On Sat, 26 Apr 2014 11:00:37 +0100, The Natural Philosopher
wrote:

On 25/04/14 23:55, Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
take away a massive steel disc brake cos regenerative braking absorbs
most of the power,

You don't need massive steel disc brakes for moderate retardation. What
you do need them for is a panic stop from 90 mph. And regenerative braking
ain't going to do that.

Oh yes it is.

you put a short across a leccy motor and it stops dead in its tracks mate.

I can certainly attest to that phenomenon from my own experience with
designing and building a controller board for a Philips solenoid
controlled bi-directional data cassette drive which used seperate cush
drive high quality permanent magnet DC motors (using proper carbon
brushes) on the tape drive hubs. This phenomena becomes ever more
extreme as you inrease the motor size.

I had to use a 27v zenner diode (via steering diodes) to provide an
rpm limited back tension during fast forward/reverse seek operations
to both limit the windage effect (to avoid ingesting a lubricating
film of air between the incoming tape and the spool of tape it was
being wound onto) and terminal speed when the lamp/photocell detected
the leader and 'hit the brakes' by shorting the motors to stop the
tape within the 10 to 15 cm length of remaining leader.

The cush drive spring extension still had to deal with some remaining
kinetic energy but the electro- braking did at least reduce this to a
managable level. My initial tests gave me a C60 end to end fast
wind/rewind spooling time of 10 seconds... and broken/tangled tapes.
The zenner speed limiter mod resulted in a 14 second spooling time and
unharmed tapes.


the disc brakes are there to get to to a final stop, not to absorb the
bulk of the energy.

Quite true. Also, they're needed to provide a static braking force to
allow the vehicle to come to a stop without creep as well as for
parking. In fact, the disks could simply form the outer portion of the
motor helping to minimise the 'unsprung mass' even further.

Whilst 'sharing' the mass of a common motor 4 ways to each wheel is
undesirable[1] from a suspension and ride handling point of view, it
does have the merit of eliminating the mass of not only the common
motor itself but also that of the heavy and bulky mechanical
transmission system. The flexible cables will add some mass of their
own of course but can be considerably lighter than their mechanical
counterpart.

The only other transmission method that occurs to me that might
compete with an all electric transmission is the use of hydraulic hub
motors in each wheel connected to a pump driven by a common motor[2].
A variable delivery swash plate type pump can provide a variomatic
auto transmission system with no need for a seperate clutch. I haven't
seen any comparative data on such a scheme though.

I think the transmission losses in a hydraulic system are on a par
with those in a conventional mechanical system. The costs are likely
to be considerably higher to produce an all hydraulic transmission
(even in mass production) for something as humble as the family car so
even assuming a similar transmission efficiency between the two
systems, it's not going to happen in anything other than specialised
vehicles (plough pulling tractors and Space Shuttle Transporters)
where the high precision of control outweighs the extra cost.

[1] The "unsprung mass" problem could be mitigated by 'active
suspension' techniques where the 'absorbed energy' from a road bump
can be recycled into the KERS battery (or even an individual super-cap
per wheel) to compensate for the energy required to push the road
wheel back down to 'follow the road contour' on the 'rebound' of the
active 'spring'.

It seems a little 'short sighted' to say the least if you're going to
discount a purely electric[3] transmission system on the basis of
'unsprung mass' issues with prototype hub motor drives alone.

[2] In this case, there'd be no need for sophisticated electronic
control of the electric motor since this could simply be started up to
run at its designed speed for whatever voltage is being generated by
the fuel cell stack. The automatic transmission functions simply being
implemented by hydraulic controls. The electric motor in this case
would simply be standing in as a substitute for the more usual diesel
engined prime mover.

[3] The Honda system is basically taking the power transfer to the
road wheels via two power conversion stages. The primary one being the
fuel cell conversion to electric power with the second being the
common electric motor driving a mechanical transmission system to
mechanically drive the road wheels. It seems only common sense to
eliminate this extra stage and distribute the fuel cell's electrical
power more directly to the hub motors via a low loss intelligent power
management and control system with a built in KERS.

The Honda Hydrogen powered car is at the 'proof of concept' stage
right now. There's plenty of time (and scope) to further refine the
system to an all electric transmission and optional active suspension
configuration later on in the development cycle.

The only remaining serious issue being the infrastructure required to
manufacture and distribute the hydrogen fuel. Distribution is mainly a
matter of logistics (the existing system for conventional fuels could
be appropriated with suitable modifications) which leaves the bigger
question of manufacture to be addressed.

It strikes me that, in all probabilty, the best place at this point
in time is to use the existing oil refineries to manufacture the
hydrogen fuel. Purpose made LFTR based nuclear power station hydrogen
production can come later on when the cost of oil makes the refinery
solution too uneconomic to sustain.

The arguments over the energy losses in hydrolysing water into usable
hydrogen fuel are, in part, a little spurious. At the present time,
the production costs for hydrogen made from natural gas is about
double that of petroleum. However, this is more or less cancelled out
by the higher tank to wheel energy efficiency compared to that of an
ICE powered vehicle.

Setting aside the very high capital costs, the energy running costs
remain pretty much equal with the main benefit being that of reduced
pollution products from the vehicle itself. The issue of carbon
emmissions by the hydrogen fuel manufacturing processes is one that
can be dealt with by the producers.

Eventually, hydrogen fuel production options will utlimately shrink
down to hydrogen production co-sited with LFTR nuclear power stations
so the carbon emmission issue will become a matter of history.

http://en.wikipedia.org/wiki/FCX_Clarity gives a good summary and
some more detail.
--
Regards, J B Good