Hybrid Car – More Fun with Less Gas

prius worth getting? - Page 5

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Posted by obob@ on November 12, 2004, 4:21 pm
I am seeing more and more Prius' in the parking lots of the local

If you do a lot of city driving, this rock solid reliable car should
pay back within a year or two.

On Thu, 04 Nov 2004 20:09:23 -0600, richard schumacher

Posted by Lou Stewart on November 13, 2004, 7:42 pm
Prius is good for city driving, but don't sell it short!  In the past few
months, I have taken my 01 from Missouri to Easton, PA and home again, to
Austin and back, and most recently to Atlanta, Mobile, Lake Charles, LA, and
home again with flawless performance, significant velocity, and good
comfort, while getting somewhere between 50 and 60 mpg.

Posted by nobody on November 16, 2004, 3:33 pm
 On 3 Nov 2004 01:54:45 -0800, Chewy2426@aol.com (Aaron) wrote:

A very good question.  Because I was considering buying a Prius, I
made some energy calculations to get an idea of how much gasoline is
"recovered" through the "regenerative" braking.

The energy recovered from braking the car and subsequently storing it
in the battery is some fraction -- less than 1 -- of the kinetic
energy of the car.  For purposes of calculation I assumed 100% would
be recovered.  The kinetic energy depends on the mass and speed of the
car and is calculated from the familiar equation KE = 1/2 * m(v^2).
I used 2890 lbs for the weight of the car as given in the Prius
brochure  (mass = 2890/32.2 = 89.75 slugs).  The results at several
speeds are given below:

20 mph -   50 Btu
30 mph - 112 Btu
40 mph - 199 Btu
50 mph - 311 Btu
60 mph - 448 Btu

For illustration, let's assume that if the Prius went 45 miles with no
stops it would get 45 miles per gallon.

To estimate the effect of recovery by regenerative braking, assume
another trip of 45 miles with stops every mile from 30 mph.  That's a
total of 45 stops which would recover 5,040 Btu.  Gasoline has a heat
of combustion of approximately 115,000 Btu per gallon, thus the 5,040
recovered Btu are equivalent to 5040/115,000 = 0.0438 gallon (5.6
ounces).  We can subtract that from the gallon burned to get
(1-0.0438)=0.9562 net gallon consumed to cover the 45 miles with 45
stops.  The mileage with 100% regenerative recovery would be 45/0.9562
= 47 mpg, or about 4.5% better.

The recovery cannot be 100% because the hydraulic brakes come into
play, there are losses in generating, transmitting and storing the
recovered energy in the battery, and further losses in extracting the
energy from the battery and utilizing it to power the car.  I have no
idea what the overall recovery factor should be, but a 50%  value
would reduce the mileage value to 46 mpg, which is about a 2.2%
improvement over no regenerative braking.

Can't estimate how much gas is saved by stopping the engine at traffic
signals. etc, because I don't know the rate of fuel consumption of the
engine at idle.  However, the Prius brochure lists miles per gallon at
51 hwy and 60 city, and one newsgroup poster (Jeffry Johnson) reported
44 hwy and 52 city.  Both sets of data provide a crude estimate of
overall mpg improvement of about 18%.  Assuming the city mileage
improvement is due to braking + engine stopping (because there is
neither on the highway) and using 3% as the braking effect  gives a
15% improvement attributable to engine stopping  (any drag effects

I pondered these results for awhile and it became apparent that the
advantages of the hybrid technology are not so much from regenerative
braking but rather from two factors: 1) stopping the engine at vehicle
stops, and 2) employing a small gasoline engine with an electric motor
for power boosts when needed. Ultimately, the gas engine has to
replace the energy expended in those boosts since gasoline is the sole
source of energy in the vehicle.

I know this is a bit long and that it does not answer the many
questions that can be raised, but it helped clarify my thinking and
maybe it will help some others.  Personally, I'm going to hold off on
buying a hybrid for a while.  Maybe a long while. Economic analysis on
the cost difference of the hybrid versus the non-hybrid when compared
to gasoline savings using current prices just doesn't justify
purchasing the hybrid. Come to  think of it, one can almost never
justify purchasing any new car on an economic basis :-)


Posted by GAN on November 17, 2004, 12:34 am
 Here is a good technical description of the Prius Hybrid System.


On Tue, 16 Nov 2004 09:33:03 -0600, nobody@home.net wrote:

Posted by nobody on November 17, 2004, 1:56 pm
 On Tue, 16 Nov 2004 19:34:01 -0500, GAN <> wrote:

Thanks GAN for the excellent reference. Recommended reading for anyone
contemplating buying a hybrid auto.


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