On Sun, 23 Apr 2017 21:37:06 +0100, Vir Campestris wrote:

On 23/04/2017 08:13, harry wrote:
Higher steam temperatures results in less efficient boilers because the
exiting combustion gases can be at no lower temperature than the stem.

This turns out not to be the case.

You can, for example, use the warm exhaust to preheat the incoming air &
fuel.


Mother nature has beaten us to this 'trick' as can be seen in the
anatomy of penguins' feet where the veins and arteries in their legs run
side by side so as to allow the veins to recover heat from the arteries
before it gets lost by contact with the ice upon which they roost or
walk. The feet themselves are little more than bones, ligaments and
tendons acting as remote controlled low temperature tolerant appendages.

In the case of the flash boiler, the hot combustion products are routed
in a contra-flow to the direction of the feed water flow from the
condenser and the boiler unit itself is extremely well insulated to
minimise unproductive heat loss through the casing.

Getting back to the original question, most electric vehicle designs
eliminate the mechanical variable ratio gearbox, electing instead to use
high power handling switching converters to control the motor speed
instead. Even when a design uses a fixed gear ratio box to better match
the loading on the drive motor, electronic drive voltage control is still
the method by which modern electric road vehicles are speed regulated.

In an ideal setup, you would have hub motors in each wheel which could
be arranged as a series wired pair on each axle to provide a built in
differential and halving of the current required to drive each axle's
worth of hub motors.

However, in order to maximise efficiency and power to weight performance
of electric traction motors, you get the best performance using a high rpm
motor[1] which basically precludes direct drive hub motor designs unless
you're prepared to sacrifice top end performance for improved battery
economy at more modest urban traffic speeds (you replace mechanical
transmission losses with much lower electric cabling losses).

[1] High rpm on account it uses less turns of heavier gauge copper in its
windings, meaning reduced I squared R losses.

--
Johnny B Good