EV Design: Wind resistance, etc

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arsharpe
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EV Design: Wind resistance, etc

Postby arsharpe » Sat Jan 03, 2009 9:45 pm

[This was placed here as it relates to general design.]

Maybe you can help understand something about mpg and speed.

As we did a long run in the Prius the other day and the wife was a sleep I took the opportunity to do some tests.

a) Reset the MPG counter
b) Set and maintain a given speed for between 13-26 miles.
c) Read off the MPG

The readings are center values of possible final variation and are worst case +- 2 mpg.

Code: Select all

MPH    MeasuredMPG  Curve[1]
----------------------------------------
55      63          63.00
60      61          61.22
65      58          59.62
70      58          58.18

[1] This is the calculated (using the above equation) and the adjusted (for hilly routes).


I fitted the following curve

Code: Select all

mpg=(speed0/speed)^(0.33)*mpg0
which fits nicely and is relatively "flat".

From my flying days air drag is at least a squared function maybe even a cubed, not a 0.33 :o

I also did some informal (undocumented) tests another time (maybe on another vehicle) that gave similar results.

Can you anyone tell me were I am going wrong ?

microman
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Postby microman » Sun Jan 04, 2009 11:26 am

You don't have sufficiently reliable data to do any curve fitting. To get meaningful results you need to drive boringly round and round a standard level test track with near zero wind and constant ambient temperature.

Under real driving conditions you would need hundreds/thousands of miles to average out wind, temperature and gradient effects and you can't do that at constant speed.

If you look on the Yahoo Prius forums you will find real data obtained by the painstaking, boring method. Or you can have an armchair version with the simulator on http://privatenrg.com/!


Brian Stanier

(2005 T3, 55.4 lifetime mpg over 3+ years)

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Jeremy
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Postby Jeremy » Sun Jan 04, 2009 9:15 pm

Power required (and hence fuel consumed) is proportional to the speed cubed, in terms of pure aerodynamic effect, but there is far more to this with a road vehicle.

The other factors will be surface wind speed and direction variation, air temperature, and, for a hybrid, the specific charge current/battery power contribution at any instance. Rolling resistance also changes markedly with varying road surface friction coefficient and speed, which in turn depends on tyre and road surface temperature and dampness

All told, you need a lot of data points to get a meaningful plot. Here's a plot of the consumption of my Prius over a period of about 2 1/2 years and 50,000 miles. Journeys were mainly an 80 mile a day commute. The effect of air temperature is very clear, greater than just about every other effect in fact.

Image

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retepsnikrep
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Postby retepsnikrep » Mon Jan 05, 2009 8:11 am

I concur fully with Jeremy on the above.

Although I don't have the nice graph to back it up I can say from my own experience in another hybrid that air temp makes upto 20% difference to fuel consumption.

It's certainly the most noticable factor during winter/summer :wink:
Regards Peter

Two MK1 Honda Insight's. One running 20ah A123 Lithium pack. One 8ah BetterBattery Nimh pack.
One HCH1 Civic Hybrid running 60ah A123 Lithium pack.

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qdos
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Postby qdos » Mon Jan 05, 2009 9:01 am

Image

Jeremy, your graph is excellent it appears to show the same shape pretty much 3 times over so I assume this demonstrates the annual change in seasons both in terms of weather and traffic.

However it's very clear that something is in decline. Is this just wear and tear on the car. eg the bearings, tyres and engine? Or is the climate really changing that much !!!! :shock:

microman
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Location: Durham

Postby microman » Mon Jan 05, 2009 9:23 am

Jeremy, I notice you have an apparent degradation too. I see more annual variation (shorter journeys than yours and probably colder here too) from low 50s now to low 60s in mid summer, but no significant change over three years - maybe a slight improvement. Odd. Or have you been driving more enthusiastically of late?

ChrisBarron
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Postby ChrisBarron » Mon Jan 05, 2009 9:45 am

Jeremy wrote:All told, you need a lot of data points to get a meaningful plot. Here's a plot of the consumption of my Prius over a period of about 2 1/2 years and 50,000 miles. Journeys were mainly an 80 mile a day commute. The effect of air temperature is very clear, greater than just about every other effect in fact.

Image


Is it really only temperature which causes such a dramatic effect ?

I was of the opinion that a lower air temperature imporved internal combustion efficiency.

It seems to me that in winter time not only is the temperature lower but we spend more time running more accessories, whilst driving on wet roads. The accessories, such as fans and rear screen demisters all add up to a couple of mpg, but more importantly I think the effect of driving on wet roads with their higher rolling resistance, as well as the 'thicker' air due to rain drops plays an important part too.

Although, I'm happy to concede that this is the British Isles and we get lots of rain in the summer months too !

ChrisBarron
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Re: EV Design: Wind resistance, etc

Postby ChrisBarron » Mon Jan 05, 2009 9:53 am

arsharpe wrote:[This was placed here as it relates to general design.]

Maybe you can help understand something about mpg and speed.

As we did a long run in the Prius the other day and the wife was a sleep I took the opportunity to do some tests.

a) Reset the MPG counter
b) Set and maintain a given speed for between 13-26 miles.
c) Read off the MPG

The readings are center values of possible final variation and are worst case +- 2 mpg.

Code: Select all

MPH    MeasuredMPG  Curve[1]
----------------------------------------
55      63          63.00
60      61          61.22
65      58          59.62
70      58          58.18

[1] This is the calculated (using the above equation) and the adjusted (for hilly routes).


I fitted the following curve

Code: Select all

mpg=(speed0/speed)^(0.33)*mpg0
which fits nicely and is relatively "flat".

From my flying days air drag is at least a squared function maybe even a cubed, not a 0.33 :o

I also did some informal (undocumented) tests another time (maybe on another vehicle) that gave similar results.

Can you anyone tell me were I am going wrong ?


I've just picked this up from another Prius owner who has studied his mpg over the years, and might be are the sorts of 'other factors' hinted at by Jeremy.

It seems also that the time between trips affects the Prius due to the way it stores hot water for later use, although on a long run like yours this should be a moot point

Effects on mpg

short trips ( under 10 min ) : -20%
cold weather driving : -10%
rain storms : -15%
into the wind : -10% ( varies by severity )
with the wind : +5%
highspeed driving ( > 70 mph ) : -10%

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retepsnikrep
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Postby retepsnikrep » Mon Jan 05, 2009 10:19 am

ChrisBarron wrote:I was of the opinion that a lower air temperature imporved internal combustion efficiency.


Chris it depends what you mean by efficiency :wink: ASFAIK the opposite is true, lower temperature increases charge density and you get higher power at cost of increased fuel consumption. That's what some people want, more power!

Higher intake temps increase the fuel combustion effciency at cost of reduced power. That's what I want, :) more mpg

I use a hot air intake on my insight to ensure temp is warm. This means engine can run in lean burn more often at a higher effciency.

The power you develop though has to be enough to propel vehicle at your choosen speed.

ASFAIK european Prius do not have the thermos bottle. Only Canadian and some american states I believe. I actually bought one from a scrap car and had it sent over, I intend to install it on my own hybrid to gain a few extra mpg.
Regards Peter

Two MK1 Honda Insight's. One running 20ah A123 Lithium pack. One 8ah BetterBattery Nimh pack.
One HCH1 Civic Hybrid running 60ah A123 Lithium pack.

ChrisBarron
Posts: 244
Joined: Tue Dec 11, 2007 11:26 pm

Postby ChrisBarron » Mon Jan 05, 2009 10:52 am

retepsnikrep wrote:
ChrisBarron wrote:I was of the opinion that a lower air temperature imporved internal combustion efficiency.


Chris it depends what you mean by efficiency :wink: ASFAIK the opposite is true, lower temperature increases charge density and you get higher power at cost of increased fuel consumption. That's what some people want, more power!


I believe that you don't have to use the power just because there is the potential to create it, and therefore a smaller throttle opening can be used to create the same power with colder air than for a wider opening and a hot air intake.

I've always been led to believe that the colder air means more oxygen, which means fewer pumping losses and that means increased efficinecy for any given power output.


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