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Driving with a Load

  #1  
Old 08-05-2004, 03:41 PM
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Originally posted by kenny@Aug 5th 2004 @ 11:55 AM
I really am not sold on the driving with the load thing.
Remember Physics 101?
The force of gravity is constant, going up or downhill.
The way I see it whatever you lose going uphill you gain going downhill. So, it's a wash.
I'm not sure about that. Well maybe not the load thing but I do think one can benefit from hills. I've thought about this a lot while driving and while sitting in Physics 101. I'm sure it's a case by case (car by car) case but think of two equal distance stretch of roads. One has a hill and one is flat. On the hilly stretch think of it as two sections, pre-crest and post-crest. Now think of the flat stretch broken into the same two sections just flat. As long as the marginl difference of the pre-crest from the flat's first section is less than the difference of flat from post crest than you should be making out. ( I hope that makes as much sense in words as it does in my head)
The empirical data that I have for this crazy-speak is this. My parents have a GMC envoy (18.5 mpg avg hghwy, and it KILLS ME) Living in Denver we go skiing a couple times a year. We drive up into the mountains, over-revving the whole way up, getting like an avg of 8-9 mpg. But on the way back down we coast the whole frickin way. I actually have to put it in 3rd gear to engine break down to save the brakes. But guess what the MPG is when we are back down the mtns? Low 20's to mid 20's. That thing would never get those kind of numbers on a flat road.

Something to think about.
 
  #2  
Old 08-05-2004, 06:32 PM
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I'm pretty sure about the force of gravity being virtually the same as you go uphill downhill or flat.
Draw a force diagram.
If it takes x additional energy to overcome the force of gravity to climb a hill, then gravity will contribute exactly x on the way down.
It has to be a wash.

Anyway, I've driven over this particular huge bridge several times.
Of course my MPG drops by the time I crest it.
But when I get to the bottom I seem to recall that it has returned to what it was before I approached the bridge.

This warrants several delibrate careful tests.

I'll be back in touch when I get data of 10 or 20 passes over the bridge.
I only take that bridge about once or twice a week (it is my alternate route when traffic is bad) so it will be a while.

BTW I have about 4400 miles on my car but the database says 3800, not sure why.
 
  #3  
Old 08-05-2004, 07:09 PM
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Neglecting friction, from a physics point of view the ONLY thing that matters is the starting and ending hight above sea level. If we assume a perfectly flat road starting and ending at sea level the total work done (neglicting friction) is zero. The story is exactly the same for a road with hills or curves, so long as you start and end a sea level.

In practice we must overcome rolling resistance, and mostly air resistance. The speed limit requires the need to use engine braking or friction brakes on the way back down a hill. The only way to get the energy you put into the system by going up the hill would be to coast down the hill at well over the limit and even this is not as efficient because the higher speed coming down would generate more air resistance, even without regard to speed limits because the drag force is proportional to the square of the speed.

A level road allows the speed to be near the limit without the need to go over to recapture the energy.
 
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Old 08-05-2004, 07:58 PM
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I don't think the energy consumed to go uphill is recovered on the down side.

Take 2 parallel Hot Wheels tracks 100 feet long on flat ground.
Take one of those tracks and put a series of humps along the way.

Take a marble on the end of each track and start them on their way using equal force.
If the energy was reclaimed on the downside then both marbles should reach the end at the same time- right? It slows down rolling uphill but speeds up on the other side.
Same 100' of track, same gravity, conditions etc
Depending on the severity and number of humps, the marble may even stall.

The track with humps require additional energy to compensate for gravity loss for both marbles to "tie" in the race. Perhaps if we could tilt the entire length of track for more gravity energy on the downside, and less loss on the upside of the humps.

Which brings me to load vs Cruise control.

With the last example in mind, it consumes less gasoline to allow the auto to gradually slow down over the length of a hill than for cruise control to maintain speed.
Given the situation in, for example a 65MPH limit, in the basin of a hill I might be going 68MPH. Upon the top of the next hill I'll slow to about 52-55MPH.

In the same situation if the driver wanted to go a maximum speed of...say 55MPH in the 65 limit then load wouldn't make sense.

I'll be back in touch when I get data of 10 or 20 passes over the bridge.
Kenny- I hope you keep in touch even before your experiment is finished!
 
  #5  
Old 08-05-2004, 08:51 PM
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Sure letting the car slow down as you climb a hill will get you to the top on less gas.

But your car is going slower at the top, so it has LESS kinetic energy.
Therefore, it will take more gas to go down (even if the thing says 120mpg), or you will be going slower as you get to the bottom of the hill.
So you will need more gas to get back up to speed than you would if you were going faster at the top.
Just because you are going downhill at the moment and that meter says 120 MPG doesn't mean the laws of conservation of energy are on hold.
The key not just how much gas you are using at the moment but also the kinetic energy (speed) your car has at the moment.

Look at it another way:
Ignore the uphill part for the moment.
Imagine two cars side by side at the top of the hill at the same moment.
One is going 10 MPH the other 30 MPH.
They both go down the hill the with the goal of both going the same speed when they hit bottom.
Which one will use more gas?
The slower one of course.

Again, I will try to do some tests on that bridge.
Some variables I can't control will be:
1. Any tail wind will help more going uphill than downhill since the hill blocks the tailwind on the way down.
2. Any headwind will hurt me more on the downhill side than on the uphill side since the hill blocks the headwind on the way up.
3. I don't know the exact elevation at the beginning and the end. To be fair they should be the same elevation.
4. The car's MPG indicator refreshes every 10 seconds. A one second or less refresh time would be more useful. A lot depends on exactly when I happen to look at it.

I could be wrong; it's happened before.
 
  #6  
Old 08-05-2004, 09:11 PM
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Hi All:

___Huge Driving w/ Load proponent here Ö

___Lakedude, Hot_Georgia_2004 brings up a good point about the Hot Wheels or Marble on the track with small hills. There is no free lunch on the friction side. There is another piece missing from the puzzle and that is dropping tranny ratios to maintain speed. This only worsens the negative fuel economy effect during climbs Ö Can you achieve the same fuel economy on the flats with lower gear ratios? No.

___In the X and Corolla, they will downshift while trying to maintain speed while in cruise up a hill. I am sure the HCH and Prius CVTís drop ratios while climbing as well Ö Watch the instantaneous sink like a stone when that happens. Another irritating item is that the supposed free coast on the downside is not free at all as the cruise is adjusting up and down and can be seen on the instantaneous of the X and the rented Buick Le Sabre I was driving last week. I am sure the Autoís of the Corolla, X, and Le Sabre as well as the CVTís of the HCH and Priusí are adjusting ratios while the non-finite cruise control is attempting to bring the target automobiles back to its exact set speed. While driving the Insight up a hill at constant speed, I see the instantaneous sometimes drop to the high 40ís with the 150 + free coast on the downhill side. I do not use assist while climbing if at all possible (4 bars max) unless someone is behind. Climbing under the driving w/ load technique usually maintains 100 mpg + up and the 150 down while slowly accelerating down the back side on a low elevation hill with a larger drop on the backside. On the really small overpasses (20 - 25í), the instantaneous simply hangs in the 100 - 110 mpg range up and down.

___Kenny, I read your post in the other thread. Driving w/ load isnít maintainable on all hills even here in Illinois as they are simply too high in elevation. I have 8 of these larger climbs on my daily commute. What I do is reach my minimum and maintain that minimum until I have cleared the apex. Once that minimum is reached and I am in a constant speed mode on these larger hills during the climbs, I have now traded off as much aerodynamic drag as possible while gaining potential in an attempt to maintain the maximum fuel economy. In other words, on the smaller hills, it is a straight up swap of kinetic for potential - the normal losses. On the larger ones, itís the same until the minimum speed is reached and afterwards, itís a race between lower aerodynamic drag at a constant slower speed for the gain in potential for max fuel economy albeit a lot lower then the normal 100 - 110 mpg. My minimums can be as low as 40 mph if nobody is around (late night driving), 45 if traffic is light, and 47 - 50 mph in heavier traffic. If traffic is too heavy, driving w/ load has to be abandoned until traffic behind is clear once again Ö On the record breaking run at the end of June, I was using the emergency flashers while climbing slopes in the southern portions of our fair state although the Interstate routes I was traveling were about as desolate as you could ask for given the distance away from Chicago. I still maintained my overall averages to mimic the minimum speed limits on the flats of course (I had to keep the police from asking questions ) and surpassed the average set by the French driver but once my own preset climb minimums were met (37 mph in light traffic while climbing during those 3 days), my aero drag during the climbs at those speeds was lower then my normal 40 - 50 mph while climbing on my standard day time commute depending on traffic.

___With all of the above, I donít know how the HCH handles NOx purge events but in the 5-speed Insight, I am massaging the accelerator quite a bit during both NOx purge and lean-burn initiation to dampen out the extremes as seen on the instantaneous to attempt to maintain a steady reading on the game gauge. In fact, the Insight climbs in lean burn quite well but usually a NOx purge event will hit near the apex just when load drops off (I know it doesnít make sense but that is Hondaís programming) and I adjust the accelerator appropriately to maximize fuel economy.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___[email protected]
 
  #7  
Old 08-05-2004, 09:17 PM
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Originally posted by Hot_Georgia_2004@Aug 5th 2004 @ 9:58 PM

Take 2 parallel Hot Wheels tracks 100 feet long on flat ground.
Take one of those tracks and put a series of humps along the way.

Take a marble on the end of each track and start them on their way using equal force.
When you put the bumps in one track the end of it will slide towards you.
So the ends of the two tracks will not longer line up.

Both marbles will still travel 100 ft on their "odometers".
But as the crow flys one track will be shorter.

I think the marbles will reach the end of their respective tracks at the same time.
Of course if the bump it too high it won't make it, but that is not relavant to our discussion of cars with engines.
 
  #8  
Old 08-05-2004, 09:27 PM
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Hi Kenny:

___The larger hills are the biggest problem of all and if the marble gets stuck between two bumps, it doesnít make it as far as the marble on the flat track.

___As posted above, I believe the autoís drop ratios so you lose fuel economy no matter flats, up hills, or down hills. A manual doesnít drop ratios of course but the Insightís small ICE really needs the coals applied to maintain speed while climbing. I do in fact drop to 4th on some of these hills if I cannot drive with load during heavy traffic. The increased aerodynamic drag while at constant speed vs. the sag and gain during a driving w/ load event can never be made up either.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___[email protected]
 
  #9  
Old 08-05-2004, 09:34 PM
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Wayne,
I donít know how the HCH handles NOx purge events
As I'm driving, holding the feed at a fixed position suddenly I'll feel a surge and the instant MPG drops by about 10-15, holds there for a second or so then climbs to its previous position. It happens several times with no pattern in my 45 mile trip.
I guess this is the NOx purge you mention.

If I'm lazy I won't disturb my feed position and let it pass.
Otherwise I'll back off to the previous FCD then feed the gas back in as it finishes.

Kenny-
I think the track example should apply since we are considering what method consumes less energy in tackling hills.
You are correct- the end of the race would be 100' for the straight track and where the end of the hilly one may be.
 
  #10  
Old 08-05-2004, 10:15 PM
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Hi Steve:

___The Insightís NOx purge event is a bit longer in duration (~ 10 seconds every 3 or so minutes) and you can see the sinusoidal damped fuel injection programming happen in real time over the first 1 to 1.5 seconds by watching the game gauge through its initiation. I usually make 2 to 3 very small accelerator adjustments in the second or so after it hits to dampen out the normal transient to maximize fuel economy. The small acceleration you described is a text book example of a Lean-Burn capable Honda ICE entering the NOx purge event but it sounds like the Insightís event holds for a longer period of time vs. your HCH unfortunately

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___[email protected]
 

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