From: IN%"[email protected]" "Electric Vehicle Discussion List" 15-DEC-1993
To: IN%"[email protected]" "Multiple recipients of list EV"
Subj: ICEV vs. EV efficiency

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Date: Wed, 15 Dec 1993 08:48:20 -0800
From: Robert Weeks <[email protected]>
Subject: ICEV vs. EV efficiency
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There has been quite a bit of discussion about EVs lately in the USENET newsgroup. For those of you that don't read this
newsgroup, I've forwarded a copy of one of the more informative postings
comparing ICEV and EV efficiencies.

Robert W. Weeks
[email protected]
(206) 527-1591


Tuomas Koskinen (of Finland) writes:

This is a real-life comparison of two ICE cars and EV-conversions that have
been converted from them. Part of this is a repost of my original post
"Electric vehicle efficiency" on 11.12.1993. I have received many questions
about this (and asks for a repost) so I decided to make an "upgraded"

Both of the cars are quite small Japanese family cars (Nissan Micra). The other
is year model -83 with 215000km (133600 miles) driven before conversion and
the other -92 with 2000km (1240 miles) driven before conversion. I don`t
know if these cars are familiar in the US, but in Europe they are. Anyway,
I have data from both of them, with the original internal combustion
engine and with the electric drive in similar driving conditions.
I have personally driven both of them, both ICE and EV -versions. I use
the older one every day. This year it will be about 16000km =10000mi (it is
53km =33mi one way from my home to the university).

I have calculated some efficiency figures based on the theoretical
power requirement to drive the car under the given conditions. However, these
are not absolutely right values, because it is quite hard to estimate
losses like rest drag of brakes, bearings drag, oil viscosity friction etc.
Anyway, the relative values between the ICE and electric drive are right.

In ICE-versions Michelin MXT tires were used (rolling drag coefficient
0.011, 2.4bar). In electrics, Michelin MXN tires were used (rolling drag
coefficient 0.008, 3.0bar). This is to compensate the higher weight of
the electric versions. All other conditions were same (summer, dry road,
temperature between 5-20 C (41-68 F), wind compensated by driving the same
road both directions. It is assumed that the ICE runs at 2000rpm (4000rpm when
driving 120km/h) and the electric motor runs at 4000rpm in all of the
conditions below.

Here is the calculation method and assumptions I have used to calculate
the theoretical power demand of the vehicle:

The force needed to move the vehicle is Ftot=Fr+Fa+Fb, where
Fr=my*N is tire rolling drag, my=tire rolling drag coeff., N=m*g, mass*9.81
Fa=.5*ro*v^2*Cd*A is air drag, ro=air thickness (1.293), v=vehicle speed,
Cd=vehicle body air drag coefficient, A=frontal area of the vehicle
Fb=50N is an approximate value of bearings drag and brake rest drag, it has
been obtained empirically by pulling the Nissan -82 with a long rope at
a constant speed
The power needed at wheels at a given speed is Pw=Ftot*v. There is also
an oil friction loss in the gearbox, which depends on the speed the motor
runs. In the older Nissan it is about Po=130W /1000rpm, in the newer it is
about Po=100W /1000rpm. So the theoretical power demand is Pt=Pw+Po.
An example:
The older Nissan, ICE version, driven at 90km/h (56mph) constant speed.
Fr=my*N=0.011*9.81m/s^2*680kg=74N (Michelin MXT -tires)
Po=130W /1000rpm =260W /2000rpm
Pt=Pw+Po=10.2kW+260W=10.5kW, the same number that is in the table below.

Now you say that I`m favoring EV:s because lower rolling drag tires are
used with them. Actually, you can make the rolling drag coefficient better
simply by putting more air pressure in the tires. In an EV, however, the higher
weight compensates the negative effects of higher air pressure. Try 3.0bar
in your car. I`m sure you will get bumpy ride until you put 500kg more weight,
and if you do, your original springs and shock absorbers will probably do
quite bad. In the EV it works well.

Here is a comparison what happens, if you had a car body with Cd=0.19,
tires with ro=0.005 and 150kg lighter body ("simulating" the GM Impact).
I also assume here, that the bearings/brake rest drag is reduced to 30N.
Lets use the ICE / electric drive like in the Nissan -92 (90km/h):

For the ICE-version:
Pt=200N*25m/s+260W=5.3kW, so the drag has reduced 47.5%
This is the same than the original power demand at 65km/h. The car used
to have a consumption of 4.5l/100km at that speed. This is 2.9l/h. Now
at 90km/h it takes 1.11 hours to drive 100km, so the consumption with the
new configuration would be 3.3l/100km. So there is 1.6l saving, which is 33%.

For the EV:
Pt=220N*25m/s+260W=5.7kW, (drag has reduced 45%)

>From the table below it can be seen that the total efficiency of the drive at
an output power like this is about 68%. So the consumption from
mains is 9.3kWh/100km. There is a 17.6-9.3=8.3kWh saving, which is 47%. The
range with the same batteries would be about 300km, which is x2 improvement!
And think: you would have an EV with a range of 300km (186mi) at 90km/h, at
50km/h it would be over 500km (311mi)!

I made this comparison to show, that
a) When the drag of the vehicle is lowered, the efficiency of an ICE gets
also lower, and the fuel economy gets only a little bit better, compare
47.5% reduction of drag leads to only 33% reduction of fuel consumpt.
b) When the drag of an EV is lowered, the efficiency of the electric drive
remains the same (actually it gets a little bit better, because battery
efficiency is better at lower power), compare 45% reduction of drag leads
to 47% reduction of consumption.
c) The range of the EV grows more than the consumption reduces (if lead-acid
batteries are used), because capasity of the battery depends on the
discharge time (capasity gets bigger when the discharge is longer).
d) You still have to have an ICE that has a peak power of about the same,
because the weight reduction is quite small, and you need an acceptable
e) Almost everyone seem to only think that EV:s are bad because thay have so
little energy - the real fact is that CARS are bad because they have so
much drag! The EV is here to reduce waste of energy. I find it actually
a good thing that current batteries have low energy - it forces car designers
to lower the drag. The GM Impact is a great piece of work showing that
this can be done. Imagine for example the flywheel battery, it has not yet
safety approval for vehicle use, but in summer 1994 Honeywell will launch
a commercial (limited) flywheel battery that has an energy density of
220Wh/kg (whole system, it has only power inlet/outlet terminals). This is
over five times more than that of the batteries used in the Nissan -92. So
a simple calculation yieds to a range of 1500km (932mi) at 90km/h if the
batteries were replaced (which is techologically VERY easy, excluding
safety, which will be solved some day). At the same time, the efficiency
would get from 68% to 80%. My opinion is that the EV is very promising.

Ok, back to the original comparison.
The electric versions have commercial lead-acid batteries, microprocessor
based controllers and ac-induction motors directly coupled to the original
gearbox. The older one (-83) takes advantage of all gears, the newer (-92)
has an automatic electric sifting and uses only gears 1 and 3. Both of them
have regenerative braking.

ICE Electric ICE Electric
Weight 680kg 1100kg 800kg 1200kg
Top speed 140km/h (87mph) 125km/h (78mph) 145km/h (90mph) 145km/h
Acc. 0-100km/h 15s 29s 15s 22s
Motor type 1 liter ac, 80kg 1 liter ac, 55kg
Motor max.power 37kW/50hp 26kW/35hp 40kW/55hp 40kW/55hp

50km/h (31mph) constant speed
Theoretical 3.2kW 3.6kW 3.3kW 3.5kW
power demand =0.23MJ/km =0.26MJ/km =0.24MJ/km =0.25MJ/km
Real consumpt. 5.0l/100km 8.2kWh/100km B 4.2l/100km 7.5kWh B
B=from battery =46mpg 11kWh/100km M =55mpg 10kWh M
M=from mains =1.8MJ/km =0.39MJ/km =1.5MJ/km =0.36MJ/km
Efficiency 12.8% 66.7% 16% 69.4%
Range 750km/466mi 135km/84mi 1000km/622mi 300km/186mi

90km/h (56mph) constant speed
Theoretical 10.5kW 11kW 10.1kW 10.4kW
power demand =0.42MJ/km =0.44MJ/km =0.40MJ/km =0.42MJ/km
Real 6.5l/100km 14.4kWh/100km B 4.9l/100km 13.4kWh B
consumption =36mpg 19kWh/100km M =47.5mpg 17.6kWh M
=2.34MJ/km =0.68MJ/km =1.76MJ/km =0.63MJ/km
Efficiency 17.9% 64.7% 22.7% 66.7%
Range 570km/354mi 65km/40mi 850km/528mi 150km/93mi

120km/h (75mph) constant speed
Theoretical 21.2kW 21.9kW 19.9kW 20.3kW
power demand =0.64MJ/km =0.66MJ/km =0.60MJ/km =0.61MJ/km
Real 8.5l/100km 20.5kWh/100km B 6.7l/100km 19.4kWh B
consumption =27mpg 28.4kWh/100km M =35mpg 25.8kWh M
=3.1MJ/km =1.02MJ/km =2.4MJ/km =0.92MJ/km
Efficiency 20.6% 64.7% 25% 66.3%

City driving (avg. speed 38km/h, 24mph)
Theoretical 2.1kW 2.5kW 2.2kW 2.4kW
power demand =0.20MJ/km =0.24MJ/km =0.21MJ/km =0.23MJ/km
Real 7.0l/100km 10.5kWh/100km B 6.6l/100km 9.4kWh B
consumption =33mpg 13.6kWh/100km M =35mpg 12.4kWh M
=2.52MJ/km =0.49MJ/km =2.38MJ/km =0.45MJ/km
Efficiency 7.9% 49% 8.8% 51.1%

With the electric drive, there are two different values for "Real consumption".
The value marked with a `B` means consumption from the batteries of the
vehicle. The `M` -value means total consumption of the vehicle from mains
power (this includes battery and charger losses). A single phase 230V/16A
charger with power factor correction is used.

Now it is easy to see that the total efficiency of an EV is
2.7 - 6 times better than that of an ICE car with the same body. These
EV:s have ordinary commercial lead-acid batteries and ordinary ac-motors
you can find almost everywhere. Also the controller is no more complicated
than your VCR and PC together. There is nothing special.

If you make the electricity from oil (which is the worst case, coal being
about equal), you can have a typical diesel motor (size > 1MW) with an
efficiency of 53%. When you combine this with a synchronous generator
(eff. 96%) you have electricity-from-oil -efficiency of 51%. Now add
distribution (eff. >90%) losses and you have a total efficiency from
oil to electricity in the wall outlet where the EV is charging of 46%. This
is 1/2.2. The electric drive is still 1.2 times more efficient even in
motorway driving (120km/h, 75mph) which is the best case for an ICE car and
the worst case for an EV. In city driving, the electric drive is 2.6-2.8
times more energy efficient, based on oil consumption. The emissions
of a large power plant are much lower than if the same amount of fuel
were burned in car-size motors. Conclusion: An EV causes many times less
pollution than an ICE-car.

OK, but what about the batteries? For example the lead-acid batteries in
the Nissan -83 last about 30000km (18600mi). The efficiency of the batteries
does not get remarkably lower during their life, it lowers a few % to the
end of the lifespan. The capasity remains constant the first 85% of the
lifespan and then reduces to 75%-80% of nominal at the point they have to
be replaced. The batteries used in this particular EV (Nissan -83) are
already recycled. Unfortunately I don`t have exact numbers about how much
energy it consumes to recycle them, but I have heard estimates in the
magnitude of 20-30 times battery energy which in this case means 200-300kWh.
This is about 4-6% of the total used energy during the 30000km.

I don`t have a written document of the recycling energy requirement.
That is because there are none. These numbers are from the local battery
dealer (TUDOR Group) that has supplied batteries for these EV:s. Even
they don`t have exact data, because the energy required depends very much on
how large series are handled. If these batteries were used widely like say
in 50000 vehicles, these numbers should apply.

-- --
Tuomas Koskinen
[email protected]

ICEV vs. EV efficiency RAD
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