Pulse & Glide vs maintaining constant speed

[YeahPete]

In my wifes honda fit, I find that neutral coasting works best, but its a massive pain in the ass and you don't want to accident shift in into reverse instead of neutral. I usually only drop it into neutral when I see a nice coasting opportunity of over a min.

[wobombat]

I normally pulse up hills and glide down them, but then I realized that putting cruise control on would do pretty much the exact same thing. Sure it wouldn't manipulate the load, rpm's etc in the most efficient manner going up the hill, but otherwise it's about the same. Why then is cruise control considered inefficient and bad for fuel economy? Or is it that you shouldn't pulse up a hill.

And considering that a hill puts a constant backwards pressure on your car, wouldn't you save the most gas by getting off that hill as soon as possible without going too fast as to lose efficiency to air resistence ? Because of this, I don't understand why DWL would give you any better fuel economy than constant speed driving, except that it forces you to slow down to a more efficient speed.

[serialk11r]

Originally Posted by wobombat

I normally pulse up hills and glide down them, but then I realized that putting cruise control on would do pretty much the exact same thing. Sure it wouldn't manipulate the load, rpm's etc in the most efficient manner going up the hill, but otherwise it's about the same. Why then is cruise control considered inefficient and bad for fuel economy? Or is it that you shouldn't pulse up a hill.

And considering that a hill puts a constant backwards pressure on your car, wouldn't you save the most gas by getting off that hill as soon as possible without going too fast as to lose efficiency to air resistence ? Because of this, I don't understand why DWL would give you any better fuel economy than constant speed driving, except that it forces you to slow down to a more efficient speed.

I actually wonder this too, the difference between cruise control and pulse and glide is pretty much that down the hill, the engine is in gear and you're feeding it enough gas to not cause the engine to drag your speed down. I'm guessing the same, DWL slows your speed down.

Hills are perfect for saving fuel, because you can glide down them, and travel up them with a pretty high load.

[euromodder]

Originally Posted by wobombat

I normally pulse up hills and glide down them, but then I realized that putting cruise control on would do pretty much the exact same thing.

The most efficient - but not always the most practical - way is to slow down going uphill.
You trade kinetic energy for potential energy (due to being higher) and can get (most of) the potential energy back while going downhill.

That's exatly what a CC does NOT do, as it tries to maintain a constant speed.
So uphill it's pouring on gas, downhill it's engine braking (throttling back).
Both are inefficient ...

I try to pulse before an incline, then coast up it and down again - works for bridges etc, but not on hills as the climb is too long.


Long climbs on CC can become highly inefficient if you "overload" the engine - which I can do on my tiny 1.6 L turbodiesel.
It WILL pull up the hill, @ 100% (relative) LOD on the ScanGauge, but it comes at a cost : diesel.
Some 20-30% more of it than when going 20kph faster !
And at triple to quadruple the "level" FC.

Thus, going uphill faster on CC, can actually be more efficient than going up slower on CC - simply because it doesn't load up the engine that much as the rpm are a lot higher.

This is a case where big US engines will do relatively better - simply because they can cope better with the temporary higher load.

And considering that a hill puts a constant backwards pressure on your car, wouldn't you save the most gas by getting off that hill as soon as possible without going too fast as to lose efficiency to air resistence ?

Drag is the permanent loss in our real-life system.
More speed = more drag , both air and mechanical
So the faster you go, the more drag you'll have.

[serialk11r]

Originally Posted by euromodder

That's exatly what a CC does NOT do, as it tries to maintain a constant speed.
So uphill it's pouring on gas, downhill it's engine braking (throttling back).
Both are inefficient ...

I try to pulse before an incline, then coast up it and down again - works for bridges etc, but not on hills as the climb is too long.

This is a case where big US engines will do relatively better - simply because they can cope better with the temporary higher load.

My car won't coast down most hills at over 50mph though, it'll lose speed, so I doubt the engine braking thing. I suppose my ultra light curb weight could have something to do with it though.

If you pulse before the incline, you'll be travelling faster before the incline and thus using more energy. If you pulse on the incline, you can use the incline to store energy and keep a lower speed, while maintaining the same average speed. Pulsing up the hills and gliding down tends to keep my speed closest to constant.

You might be right about the bigger engines though, especially if you say your engine hits 100% load. I only need 50% load to maintain speed up hills, but my 5th gear is very short. I can't see it ever being more than 75% on any car sold here.

[user removed]

Everyone is right, but it depends on the vehicle and the hill, not even close to a written in stone tactic.

If you can coast down the hill at your desired speed, or even a lttle less, you have the best possible scenario for mileage. That hill makes your car a hybrid with gravitational energy storage. Pulse up the hill and engine off coast (or engine on if you fear EO) and glide down the hill. You mileage uphill will be low, maybe less than half of your average, but downhill you will be getting literally hundreds (infinite if EO) of MPGs.
Every other hill from way too steep to way to shallow requires some modification of the tactic and it also depends on your vehicle, since some will coast at higher speeds down the same hill compared to others.

regards
Mech

[ps2fixer]

Originally Posted by YeahPete

In my wifes honda fit, I find that neutral coasting works best, but its a massive pain in the ass and you don't want to accident shift in into reverse instead of neutral. I usually only drop it into neutral when I see a nice coasting opportunity of over a min.

Just wanted to point out that most shifters enable you to go into neutral from drive with out pressing in the button. Same goes from 1st -> 2nd -> drive -> neutral, but not in reverse (stops at drive). Might be something good to check on your wife's car with the engine off. I think it is a safy feature more than a P&G assist when they designed it .