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Re: Ford Focus EV

Posted: Sat Jan 08, 2011 9:31 pm
by EVDRIVER
drees wrote:
evnow wrote:A 10% weight increase will roughly decrease efficiency by 5% (don't have link).
Impossible to make generalizations like that without knowing the drive cycle...

In the city weight can make a big difference. On the highway, it can be nearly negligible.

On hills it's a killer.

Re: Ford Focus EV

Posted: Sat Jan 08, 2011 9:56 pm
by AndyH
EVDRIVER wrote:On hills it's a killer.
I understand that you're against weight - and agree. And yet, if we take a Leaf that can run 100 miles and THEN add 300 lbs, there's going to be a performance hit. But if we take a Leaf at it's weight and 100 mile range, and put it along side the Focus and it's 100 mile range, then the weights don't seem that significant at that point.

Sure - more weight means more structure and more battery and more power and larger motor, I get the gist of the design spiral and the desire to 'add lightness'. But is it really accurate to take two fully developed cars with apparently similar performance and single out weight?

What am I missing, EVD?

Re: Ford Focus EV

Posted: Sat Jan 08, 2011 10:05 pm
by evnow
drees wrote:
evnow wrote:A 10% weight increase will roughly decrease efficiency by 5% (don't have link).
Impossible to make generalizations like that without knowing the drive cycle...

In the city weight can make a big difference. On the highway, it can be nearly negligible.
True. But this is to keep things in perspective. A 10% weight increase will not decrease the range by 50% for sure.

Anyway, when EPA / Ford publishes some range info we will know.

Re: Ford Focus EV

Posted: Sat Jan 08, 2011 10:24 pm
by EVDRIVER
AndyH wrote:
EVDRIVER wrote:On hills it's a killer.
I understand that you're against weight - and agree. And yet, if we take a Leaf that can run 100 miles and THEN add 300 lbs, there's going to be a performance hit. But if we take a Leaf at it's weight and 100 mile range, and put it along side the Focus and it's 100 mile range, then the weights don't seem that significant at that point.

Sure - more weight means more structure and more battery and more power and larger motor, I get the gist of the design spiral and the desire to 'add lightness'. But is it really accurate to take two fully developed cars with apparently similar performance and single out weight?

What am I missing, EVD?

This is a classic trap, comparing a heavy EV to a very heavy one. Besides aerodynamics weight is a HUGE factor in efficiency. Anyone wonder why all the major auto makers are racing to cut weight to make the future EPA numbers? In an EV this makes even more of an impact. If you drive in an EV with a respectable weight of say 2700 lbs and watch actual kw usage and then add weight in 100 lb increments under the same conditions you will become very aware of the impact in efficiency and in your foot. Put a hill in the mix and is exponential. This and aerodynamics is what it is all about in an proper Ev design. The pack is a separate issue on its own, just look at why so many converters use the lowest weight car available, why Tesla builds the way it does.

An EV is FAR more sensitive to fluctuations in efficiency than an ICE because of the available energy storage density between gas and our present battery tech. This is only evident when people go up hills or see detailed consumption data while driving and when they see the difference in weight variables. The mods people made to cars for aero gains make even greater strides on an EV. Even the viscosity of the gear oil in an EV can make an impact on range, also the cut on the gears. The leaf gears are set for noise and compromise on efficiency, these things do make a difference particularly when added together. If you own a compact car, add 300 lbs and see how it does up hills and how the economy changes while driving the same on hills and on the city, the rules don't change because the car is already heavy.

Re: Ford Focus EV

Posted: Sat Jan 08, 2011 10:28 pm
by evnow
But low weight or very low cd don't come free. There are always compromises to be made.

Re: Ford Focus EV

Posted: Sat Jan 08, 2011 10:32 pm
by smkettner
We need the price point of the Focus to see if it compares

Re: Ford Focus EV

Posted: Sat Jan 08, 2011 10:44 pm
by TimeHorse
EVDRIVER wrote:Put a hill in the mix and is exponential.
No, linear:

Code: Select all

y = mx + b
So just drive as is up the hill at a fixed speed and measure kW; then add 300 and measure the same run's kW. y is your kW, x is your weight (say x=0 corresponds to curb weight) and compute b and m; you then know what any weight increase will do because w/o acceleration, the penalty's linear.

Code: Select all

U = mass * g * x * sin(theta)

Re: Ford Focus EV

Posted: Sat Jan 08, 2011 11:45 pm
by smkettner
Does not the extra weight get recovered somewhat on the way down?

Re: Ford Focus EV

Posted: Sun Jan 09, 2011 12:04 am
by evnow
smkettner wrote:Does not the extra weight get recovered somewhat on the way down?
Yes. Infact Ford was boasting "upto 95%" regen recovery.

Re: Ford Focus EV

Posted: Sun Jan 09, 2011 4:39 am
by Smidge204
evnow wrote:ps : Timehorse, do you have links to how EPA calculates EV range ? I know they are trying to change stuff - not sure what exact method they used.
For what it's worth, I put in a FOIA request for Leaf/Volt testing details just before Christmas. I got an e-mail on Dec 23rd confirming they'd received it but haven't heard anything else yet. Part of the request was the specifics on the test protocol used. Naturally I plan to make a topic about it once I get an answer... until then I decided it's not worth getting in an argument about.

=Smidge=