Re: Do electric cars need to be geared up/down ?

"Curt Welch" <curt@xxxxxxxx> wrote in message news:20080820133029.777$TU@xxxxxxxxxxxxxxxxx
"pogo" <pogo@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

I am still stuck with the mental picture that once a motor gets spinning
(ie - after the car is moving from a dead stop) you would then need less
power to keep it spinning, so would perhaps 2 gears be able to in ANY way
maximize the life of a single charge for a battery in an electric car for
a given amount of mileage ?

No, gears will _always_ cause the battery to drain faster because they add
additional energy loss in the power train due to friction. AKA, they cause
the battery energy to be wasted heating up the gears. In the designs that
John mentioned which used 2 gears, it seems they were added to create a
"sports car" design to allow for max 0-60 acceleration while maintaining
a maximum top-end speed. But they did it at the cost of lower range (faster
battery drain).

Electric motors translate electrical energy into mechanical energy. You
can completely control the drain on the battery by controlling the voltage
(PWM duty cycle) you put into the motor. When the car gets up to speed and
you want to stop accelerating, the controller reduces the duty cycle of the
PWM which reduces the drain on the battery to the level needed.

To increase battery life, you have to reduce wasted energy - energy that is
is used to heat things up instead of move the car. Gears heat up due to
friction loss. Every bearing in the drive train wastes energy, and every
gear you add adds enough bearing or two. Belts heat up and waste energy as
they flex and as they rub against the pulleys. The wires in the motor have
resistance which cause energy to be wasted as heat.

Electronic controllers are not 100% efficient either. They waste energy as
heat as well (out the heat sinks). It's always a design trade off to pick
what is most important for any given application. In general, because of
advancements in power electronics, it's more optimal to control energy flow
and torque with the electronics than to use mechanical systems like gears
these days.

Also, keep in mind, that when we are talking about cars driving at highway
speeds, there is a huge amount of friction due to air resistance at those
speeds so to keep a car moving at those speeds actually burns up a lot of
energy (battery life). You don't actually get to "back off" all that much
anyway when you get "up to speed". They energy loss due to air resistance
goes up with the square of the speed (if I remember correctly) which means
for each 5 mph faster you want to go, you have to add increasing more
amounts of more power.

If you want to extend battery life, drive at 15 mph instead of 70 mph.
That will do more for you by orders of magnitude than everything else
combined. I don't know the actually numbers, but you could probably go 10
times as far at 15 mph than you could travel at 70 mph (however, it will
take very long to get there). The faster you drive, the more the energy of
your battery is wasted heating up all the air and the less it's used to
make you move.

--
Curt Welch http://CurtWelch.Com/
curt@xxxxxxxx http://NewsReader.Com/

Excellent explanations ! Now *this* makes sense to me!
Thanks!
.