Re: Electric bikes?


"Ben C" <spamspam@xxxxxxxxx> wrote in message
news:slrnhaffah.31q.spamspam@xxxxxxxxxxxxxxxxxxxx
On 2009-09-09, Brendan Halpin <brendan.halpin@xxxxx> wrote:
[...]
A 20kg electric bike[1] should surely do an order of magnitude less
than
a 1.5 tonne car, though it will probably require some food-based
energy
input from the rider too.

Yes, just think of the power. A car has about 100bhp which is around
75kW. A reasonably fit bicycle rider puts out only 0.15kW.

So although the engine in most cars is actually slightly more
efficient
than the human body at converting chemicals into energy, it's
consuming
energy at about 500 times the rate.

Of course the reason the car needs all that power is because it's so
massive, as you say. We also need them to go fast.

[...]
[1] I also wonder why it wouldn't be much more efficient to have
little
petrol motors -- how much loss is there in using fossil fuels to
generate electricity, transmit it, charge it into a battery with it
and
then let it out again?

Yes petrol motors are quite efficient, and you can have a huge range
from a small fuel tank, but they don't scale well down to such low
powers, so, as you say, you might need a battery to store energy and
then run the engine intermittently.

Used to be able to get little petrol motors (around 25cc???) that
attached over the front wheel of a pushbike, or could be built in to
the rear wheel. Don't know if they would be legal today..... but,
hey, that never worried cyclists.....


.

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