An 'Ernie' trip - 75 mpg...

watercop

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An \'Ernie\' trip - 75 mpg...

My latest tank:

855.8 miles
11.989 gal

71.38 mpg


Approx 700 of those miles traveled on a round trip to Opelika, AL on back roads at 1600 rpm / 51 mph. I was able to travel at night so managed to get by with very little AC. Given that about 150 miles before the trip were several days of daytime local driving at an estimated 55 mpg the trip itself was very likely at 75-76 mpg. I calculated that via a weighted average

I believe 75 mpg is approaching an upper limit for real world results, at least in terrain with any hills. Certainly I could drive more slowly, maybe as slow as 40 mph / 1200 rpm, but not on any grade at all. I would have had to downshift regularly, which I'm not willing to do.

I chose extremely lightly traveled back roads so as not to inconvenience too many other drivers. I don't recall hearing any shots fired.

At 1600 rpm the engine still pulls smoothly and strongly enough to take the hills on the back roads of western Georgia and central Alabama. I did do some momentum management, coasting down some of the steeper grades and feathering up the upgrades. Mostly I let the cruise control have it.

I shouldn't have to say this but both the immediate prior fill and this one were complete - clear fuel to the top of the neck with the car sitting flat. The approx 12 gallons added was consistent with my fuel guage which indicated 5/16ths remaining.
 

gdr703

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An \'Ernie\' trip - 75 mpg...

A m a z i n g.

This all leands credence to the rule of thumb:
50mph = 70mpUSg
60mph = 60mpUSg
70mph = 50mpUSg

(or better)

Congrats
 

ofhs93

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Re: An \'Ernie\' trip - 75 mpg...

Very cool WC...Im sure you choice of oil and top gearing have a lot to do with this as well
The rest of us "normal" folks would probably ended up with maybe 60-65 mpg over that long trip stretch.
 

daBooj

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Re: An \'Ernie\' trip - 75 mpg...

cool! You got me beat.

me... Just got back from my week of trips up and back to and from philadephia all week long from (and to) frederick. I was hoping to do something awesome, but forgot I would have my boss with me. Speeds were higher than I would have liked, and I had to run the ac for several hours

pulled down a measly 63.5mpg if I did the calc correctly this morning.

go P Diddy, go! Perhaps I'll get to 70, soon enough. Perhaps I'll challenge my signature! (yeah, not likely)
 

ofhs93

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Re: An \'Ernie\' trip - 75 mpg...

So...did you gloat all the way to and from Philly to your boss about the mileage?
 

watercop

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Re: An \'Ernie\' trip - 75 mpg...

gdr - I think your numbers are about right for a normally geared and lubricated car...hey, I had to do something - you've dusted me the last couple mpg contests!

vf - thanks

ofhs - thanks, also, knowing the terrain around Scranton, I'm not sure I could hold hills in that area at 1600 rpm

dB - no way I can drive much slower than 65 on busy interstates, especially in the northeast or major cities - it simply isn't reasonable or prudent to hold an entire lane 10-20 mph below prevailing speed. The surface roads in the northeast are too signalized for low speed high mpg runs
 

toronto_TDI_LT1

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Re: An \'Ernie\' trip - 75 mpg...

Wow thats amazing i could never get number like that to much traffic around here.In total i miust have been in traffic for 3 hours driving home from work this week.
 

Ernie Rogers

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An \'Ernie\' trip - 75 mpg...

Good job, Watercop. Okay, you can drive fast now. (I don't want you to make me look bad.) I think we know that you would beat me in any fair race.

I have a wild idea that maybe hills can actually give us better mileage. The car is more efficient going up the hill. Maybe we don't have to give back all of the gain when we go down the other side.

Next topic-- Speed is a dominant factor in our high mileage numbers and speed is related to aero drag. It is apparent that the person that can seriously cut his drag will be the one that climbs above the pack. We currently all have about the same drag coefficient, near Cd = 0.30.

What are some simple (and not simple) tricks we might do to reduce drag further? Shall we make a list?

Ernie Rogers
 

whitedog

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An \'Ernie\' trip - 75 mpg...

I have a wild idea that maybe hills can actually give us better mileage. The car is more efficient going up the hill. Maybe we don't have to give back all of the gain when we go down the other side.
This is a very interesting statement. I'm not sure which way is right here, but lets explore all of the space here.

I belive that the statement. "The car is more efficient going up the hill." Is a highly educated guess, but I'm not sure one way or the other of it's validity.

So I think the first thing to do to explore this is to determine if that is, in fact a true statement. Is the car more efficient going up hills? How can this be determined? I have used rolling hills to my advantage in acheiving an apparent 52 MPG trip by not using cruise and allowing the speed to drop going up hills, then accellerating going down the other side when gravity was my friend. From this, I believe that hills can be a MPG friend, but I don't really have anything to back up my guess.

On an unrelated note: efficient... so much for "I before E, except after C."
 

watercop

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An \'Ernie\' trip - 75 mpg...

"...more efficient up a hill..."

There may be something to this. I believe it has to do with the fact that the engine is much more efficient at power levels and uses vanishingly small amounts of fuel at idle.

Suppose for example the steady state percent power needed on flat roads is 20%. That is a guess but likely fairly close. We can see from the BMEP (I think that's correct) graph that efficiency at such low load is fairly poor. If we found a way to operate intermittently at say 60% power 1/3 of the time overall efficiency would rise.

I have heard of an extreme mpg competition where gassers were operated intermittently - periods of full power followed by coasting (or maybe complete shutoff - not sure), rather than steady state.

I have experienced decent mpg results in hilly terrain. I really enjoy driving the Blue Ridge Parkway if I'm in the area with some time on my hands. Parts of it are extremely hilly. I assumed that my mpg would take a hit but it doesn't seem to. Granted speeds are lower (supposed to be, anyway - limit is 45) The only trouble with the BRP is that sections of it are mountainous - curves and grades requiring braking. Any braking, of course, is a complete mpg writeoff. Some other member here described it better than I - "Brakes are a perfect mechanism to convert Diesel fuel to brake dust and heat."

This issue begs a test trck test - try operating at steady state speed of 40 mph for a couple hundred miles, then try accellerating to 50 as quickly as possible and coasting down to 30 out of gear (or in gear - try it both ways and settle THAT argument as well) and then reaccelerate to 50, coast down to 30 etc. etc.

This is the idea behind hybrids - if the components could be assembled to put our engines on a short duty cycle hybrid one might see great mpg. What I mean by that is an electric car with a battery just large enough for 10 or so minutes of operation. The TDI motor is coupled to an alternator and runs at 1900 rpm / max power for long enough to charge the battery from say 20% charge to 80% charge, and then cuts out completely until the battery gets back down to 20%

Unfortunately the alternator and power electronics needed would be hellacious, though the battery wouldn't have to be really big
 

gdr703

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An \'Ernie\' trip - 75 mpg...

. . . hills can actually give us better mileage.
Mabe we don't have to give back all of the gain when we go down the other side.
I agree, hills can be used to better the mpg.

If the engine is more efficient when going up the hill, it is less efficient when going down the hill.

Drive up the hill with a heavy right foot in 5th. The engine is making power efficiently, and at the top of the hill you have all that stored potential energy.

Now you must use it wisely when going down hill.

Do not *drive* down the hill, do not use the brakes.

Try to use a heavy right foot whenever possible. Use no right foot, and avoid using a light right foot.

cheers from hilly Vancouver.
 

Ernie Rogers

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An \'Ernie\' trip - 75 mpg...

Watercop, you've got it!

You said,
What I mean by that is an electric car with a battery just large enough for 10 or so minutes of operation. The TDI motor is coupled to an alternator and runs at 1900 rpm / max power for long enough to charge the battery from say 20% charge to 80% charge, and then cuts out completely until the battery gets back down to 20%
Driving up a hill is exactly like storing engine power into a battery (a gravity battery
), and then coasting down the other side is equivalent to running on the battery.

Ernie
 

whitedog

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An \'Ernie\' trip - 75 mpg...

Driving up a hill is exactly like storing engine power into a battery (a gravity battery
), and then coasting down the other side is equivalent to running on the battery.

Ernie
So in that battery, what is the specific gravity?
 

saGhost

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2003 Jetta GLS Alaska Green ESP [sold] 2012 Chevrolet Volt
An \'Ernie\' trip - 75 mpg...

. . . hills can actually give us better mileage.
Mabe we don't have to give back all of the gain when we go down the other side.
I agree, hills can be used to better the mpg.

If the engine is more efficient when going up the hill, it is less efficient when going down the hill.
I always used to believe that, too - and I still do, for gas engines. However, a couple of years ago, we came across a BMEP map - and I tried to prove it arthimatically. Somewhat to my surprise, it didn't work - and some talented gentlemen went on to demonstrate that our combustion is almost perfectly scalable... The only benefit a TDI gets from high "throttle" settings is proportionally more fuel spent on BHP instead of overcoming friction - so if you coast down the hill in gear, the consumption is the same as if you'd driven on the level the same distance. On a gas engine, with throttling losses, it's a different story, of course. Anyone really curious can go read this.
Walter
 

watercop

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An \'Ernie\' trip - 75 mpg...

"The only benefit a TDI gets from high "throttle" settings is proportionally more fuel spent on BHP instead of overcoming friction - so if you coast down the hill in gear, the consumption is the same as if you'd driven on the level the same distance."

Ah ha! A decisive hit for we neutral coasters! We have long suspected that dragging the engine mechanicals along for a high rpm ride consumes more power (even at zero fueling) than merely letting the engine putter along at idle while we free fall downhill either in neutral or clutched!

Woo-Hoo!
 

petec1

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May 25, 2004
Re: An \'Ernie\' trip - 75 mpg...

I think that you can get better mileage on hill assuming the down hill sections are not too steep. Steep down hills of any length mean one of two things. Either you ride the brakes which throws away potential energy or you reach speeds where all that potential gets thrown away on wind drag or possibly even both. Either way, you are throwing away energy stored as you trudge up the hill.
 

Sleet

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An \'Ernie\' trip - 75 mpg...

actually last I checked it was negative (-9.8 or -32.2 depending on the system
) but then I only studied electrical engineering and dynamics was a far as I went on the ME route...
 

2004PassatTDI

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Re: An \'Ernie\' trip - 75 mpg...

Petec1

So basically, the ideal hill accent would be the one in which the engine was making its power most efficiently, and at a rate where wind resistance was lowest. Would the ideal hill be a perfect vertical accent (bear with me here)? In this way, the fuel used would be converted to about as much potential energy as possible. Air resistance as it relates to the accumulation of the maximum potential energy attainable would be at its lowest point.

The ideal hill to descend would be one where the downward slope would be just enough to get the car moving. This would also be the longest.

I’m no squid here (clearly), but if you know how much energy is required to lift your car X amount vertically, and you know how much energy you can store in your tank, while running your car at its most efficient state, then when you run out of fuel, you can coast down the ideal hill. Assuming that you hit the apex of the ideal hill 1 moment before you run out of fuel.

Ernst
 

saGhost

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An \'Ernie\' trip - 75 mpg...

We have long suspected that dragging the engine mechanicals along for a high rpm ride consumes more power (even at zero fueling) than merely letting the engine putter along at idle while we free fall downhill either in neutral or clutched!
umm... no. Well, maybe. This isn't quite the question I was trying to address... My understanding is that frictional engine losses are mostly proportional to engine speed [not positive of this - if someone knows better, say so.] Making the [mostly demonstrated] assumption that combustion scales linearly, that suggests that twice as much fuel is expended *at some point* to sustain 1900 rpm vs 950 rpm idle for a given period. But this is where it gets interesting... if the hill is steep enough to hold speed in gear, then it's energy you've already expended... as (jokingly) alluded to earlier, the car is changing energy forms - trading potential energy (from the height of the hill) into kinetic (car motion) - into heat (friction in the engine.) This isn't that different from what a hybrid does - except that it depends on outside influences to make it possible.

By contrast, the neutral situation is a little different... The total energy required is lower [less frictional loss] - but the conversion isn't happening quite the same... on the same hill, the car in neutral will accelerate - potential energy converts to kinetic, but the friction is carried by fuel instead - until the remaining loads (basically aero drag, since the rolling resistance is far less variable with speed,) equal the potential energy transfer.

Which one burns less fuel? A good question, which depends on a much more detailed situation than the hill I've suggested. The neutral model clearly burns more fuel in a given period of time - but that since it accelerates, it takes less time to cover the distance. To make an impartial determination, you'd have to take GDR's power required model (with reference to a BMEP/BSFC map,) add a standardized hill (grade, length up, length down,) and assign a speed (which is variable for the neutral case.) My professional opinion (SWAG) is that it varies based on speed (and gearing, of course) - at lower speeds the neutral model prevails, while higher speeds favor the in gear (but the gearing I would guess the other way - high RPMs favor neutral, while low RPMs favor in gear) - but remember it's a SWAG - the only way I see to answer conclusively is the project described above...
Walter
 

saGhost

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Re: An \'Ernie\' trip - 75 mpg...

This suggestion with the ideal hill is interesting... TANSTAAFL clearly applies, but by designing a very special asymetric hill, you should be able to get something. If the hill is so steep that you can maximize throttle at optimal rpms (~1700) at low speeds and hold it (hence the low wind resistance) - and then is sloped to exactly hold 1000 rpms in 5th for the downhill (so you get full accessory function, but the fueling's cut) - you can probably gain a bunch from the hill.

The point I was trying to make earlier is that if you go up and down the hill at the same speed in the same gear, the consupmtion is the same as if you drove over the flat (until the hill gets steep enough to accelerate the car in gear with the 'throttle' closed.)
Walter
 

cartog

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Re: An \'Ernie\' trip - 75 mpg...

Just from playing a bit with a couple meaningful hills on my commute, I would think we need to crest the ideal hill with enough fuel to idle for the 20 hrs or so we would expect to coast down. It wouldn't be fair to just shut the engine off


I've been leaning towards the coast-in-neutral camp after seeing that the same smallish hill which just allows me to keep speed (60 mph indicated) while overrunning will accellerate the car from 40 mph to almost 70 mph and coast on through the low-angled foot of the slope as well as a while beyond before I have to engage the clutch again at 60. Thus, the downward slope which would keep the car going in gear is much steeper than the one which works in neutral (at a given speed). I would expect that even limiting your maximum height due to the need for fueling all your idling time, you come out ahead with the much more favorable 'glide ratio'.
 

gdr703

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Re: An \'Ernie\' trip - 75 mpg...

- you've dusted me the last couple mpg contests!
I'm still trying to catch up with you. I seem to recall you did a trip Jax Fl to Richmond Va a little while ago, that was 73.22 mpUSg - 1134.9 miles on 15.5.
This latest trip of yours is almost up to the same [wow]standard!

cheers!

Now i go away and look for thinner oilz, and better gearz.
 

Fix_Until_Broke

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Re: An \'Ernie\' trip - 75 mpg...

Can someone (maybe volutering myself here) put together a model of this in Simulink or Easy5? There are far to many variables to account for "mentally". I would keep it very simple, with only the most basic variables (vehicle speed, drag coefficient, BSFC of engine, engine speed) at first. More variables can be added as confidence in the model grows and real (or simulated) scenarios are presented. For example various grades can easily be input in a model that otherwise gives believable results on level ground.

In all modeling, the first rule is that all of the results are wrong, and the second rule is that your assumptions (known and unknown) define the validity of the model. But, at least it would be something to work from and build on that can be more easily evaluated by our peers. No offense to anyone, but there are a lot of theroies and spreadsheets floating around (mine included) that may be creating more misconceptions than they are curing. If a group of open minded people can develop and criticize a model in theory, we have a large number of test vehicles/data to support or disprove theories (and people willing to do it), some hard (as it gets anyway) evidance and theory can be presented. This will lead in to more ideas and round and round we go.

Anyone game?
 

whitedog

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An \'Ernie\' trip - 75 mpg...

I always thought it was 32Ft/sec^2.
Fill in the blank: My ______ is longer than your foot!
I thought they were the same length? *glances at foot* nevermind.

Now back to this pretty cool, mileage-geek thread.

It appears that someone has opened up a very nice can of worms here. Lots of well thought out posts (other than mine) and lots of theories. My favorite is the part about all modles are wrong since they are based on assumptions. As that member mentioed though, there are lots of folks here with enough interest in this to get some facts to replace the assumptions. My question at this point is what facts would need to be gathered and what suggestions do we have for gathering that data? Certainly we can't build the perfect hill but working with what we have, I believe that some information could be gathered.

So, Suggestions?
 

Fix_Until_Broke

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Re: An \'Ernie\' trip - 75 mpg...

Again - lets keep it simple to start, an engine working against a constant load on level ground. We can/will add more compexity (rolling resistance, acceleration, grade, engine load, etc). We can model the perfect hill even if we can't build it.

Getting way ahead of myself here, but for model validation and or model inputs, maybe someone could log vag com data of engine load, fuel delivery and vehicle speed while driving on a known stretch of road which could be plotted with a GPS to record distance and altitude. Integrating vehicle speed to get position we can synch/align this data with the distance and altitude to get fuel consumpton due to acceleration, grade, etc.

All easy to talk about - the work is in executing it.
 
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