Re: Electrical cars

"Peter Hill" <peter.usenet1@xxxxxxxxxxxxxxxxxx> wrote in message
news:7unvc31v3b8r95ppqbg4m0kuv7h5ol62qg@xxxxxxxxxx
On Fri, 24 Aug 2007 10:23:28 +0100, "Mortimer" <me@xxxxxxxxxxx> wrote:


I forget the exact figures (and I'm not sure where to find the energy
contained in a litre of petrol/diesel) but I remember being told in a
lecture by the engineering prof at university that a petrol pump delivers
energy at rate of many megawatts - imagine achieving that with an electric
charging system!

Petrol 31MJ/L

60 litre fill in 5 min is 12L/min = 0.2L/sec
Energy transfer rate = 6.2MJ/s = 6.2 Mega Watts.
and that's a slow fill allowing for time to pay for it.

A household domestic electricity supply rated at 100Amps can deliver
100 x 240 = 24 Kilo watts.

The fuel pumps energy transfer rate is about 260x the max supply to a
house. If you wanted to put as much energy into a car from a domestic
supply as you do at a filling station it would take about 22 hours.
Your freezer, fridge and everything else in the house would have to be
switched off to charge it in that time.

Engine is only about 25% efficient overall while a battery/motor
system is close to 100%. So for same range it needs a domestic supply
running at full rated load for 5.4 hours to charge the car.

But a large number of people in UK don't need 400 miles per charge. A
lot only do 12000 miles/year = 48 miles/day (2 days off). AM School
run, drive to work/shops, pm school run. So a 6.5 hour charge at
1/10th the rate each night will suffice.

I wonder how the cost of the fuel compares?

60 litres of petrol or diesel costs about £55.

If an electric car needs to be charged at 24 kW for 5.4 hours, that's 24x5.4
kWhr. I think 1 kWhr costs about 8 pence.

So it would cost 24x5.4x0.08 = £10.40 to charge the car.

Interesting: before I did the calculation I was expecting the electric car
to cost more than the petrol car. But of course, who knows what surcharge
our beloved Chancellor would slap on electricity that is used for
transport...


What actually is the typical range for an electric car of equivalent
performance to a normal family car? Is 400 miles feasable? The difficulty
comes if you ever need to make a journey of more than the maximum range,
because then you have to schedule in an overnight stop in order to recharge
the battery, whereas a car has unlimited range because it only takes a few
minutes to buy another 600 miles range.


As a matter of interest, do you have the MJ/litre and efficiency figures for
diesel? Does diesel have more energy per unit volume? My present 90 hp
diesel car achieves about 50 mpg, compared with about 35 mpg for the
previous petrol car with about the same power, so (if the energy content of
both fuels is similar) it's about 40% more efficient - it travels 140% of
the distance that the petrol car could on the same volume of fuel.

What sort of acceleration and top speed can electric cars currently
achieve - can they compete with 0-60 in (say) 12 seconds and top speed of
maybe 80-90? And can they climb steep hills such as 1:3 or 1:4?


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