From: gary@ke4zv.uucp (Gary Coffman)
Newsgroups: sci.energy
Subject: Re: Electric Cars Energy Usage: Summary
Date: 10 Apr 92 00:24:17 GMT
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group

In article <43520143@hpindda.cup.hp.com> gph@hpindda.cup.hp.com (Paul Houtz) writes:
>A great deal is made of the following calculations, which basically try
>to indicate how many kilowatt hours per mile my car should use, vs
>my own evidence.
>
>In particullar, Mr. De Armond, Mr. Coffman, and Mr. Cage attempt to put 
>forth the notion that these calculations are based on undenyable physical
>principles, when in fact, there are a considerable number of questionable
>assumptions involved.

There are assumptions involved, whether they are questionable is something
else. Let's see if there is any basis for the assumptions.

>I can only go by my own results, however I would like to point out some
>of the inconsistencies in this posting:
>
>>gary@ke4zv.uucp (Gary Coffman)
>>In article <+vnh4=@dixie.com> jgd@dixie.com (John De Armond) writes:
>>>bg055@cleveland.Freenet.Edu (Stewart Rowe) writes:
>>>
>>>
>>>>This attempts to summarize the various postings on this and other
>>>>boards regarding the question: Do electric cars reduce the emission
>>>>of carbon dioxide compared with gasoline-fueled cars?
>>>> 
>>>>I. Power requirement of electric car:
>>>> 
>>>>Paul Houtz says his converted Ford Tempo goes 50 miles on an 8 KWH charge.
>>>>This figures to   ** 0.16 ** kwh/mile
>>>>John DeArmond quotes Kornhaus as saying Houtz uses
>>>>                ** 1.5 ** kwh/mile
>
>   Okay, let's take this figure.  1.5 kwh/mile.   This is what the 
>   anti-electric crowd would like you to believe as "reasonable".

No, it's the number *published* as the result of long term tests by
Southern California Edison, an outfit that *wants* electric vehicles
so they can increase their sales of electricity. Here's the citation:

 
                GM Gas  Electric        Chrysler        Electric
                Van     G-Van           Gas mini Van    TE Van
                ------  --------        ----------      --------
 
wheelbase   :   125"    125"            112"            112"
gross weight:   8600lb  8600lb          3975lb          5548lb
Cargo volume:   256cuft 256cuft         133cuft         133cuft
Payload     :           1800lb                          1200lb
range       :           60mi*                           120mi**
top speed   :           52mph                           65mph**
0-30mph     :           13sec                           7.5sec**
0-50mph     :                                           14 sec**
fuel efficiency: 10mpg  1mi/kWh         16mpg           2mi/kWh
energy consump : 14,400  10,800 Btu/mi   9000Btu/mi      5400Btu/mi
 
*simulated urban driving
**projected  
     
The above efficiency figures do not include losses of the charging 
system; another figure based on real operational data by SCE (Southern
California Edison), the G-Van, "During a seven-month demonstration 
period, each vehicle was driven an average of 1945 miles, and 
used 1.5 kWh of electricity per mile for charging".(p.6, Research 
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Newsletter, SCE, Vol 20, no. 1; 1st quarter 1991)  
 
-------------------------------------------------------    
                                                 
>  I travel on average about 32 miles per day in my car from Solar Electric.
>  I judge my kwh consumption by figuring the average amperage registering
>  from the charging ammeter over an 8 hour period, times the 8 hours, times the
>  110 volts from the outlet.

This is not a good assumption. Using charging data from the Gates Battery
Handbook, it can be seen that the rate of charge changes radically as the
battery specific gravity changes. This means that the charge current for
lead acid batteries is *constantly* changing during the charge cycle.
You really need watt-hour meter readings to confirm energy consumption
over the charge cycle.

>   For the sake of argument, however, I will use the "1.5 kwh/mile" figure
>   and do some calculations:
>
>    32 miles * 1.5 kwh = 48 kwh per day
>    30 days * 48 kwh per day = 1,440 kwh per month.
>
>   This figure is double what my normal PGE usage is, around 600 - 700 kwh
>   per month.   If I ADDED this to my former usage, the amount is nearly
>   2200 kwh per month.   This is what I SHOULD be seeing, if this figure is 
>   correct.   Is this what I am seeing?   Of course not.
>
>   Then what ARE my results.   The bill has increased (as near as I can make
>   out with varying usage from other appliances) about $10 to $22 per month,
>   or 280 - 310 kwh per month, and divided by 30, this comes out to around
>   5 - 8 kwh per day.   The calculation is not completely accurate, as I am
>   using $.07 for the rate, and it is actually slightly higher than this, 
>   but there is a discount for the first 330 kwh used, so the formula is
>   complex.
>
>   Obviously, however, from my direct experience, the 1.5 kwh per mile
>   figure is completely wrong, so far off as to be laughable.

It *is* the value for the long term tests done by SCE for a *van*. I've
said before that a passenger *car* should do better. See below.

>>>Thanks for the nice summary.  I just want to clarify one thing.  I am
>>>very uncomfortable with the power per mile figures we're using.  I 
>>>used the 1.5 kwh/mile figure only to be consistent with others' calculations.
>
>   Good for you.   It is obviously WAY off.
>
>>>Coastdown tests I've done on a few cars indicate that from 12 to 20 hp 
>>>are needed to maintain 50 mph on level ground.  This is also consistent
>
>   Yet I have seen figures in the 2 - 4 hp range for 50 mph.   So, I am
>   not clear on your reference.   Perhaps you could post a source for 
>   such numbers, then we could all be satisfied.

I've *never* seen numbers as low as 2-4 hp for a car at 50 MPH, except
going down a steep downgrade, or in a specially built steamliner like
the Sunracer. Why don't *you* post a source for this?

Anyone can do a coast down test, it's not rocket science. The procedures
and formulas are widely published. Two sources are the SAE Automotive
Handbook and the Bosch Automotive Handbook. All that is required is a
stopwatch and a level road on a windless day. This gives you an absolute
minimum number for steady speed power consumption for any particular
vehicle. Road & Track magazine publishes the results of these tests on
every car they review. The test is quite sensitive and can show the
difference between a dirty car and one that has recently been polished.

>>>with numbers I've seen published in auto magazines.  If these numbers are
>>>true, that would indicate (at 746 watts per HP) that between 9 kw
>>>and 15 kw continuously would be required.  At 50 mph, that would be 
>>>between 0.18 and 0.3 kwh/mile.  working backward, given 85% efficiency
>
>   I would be interested in how you came to the conclusion that 15% of
>   the power is lost between the batteries and the wheels.   For instance,
>   you have not even specified what the drive wheel configuration is. 
>   This statement is so innaccurate as to be ludicrous.  Are you talking
>   about a front wheel drive vehicle or a rear wheel drive vehicle?

As the old saying goes, wheels is wheels. :-) It makes no difference
from a power consumption standpoint which end of the vehicle does the
propulsion. Mechanical losses can be slightly higher for a front wheel
drive car because of the extra CV joints required to allow the vehicle
to make turns, but the difference is negligble.

85% efficiency is actually generous considering that you are using an
automatic transmission after the motor. Batteries can release their
energy in the high 90 percent range when the discharge is at the 10
hour rate. Source, Gates Battery Handbook. And power controllers are
also good at greater than 90% according to the Bosch Automotive Handbook.
But auto transmissions are less good. Here is a chart from Hot Rod
Magazine:

	=====================================
	  TRANSMISSION     |     HP LOSS
	-------------------------------------
	  Powerglide       |     18
	-------------------------------------
	  TH350            |     36
	-------------------------------------
	  TH400            |     44
	-------------------------------------
	  Ford C6          |     55-60
	-------------------------------------
	  Ford C4          |     28
	-------------------------------------
	  Chrysler A904    |     25
	-------------------------------------
	  Chrysler 727     |     45
	-------------------------------------

Now these numbers are for *accelerating* through the gears, but are
indicative of the relative losses of the various transmissions. Steady
state numbers are better, but there are *still* losses. If we take a
generous figure of 90% for transmission efficiency at steady state,
then total efficiency from battery through controller to motor through
transmission to wheels is on the order of .9*.9*.9=.73 or 73% efficiency. 

>>>from battery to wheels, that would be 0.21 to 0.35 kwh/mile.  Using 
>
>   So this assumption is entirely meaningless...

Not at all. It's based on data from several sources that all show close
agreement.

>>>70% charging efficiency, it works out to 0.35 to 0.44 kwh/mile.
>
>   And I have to question this assumption, as well.  I have no 
>   idea what the charge efficiency of MY charger is.

But we can assume that it's no better than the *best* charging systems
recomended by Gates in their battery handbook. *They* say 70% from
the wall to usable chemical energy in the battery.

>>>As a check, I went back and looked at this from another angle.
>>>Consider a car that gets 25 mpg.  Using 18,500 BTU/lb for the 
>>>energy content of gasoline and 6.2 lbs/gal for density (_Internal
>>>Combustion Engine Fundimentals_, John Heywood), I calculate a 
>>>gross energy usage of 1.344 kwh/mile.  At 30% efficiency, that 
>>>works out to 0.40 kwh/mile net.  This compares very good with the 
>>>0.35 to 0.44 kwh/mile figure estimated in the previous paragraph.
>
>   So, you are using the efficiencies of a gasoline engine to
>   determine if the your calculations regarding an electric engine
>   are sensible.   Scientifically speaking, this is total hogwash.

Scientifically speaking, this is exactly the kind of confirmation
you want. It shows that a similar vehicle with a totally different
propulsion system still gets about the same numbers. It's a sanity
check that shows you've been using reasonable numbers and reasonable
assumptions as to vehicle power requirements. That's what you are
looking for, the amount of power needed to propel the vehicle. That
sets an absolute floor on the energy consumption of the vehicle.
No matter how you power it, it still takes a certain amount of
power to make the vehicle move. Coast down tests, gasoline mileage
figures (with engine efficiency included), and kilowatt consumption
all reduce to the power required to move the vehicle. If all the
numbers agree, you have good confidence in your data. Everybody's
data, coast down from Road & Track, SCE's long term tests, calculations
from fuel economy and efficiency of gasoline powered versions of the
vehicle, all converge toward the same set of numbers. Only *your*
claimed numbers are at wide variance with the others. What do *real*
scientists do when their data set plots to a nicely converged value
except for one data point? They suspect that the rogue data point is
the result of measuremental error.

>>John, my calculations back this up, using Paul's Tempo as a model I
>>get .46 kWh/mile *rear wheel* at 65 MPH. But please note that that 
>
>   Now, you have switched from 50 MPH to 65 MPH.  Interesting.  
>    
>   And, you indicate "rear wheel", which makes me believe you are 
>   talking about a rear wheel drive vehicle, and speaking of the end
>   of the drive train.   
>
>   This gives you an idea of how totally off these calculations are.

As I said before, wheels is wheels. The fact that calculations show
a shift from  .3 kWh/mile at 50 MPH to .46 kWh/mile at 65 MPH is
another confirmation of the validity of the number set. Power requirements
for a vehicle vary with the square of the velocity and linearly with
velocity for rolling friction. So let's see, .3*65^2/50^2=.51, so .46 
is actually generous.

>   To give you the benefit of the doubt, I *never* drive my car at
>   65 MPH, except downhill, not because it won't go that fast, but 
>   because I see no need.  It is also illegal on all the freeways
>   within my 30 mile one-way range.  I make no apology for this.  It
>   is a simple fact for electric cars.   If you want to argue about
>   limited range, then you will finally be on FACTUAL ground.
>
>   So, I have *no* idea what the efficiency is at 65 MPH.  It wouldn't 
>   surprise me if it is *terrible*.  My vehicle can hardly maintain 65 MPH 
>   on any but the slightest of inclines.      

Well that sets an upper limit on the power of your vehicle. Let's work
some numbers. Assuming that your vehicle can only sustain a maximum of
65 MPH on level ground, no wind. We know that vehicle power Pw in kilowatts
is equal to the total driving resistance Fw in Newtons times velocity V in
km/hr, divided by 3600.

(1) Pw=(Fw*V)/3600

We also know that Fw is the sum of rolling resistance Fro, climbing
resistance Fst, and aerodynamic drag Fl.

(2) Fw=Fro+Fl+Fst

Now since we stipulated level ground, we can ignore Fst. Fro is calculated
based on vehicle mass and road surface. It's equal to the coefficient of
rolling resistance f, times the gravitational constant g, times the vehicle
mass m.

(3) Fro=f*g*m

Aero drag is the primary resistance at speeds over 30 km/hr. It's calculation
is a bit more complex. 

(4) Fl=0.0386*Q*Cw*A*(V+Vo)^2

Where Q is the air density 1.202 kg/m^3, Cw is drag coefficent, A is vehicle
frontal area, V is velocity and Vo is wind velocity. Since we assumed no
wind, we can drop out Vo giving us,

(5) Fl=0.0386*Q*Cw*A*V^2

Now lets crank the numbers and see what we get. First we will calculate
rolling resistance. The value of the coefficient of friction for pneumatic
tires on asphalt or concrete is 0.015. We'll be generous and place the
mass of the car at 1545 kg including batteries and driver. Now using 
equation 3 we have:

Fro=0.015*9.81*1545=227.41 Newtons

Now figuring aero drag, we will assume a Cw of .35 for the Tempo and a
frontal area of 2 m^2. This agrees with Road & Track. The velocity is
65 MPH or 108 km/hr. So using equation 5 we get,

Fl=0.0386*1.202*.35*2*108^2=378.82 Newtons

Now from equation 2 we get total driving resistance of

Fw=227.41+378.82=606.24 Newtons 

Substituting in to equation 1,

Pw=(606.24*108)/3600=18.19 kilowatts or 24.38 horsepower

Now that's the *required* power at the driving wheels. We now have to
back solve through the drivetrain efficiency to get battery requirements.
If we grant John's generous 85%, that means we need a battery power of
28.68 horsepower or 21.4 kilowatts. That means you could sustain 65 MPH
for only .37 hour if your battery pack is 8 kWh for a range of 24.3 miles.
Of course at such high discharge rates your battery wouldn't be nearly as 
efficient as it is at lower discharge rates, so your range at 65 MPH would 
be much less, especially if we were to consider wind and hills. 

Now let's look at *current* requirements to supply 21.4 kW. At your stated
bus voltage of 108 volts, that's a current flow of 198 amperes. Now I
recall you stating that your current flow was a maximum of 60 amperes.
That's only 6.48 kilowatts. Based on that, we can conclude that your
Tempo can never reach 65 MPH except down a steep hill with a tail wind.
*Or* that you have inaccurately reported current consumption. I'll leave
it as an exercise for you to back solve the equations to determine your
maximum achievable speed with a 6.48 kilowatt maximum power.

>>calculation was for level ground, steady speed, no wind, and *no hills*. 
>>In real world driving with accelerations and hill climbing, and no 
>>regenerative braking, the situation is at least three times worse 
>>according to Southern California Edison's fleet experiments.
> 
>    I would very much like to see you post information about these fleet 
>    experiments, Gary.   Since you know about them, it is likely that you 
>    have the information at hand.  Please post it.  

I have before, and again in this message, see above.

>Basically, the calculations that Cage, Coffman, et al, have posted in an
>attempt to discredit my figures about my electric car are such a mish-mash
>of assumptions and occaisional fact that they are useless in making any 
>predictions about my energy usage.

We've all used the well understood and accepted formulas to attack the
problem from several different directions. In each case our numbers have
converged nicely. Your irrelevant complaints just show you don't understand
the physics of vehiclular dynamics or have a clue about electrical drive
systems.

> I believe that the results I am getting represent REAL reductions in
> per mile CO2 output and fossil fuel usage, and nothing posted so far
> has successfully disputed my results.

Well you are free to *believe* anything you want. That still doesn't
make it true. The physical reality of the equations is undoubted.
Every vehicle dynamicist uses them in his work. We've plugged in numbers
that *you* have supplied. And we've plugged in numbers that SCE, Ford, GM,
Gates, Bosch, the SAE, and Road & Track have published. We've back solved 
from equivalent gasoline powered Tempo data. All the numbers converge 
*except yours*. Unless the conversion shop where your car was changed from
gasoline to electric power has a secret that violates the known laws of
physics, you're feeding us bogus information.

Gary