print the numbers we read off the trip computer's display: We use the gallons pumped into the tank divided by the mileage on the odometer—which we check against a handheld GPS.

But you'll notice: He doesn't "just print the numbers," he just doesn't print the numbers!

Anyway, I'll put my car in gear before I'll use the brakes to slow down. Using zero fuel to slow down makes sense--especially if coming to a stop. Otherwise, wasting momentum is wasting fuel.

I would just say to the author that my "mileage on the odometer divided by the gallons pumped into the tank" over the past 14 months shows no doubt that he is mistaken--even though Scangauge agrees with me.


-Whitey

[ecomodded]

It would be a good idea to use the engine braking instead of the brakes once you are in a High enough rpm to trigger the fuel cut off.
Then put it back in neutral when the rpm's drop too low for fuel cut off.

Below is a tip from the linked article that scangauge owners can do to find their engines rpm fuel cut off.


quote from original article:

I use a Scangauge II for a lot of diagnostics and general tinkering. It's a great tool—but I've learned not to trust the economy or gallons-used function too closely. That's why the Scangauge has a function that allows you to tell it what the engine-idle cutoff is for your particular car to get somewhere closer to the truth.

Read more: Coasting in Neutral or Gear to Save Gas - Coasting and Fuel Economy - Popular Mechanics

edited to add:

I made a error, he said to enter the fuel cut off rpm, not if you could find it. I suspect you can find it by watching the fuel injector duration readout and notice what rpm it cuts out at.

[Fat Charlie]

Originally Posted by Whitey

...wasting momentum is wasting fuel.


-Whitey

Sorry, but that just needed to be said again.

[UltArc]

I haven't been too up to date on here, and this thread is old, but coasting today in my wife's 2008 Ford Escape 4 cylinder auto had roughly a 25% loss in efficiency by putting it in neutral. It went from 80s to 60s. Watching more carefully, rpm went from 900ish to over 1200 in neutral.

I just setup a grille block, so put my scanguage in and was checking it out, I couldn't believe how efficient they are. 31 to 33 mpg with the grille block, 27-29 without. City driving, me driving. My wife seems to average around 24.

Just an observation.

[Xist]

I made the 165-mile trip to and from my parents' house almost every week this summer. The best mileage going was 28 MPG and the best returning was 40.4 in my Forester. According to usclimatedata.com, there is a 5,147-foot climb overall. A decade ago I used to coast down a canyon in my Prelude and coast back up the other side. With my Subaru I usually coast in gear. I am not taking those corners quickly. With perhaps a 5% grade I can maintain speed in neutral, while I would need to accelerate in gear. With perhaps a 6% grade I can maintain speed in gear, but I have accelerated 40 MPH in neutral time and time again.

I do not know how long these slopes are, but the difference of 0.3 GPH and 0 downhill is negligible compared to coasting up the other side and driving in gear. There are other hills that are relatively flat at the bottom. How far would an additional 40 MPH of momentum take me on level ground?

As I have mentioned in other posts, the highway patrol like to sit and wait for me on those hills, and once I hit a deer or an elk. Mythbusters drove cars into a rubber buffalo at different speeds. At high speeds the buffalo removed the top half of the car.

[meanjoe75fan]

It seems this is a largely theoretical discussion, in that you're discussing the optimal means of descending a hill in which:
1. The slope is gentle enough that the PSL (or other maximum speed determined by driver) won't be exceeded by gravity.
2. Hill.has no steep curves/switchbacks.
3. Hill has no traffic-control devices at bottom that would make momentum conservation moot.
4. Vehicle is equipped w/ DFCO and is observed to engage for the lenght of the hill.

So YES, in this very special case of a "perfect" hill, coast in neutral. For most "real-world" hills, leave it in gear (DFCO) and get less fuel burn, "free" alternator and accessories, less brake-pad wear, etc.

[PaleMelanesian]

Originally Posted by meanjoe75fan

It seems this is a largely theoretical discussion, in that you're discussing the optimal means of descending a hill in which:
1. The slope is gentle enough that the PSL (or other maximum speed determined by driver) won't be exceeded by gravity.
2. Hill.has no steep curves/switchbacks.
3. Hill has no traffic-control devices at bottom that would make momentum conservation moot.
4. Vehicle is equipped w/ DFCO and is observed to engage for the lenght of the hill.

So YES, in this very special case of a "perfect" hill, coast in neutral. For most "real-world" hills, leave it in gear (DFCO) and get less fuel burn, "free" alternator and accessories, less brake-pad wear, etc.

I think you're looking at it backwards. Don't look for the perfect place TO coast, look for the special cases where you should NOT coast. In other words, make coasting your default, and break that only when necessary (like the cases you mention above).

1. Except for long / steep downhills, just start the coast at lower speed so as to not exceed the speed limit.
2. Again, start slow at the top, and use engine braking to control speed. Try to fit some neutral in between the curves if possible.
3. Again, start slow. If it's a STOP, DFCO. If it's a light, DFCO just enough to hit it on green.
4. If you don't have DFCO, why are we even talking about this? Neutral!

[wdb]

Originally Posted by reb01501

I don't understand the point you're trying to make. You start by questioning whether engine braking is occurring (and why is Moore's law relevant?), and then go on to describe you actually do observe it occurring. Seems to me you've answered your own question.

Sorry for the oh so late reply. Moore's Law applies due to the greater amount of computer intelligence being used to control automatic transmissions today, as opposed to a few years ago. The point I was making is that I notice different levels of engine braking under different conditions even when I do nothing other than lift my foot from the gas pedal. Specifically it seems to me as though there is more engine braking occurring when the car is descending a hill than when it is rolling on level ground. Is it jus my imagination, or are the engineers perhaps using data to change the transmission's response under different conditions?