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Discussion Starter #1
Different cars with their own C-drag, tires etc. have a sweet spot where they get the best fuel economy in the real world. It would make sense that with the .26 C-drag, the sweet spot might be 60-65 mph rather than 55 or lower. It could be anywhere from 40-65, I'd guess.

Does anyone know this (more than speculation)?

A Japanese owner made a good point--although many of us love 'stealth' mode, in fact, being powered mostly by the engine is more efficient than using the engine to generate electricity (with losses) and then using the electricity to move us (with more losses). His point was that accelerating with the engine was more fuel-efficient than straight electric-only acceleration. Interesting if true. This could affect how I drive to maximize overall fuel economy.

One way is to view the electric part as a way to balance and use the engine power more efficiently, rather than as the main mode of powering the wheels. We shouldn't be all-electric prissies, but instead pragmatists. The Prius has a great overall cost/efficiency/room/performance/extremely low total pollution combination that should be appreciated for itself.
 

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Hi Mel,

Dan at Yahoo Prius-2G group posted his test results...
http://autos.groups.yahoo.com/group/Pri ... ssage/4945

At 45 degree F,
- 38 MPH, 72.2 MPG
- 35 MPH, 77.9 MPG

At 73 degree F,
- 35 MPH, 93.6 MPG

The test was done at flat road.
But, you will be able to achieve better mileage on slight up and down hill road.
I think the Route 1 - Pacific Coast Highway in your area is good place to try your own test.

Enjoy,
[email protected]
 

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Discussion Starter #4
I'm not sure I trust the Prius mileage calculation onscreen

Thanks to Ken for the link. After reading it all, it appears that the numbers are based on the Prius' display numbers, rather than having done a full tank test. The simulation is interesting, but it's not clear that it takes conversion inefficiencies into account. I'd like to see the A/C on/off comparison. The best test of the simulation would be a test track full tank run at one of the speeds, but who's got a test track and all day to do it?

I'm not sure I trust the Prius display numbers as very accurate. The power sharing means that much of the time, you are putting some power into the wheels, and some into the battery. If you run in 'stealth' or all electric mode, does the mileage factor in the losses from generating and then using the electricity? That might inflate the apparent results, but not the real ones.

I'd be interested in comments from others on what they know of how the Prius comes up with the display numbers, and how accurate they are.
 

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Hi Mel,

The display value is relatively accurate, less than a few percent and basically no dispersion. Dan did the test with his own car and the relative data is very accurate. Maybe there are some dispersion car by car.

I think a full tank test by constant speed is not practical. (1,000 miles drive)
His test starts and ends with same battery level, so the result is still very vailable.(Please follow the threads on the link.)

Regards,
[email protected]
 

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Re: I'm not sure I trust the Prius mileage calculation onscr

hinalea said:
...I'm not sure I trust the Prius display numbers as very accurate...I'd be interested in comments from others on what they know of how the Prius comes up with the display numbers, and how accurate they are.
Well, the Prius display numbers only need to be accurate when compared to other Prius display numbers on the same car. This may sound confusing, but follow me for a moment, and all will be revealed.

If my MFD says 45.5 MPG, and in reality I only got 44.1 MPG, it doesn't really matter all that much. As long as when my MFD says 48 mpg, it's better than when the MFD said 45.5 MPG. So I can still use the MFD to see what type of driving is most conserving.

"Huh?" you say.....

The MFD is telling me all I need to know about how to get the "best" mileage as long as the MFD is accurate with itself under all conditions (uphill/downhill/AC on/off, etcetera).

Um....er..I know I did an awful job of explaining that. Someobody help me, please?
 

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Discussion Starter #7
Clarifying my question

What I'm asking for, BIF, is if anyone knows whether the display 'mileage' number factors in losses due to electric generation/conversion or not, and whether it is just estimating fuel consumption or actually accurately measuring it.

In the long haul, you can tell the overall average mileage as miles driven divided by gallons consumed, and that is certainly accurate (assuming you can tell when the gas tank is 'full', which seems especially hard for the USA Prius). It's just that my 'instant' display bounces all over the place, depending on whether you're on ICE or electric or blend, and leads me to doubt how useful it is.

So my specific technical questions are:

How does the Prius measure fuel consumption? (Estimate or fuel line measured?)
How accurate is this?
How does the Prius arrive at the current mileage number in the display when you're in mixed mode (some ICE to wheels, some to battery)?

(I realize that this is pretty technical, and may not be known outside of Toyota)
 

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I think that we should:

1) treat the "instant" as an estimate.
2) treat the "consumption" figure as a less-volatile (but probably not anymore accurate) estimate, but still just an estimate.
3) treat our calculated (at tank fillup time) number as a somewhat more accurate estimate (but still imperfect), unless major temperature variances have happened between fillups, in which case, it's a potentially less-accurate estimate.

No matter what we do, we're still taking a SWAG. I don't need to be anal-retentive in any of my recordkeeping, so I can live with it.


(S.W.A.G. = Silly, Wild-Assed Guess)
 
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Hi Hinalea:

___You might be mixing up efficiencies of the individual components vs. efficiency of the system (automobiles fuel consumption) as a whole? I see this in other forums as well with statements like my car gets its best mileage at such and such a speed (usually in the 60s) and this is absolutely and total BS. The instantaneous is where you can see real time fuel economy and with any number of automobiles, a low RPM in the highest gear at the lowest possible speed and lowest possible load (no ascent) usually gives maximum fuel efficiency. This is in contrast to an ICEs best thermal efficiency at a given RPM and/or intake air temperature, highest % of power transfer of the CVT at a given RPM or load, Pack current transfer at a given temperature or demand, or any other number of power and fuel consumption variables from any and number of components that have a direct correlation to an automobiles fuel economy In other words, the Prius fuel economy at the various speeds discussed in this very thread whether estimated from the instantaneous or calculated tank over tank is probably so close to actual that you can call it factual real world data. Drive an 04 Prius at 32 - 35 mph with no wind, on hard flat surface, with temperatures in the 80s or 90s and you should see numbers approaching 100 mpg. A properly setup Insight maxxes out its instantaneous at 150 mpg under these conditions as well but this is not sustainable over time given the load is continuously changing with road surface hardness and condition, elevation changes, real world highway speeds, wind, temperature, RH, your ability to control the throttle position in the smallest of increments to match the most efficient RPM/load/fuel efficiency and so many other external variables I couldnt even being to imagine. In other words, it doesnt matter how efficient a particular component is at a given place on a curve, it all comes down to the entire system (an automobiles actual fuel economy w/ distance traveled/fuel used) as to where the best efficiencys appear and I can bet it is probably in the 30 - 35 mph range w/ the ICE running and minimal draw off the pack for the 04 Prius as well? If you use A/C, your fuel economy will fall.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___[email:3aydze7z][email protected][/email:3aydze7z]
 

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Mel,

I'm not BIF nor Toyota personnel, but I would like to comment...

> How does the Prius measure fuel consumption? (Estimate or fuel line measured?)

The engine ECU controls the fuel injection timing and duration. The fuel consumption is proportional to it's duration. The Skid Control Computer knows the tire rotation. Then, the MFD can calculate the mileage from their data.

> How accurate is this?

It is about 3%. Please refer to...
http://eshy.s22.xrea.com/cgi-bin/nenpi/ ... %83I%83%93
(Sorry in Japanese, but you can read numbers, or Babel Fish site will help you for translation.)
Please note that the dispersion is very limited.

> How does the Prius arrive at the current mileage number in the display when you're in mixed mode (some ICE to wheels, some to battery)?

It just calculates the mileage from it's fuel usage and miles driven during a certain period. It displays 99.9 MPG (max number) if the engine does not run.

Hope this helps, but poor English...
[email protected]
 

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Discussion Starter #11
Good comments

I appreciate the comments; Ken's have been especially useful.

In the long run, of course, it doesn't matter. In the long run, nothing matters :) In the short run, the instantaneous mileage readout is practically meaningless, as far as I can tell--bouncing from 26 to 48 to 62 to 81 to 99 from moment to moment. The only one that seems to mean much is the trip fuel mileage calc, averaged out over a long period. That might be pretty close, and shed some light on the real world fuel average. It would appear to be higher than Wayne G. speculates above, maybe in the 50s or better at 35-40 mph, if the calcs Ken refers to are halfway accurate. Why Wayne hangs around the Prius site when he's such a big Honda fan, however, is a mystery. The Honda sounds like a fine, very fuel efficient car if you can get by with its limitations. For those who can't, the Prius is a great alternative.
 

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Re: Good comments

hinalea said:
....Why Wayne hangs around the Prius site when he's such a big Honda fan, however, is a mystery....
hee hee...maybe he's secretly on a waiting list for a Prius!

:p
 

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:roll:
How the Prius calculates instantaneous fuel economy. I'm not sure how the Prius does it. But it probably uses one of the two following methods.

Method one: Fuel consumption is measured by inline positive displacement fuel flow meter. So if the computer calculates fuel rate of consumption. Lets say 1 gallon per hour. And if the vehicle is traveling at 50 MPH. We can now calculate that the car is getting 50 mpg.

Method two: Fuel consumption is calculated by the computer. When a fuel injector fires it delivers fuel to the engine. This is done at a constant flow rate when the fuel injector is on. To very the amount fuel being delivered to the engine the computer regulates the time the injector is on. Since the computer knows the fuel flow rate of the injectors and how much time the injector is on. The computer can now calculate fuel rate consumption. Since we now know the fuel consumption rate, and speed the vehicle is traveling we can now calculate the MPG.

:idea:
Method two is cheaper to do, since fuel flow meter cost a lot.
 

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Discussion Starter #15
Fuel flow calcs

Useful information! If I understand this right, then you can reset the fuel calc at fuel up, and you can derive what the car thinks it consumed in gallons since then by dividing the miles travelled indicated there by the mpg. Isn't algebra wonderful!

I'm going to try seeing how well that correlates with the gas I put in the tank. It would be interesting to move from ten little bars on the fuel display to an idea of just how much gas is still in there. I like to know if I must go out of my way to gas up near the bottom, or can squeeze out another day without the dreaded, catalytic converter wrecking running out of gas event. Or if I'm 120 miles from gas, what are my chances?

I owned a '58 VW bug as a student in Germany that had no gas gauge at all. You ran it until it ran out, and then flipped a little lever that dumped a small reserve gas supply in, who knows, maybe a few liters, but enough to last you till you could gas up.
 

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Question on optimim speed for high mpg

I have developed a simple physics model for Prius gasoline consumption vs. speed, and using a drag coeff of 0.26, frontal area of 2.5 meters squared, engine thermodynamic efficiency of 40%, and a rolling resistance of 0.0105, I get the following mpg:

30 mph 80 mpg
35 78
40 75
45 71
50 67
55 62
60 57
65 53
70 48
75 44

How does this agree with actual measurements under ideal conditions?

PriusPhysics
 

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PriusPhysics,

Your math is pretty good based on my experience. I recently drove my 2004 Prius for about 30 minutes to warm it up and then did a mpg experiment on a very level stretch of interstate between Baton Rouge and Hammond, Louisiana. I had A/C and lights, radio, etc., off and started with the battery at 2 bars from top. I drove with the cruise control on for several five-mile stretches at different speeds for each stretch. I reset the mpg readout between each five-mile stretch. I had 92 octane gasoline in my car for this experiment. The following is what I observed. The numbers are very close to yours.

MPH MPG from readout
50 67.5
55 62.3
60 57.0
65 52.5
70 50.0
75 46.5
80 39.0

I wished I had used regular 87 octane which is what most people use. Oh well, I am about to repeat the experiment with 87 octane in the next day or so.
 

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More on physics model for optimum mpg

Below are listed the calculated mpg for the 2004 Prius using the physics model with input parameters below:

The physics model for calculating mpg assumes
drag coefficient = 0.26
frontal area = 2.5 square meters
air at sea level, 68 deg. F
rolling resistance coefficient = 0.0105 (from Green Seal Inc. report)
gross vehicle mass = 1500 kilograms
accessory power = 2000 watts (guess)
engine thermodynamic efficiency = 0.4 (wow!)
drivetrain efficiency = 0.90
gasoline energy = 1.28E8 (128,000,000) joules per gallon

Results
25 mph 79 mpg
30 80
35 78
40 75
45 71
50 67
55 62
60 57
65 53
70 48
75 44
80 41

For example, at 70 mph, the power required is
air drag power = 13,278 watts
rolling resistance power = 5,372 watts
accessory power = 2,000 watts
total power = 20,650 watts


Many of the constants in the Excel spreadsheet are empirical guesses. Anyone who has mpg vs. mph measurements that can be used to improve the calculations should post them. In any case, the Prius performance is very close to the maximum thermodynamic limit.

[email protected]
 

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Discussion Starter #19
Test request

Buggman--

Thanks for your very careful and interesting test.

Could you monitor approximate headwind/tailwind next time, tire pressure cold before you start, and if time allows, on one of the speeds, like 55 or 65, try air conditioning on, noting outside temp and inside temp setting? I imagine Priusphysics could approximate the aircon power consumption.

I recently upped my cold tire pressure to 40 front, 38 rear, and am trying to see how it affects my mileage. However, as my car is driven under mixed conditions, that is not easy to separate reliably.
 

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Re: Test request

hinalea said:
Buggman--

Thanks for your very careful and interesting test.

Could you monitor approximate headwind/tailwind next time, tire pressure cold before you start, and if time allows, on one of the speeds, like 55 or 65, try air conditioning on, noting outside temp and inside temp setting? I imagine Priusphysics could approximate the aircon power consumption.

I recently upped my cold tire pressure to 40 front, 38 rear, and am trying to see how it affects my mileage. However, as my car is driven under mixed conditions, that is not easy to separate reliably.
Hinalea,

I forgot to add that there was no wind that day that I remember. I did not notice any when I had gotten in the car about 30 minutes earlier. Also, my tire pressures were 39 psi front and 37 psi rear. I tried the half way point between the 35/33 (front/rear) psi dealer recommendation and the 44/42 psi recommendation from some of the posters.

Does the 44/42 psi settings make the readout mpg higher or lower than reality? The real mpg would go up, but I guess the readout mpg might stay close to the same. Rolling resitance will decrease but computed miles will be less than reality. Does this make any sense? Someone clear this up.
 
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