Hybrid Car – More Fun with Less Gas

Question about MPG - Page 5

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Posted by NeoPhyte_Rep on August 15, 2006, 12:23 am

Steve Pardoe wrote:

Point of clarification:

The Prius does not wait for the application of the brake to begin the
regeneration cycle.  I've seen the display indicate regeneration at 50
miles per hour when the conditions are adequate.  This is why steady
driving is a good thing.  If the electronics detects a request for 50
steady mph and there is a down slope of as little 2%, the controller may
turn off the engine, turn on the motor, and may even begin regeneration.
  If the down slope is even more severe, but does not require braking to
control the speed of the Prius, the electronics will still take
advantage of the situation.

Your second point is offset somewhat by the continual running of a
non-hybrid to support the ancillary requirements.  The Prius
air-conditioning is run off the traction battery, thus allowing complete
shutoff of the engine at any complete stop.  Given that some of our
intersections require minutes of stopped cross traffic, that can be a
major savings.

Probably the reason we don't have hybrid diesels for the U.S. is the
high sulfur content of the petroleum we receive from our sources as
opposed to the lower concentration in the supplies sent to Europe.
There is a new requirement to lower that content at the refinery that
should lead us to diesel hybrids as the best of both technologies.

In fact, I hope someone takes a close look at a serial hybrid (the Prius
is a parallel hybrid in that both sources directly drive the wheels).  A
constant rotation diesel can be more efficient than a variable speed
diesel.   It would seem a diesel that charges a battery or capacitor
that runs the motor would be the ultimate combination.  And then, if we
plug it in for the first twenty or forty miles of power requirements, we
may have the best energy balance compromise.

Posted by Steve Pardoe on August 15, 2006, 1:35 pm

What? If the Prius is charging its battery while the engine is running, but
you're not applying the brakes, that's got to be generation, not

Again, if the (electric) motor is turned on, it's hard to see where the
regenerative energy is coming from!  Either the car is slowing and
converting some of the energy (which in a conventional car would be lost as
heat from the brakes) into battery charge, or it's drawing power from the
battery to run its electric motor. It can't be doing both at the same time.

I presume this refers to cruise control 'braking' the car?

But surely no _net_ saving at all, since all the charge taken from the
battery during rest has to be replaced by energy from the gasoline engine at
some point (unless you believe in perpetual motion)?

Plus of course the fact that until recently gasoline in the US has been so
cheap and plentiful that there's been no incentive to look for alternatives.

Yes, though it will always be more efficient to use the internal combustion
engine (gasoline or diesel) directly, rather than using it to drive a
generator to charge a battery to drive an electric motor to drive the
transmission, since you have significant conversion losses at every stage.
As I understand, it the main advantage of the battery in the Prius is that
it boosts short-term acceleration (especially in the urban cycle) without a
corresponding increase in instantaneous power demand (and hence emissions)
from the IC engine.  Energy recovery from regenerative braking is a bonus,
but only of secondary importance.

Good debate, thanks for your input.

Posted by Bill on August 15, 2006, 4:37 pm

The motor/generator used to propel the Prius is a motor when accelerating, a
generator when decelerating.   Applying the brakes with light-to-moderate
pressure increases the charging rate which in turn increases the generator
load.  Sort of like this:

Take foot off accelerator pedal = 1/3 charge rate.
Apply light pressure to brake =  2/3 charge rate.
Apply moderate pressure to brake = full charge rate.
Apply heavy pressure to brake = mechanical brakes engaged.

Yes it can.  The "(electric) motor" is an (electric)  motor/generator.  Take
your foot off the accelerator pedal and it reverts from motor to generator.
The generator is driven by the front wheels, not by the gas motor.

With or without the cruise control engaged, the motor/generator reverts to
generator whenever kinetic energy can be recovered.

Actually the energy taken from the batter during rest  was kinetic energy
recovered as the car coasted and/or braked to a stop.  With a good tail
wind, some of it is actually wind energy.

Steve, your last paragraph doesn't describe the Prius drive system.  I
believe Toyota calls it "hybrid synergy drive" because the gas engine, the
electric motor/generator or both simultaneously propel the car.  Remember,
energy is recovered from both braking and coasting.

Posted by Steve Pardoe on August 16, 2006, 2:08 pm

Yes, I understand that.

<snip more details about regeneration which I also understand>

...but surely, surely, you see that all that energy came from the gasoline
engine in the first place, which was my point?  Sure, in a conventional car
slowing / braking energy is wasted, but even with a Prius there is no free

Oh, come on...  ;-)

...and it all comes from the gasoline engine to start with!  Look, all I'm
trying to find out (as per my first post) is what mileage drivers such as
you actually get per (US) gallon you put in the tank, as opposed to what
Toyota's dash gauge (whether or not we agree it's a trip computer) tells
you.  I don't think the Prius "knows" how much gasoline you put in at any
particular top-up, so the only reliable way (as the OP asked) is to keep a
log and do the math yourself.  Looking at the table at GreenHybrid.com it
seems most drivers are relying on the dash indicator, "by display" in the
jargon.  You may say that "Frankly, both methods are so close as to render
further discussion pointless" but I'd like to see some evidence to back that
up.  That's all.  I've told you what I get in my European diesel car, but
(unless I missed it) you haven't told me what you _really_ get in your
Prius.  All I get in this forum are explanations of how the Toyota system
works - which I accept is very clever - but that's not my point.  I'd be
interested in a car that was truly as economical as claimed but not if (in
fact) it's easy to get the same economy with a much cheaper diesel engine.
I'm just trying to make a fair comparison, is all.


Posted by Bill on August 16, 2006, 4:34 pm

Do you have any idea what a 30 mph wind does to mileage?  Where I live, wind
is the rule, not the exception.

  I don't think the Prius "knows" how much gasoline you put in at any

I calculated my own mileage for the first few tanks.  My calculated mileage
was essentially the same at my displayed mileage plus or minus a couple of
percentage points.  My worst tank was 48 mpg.  My best tank was 55 mpg.
There.  You have it.

To see what our EPA is contemplating, go here:


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