Inertia... IS the enemy!
The
single most influential design factor for a race car is weight or
more properly; mass.
The greater the mass, the harder it is to accelerate, change direction
and slow the car.
Imagine
that you are designing a race car in a world where each gallon of
gas weighs 237 pounds.
How much emphasis would you put on getting good gas mileage?
Since
energy storage (batteries) is a larger part of the weight of an
electric vehicle than gas is on a standard race car, even small
changes in efficiency have a far greater effect on the weight required
and hence, the competitiveness of the electric race car.
To
illustrate the scale of the difference, here is a comparison of
a similar design choice between an ICE race car and an electric
race car:
Consider
a gas powered, less than 2.5 liter, production based GT racer.
You have an opportunity to make it 10% more efficient.
The car gets 5 miles per gallon while competing in 30 mile sprint
races.
This means the car uses 6 gallons of gas during the race.
The gasoline weighs 6 lbs per gallon for a total of 36 lbs.
Since the car starts with a full tank and finishes with an empty
tank, the actual gasoline weight carried during the race is ½
of the 36 lbs or 18 lbs.
The
effective weight of the gasoline saved by being 10% more efficient
is 1.8 lbs.
Let's
look at the same situation with the Electric Imp.
We use around 1.25 kWhr per mile.
This means 37.5 kWhrs to complete a 30 mile race.
It takes around 19 lbs of our cutting edge Kokam High Power cells
to supply each kWhr at the 450 amp draws we need to be competitive.
This adds up to a 712 lb pack.
We carry the full weight of the pack from start to finish.
The
same 10% efficiency improvement which saves the gas car 1.8 lbs
will remove 71.2 lbs of batteries from the electric car.
Every
component on an EV should be judged by its' actual weigh and the
efficiency penalty weight.
When we compare two motors of equal weight but one is 1% less efficient,
it can be considered 7.12 lbs heavier under these race conditions.
Efficiency
can be speed dependent.
One motor might be more efficient at low speed while the other is
more efficient at high speed.
Then which is the most efficient motor will depend on whether it
is a high speed or low speed track.
The
efficiency penalty will be different for different race lengths
(greater for longer races), and different batteries (greater for
heavier batteries).
The weight per kWhr will be also depend on amp draws (see Peurket's
equation).
