10% Mileage increase... free...

[Tiger]

Darn it! All of you! Just go out...buff you car, clay it and give it a good wax, and you will save on gas! Lower air friction on car body is higher fuel mileage and feeling of more power!

Hypermiling technique does work... but some of those techniques are insane. And don't do this on packed highway because you want to be an *******. Most of us are smart enough not to do this on jammed highway... and make sure no one is behind you.

Easiest hypermiling technique is learning the art of coasting... and also staying in the 'vacuum' of other cars, SUV and trucks.

[Robin-535im]

dude - science says no.

driving at a consistent speed will take much less fuel than constantly slowing down and accelerating, it's simple physics!

Text is cheap - educate us with this science!

Simple physics says it makes no difference. I think, and maybe I would do the work to prove it if I had the time, that Medium physics will say that "pulse and glide" (thanks to Tiger for the term (http://metrompg.com/posts/pulse-and-glide.htm) is mathematically more efficient due to the reasons I mentioned already.

Like I said - I didn't believe it either until I did it myself.

[repenttokyo]

Text is cheap - educate us with this science!

Simple physics says it makes no difference. I think, and maybe I would do the work to prove it if I had the time, that Medium physics will say that "pulse and glide" (thanks to Tiger for the term (http://metrompg.com/posts/pulse-and-glide.htm) is mathematically more efficient due to the reasons I mentioned already.

Like I said - I didn't believe it either until I did it myself.

it takes more energy to accelerate mass than it does to keep mass at a specific speed. Therefore constantly accelerating a mass back up to speed X takes more fuel than constantly cruising at speed X.

common sense says that this is also an unsafe way to drive. if you are costing back down to a slower speed, then you aren't using your brake lights. Particularly at night, it will be quite difficult for drivers coming up behind you to know that you are slowing down.

In addition, once they pull up beside you to pass and you start to accelerate back up to whatever speed you are aiming for, you create yet another frustrating and potentially dangerous situation, particularly on a two lane road.

If this were any kind of revelation in terms of fuel mileage, it would have been widely adopted and well known for decades. In reality, the opposite is true: constant speed changes use MORE fuel, and keeping a constant speed is one of the best tips for maintaining fuel economy that exists.

[mikell]

I'm sure one of our mathematically able posters can explan this more thorpoughly, but isn't most fuel consumption at speeds over 35 or so mph mostly overcoming aerodynamic drag? And, if memory serves, drag increases as the square of speed (or something like that). So, you're spending big fuel to get up to top speed overcoming both inertia and drag, then drag robs you of that momentum. . .

Seems like a lot of work and risk (see below) and not too much fun, IMO.

because constantly slowing and accelerating for no damn reason is a really *******d thing to do in traffic.
it's a damn good way to spend a few minutes taking a field sobriety test after you get reported as a possible dui. ( i know I call people in who drive erratically like that!)
Or be cited for failure to control speed or obstructing traffic ( i would hope).

Yah, this occurred to me also - that kind of erratic driving is an invitation for a roadside visit with THE LAW, who'll want to see whether you're driving "substance enhanced"

[Blitzkrieg Bob]

I just plan my route to drive down hill both ways to save gas

[repenttokyo]

I just plan my route to drive down hill both ways to save gas

sometimes I turn around halfway to my destination when I realise I just don't want to spend the gas.

[Robin-535im]

it takes more energy to accelerate mass than it does to keep mass at a specific speed. Therefore constantly accelerating a mass back up to speed X takes more fuel than constantly cruising at speed X.

Your statements are true but you're not quite thinking of it the right way...

Neglecting losses and nonlinearity of engine efficiency with RPM, the work done to get your car to a certain speed (i.e., impart a certain kinetic energy) is constant. That work is done by burning gas - same work done, same gas used. Accelerate fast and you will consume gas faster, but for less time.

The key is the difference in efficiency between going at a constant speed vs. coasting + accelerating. Read the links I posted last time, they explain it better than I can do off the fly. The physics are sound, and (obviously,) counter intuitive.

common sense says that this is also an unsafe way to drive. if you are costing back down to a slower speed, then you aren't using your brake lights. Particularly at night, it will be quite difficult for drivers coming up behind you to know that you are slowing down.

In addition, once they pull up beside you to pass and you start to accelerate back up to whatever speed you are aiming for, you create yet another frustrating and potentially dangerous situation, particularly on a two lane road.

All true and I agree 100% as does probably everyone on this board. Those are good points to make and I, too, feel very strongly that people need to consider the impact of their driving habits on those around them, both for safety and for road-rage reasons. Good mileage is never a reason to drive in an unsafe or antagonistic manner.

If this were any kind of revelation in terms of fuel mileage, it would have been widely adopted and well known for decades. In reality, the opposite is true: constant speed changes use MORE fuel, and keeping a constant speed is one of the best tips for maintaining fuel economy that exists.

Again - the key is coasting, not decelerating. If you just let off the gas and the engine slows you down, your statements are correct. That's why the coasting part is important.

Read up on "pulse and glide" and you will see that it IS widely adopted among fuel-saving nerds. We're talking 50 - 80 MPG from gas engines.

[pingu]

You can't just neglect non-linearities when it seems convenient.

By accelerating and gliding, you achieve a given average speed but during the "pulses" you've got extra aerodynamic drag (I think drag scales with either the square or the 3/2 power of the speed). So you expend more fuel during the pulses than you save during the "glides".

Also, you mentioned (I think in an earlier post) that the fast pulses make up for the "glides" to give the same average speed. Not true. For example, if a car were to drive for 30 miles at 50mph (36 minutes) and for 30 miles at 70 mph (25.7 minutes) then the average speed would be lower than 60 mph (60 miles in 61.3 minutes actually comes to 58.3 mph). This is because at 50 mph it takes more time to cover the distance and thus the slower speed has a disproportionate (I'm using that word loosely!) effect on the average speed.

On top of this, you don't in any case want to coast in neutral as you're wasting fuel compared to coasting in gear (by coasting in gear, I mean coasting in a gear that will achieve idling speed - any faster than this and you're turning the engine unnecessarily fast). If you coast in neutral then you're burning fuel to keep the engine at idling speed. Burning fuel is a lot less than 100% efficient. By contrast if you coast in gear, you're converting some of the car's kinetic energy into rotational energy in order to keep the engine turning - this is a purely mechanical conversion as opposed to a thermodynamic (fuel burning) process. If you coast in gear then the injectors will be switched off by the DME - no fuel used. Of course, you could coast with the engine switched off...

[Morgenster]

"Pulse and glide" is only more efficient due to the inherent inefficiency of traditional power trains. With a perfectly efficient power train the pulse and glide system would be unnecessary to achieve that.
I'll pass on that and stick to driving steady speeds at a more moderate pace.

[Robin-535im]

You can't just neglect non-linearities when it seems convenient.

It was really to simplify the argument but the facts are the same. In fact - it's the second order effects (engine friction & efficiency as a function of RPM) that make this work. In the truly ideal case, without drag or friction, it's a simple energy balance and both approaches are equal.

By accelerating and gliding, you achieve a given average speed but during the "pulses" you've got extra aerodynamic drag (I think drag scales with either the square or the 3/2 power of the speed). So you expend more fuel during the pulses than you save during the "glides".

Both comments are technically true (drag force is proportional to velocity squared). If you saved more fuel during the glides than you spent during the pulses, your gas tank would overflow.

The pulse & glide technique gives you higher MPG than going a constant speed. Really. Try it! Read up on it! I'm not just whistlin' dixie...

Also, you mentioned (I think in an earlier post) that the fast pulses make up for the "glides" to give the same average speed. Not true. For example, if a car were to drive for 30 miles at 50mph (36 minutes) and for 30 miles at 70 mph (25.7 minutes) then the average speed would be lower than 60 mph (60 miles in 61.3 minutes actually comes to 58.3 mph). This is because at 50 mph it takes more time to cover the distance and thus the slower speed has a disproportionate (I'm using that word loosely!) effect on the average speed.

Think about it from a dead stop though - if you mosey on up to 55 MPH you spend some time at 5, 10, 15, 20, etc. along the way. If you floor it up to 55 MPH, you spend a lot less time at the slower speeds.

Now on the highway at "steady state", oscillating about a middle speed, that's where the math gets non-linear because your drag force changes with v^2 and your acceleration is a function of the torque curve. It's hard to say what the average speed would be as a function of all these things without actually modeling it. Probably it's an epsilon below the middle speed since you slow down at a faster rate the faster you go, so you'll spend more time at the lower end than at the upper end of the range.

Kudos for doing the math - illustrates the point very well. I think it might be more accurate to assume you spend the same amount of time at each speed though, not that you travel the same distance at each speed. So for example, 1/2 hour at 50 MPH plus 1/2 hour at 70 MPH = 60 Miles in that hour = 60 MPH. Probably the truth is somewhere in between the two calculations.

On top of this, you don't in any case want to coast in neutral as you're wasting fuel compared to coasting in gear (by coasting in gear, I mean coasting in a gear that will achieve idling speed - any faster than this and you're turning the engine unnecessarily fast). If you coast in neutral then you're burning fuel to keep the engine at idling speed. Burning fuel is a lot less than 100% efficient. By contrast if you coast in gear, you're converting some of the car's kinetic energy into rotational energy in order to keep the engine turning - this is a purely mechanical conversion as opposed to a thermodynamic (fuel burning) process. If you coast in gear then the injectors will be switched off by the DME - no fuel used. Of course, you could coast with the engine switched off...

Okay - not quite sure I understand this part... but look at it this way. Drive down a flat road at speed. Then either a) let off the gas with the car in gear, or b) let off the gas with the car in neutral. The engine will use an idles-worth of gas either way but in neutral you will go really far. In gear you will slow down much faster. If you did both options a) and b) for the same time interval (on separate trials), your mileage would be higher for b) and the gas spent (amt. of gas to idle times the time you were idling) would be the same - hence higher MPG.

Some people (dangerous people) actually turn the car off during the glide. I would never do that unless I had a death wish... but that is the optimal situation for MPG savings.