[SGVLUG] OT: Hybrid efficiency (was:New Linux Lug)

[David Lawyer]

I'm back.  From what I recall, Dustin didn't study good "Brake
Specific Fuel Consumption Maps", nor plot load lines on them.  If he
had done this, he would understand what I'm talking about and not have
raised all the objections he did.  But it's not easy to find such Maps
and there may not be many (if any) good maps since the ones I've seen
only cover a small range of efficiencies (or specific fuel
consumption).

I have a few efficiency maps, but none of them are very good since
they fail to show the regions of negative engine torque (braking with
the engine).  Every time you let up on the accelerator of a non-hybrid
you are braking with the engine:  the engine is producing negative
power output but it's often consuming fuel.  One might consider this
to be negative efficiency.

Back to efficiency maps.  These go by various names such as "engine
performance ...".  One I have has engine rpm on the x axis and engine
torque on the y axis.  In the body of the map are various contours and
curves.  A lot more complicated than a topographic map.  There are
iso-efficiency contours, and iso-power curves (hyperbolas since
P=torque x angular_vel.).  There should be load lines for various
grades and gear rations.  For example, in high gear at a given rpm,
the car's speed is determined by the rpm and there's a certain
vehicle resistance at this speed which determines torque (for steady
state speed).  Change the grade and these load lines will all change
due to more torque required for going up hill.
gear ratio, and this curve changes.

Then trace along a load line to note efficiency at various speeds.  In
high gear, one I plotted years ago for a Ford OHV V-8 goes from about
10% at 15 mi/hr to 20% at 75 mi/hr.  But at 15 mi/hr, high gear would
likely lug the engine, so shift down to 2nd gear and get significantly
worse efficiency than 10%.  If you are cruising along at say 15%
efficiency, pressing the gas pedal to the floor might about double the
efficiency.  This savings will greatly exceed the loss due to varying
v^2.

As cruising speed increases so does engine efficiency but aero drag
increase also.  The result is that at slow speeds, going faster saves
energy (the increase in efficiency is more than the increase in
aero-drag) but at higher speeds going faster uses more energy (due to
aero-drag more than cancelling out the increase in engine efficiency).

So what is the average efficiency of the typical auto?  For city
driving, it's pretty low, since in spite of lower drag, mileage is
worse.  It may be under 10%.  This is about the same as the Prius
charge-discharge cycle when electricity is generated by the engine.

But back to the Prius.  It doesn't always get only say 9% efficiency
using the engine powered charge-discharge cycle.  When you're cruising
and the battery is being charged, the torque on the engine is more due
to generator load, thus increasing engine efficiency.  Then the
battery is also charged by regenerative braking, recovering energy
that would be otherwise wasted in non-hybrids since almost no one
tries to recover this energy by coasting.  Furthermore, with the high
pressure tires, the rolling resistance of the Prius is perhaps only
half that of a non-hybrid (and even less due to lightweight
construction such as alloy wheels).   Also, the Prius engine shuts off
when needed, avoiding the negative efficiency regions of non-hybrid.
Thus the observed high miles-per-gallon of the Prius should come as no
surprise in spite of a low overall efficiency of charging the battery
from engine power.

Someone needs to research all this is more detail and present the
results to the public.  I could do it but I think that other tasks
have higher priority, like publishing all my unpublished stuff on
railroad energy.  Some of this (a third of it) is on my website already
at www.lafn.org/~dave.  The other 2/3 is in my file cabinet (and
bookcase full of Russian technical books on railroads) and much
of it needs a lot more work before releasing it.

Railroad energy efficiency is a little similar to auto efficiency.
They've had regenerative braking on electric railroads for a much
longer time than hybrids.  And coasting to save energy was almost
standard operating procedure in communist USSR.  The USSR used to be
first in railroads, hauling more railroad freight than the rest of the
world combined and publishing books that had no equal in the US.
Now, in Russia today, the railroads are only a ghost of their former
selves.

I just checked the Internet and find that a lot of people are quoting
stuff I've written on my website regarding transportation energy.  I
just don't have time to check up on all the times I've been quoted.
I've also been quoted without attribution which isn't legal but I
guess it happens all the time.  But in one of them, someone claimed
that the hybrid charge-discharge cycle is only 20% efficient when the
batteries become old.  I'm sceptical of it being this low.  But if
it's true, then my estimate of 9% efficiency for the
engine-generator-batteryCharge-batteryDischarge-electricMotor cycle is
way too high.  Could it be like say 6% ?

On the topic of the "Power Curve".  This is not a very useful
curve to look at when trying to determine thermal efficiency.  People
have used the term erroneously on the Internet to imply that it
determines efficiency.  Actually, on one of my efficiecny maps, one
may see that along an iso-power curve, the efficiency may vary by a
factor of 5 (5% to 25% with the same power output).  The power curve
is a curve of power (y-axis) vs rpm (x-axis) all at maximum torque.
But who drives much at max. torque except me when I'm accelerating
prior to coasting :-).

			David Lawyer