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-   -   Hills are a good thing. (

kgwedi 08-10-2010 01:10 PM

Hills are a good thing.
I do P&G (engine off coasting) with my Chevy Metro. I have a fuel injector cut off switch to stop all fuel while coasting.
I see P&G from an energy point of view. Accelerating increases your Kinetic energy by using the Potential energy in your fuel. Coasting uses that Kinetic energy to overcome drag and friction.

I see hills as a good thing, because they trade the Potential energy in the fuel for an equivalent increase in Potential energy from the increase in height. Whether you use the energy in the fuel to accelerate, or go up a hill, there is still the same amount of energy transfered. Going downhill changes that Potential energy into Kinetic energy to overcome drag and friction.

Also the same thinking goes for stop lights when there is no traffic to force your speed. The energy I use to accelerate is used while coasting to the next light. I plan on arriving at the next light at the minimum speed so if it is a Red light I just sit and wait for green, when I'll start the engine and accelerate again. In my town I accelerate to a speed that will let me coast engine off just to the next light.
My city mpg is in the high/mid 50's and my back road long distance mpg is in the high 50's. What really hurts is slow creeping freeways.

Does this make sense to anyone?

gone-ot 08-10-2010 02:10 PM

...for "one way" trips.

kgwedi 08-10-2010 02:30 PM

The same physics is involved on any trip of any distance and any amount of hills.
I try to go up hills, (or accelerate) at 80% LOD on the ScanGage, and I decelerate, (or go down hills) EOC.

RobertSmalls 08-10-2010 04:05 PM

Hills make P&G easier and more practical, so P&G on hills outperforms DWL on flat land. But if you're not going to engine-off coast down the hills, hills are bad. They're also bad when you need to accelerate up one.

Wind is bad 3/4 of the time. Overall, hills and valleys reduce ground-level wind, which is good.

Bottom line, depending on your hardware and your technique, you may be able to use terrain to your advantage. Not me, though: my car has excellent gearing and lean burn for flat land cruising, where I outperform hills by 10-20mpg.

kgwedi 08-10-2010 04:32 PM

Thanks for that. I didn't know engine tuning would help that much.
You are correct that wind is usually a bad thing.
I also think that weight is important. A high Sectional Density is better in the long run than light weight. It will extend the coast down distance. That is why in gliding competitions the gliders carry up to 500 pounds of water ballast.
I tried doing a coast down to figure my Cd ratio and found that a 6 gallon water can in my Metro changed the results significantly. You must really have an accurate weight to do those calculations.

VegasDude 08-10-2010 06:43 PM

If the backside of the hill is steep, you will be wasting all the potential energy in engine braking and air resistance. You would do better if there was no hill at all. Consider three scenarios: Firstly a perfectly flat drive. Second: You drive up a 2% grade and coast down a 2% grade which allows you to EOC at 55 MPH. Third: you drive up a 2% grade for and then down a 4% grade which accelerates you to 75 mph before the air resistance becomes too much.

In the second scenario very little extra fuel is used (if any), but in the third scenario you are essentially buring the extra gas to go 75 on the way up and there is no way around it. Bottom line, the steeper the hill the worse you FE.

kgwedi 08-10-2010 07:16 PM

That's a very good point. Thanks VegasDude.
I will lose out by too steep a decent.
I guess the determining factor on whether a hill is good or not, is whether you will exceed a chosen speed (say 50 mph) during coasting down a hill. With that speed being what you normally use as your fastest speed on a P&G.
I learn so much on this site. Thanks again.

RobertSmalls 08-10-2010 07:29 PM


Originally Posted by kgwedi (Post 188272)
I guess the determining factor on whether a hill is good or not, is whether you will exceed a chosen speed (say 50 mph) during coasting down a hill.

You must also be able to climb the hill efficiently, e.g. in top gear. My car suffers more than most on hills because it does so well on flat land due to the small displacement and tall gearing.

Regarding the 500lbs of ballast: that will add to your rolling resistance, and raise the stakes if you have to brake. Hills don't make P&G any more effective, just easier to do.

user removed 08-10-2010 07:48 PM

Depends on the hill. Ideally (without Roberts Lean burn Insight) You can climb the hill at a higher BSFC than on flat ground. Then you can coast downhill with the engine off and use no fuel.

As long as you monitor your uphill consumption and keep it as low as possible, you will do better than you would on flat ground.

You can also (if the hills are just the right grade) avoid the exponential increase in aero drag that you would suffer when you pulse to a higher than average speed on flat ground.

I don't have hills in my local area that are steep enough to maintain average speeds over 40 MPH downhill.

If you can coast at the posted speed limit downhill, and the uphill grades are about the same, you are lucky enough to have a hyper-milers ideal terrain.


skyl4rk 08-10-2010 09:48 PM

I look at hills as energy batteries that can be very efficient if you approach them correctly. My goal is to be rolling at my most efficient speed (about 40 mph) at the bottom of the hill. I often crest a hill just crawling and hoping to make it over the top.

This works when you can look in the mirror and see no traffic.

Hills in my area are fairly small.

I hate stop signs at the bottom of hills.

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