Prius Heating/Cooling & Fuel Economy

I had one installed (here in Victoria). I think it was $150 installed. Since it almost never snows here, I did feel a bit awkward. But I used it in the Spring and Summer and I think it helps. Just get a programmable timer and a big sign for the windshield with "UNPLUG ME"

 

Attempted tests on my Prius, in the style of my A4 tests

I finally had a try with the Prius yesterday. The test site was a stretch of the main highway that goes past the VLA, some miles west of Magdalena NM.

My primary conclusion was a sort of null: my test methods failed to give sufficiently reproducible results to provide useful distinction among the five air conditioning/windows options I wished to compare.

I arrived at the test site after many minutes of cruising at a stable air conditioning setting which was comfortable for me (two dots above L on the fan, 74 commanded temperature, recirc on, both outlets selected).

My intent was to start from about a half mile before the test zone, accelerate hard to 70 mph, engage cruise control several seconds before the zone beginning, hit the mpg reset button at beginning of zone and log the mpg average at the end of my 1 mile zone coasting down to a turnaround a half mile later. I had a gps giving distance to one zone end to help me gauge things, but actually used two signs as the exact endpoints.

As before I intended to combine runs at the same condition in both directions.

The difficulty with reproducibility came, I think, not from wind variation or speed setting error but from the varying state of the battery regarding both charge and temperature, and the varying response of the car's control system in choosing electrical power or charging on various runs.

At arrival, I had cruised for a couple of hours in New Mexico strong sunlight, but with very little regeneration going on. The battery temperatures I could read on my scan gauge were in the low 90s F. I think the state of charge was near 53% (it had stayed very close to that during my trip down the Interstate at 75 mph) By the end of five cycles (10 start/stops), the battery temperature was approaching 130 F (no fan, I don't know what temp turns that on). Battery currents were high during both acceleration and deceleration, staying over 100 Amperes for much of the accel, and starting at something higher than I expected on initial decel from 70 (about 40 A, I think).

Long post, two actual points:

1. Comparison testing of configurations, always somewhat difficult, is made much harder on the Prius by the extra variables presented by non user-controllable state in the hybrid system.

2. I won't actually be attempting to post my own Prius data on this question after all.

I continue to believe that the balance of advice out there in the general marketplace of ideas is mistakenly urging people to prefer air conditioning over partially open windows for economy, while the opposite is almost always actually more economical where comfort is adequate.

One other point: on the way back home I cruised comfortably at 65 mph with no A/C and the two passenger windows cracked about an inch. All this in full New Mexico September sun and indicated ambient air temp about 87F. The battery temperatures, still remembering their few minutes of violent exercise, continued to drop, and got down to 95F with no help from cabin A/C.

So I'm not impressed by the argument that use of cabin A/C in preference to partial windows is essential for Prius traction battery health either.

I did notice that the overheated battery temperature dropped very little while parked for half an hour, and that the rate of drop while driving picked up at higher speed. It seems that the provision of passive air flow from the cabin flowing over the battery and exiting outside works pretty well when the car is moving fast enough to provide the required suction at the outlet.