A/C and fuel economy.

I'll need to try this when it gets cooler because we get around 108-114F here and I need the A/C on but I want to see if my MPG increases after giving it a try.

 

We usually turn the vent system off completely, just adjust window opening. The fan just seems to be pumping heat in, defeats the purpose. It's nowhere near your temps here, but it's been regularly over 80F, a lot of the summer.

The AC definitely impacts mpg, I'd guesstimate between 5 and 10 percent.

 

That's a huge jump. Wouldn't have expected it.

After experimenting with AC on/off when I first got the car, I found it did not have a great effect on gas mileage for my normal 40 mile commute, which is half highway and half rural/city. It dropped from an average of 60 mpg to 58 with AC on. I was so thrilled to be getting mileage in that range that a difference of 2 mpg didn't phase me. But at that point (this is my first hybrid) I hadn't learned any of the driving techniques for increasing mileage and was just getting to know the dashboard gauges. Now I get 60 with AC on pretty regularly. But I drive in ECO mode all the time. Was your test with ECO on or off?

The only time where I saw AC use dramatically effect mileage was when stuck in a traffic jam for 30 minutes in 90 degree weather. The AC runs electrically so normally it doesn't need the ICE to keep it going. But in this jam, going maybe two mph when we were moving, and not moving most of the time, the AC was chewing up battery at a fast pace and the ICE had to keep coming on just to charge the battery to feed the AC use.

Your difference of 52 to 62 mpg running at 55 mph seems too much, but at 100 degrees, who knows. If that wasn't with ECO on, it would be interesting to see the difference.

 

I normally run the A/C with the HVAC system set to "recirculate" on the basis that it doesn't have to work as hard with the pre-cooled interior air as it would with the hot outside air. There's still a small percentage of outside air that enters the system even in "recirc" mode, but it's pretty I haven't done any formal testing, but I've never noticed the A/C sapping my fuel economy.

 

no, not normal, but i'm also not sure it's a very scientific study.

 

We don't use the AC much to speak of through summer. I know, stoics, lol. Mainly just rely on windows. If and when the AC's on, the only time I'd set it to recirc is following a smoky diesel.

Most of our AC use is in the rain, to keep the windshield clear of fog. If you set mode to Heat/Defog the system will use AC, sometimes, without the AC button being pushed *. I'll try that tactic first, and if it's still fogging, push the AC button expressly.

* Monitoring the AC compressor wattage, with ScanGauge

 

Although I have had my C just 3 weeks, I had an "original flavor" Prius (2001) for 16 years. The strategy I developed to maximize both comfort and mpg was to always turn off the A/C before pulling away from a stoplight/sign, so as to maximize how far I could get without the ICE coming on. Once the engine came on, I would turn the A/C back on as well. I also frequently waited to do that for a bit longer if it wasn't too hot out. This should work as a strategy with the C as well.

 

Maybe a PassMaster would work for that, if you can still find them. It's a vacuum switch, open unless your intake manifold has decent vacuum. Under heavy acceleration vacuum drop, switch becomes open. You splice it into compressor power wire.

It might throw a code tho, with today's cars.

 

I'm not that sophisticated - I just manually press the A/C button off and on. Works fine.

 

Well, ferrying someone around all day, with the AC on more-or-less constant, manage to keep the displayed liters per 100 km at 4.2, where it was at the outset. Still that's on a tank with about 150~200 km since the trip meter was reset. But I'm just not a big fan of AC, still.

 

If my understanding is correct .... the ICE does not power the A/C compressor. From that standpoint, it should make no difference.

But I agree, there is a fuel efficiency hit when the A/C is on ... and I can only assume the ICE must run more to replace the electricity the compressor use. OTOH, the vehicle is not as "slippery" with the windows open except at very slow speeds.

Personally .... 100 degrees in humid Louisiana .... I don't care how much it drops the fuel efficiency .... it is worth running the A/C. I lived in Louisiana as a child (before A/C) .... never again without A/C.

 

That is more or less what the air-conditioner ECO mode does. Also, running the drive in ECO mode will also do something similar

 

I have owned and driven a Toyota hybrid since late 2009 ...... and never once driven in anything buy ECO (occasionally EV, but not very often). Am I missing something?

 

Correct. But the A/C does use the traction battery, so turning it off before acceleration leaves you more energy available from the battery, allowing you to get to a higher speed before the ICE comes on, thus saving gas.

As to the functioning of the eco modes equalling the strategy I employed with my 2001 Prius, I have not had the new C long enough to know if that is exactly the same thing or just almost. I would be happy to be enlightened.

 

then this begs the question. if outside temperature is 100° and say temperature inside car is 130° would it be better to not use recirculating until it get cooler inside car then switch on recirculating?

 

same. we could be missing an unbelievably exciting experience!

 

I leave all the doors wide open with A/C on for the first few minutes and then close them back and use recirculating if needed.