10% Mileage increase... free...

[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.

[pingu]

Not that I've tried this method but I'm a big cynic - and also, even with petrol at £1.10 a litre, I only do about 10 miles a week so doing 95 mph is no drag (pun intended - sorry!) to me.

"Technically true" seems to me the only way of being true or not true when discussing something technical. We're not discussing an English essay - there's only one truth. (I presume you were joking about the fuel tank overflowing!) I can't see how you can avoid the extra drag (due to speed squared) that results from going alternately fast and slow, when compared to going at a steady speed. Also, I can't help but think that if this fast/slow method really worked then all manufacturers would build it into cruise control so as to massage/improve their mpg rating when being assessed.

You got me thinking when you pointed out how much quicker a car slows down when it's in gear compared to when it's in neutral. I think this shows just how much internal friction an engine has. If you slow down in neutral, the rate of loss of speed is less than it would be in gear. But when you slow down in neutral, you don't see the fuel that you're burning (at a low efficiency) in order to keep the engine idling. If you slow down by coasting in gear, the rate of loss of speed is higher (i.e. you slow down faster than you would do in neutral) but you don't use any fuel while slowing down as the DME turns the injectors off. So I don't think it's true to say that you use an idles-worth of fuel either way.

You're probably right about it being the same time, rather than distance, at each speed (in my example with the 50 mph and 70 mph).

Anyway, an interesting thread! Cheers!

[attack eagle]

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.

depends on the vehicle... an engine with a torque heavy lower rpm would benefit from a low rpm slower acceleration, but a car with low torque (like all bmws, except maybe the v12s) would benefit from getting up to peak torque (max VE) more quickly in first and second.

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.

0 gas when off the loud pedal and over 1000 rpm.

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.

Or unless you have a car with no power steering & non vacuum boosted power brakes, like many compact 60's-70's cars or the electrical systems that substitute for them on hybrids.
In that case you can run just as safely with the engine off. I've coasted down a mountain in the mgb with the engine off (cracked head).

In anycase, 2 mpg = 24 miles MAX gain per tank, for a lot of frustration over many hours of driving... all to save $4. Skip the coffee, or take your own drinks for lunch/breaks and you have saved just as much... especially at 1.25 for a fountain drink or a vending machine 16 oz.

I'm just frustrated that I have to pay twice and fill up twice using my debit card to fill the tank... cause it shuts off at $50 or $75 depending on station.

Tip:
Avoid gas with 'air quality' additives like ethanol or mbte etc and you'll pick up 2-3mpg a lot easier...


The closest I come to the hypermile idiocy is to slowdown in gear for lights, and try to time my route so I hit the lights when they are green. htat is worth 3-5 mpg itself.

there are a couple that are out of sequence and close together (1 block) so I clear the first intersection quickly (so other traffic isn;t held up) then decell for the next light.