If you have an interest, I can work up a spreadsheet. The independent variables are:

1) altitude change (feet or meters)
2) distance of climb (miles or feet)
3) combined vehicle and passenger weight (lbs or kg)

Excellent data!! Now we have some empirical facts and a baseline.

I too didn't think to check the battery state of charge level but the MFD is so crude. Still, it would be better if we had the battery SOC to understand what it contributed.

I suspect you are seeing the infamous 'variable sized' tank problem. If you check my mileage, always pump calculated, you'll notice there is considerable variance.

My last test showed no improvement in stability of the calculated fuel efficiency. I changed my fill-up protocol to: (1) fill the tank, (2) drive 3-5 miles reaching a minimum of 50 mph, (3) pulled in to top off the tank, and (4) treat the two as a single fill-up. However, the MPG variability remained.

My next test will use a low-pressure gage to measure the pressure in the space between the bladder and the tank. If filling up causes an increase in pressure, this would 'shrink' the effective tank size. The question then is how can we get those fumes into the carborator to reduce the pressure? BUT THIS IS AN UNTESTED HYPOTHESIS.

Bob Wilson

 

138 feet over 0.4 mile. The "curb" weight is listed as 2,950 lb + 190 lbs = 3,140 lbs.

Regarding the fill-ups. I filled up the first 2 tanks with 1 bar remaining. I filled up the last 2 tanks with 3 bars remaining. Could this cause the bladder to behave differently?

 

Hi,

The only data that seems anomolous is your 45 mph run. However, I also think there may be some other factors, temperature and tire pressure, that could change the results:

Engineering Contants Value Units
feet/meters 3.28084 ft/m
pounds/kg 2.20462 lbs/kg
gas J./gal 121,000,000 Joules/gal
Watt/hp 746 W-hr/hp
gravitation 9.80665 m/s/s

Value Units
Altitude 138 feet
Distance 0.4 miles
Weight 3,140 pounds
Energy 587,504 Joules

Speed MPH MPG Gal. Gas Joules Effic. Wheel HP Sec. J/sec.
40 18.8 0.021277 2,574,468 22.8% 21.9 36.0 16,319.5
45 19.6 0.020408 2,469,388 23.8% 24.6 32.0 18,359.5
50 19.1 0.020942 2,534,031 23.2% 27.3 28.8 20,399.4
50 18.5 0.021622 2,616,216 22.5% 27.3 28.8 20,399.4
55 16.9 0.023669 2,863,905 20.5% 30.1 26.2 22,439.4
60 15.4 0.025974 3,142,857 18.7% 32.8 24.0 24,479.3

Altitude 525 feet
Distance 1.5 miles
Weight 3,000 pounds
Energy 2,135,416 Joules

35 22.4 0.066964 8,102,679 26.4% 18.6 154.3 13,840.7
45 22.5 0.066667 8,066,667 26.5% 23.9 120.0 17,795.1
55 20.8 0.072115 8,725,962 24.5% 29.2 98.2 21,749.6
65 18.7 0.080214 9,705,882 22.0% 34.5 83.1 25,704.1
70 18.8 0.079787 9,654,255 22.1% 37.1 77.1 27,681.3
75 20.0 0.075000 9,075,000 23.5% 39.8 72.0 29,658.6
75 19.5 0.076923 9,307,692 22.9% 39.8 72.0 29,658.6
80 20.9 0.071770 8,684,211 24.6% 42.4 67.5 31,635.8
80 20.9 0.071770 8,684,211 24.6% 42.4 67.5 31,635.8

I would recommend re-running your 45 mph test. Also, my tires were at a cold pressure of 53 psi. I'm still 'tuning' my new tires so results are likly to change. Also, need to check the oil level. My morning test runs were close to 70F but the afternoon test at 80 mph was much warmer.

I'll try to attach a link to the spreadsheet so you can easily recalculate as needed. This may take a few minutes to work.

Bob Wilson

 

Although I was a biology teacher in my other life (i.e. before I retired), I think all the experiments you guys set up are kinda neat. However, I don't see much value (except as an intellectual exercise - and nothing wrong with that). I tend to look at the results on a tank by tank basis.

What I have noticed is that, even over the same route, mpg changes. But what I would first check is tire pressure. I don't know if pressure decrease in hyperinflated tires would be more detremental to FE than a similar (percentage-wise) drop from tires at recommended pressure.

That said, I've improved my FE tank-over-tank, and I haven't checked my tires in 2 months. Not very smart, I agree. But, there you have it.

Who ever suggested turning off the A/C - well I've done that. Easier to do in the morning or evening than during the day, but it certainly helps.

centrider

 

Hi,
To the extent that any trip is a series of shorter segments strung together, these experiments let us manage our speed on different segments to optimize MPG. It is a technique common to calculus.

I'll be driving 750 miles to Washington DC in October which includes crossing the Smokies. What I'll do is adjust my up-hill speed to 50-55 mph, finding a nice truck to follow. Then on the back side, back to 65 mph. I'm expecting to get better than 52 MPG for the 1,500 miles, round trip.

GOOD LUCK!

Bob Wilson

 

I noticed that if I can find the, "sweet spot" on any segment of a trip I make weekly, and I can hold it over hill and dale - my mpg is between 55-99 mpg. The problem is that, on the next trip, conditions change (traffic, the Prius god - who knows), I can't hit that magic spot.

Even within my neighborhood, 1 block or so from a supermarket to my house, sometimes I coast along at 99 (on battery), and at other times I won't exceed 20 mpg (engine working hard), the latter beyond my control.

Even with all that, over a 100 miles driven last Sat. I still avg 51 mpg, which ain't hay and without, as I indicated, hyperinflating my tires, and pretty much let it roll at 64 mph on cruise control.

Oh well, I still enjoy reading your experiments, and I still pick-up tidbits here and there from you.

Thanks,

centrider

 

There are non-linear effects as shown by my fillup MPG performance:

But inspite of the non-linear fuel tank, there are reproducable performance rules. All it takes is study and practice.

It takes study but over time, one learns what triggers poor MPG and what avoids it. But the lessons never end.

That is the goal. We'll probably never fully understand how to achieve the limits of these vehicles. But we get snippets and experiments are how we find what works.

Bob Wilson

 

I did the run again at 45 mph and got 19.6 mpg, a considerable improvement.

 

Excellent! Once again, a chart shows the data:


It looks like the optimum climb speed for efficiency is 50-55 mph where we see the 'knee' in the curve. But time is also important and 55 mph provides a 10% time savings.

For the NHW11 Prius, it looks like things change around 65 mph with an apparent improvement in fuel burn on the way up. However, we know it starts drawing more traction battery power above 65 mph and the short-term hill advantage is probably erased by the lower MPG on the backside as the battery is recharged.

Bob Wilson

 

Hi Bob,

Thank you for the nice report.
I believe the above energy is based on mgh potential energy.
I think you also consumed other energy against the driving resistance moving hirizontally.
As I posted at yahoo technical group, one of the approximate value of driving resistance for NHW-11 Prius is...
N = 190 + 0.42 * V ^2
http://homepage.mac.com/inachan/prius/current.html
So, it becomes...
Speed MPH MPG Gal. Gas J-used J-hiriz J-total Effic.
35 22.4 0.06696 8,102,679 706,015 2,842,031 35.1%
:
80 20.9 0.07177 8,684,211 1,754,501 3,889,917 44.8%

Where did you get the gasoline usage value?
I think we need longer distance and more fuel usage to get more accurate numbers.

Ken@Japan