Optimizing Accel and Decel

[VOLDAR]

Originally Posted by kam327

Gotcha. I think what gets lost there though is you have to continue at cruising speed further before coasting in gear in order to reach a stop at the same speed as if you had started coasting in nuetral earlier. So you may burn 1 gal/hr for 20 seconds and then burn no fuel for 40 seconds. As opposed to burning 0.25 gal/hr for 60 seconds while coasting in nuetral. You can see in that very simple, generalized scenario coasting in neutral has a clear advantage - about half the fuel burn of coasting in gear. That's what my own simple test reinforced.

Great. I am not a "hypermiler" and I don't coast in neutral with my DCT, but it is a good finding
I did coast in N from time to time when I was driving MT, but never with an AT. I always thought that the AT are not made for D to N and N to D switch endlesly.

[kam327]

Originally Posted by VOLDAR

I always thought that the AT are not made for D to N and N to D endlesly.

Oh yeah, I wouldn't go from N back into D while at speed. This is just for when you know you're going to have to stop well in advance. I wouldn't do it before light which could turn green before you come to a complete stop.

[YesMAN]

Interesting analysis. I'll probably still gun it all the time though.

That's counterintuitive about the coasting in neutral vs in gear. Not sure how practical coasting in neutral will be where I drive, but good to have the info rattling around in the back of the noggin nonetheless.

[jsr72]

Originally Posted by kam327

Well you're right it's very strange. Actually there's 3 CPC tires listed in that size. Two are listed as LRR.

One of those LRR tires is an XL tire. The other is an SL.

[dbenney]

Originally Posted by kam327

But on more rural roads with a known stop sign way up ahead, neutral is the way to go.

You will always get better avg mpg coasting in N vs D or S... this is due to RPM. Coasting in N is like putting the clutch in on a manual trans. As soon as you put it into N your RPM drops to idle, where as if you leave it in D or S your RPM is matched per gear and vehicle speed programming. as you slow down the auto will downshift raising the RPM... the higher the RPM the lower the MPG and vice versa.

[Fresh1]

Originally Posted by dbenney

You will always get better avg mpg coasting in N vs D or S... this is due to RPM. Coasting in N is like putting the clutch in on a manual trans. As soon as you put it into N your RPM drops to idle, where as if you leave it in D or S your RPM is matched per gear and vehicle speed programming. as you slow down the auto will downshift raising the RPM... the higher the RPM the lower the MPG and vice versa.

I think the OP wanted to test the effectiveness of fuel cut-off during deceleration. This is why it's strange that the fuel consumption was higher in gear. If the fuel is cut off (regardless of engine RPM), it should burn NO fuel - vs. engine at idle speed in N. Anyone know what's goin' on? ...

[Fresh1]

Originally Posted by Fresh1

I think the OP wanted to test the effectiveness of fuel cut-off during deceleration. This is why it's strange that the fuel consumption was higher in gear. If the fuel is cut off (regardless of engine RPM), it should burn NO fuel - vs. engine at idle speed in N. Anyone know what's goin' on? ...

Well, here's a theory (to answer my own question ;) ...
Maybe when decelerating in gear, the vacuum generated by the piston downstroke sucks in blowby gasses back into the cylinder/combustion chamber, and then sends them out through the exhaust during the upstroke. So, maybe the Scanguage just "thinks" more fuel is being burned at this time, but it isn't? ...

[pozi240]

Originally Posted by Fresh1

Well, here's a theory (to answer my own question ;) ...
Maybe when decelerating in gear, the vacuum generated by the piston downstroke sucks in blowby gasses back into the cylinder/combustion chamber, and then sends them out through the exhaust during the upstroke. So, maybe the Scanguage just "thinks" more fuel is being burned at this time, but it isn't? ...

^ This
I think you have yourself the answer, our F150 (With the "gas guzzling" 5.4) has this technology (as opposed to the multi cylinder displacement crap that GM and Dodge use) and it works. When deacelerating in gear in our truck, it effectively "shuts off" fuel to engine and uses road speed (and the transmission) to continue to propel the vehicle forward while coasting (as long as possible anyway). To keep the engine "running" without new fuel, it uses compression and EGR system. We do notice a difference by "coasting" in gear to stop signs etc... I bet the Scanguage is not configured with those parameters and is merely measuring the amount of vacuum in the intake.

[kam327]

Originally Posted by Fresh1

I think the OP wanted to test the effectiveness of fuel cut-off during deceleration. This is why it's strange that the fuel consumption was higher in gear. If the fuel is cut off (regardless of engine RPM), it should burn NO fuel - vs. engine at idle speed in N. Anyone know what's goin' on? ...

It can be explained very easily. The main assumption here is that you have to travel further at speed before coasting in gear in order to reach the stop sign at the same speed you would reach it coasting in neutral, since you'll decelerate more quickly in gear than in neutral.

The Focus burns say roughly 0.3 gal/hr at idle, and roughly 1.3 gal/hr at 45 mph (I'd have to confirm these numbers but rough order of magnitude they're pretty good).

In the coast in neutral scenario, you're burning 0.3 gal/hr for the full 60 seconds to the stop for a total burn of 0.005 gal. In the coast in gear scenario, you're burning 1.3 gal/hr for say the first 20 seconds and then 0 gal/hr for the next 40 seconds for a total burn of 0.0072 gal, a 44% increase over the coast in neutral scenario.

Originally Posted by pozi240

^ This
I think you have yourself the answer, our F150 (With the "gas guzzling" 5.4) has this technology (as opposed to the multi cylinder displacement crap that GM and Dodge use) and it works. When deacelerating in gear in our truck, it effectively "shuts off" fuel to engine and uses road speed (and the transmission) to continue to propel the vehicle forward while coasting (as long as possible anyway). To keep the engine "running" without new fuel, it uses compression and EGR system. We do notice a difference by "coasting" in gear to stop signs etc... I bet the Scanguage is not configured with those parameters and is merely measuring the amount of vacuum in the intake.

Yes of course there is a difference by coasting in gear versus speeding up to a stop and then slamming on the brakes. The point is that theoretically you can do slightly better by coasting in neutral.

[suss6052]

Originally Posted by kam327

It can be explained very easily. The main assumption here is that you have to travel further at speed before coasting in gear in order to reach the stop sign at the same speed you would reach it coasting in neutral, since you'll decelerate more quickly in gear than in neutral.

The Focus burns say roughly 0.3 gal/hr at idle, and roughly 1.3 gal/hr at 45 mph (I'd have to confirm these numbers but rough order of magnitude they're pretty good).

In the coast in neutral scenario, you're burning 0.3 gal/hr for the full 60 seconds to the stop for a total burn of 0.005 gal. In the coast in gear scenario, you're burning 1.3 gal/hr for say the first 20 seconds and then 0 gal/hr for the next 40 seconds for a total burn of 0.0072 gal, a 44% increase over the coast in neutral scenario.





Yes of course there is a difference by coasting in gear versus speeding up to a stop and then slamming on the brakes. The point is that theoretically you can do slightly better by coasting in neutral.

The question is what if you're going to have to stop anyway, why would you hold the higher speed longer before coasting in gear vs coasting in neutral to try and maintain a higher speed at the end of the coast?