[ausev] Series hybrids

[Christopher Robison]

On Mon, 2007-12-03 at 21:08 -0600, John Penry at Texrocks wrote:
> The series Hybrid would be more efficient, from the documentation I have 
> found.
> Using fuel to generate electricity is more efficient than an ICE engine.
> Parallel only uses the electric up to 30 or so mph, then the gas engine 
> takes over, so they are only using the electric for the power in lower 
> speeds, and letting the ICE take over when only 60 or 70 hp are needed.

While I don't think there's an absolute answer to this question since it
is so heavily involved with design details beyond the simple distinction
of series vs. parallel, there are pros and cons to each that are worth
noting. In the end, I don't think the inherent differences are large
enough to dictate design beyond other considerations (modularity,
maintenance, flexibility of vehicle design, etc).

With a series hybrid, you can certainly keep the engine at a consistent
RPM, and that does increase efficiency. However with modern highly
computer-controlled engines it doesn't make as much difference as you
might think, and you have two large drawbacks with a series design that
work against that advantage.

The first which I've mentioned before is weight and volume. For a pure
series hybrid, that can't be assisted by parallel (direct engine drive)
capability, your generator, which is separate from the motor, must be
sized to supply greater than average HP required for the highest
expected driving speed. (This size can be reduced with higher RPM and
greater output voltage). This is possible, but it's not an off-the-shelf
item. Weight isn't much of a drawback for cruising, but it hurts with
stop-and-go driving.

The second is energy conversion. Instead of directly pushing the car,
where the losses are minimal as energy flows through a spinning shaft
and some gears, you're instead converting the mechanical motion of the
engine to electricity in the genertor (lossy, dissipates heat) and then
back again through the motor (more heat loss). The efficiency losses of
each are multiplied together. Charging and discharging a battery incurs
even more loss but we can leave that out since it's common to both
systems.

I think equal efficiencies are achievable with series and parallel
designs; but in order for the series design to be competitive, it needs
to be engineered specifically for the purpose. You won't get there with
inexpensive off-the-shelf parts you can throw together yourself. With a
limited budget, a parallel hybrid design (a "through-the-road" hybrid or
a pusher trailer) would be more efficient, though less useful beyond
moving your car. You wouldn't be able to use your generator trailer to
power your house in a blackout, for example.  :-)



> I believe a series hybrid would get much more mpg than a parallel.  Think 
> locomotives.

Locomotives are series hybrids simply for the reason that maintaining a
mechanical linkage to the wheels is not worth what it would require.
Since you have to use electric motors to begin with to get that much
mass moving from a stop (can you imagine the clutch you'd need to do it
with an engine?) there is no reason for the extra mechanical complexity
and mass of a mechanical drivetrain and 50-speed (?) transmission, when
it won't win you enough in terms of efficiency, if any at all.

And for what it's worth they don't contain batteries or work like hybrid
cars we're familiar with, since that really has no benefit for
long-distance runs. The Green Goat
(http://railpower.com/products_hl.html) is a new exception, an example
of a hybrid locomotive engine that actually has batteries, but it's for
switchyard use where such a hybrid design is very effective.



-- 
Christopher Robison
chris at ohmbre.org
http://ohmbre.org          <-- 1999 Isuzu Hombre + Z2K + Warp13!