Fuel economy of Prius at Idle

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

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 :-)

Chas

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

Correct. In addition to that, the daylight running lights were off. The trick is to set the parking brake and restart the car. This will keep the daylight running lights off until the car is put in gear and/or the parking brake released.

Bob Wilson