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Old 05-22-2008, 11:32 AM   #11
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But to look at it another way. Assume you have a hill to climb. Our test starts and ends at the same elevation. For the sake of argument, let's examine two scenarios:

#1: Constant speed 60 mph, we make 17 mpg going up the hill and 38 mpg going down.
#2: Constant TPS, say we slow to 55 mph at the top of the hill, and accelerate back to 60 mph by the bottom. Let's say we can maintain 26 mpg going up, and get 35 mpg going down.

In scenario #1 we average (17+38)/2 or 27.5 mpg at 60 mph.
In scenario #2 we average (26 + 35)/2 or 30.5 mpg at say 58 avg mph.

These are roughly the numbers I was getting on a trip I took in a rented Pontiac G6 GT, experimenting with constant speed vs constant TPS. I was able to get the total average for the trip up to 32.5 mpg by the time I finished my trip, using the constant TPS approach. It's rated at 18/29.
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Old 05-22-2008, 11:37 AM   #12
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as long as I am staying in OD gear, wouldn't it still make better sense to use momentum even if you get deceleration that you have to make up somewhat on the downslope?
Not necessarily.

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aren't you loading the engine more by keeping steady speed on the upslope [compared with letting the car slow down]?
Yes, but that can be a good thing. Assuming you're not downshifting, i.e., you're using low revs, then a gas engine is typically more efficient when operating with a larger throttle setting. That doesn't mean it's using less gas, as measured absolutely. It just means it's using less gas per unit of work produced.

More on this subject: http://autospeed.com/cms/A_110216/article.html

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I barely passed physics. That's why I am an archaeologist.
I have a lot to learn about both subjects, so you're probably ahead of me, on balance.
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Old 05-22-2008, 11:38 AM   #13
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Right, for me, the numbers were silly-like 9 uphill then 55 mpg or so downhill. The downhill didn't change much when I dropped the speed and got close to 26 mpg uphill. So then would it just boil down to total trip time?
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Old 05-22-2008, 11:39 AM   #14
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That was a response to the previous post by samandw
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Old 05-22-2008, 11:41 AM   #15
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Yes, but that can be a good thing. Assuming you're not downshifting, i.e., you're using low revs, then a gas engine is typically more efficient when operating with a larger throttle setting. That doesn't mean it's using less gas, as measured absolutely. It just means it's using less gas per unit of work produced.
Well, see, I had figured on this too. Does the fact that I have a turbo car make a difference? B/c the less vac I make the more fuel I seem to use....
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Old 05-22-2008, 12:18 PM   #16
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Here you go.

Your mileage only reflects your distance, not your elevation.

An 8% grade 8 feet up per hundred, you are travelling 100 feet per second horizontally.

You are lifting your car using the road as a lever, exactly 8 feet per second.
2200 pounds (car weight) times 8=17600 pounds (one foot every second)divided by 550=32 horsepower

That is what you are storing per second for as long as you climb.

Whats really sweet about it is you can get it back, when you go over the top of the hill, but it only cost you 50% more fuel than driving level (better place in the BFSC map). Use a gear that keeps you closest to staying in the sweet spot of the map.

Lets say it takes 12 hp to maintain your level speed but 32 more to climb the 8feet per second. you are lifting your car up at a rate of 480 feet per minute like it was an airplane, both gaining altitude.

Now follow me closely, if the grade is less than one too steep to maintain speed in any gear, the gear the gives you a mileage equal to two thirds of you level mileage is best minimum speed. The choice is the gear that gives you the best mileage as long as it doesnt take a lot longer to climb the grade.

The total amount of power it takes to climb the grade (vertically) is the same regardless of how long it takes, but your engine wants to get the job done quickly, and rewards you with better overall mileage when you get the most of those HP for the absolute least amount of fuel. Up to about 75% of full throttle is OK, past that you migh try another gear.

Use your mileage difference to find the minimum peak mileage, then gradually increase your speed to the best balance between what it cost you per second and what the total cost would be. Your mileage will go higher at lower speeds, but it takes you longer to finish the climb, offsetting penalties so to spesk.

It may be more efficient to use a lower gear on steep grades. shallow grades are a hypermilers dream.

I use my local knowledge to always accelerate up a shallow grade, then cost down the shallow downslope. If the grade is fairly steep and sustained I woud actually reach my highest pulse speed at the bottom then gradually lift the pedal as I try to maintain the same speed regardless of the fact I am going uphill or downhill. Constant speed will give you the lowest overall drag, by avoiding the higher speeds exponential increase. Vertical climb no more than 60 feet in this example.

If I could design a hypermiling road, the approach grade would be 3 times as steep as the downslope, which would allow you to pulse up the hill and maintain a constant speed downhill with your perfect slope.

Then your 9 MPG would be 36 average up one side and down the other., if you shut the engine off on your coast. Then the downhill mileage would be infinite.

regards
gary
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Old 05-22-2008, 12:21 PM   #17
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Right, for me, the numbers were silly-like 9 uphill then 55 mpg or so downhill. The downhill didn't change much when I dropped the speed and got close to 26 mpg uphill. So then would it just boil down to total trip time?
Yes, you'd want to compare total trip mpg to give you an accurate idea of which method yields better mpg.
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Old 05-22-2008, 12:31 PM   #18
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I use my local knowledge to always accelerate up a shallow grade, then cost down the shallow downslope. If the grade is fairly steep and sustained I woud actually reach my highest pulse speed at the bottom then gradually lift the pedal as I try to maintain the same speed regardless of the fact I am going uphill or downhill. Constant speed will give you the lowest overall drag, by avoiding the higher speeds exponential increase. Vertical climb no more than 60 feet in this example.
That's an extremely interesting idea. I think a series of tests is in order over moderately hilly freeway.

Test 1: constant speed
Test 2: decelerate up hills, accelerate down
Test 3: accelerate up hills, EOC down.

I wonder if the real answer depends on the steepness of the upslope vs the downslope.

Maybe I'll make an excel spreadsheet to find an answer. Anybody know of a car I can find weight, CdA, and BSFC vs. hp?

Imagine in the future your cruise control syncing with your GPS to optimize fuel economy while maintaining a target average mpg. That would be sweet!
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Old 05-22-2008, 12:38 PM   #19
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Okay R.I.D.E., here's the thing. I live in Louisiana, so these are TINY hills in comparison to what most folks think of. If I start at the bottom of the hill at 55 mph, the lowest I let it drop to is maybe 48-50 mph. It takes maybe 7-10 seconds to go from base to crest of one of these Everest-sized monsters. Maybe that will clear up my circumstances a tad.
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Old 05-22-2008, 01:02 PM   #20
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bkrell, always used the slightest upgrade to apply a decent load to your engine. On very shallow grades you want to hit your peak speed at the top, then coast as far as you can to your minimum speed, downhill, then repeat.

Regardless of everything else your highest speed at the top gives you the longest glide.

If the grades are far enough apart you may have to pulse again between them (or more than once), but develop your timing for peak speed at the peak. Highest sustainable load in highest gear without downshifting, neutral coast.

The hill I would reach top speed at the bottom would be one that can catch me below my power band and peak efficiency, then I would have to downshift. That would never happen on your slight grade.

I had a Valiant that would only go 45 up a certain steep grade, downshifted to second floored it and shifted to third, still floored, and watched it slowly loose speed to 30, back to second and repeat. I could have left it in second floored for 8 minutes it took to climb 2500 feet, but it was better to average 40 mph by peaking in second and then going to third even thoug it woud not maintain speed. A couple more gears than a 3 speed would have been nice.

Same as the truckers in parts of West Virginia.

regards
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