Drive train efficiency (including motor)

[Ben Haines <b.haines@student.unsw.edu.au>]

  From  my experiance from working on the University of NSW's solar car in 
Australia, The hub motors typical used there are peak 98% efficient (DC 
brush less).  Being a solar car with DC power from batteries and the solar 
cells themselves the motor controller uses DC input and is typical 95% and 
possibly higher in efficiency. Considering it is a hub motor which removes 
the transmission losses it makes this a very good setup in terms of 
efficiency.

The one lilting factor is that most of these will only put out about 6kw 
without stretching it to much but have run about 20Kw  through it for short 
periods.  I realise this isn't very practical in terms of using it for an 
EV conversion considering that the solar cars are in the order of 300Kg 
(with driver) and thus don't need a great amount of torque for starting 
where as standard cars are typicaly 1-1.5ton.  With these motors being 
direct drive this limits their torque and the cars are also designed for 
more highway racing than stopping an starting as with the city. but in this 
configuration they are capable of equal IC acceleration (for a standard 
family four cylinder) especially for general use.  Due to this reduction in 
power the aerodynamics play a significant role governing the power required 
to travel at a given speed, and if you see many of the solar cars their 
shapes are very slippery.  most can do 100Kph on 2Kw and the better ones 
can do it on 1600 watts. (remembering no change of gears).  we have ben 
able to glide on no power to the motor for 9mins from 120or 110kph to 100Kph.

My question is I read so  many of using normal car drive trains and gear 
boxes and It seems a waste to be losing a considerable amount of your power 
in the drive train.  rotating gears/differentials and clutches all add to 
rotating mass and lose energy.  has anybody given much thought to this or 
even to improving the aerodynamics of their cars instead of simply upping 
the capacity of the batteries.

I'm interested to hear about any good ideas people have seen in terms of 
drive train design ?? because if there is one thing that will increase your 
range, it is the overall system efficiency.

Has anybody tried two hub motors the rear wheels, one on each wheel?? I 
have seen a prototype Honda EV 4WD with a hub motor on each wheel.

love to hear your thoughts

Ben Haines

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Thanks for your time and interest.