Does coasting in Neutral Work

[popimp]

On my trip to Travis AFB, CA from Biloxi, MS, I questioned if Coasting in Neutral with the engine on really works on my van. The first 2 legs of the trip I coasted at highway speeds. The SG2 reported a 31 mpg tank but when it came time to fill up I only got 26 and 25 mpg. So I'm curious as to why the SG2 would calculate more MPG with a coast?

[Danronian]

Technically, if the motor is at idle (around 600rpm) at 65mph instead of humming along at 2500rpm at the same speed, it simply must take less gas at the lower rpm.

I'm guessing something else must have played a factor in the low mpg.

[SVOboy]

It might not work on the van, especially since it's an auto, and especially at highway speeds, but it's hard to say. I suggested trying a few commutes using and not using it to get a rough feeling for it.

[brucepick]

I saw in your gaslog you had some hills. Just some? Not on the whole route?

I also saw you had good results with neutral coasting back in July. Did you change something this time?

I've been pulse/gliding on highway hills w/auto tranny. I limit my throttle to just enough gas to stay in top gear going up. Dropping into a lower gear kills the mpg right away. Staying light on throttle causes speed will drop but that's what I do. Once over the top I goose it up to 60 or so. Then coast in neutral.

If aero drag and tire rolling resistance are bad enough, even a decent hill will not be enough to maintain speed. Pretty soon you need to give it gas again.

[popimp]

I think I agree with SVO on this one. Why? I broke my record with pulsing and gliding (engine on) going to Houston 26.51. My previous best was 26.02. From Houston to Biloxi there are very little hills. The SG2 reported a 31mpg trip going to Houston. I didn't change anything. The van had about 4500 miles when we left so it's not a high mileage issue. I also got the best fill using cruise control to Phoenix averaging 29 mpg with deflated tires in Las Cruces from 44psi to 36 psi. Brucepick: Maybe my July numbers are just me being more aware of my driving?

[StorminMatt]

Originally Posted by Danronian

Technically, if the motor is at idle (around 600rpm) at 65mph instead of humming along at 2500rpm at the same speed, it simply must take less gas at the lower rpm.

Yes, an engine idling at 600RPM WILL cosume less gas than one humming along at 2500RPM. BUT, the engine will then be faced with the task of accelerating the vehicle back up to speed again. And this can't be too good for economy. I would think that a steady speed would be best for fuel economy.

[GasSavers_Red]

Perhaps simple error on the part of the SG?

[cfg83]

StorminMatt -

Originally Posted by StorminMatt

Yes, an engine idling at 600RPM WILL cosume less gas than one humming along at 2500RPM. BUT, the engine will then be faced with the task of accelerating the vehicle back up to speed again. And this can't be too good for economy. I would think that a steady speed would be best for fuel economy.

People can describe it better than me, but the idea is that during the acceleration, the engine is operating closer to it's "maximum operating efficiency" in the BSFC map (Brake Specific Fuel Consumption). Sooooo, for the acceleration, your getting more bang for the buck. For the coasting, you're obviously getting great (below RPM for current speed) MPG. When you add the two up, you get better total MPG.

Here's something that talks about BSFC :

BSFC maps and fuel economy
http://www.cleanmpg.com/forums/fuel-...nomy-6069.html

Reply to Wayne-
I think my engine's maximum thermal efficiency is at about 120 Nm and 2200 RPM, corresponding to about 37 HP. The thermal efficiency is about 31%, corresponding to 260 grams of gas per kilowatt-hour of brake (flywheel) power output (1 gram of gasoline contains about 44,000 joules of energy). However, if I could drive at a steady speed of 35 mph using only 8 HP (6000 watts) at 1500 RPM (5th gear), the thermal efficiency is probably closer to 24%, corresponding to 340 grams per kWh. This corresponds to 58 grams of gas per mile, or 48 miles per gallon (using 2800 grams per gallon).
Retired physicist.

In the above, the thermal efficiency is better at higher RPM, so the engine is doing more efficient "work" (right?!?!?!), so the pulse is not as bad as it would seem.

Example : There seems to be a Prius "sweet spot" where you can simulate the P&G we are talking about :

1 - Driving the city streets
2 - Pulse up to but not greater than 40 MPH
3 - Take your foot off the accelerator pedal and start a Glide.
4 - Wait until the car slows down to maybe 25 MPH.
5 - Repeat steps 2 to 5.

During the glide, the Prius turns off the engine (and I think it disengages the engine from the tranny as if you are coasting in Neutral?!?!). This means the glide portion will be "infinite MPG". Using this technique, Prius owners have been reporting upwards of 80 MPG.

NOTE : This means that for P&G, aerodynamic efficiency has a bigger impact because :

better aerodynamics => shorter pulse = less fuel used during "bad MPG portion" of P&G
better aerodynamics => longer glide = greater "high MPG portion" of P&G

Conclusion : If your car is a box, P&G will not be as effective.


CarloSW2

[Rick Rae]

There are two things being discussed here. One is traditional Pulse and Glide, which CarloSW2 et. al. touched on. Generally it does work, but there's been some discussion as to whether torque converter losses make it less useful (or useless) for automatics. I saw some improvement when I was experimenting with it, but I wasn't really comfortable with the potential wear and tear from constant shifting, so I've backed off on that.

The other is taking advantage of hills and coast-downs to lights and lower speed stretches of road. brucepick's message focuses more on that, and I use pretty much the same approach he does. Without a doubt this makes a huge difference for my car, especially on routes I know well. I can't think of a reason it shouldn't work for you.

I don't know why your SG over-reported. I've seen mine be bang-on one fill and off by a few percent the next, so you're not totally alone in that. Since the SG always estimates high for me, I think part of it is fuel injector cutoff -- I've been learning to take better advantage of that lately, and the SG doesn't account for it. Your difference is too large for it to be that alone, though.

Rick

[brucepick]

Another way to describe how + why pulse and glide works -

At any given engine speed -
With standard tranny, or with an auto in lockup mode, the engine is more efficient at half or 3/4 throttle than at a low throttle or idle. This is because at low throttle, a higher percentage of the fuel is being used just to keep the engine spinning. At a normal neutral idle, ALL the fuel is used just to keep the thing spinning.

In fact one guy here did some tests (look in "Experiments") and found that the amount of fuel used in neutral was almost directly proportional to the rpm.

So you take advantage of this by giving it gas to get up to say 60 mph, then into neutral and coast till you hit 50. Back in gear and do it over again.

You can see there's a loss while in neutral if the engine is still running. Some don't call it Pulse and Glide unless it's a standard and you can kill the engine while coasting. I'm stuck with auto for now, so I P&G as best I can, coasting in neutral when I can.

I don't want to battery-restart the engine repeatedly so I rarely kill it while rolling. Starter replacement is a real bear on my particular model.