Does coasting in Neutral Work
 

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?

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.

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.

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.

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?

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.

Perhaps simple error on the part of the SG?

StorminMatt -

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
https://www.cleanmpg.com/forums/fuel-...nomy-6069.html
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

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

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.

I'd say for all you auto trannies out there that, unless you live in an area that is really flat, to just use the hills as P&G and the rest of the time leave it in drive. The fuel savings are not going to be wroth the extra wear on the tranny going from D to N to D all the time.

As for why P&G works, think of it this way: The amount of fuel pumped into the engine is not linear to the amount of power produced. So if you feather foot the gas pedal, you are using more fuel per horsepower produced than if you press the gas pedal down hard enough to be in the engine's "sweet spot" for efficiency. In other words, at 75% or near full throttle (closer to the engine's ratio of fuel efficiency to power sweet spot), you are going to produce the most horsepower per unit of fuel used. I don't know exactly what this sweet spot is on most cars, but the point is, holding constant speed, while requires little power, isn't requiring *that much less gas to maintain*: it's an inefficient use of fuel for the amount of power it's producing. So when you are accelerating moderately you are only using about 2 1/2 times more fuel than when you are maintaining constant speed, but it's allowing you to spend much more time coasting, which uses about 5-10+ times less fuel than constant speed. That is why P&G becomes less effective in situations where the car is slowing down quickly during the glide. If the car is slowing down quickly during the glide, it means it is requiring more power to maintain constant speed, and therefor is closer to the power/efficiency sweet spot which apparently is somewhere above 60% full throttle (although I think it is less than full throttle) Anyone know where abouts the sweet spot is?

To conclude the argument for P&G....

So instead of driving at a constant speed that is inefficient for the amount of power it is producing, keep the engine in its most efficient power to fuel ratio for as little time as possible, and try to have the rest of the time driving in neutral!

It's not practical to do it for long trips! I got the same mpg using the engine on coasting technique at a 60mph average as I did with setting it at CC to 60 mph. I can say that speed does decrease mpg though. At a 70 mph cruise I only got between 24 & 25. So on this end I don't think it works. I'm gonna try and get together with Red and do some more testing.

Has anyone tested this for the VX? I've been coasting down all hills in neutral lately, and it has netted me my best avg mpg (45-48) since owning the car, but I wonder if leaving it in 5th would be better...

For this car, the reason I don't resort to shutting it down, is because it takes most wideband sensors around 10 minutes to warm up, so I don't want to avoid being in lean-burn if I can help it.

Only time I coast in neutral is when the engine is off, now that helps.

Usually before well known intersections, the light turns red, gear lever to N, engine off, coast to stop. It helps to be familiar with the light pattern, turn it on when the opposing light turns yellow (and mine is about to turn green), or if I have cars in front of me, by the time the 3rd car in front of me starts to move it is high time.

A properly maintained and well tuned engine helps as well, very embarrassing if it won't start now, preferrably it should start with just a 'kick' of the key, 1-2 revolutions at the most.

But your o2 sensor isn't going to cool down in one or two minutes. If my CE light is any indication, it takes 5 minutes of the engine off for it to cool down.

Part of the problem is that the engine still is burning probably 0.5gph at idle so you do not save much fuel coasting then the added automatic tranny losses offset the savings. Best to keep a light foot on the pedal and keep the body surface clean. Think of it this way, if you turned off the engine for an hour you would save 0.5 gallons of fuel at idle so coasting for an hour would save you ??? . . . not much.

Isn't the question how much I save in that hour of coasting VS what I'd burn at 55 MPH?

Yeah, that's what I'm wondering. Here's an example of what I'm wondering:

Going down a long hill, where no throttle is needed (a few minutes of coasting), would it be better to be in 5th gear, which would have the motor at about 1500 RPMs or would it be better to be in neutral at 600 RPM?

For the VX, I'm wondering if idling wastes more gas than being in lean-burn?

As soon as this tank is over (and I get a base-line for my car with my shift light now functioning and following those points), I'm going to stop coasting in neutral and keep it in 5th instead. If there is a difference in MPG, I would assume the VX would prefer to be in gear going down hills.