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

[David Lawyer]

On Tue, Feb 21, 2006 at 12:03:20PM -0800, Emerson, Tom wrote:
> OK, being a hybrid owner, I suppose I should interject my observations...
> 
> > -----Original Message-----
> > Behalf Of David Lawyer
> > On Mon, Feb 20, 2006 at 10:08:47AM -0800, Dustin Laurence wrote:
> 
> > [David:]  Charging batteries is only 36% efficient in the Prius II.  

It's the recovery of the kinetic energy of the car to do "useful
work" which is only 36% efficient.  "useful work" is overcoming
vehicle resistance: aero drag, rolling resistance, and grade
resistance. 36% is the combined efficiecny of 4 steps: electricity
generation, battery charging, battery discharging, and electric
motoring.
> 
[snip]
> Long story short, I believe the system *could* recover more energy,
> but doesn't.  Why it doesn't do this is more likely due to political
> or marketing reasons than technical, but this borders on conspiracy
> theories...

The reason it doesn't is due to the physics of motors/generators and
the chemistry of batteries and not conspiracy.  There are resistance
loses in the motor windings and hysteresis losses in the motor iron.
In Soviet Russia, only about 60% or so of the regenerated energy on
electric railroads was recoverd.  It's better than 36% because there
are no batteries involved and the rolling resistance of trains is so
low that locomotives can use heavy motor-generators  with a lot of
copper and iron to reduce loses.  No consipiracy!

> [dustin:]
> > > Most of the drag at highway speeds is wind resistance, and the
> > > difference there is essentially zero: notice that some cars can
> > > be purchased in either configuration.  It is the same body 
> > shape and size, thus the same drag.  [...]
> > > a hybrid is better there (I suspect less ventilation is needed). 
> [side note: on the first generation Prius there is a battery exhaust
> port specifically to ventilate the waste heat generated by the
> charge/discharge process, but the inlet comes from inside the
> passengar compartment, so I have no idea how this affects your
> argument; the Prius II apparently doesn't have this "port", or else
> it vents to "underneath the vehicle" just to avoid an external
> blemish -- anyone with the new version around here care to comment?]
> 
> > > Which lists the additional weight for a Prius (a fuzzy concept
> > > since there is no direct-drive Prius) as 200 lb.
> 
> Actually, there is, or at least, there appears to be: the Ford Focus
> from the same timeframe has a nearly identical exterior size and
> shape as a Prius I[judged by viewing a Focus that parked next to me
> one day]  There isn't much difference in effective "engine weight"
> as the electric motor essentially weighs the same as the equivalent
> portion of the gasoline engine it replaces, however since you're
> lugging around 6 or 8 extra "batteries", you do have to account for
> that, and 200 lbs sounds about right for a half a dozen "regular
> automotive batteries"
> 
> [David:]
> > > > But worst of all is the poor efficiency of regenerative
> > > > braking.
> > > 
> [snipped long treatise on kinetic energy...]
> 
> Another observation: near my sister's house there is a good sized
> "gully" that the road dips down into and back out -- at least a 100'
> drop or more on each side, with a stop sign at the top.  I have, on
> occasion, started out from a dead stop at the top of one hill,
> *added* a little energy [from the batteries] and "kicked it into
> neutral" and coasted to a stop at the top of the second hill.  (very
> tricky to judge "just enough" to add at the start -- usually I'm
> still going approx 15 mph and have to actually use the brakes)  I
> haven't really tried using "just the energy in creeping" (*) because
> there is enough traffic on the road that I'll likely get a
> "suggestion" from another driver on where I would find the gas
> pedal...
> 
> (*)  "creeping" -- the prius is specifically designed to mimic
> "engine creep" as found in a conventional vehicle.  Technically, the
> car shouldn't "add" any energy to the system if you don't have your
> foot "on the gas", but it does since "people are used to cars doing
> this" in the first place [I think it even mentions this fact in the
> handbook...]
> 
> Note I said that I "kicked it into neutral" -- if I didn't, the
> system would automatically attempt to bleed energy from the downhill
> drop and feed it to the batteries, however the amount "bled" from
> the system is greater than what it applies as "creep", so I'm nearly
> certain that the car would indeed "stop" on the way up, and might
> even start rolling backwards if I were to let it.  I don't think it
> is possible to quantify whether the energy bled from the downhill
> drop, when stored in the battery and then re-applied to the motor on
> the way out would be enough to reach the top of the second hill (due

Well, if the conversion is only 36% efficient, you would only reach
36% of the way to the top.  Except that it would be less than 36% due
to vehcile resistance losses, and it would be more than 36% due to
full recovery of the kinetic energy that the car had at the bottom of
the grade.  These two factors work in opposite directions and might
almost cancel out, resulting in getting about 36% of the way up the
hill with the battery having the same charge as when you started down
the hill.

> to the difficulty in applying "just enough gas" to use the electric
> motor without the computer automatically starting the gas engine to
> help push up the hill)
> 
[snip]
> [David, then Dustin]
> > > > I think auto manufacturers should be required to provide 
> > efficiency
> > > > curves for their engines so that drivers can select efficient
> > > > operating points.
> > > 
> > > Most drivers can neither select efficient operating points nor
> > > understand the efficiency curve.
> 
> > > > ...Why not a display on the dash that would show efficiency?
> > > 
> > > I believe a Prius has this (Tom can tell us for sure).  
> 
> yes, as does the Honda (but I found the honda to be more annoying
> and distracting than the toyota, which actually became a deciding
> factor in my choice)  On the Honda, there is a LED bargraph type
> display that sways back and forth from "charging" to "discharging",
> and it is placed near the speedometer/odometer; On the Toyota, you
> have to pull up a seperate screen to show the relation of battery
> usage: battery usage when the engine isn't running, battery charging
> due to "coasting"

You mean braking, not coasting.  I have frequently seen coasting used
when what is meant is braking.

> battery charging due to excess engine capacity,
> battery assisting due to lack of engine capactity, and so on.
> 
> As I mentioned earlier, the toyota also has a display that
> summarizes your "fuel effeciency" over 5-minute periods, and even
> shows the amount of recovered energy (as "lightbulbs", or something
> vaguely resembling such -- the screen's resolution isn't all that
> great...)  There is also an instantaneous display.

Does it show the 9% overall efficiecny when you are using the electric
motor?  It needs to show instantaneous engine efficiency so you can
know just how far down to press on the accelerator pedal.  So I don't
think it has the type of energy-efficiency indicators that I had in
mind.

> 
> The truly "geeky" members of the Prius community, however, have gone
> so far as to install meters showing the actual current draw and have
> learned to drive "just under" the point at which the "demand" for
> speed exceeds what the electric motor alone can produce  (see the
> web-based Yahoo discussion group "prius-technical-stuff" for
> details)

These people fail to understand that energy in the battery which was
supplied intitially by the engine is only 9% efficient.  Thus one
should try to aviod letting the engine power charge the batteries.
They can likely get more efficient energy directly from the engine,
provided they apply sufficient torque to the engine.

			David Lawyer