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Fuel economy of Prius at Idle

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Posted by techman41973 on February 22, 2008, 5:52 am
I am on the road most of the time for business and I am considering a
2008 Toyota Pruis. Often my car becomes my office for up to a few
hours a day. The engine must run to keep the car cool or warm,
depending on the outdoor weather of the region I am in.
My Accord burns about 4/10 of a gallon per hour. I would assume that
the prius would burn far less since it would stop the engine
periodically. Does anyone have any data on the idle fuel economy of
the prius with the heater on on a cold day and/or with the air
conditioning running on a hot day?

Posted by Michelle Steiner on February 22, 2008, 6:51 am
In article
 techman41973@yahoo.com wrote:

The air conditioner runs off the high-voltage battery, so the engine
will run only when it needs to recharge the battery.

Support the troops:  Bring them home ASAP.

Posted by Chas Gill on February 22, 2008, 9:44 am

Cabin heat is, however, supplied directly by the engine (via the circulating
coolant - a conventional heater, in fact).  So, in the Winter, the engine
will run periodically to maintain cabin heat and in the Summer periodically
to recharge the HV battery and maintain cabin "cool".  As the laws of
conservation of energy apply here a first guess at what is going on would
suggest that heating and cooling the Prius will cost no more or less than a
conventional car.

However, in the case of cooling, whatever charge is in the HV battery
resulting from regenerative breaking, etc just prior to the idling phase
will, of course, contribute to the cooling energy requirements, thus saving
a bit of gas.  Furthermore, the efficiency of the cabin heat controls and
their influence on the decision to run the engine will affect the gas
consumption required to maintain heat.

My direct experience is that the engine runs infrequently to maintain cabin
temperature in all but the coldest (UK) conditions and resembles the action
of a domestic boiler, i.e. it responds to the heat demands of the cabin
thermostat, rather than running all of the time and just dumping the excess
heat to atmosphere (via the radiator).

However, running the aircon at idle in a typical British Summer (getting
hotter these day, people.....) the HV battery rapidly discharges and then
calls upon the engine to maintain the charge.  Once this happens it is the
gas that is providing the energy for cooling, via an electricity generating
system, which has to be less efficient than a direct mechanical cooling
compressor turned by the engine.

So, on balance, I would suggest that it is significantly more economic to
keep the Prius cabin warm than that of a conventional car, but probably only
marginally more economic (depending on the elapsed idling time) to keep it
cool.  Over long periods (more than about 15-20) minutes it is probably more
expensive to cool a Prius than a conventional car.  However, I have no
numbers to back this up!

Contributions to the debate welcome :-)


Posted by Bob & Holly Wilson on February 22, 2008, 10:58 am

I can't answer for AC but I have done idle experiments with my 2003
Prius and found:

0.06 gal/hr - no load, just idle in 40-50F weather
0.25 gal/hr - with 1 kW load in same weather


Bob Wilson

Posted by techman41973 on February 22, 2008, 11:04 pm
 Thanks Bob. When you say "no load" can I assume that the heater in the
pruis was turned off?

On Feb 22, 2:58am, bwilson4...@hotmail.com (Bob & Holly Wilson)

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