Hot Weather Surprise!

Don't those numbers just make you grin?

 

The AC didn't cause that mpg difference. The route did. If you could run the same route reversed I better it changes the outcome. Even mild grades or road surface condition can affect mpg.significantly. Nice numbers though!

 

There's more at play than just the A/C. Humidity definitely contributes to higher efficiency, even in the winter. The influence of route & traffic are big factors too.

That being said, DON'T DO IT AGAIN. Allowing the battery-pack to get extremely hot will shorten its life. It should remain at a comfortable temperature, which is why the vent shares the same cabin air as you. Cooking the chemicals is something best avoided. If nothing else, it's a good excuse to indulge with the A/C... which is surprisingly efficient anyway.
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18 miles @ 51 MPG = 0.353 gallons = 5.648 cups of gasoline
18 miles @ 76 MPG = 0.237 gallons = 3.792 cups of gasoline

The return trip used an extra 0.116 gallons (1.856 cups) of gasoline.

If we assume for the sake of this discussion that 85% of the increase was do to the A/C (leaving 15% for other factors such as grade, headwind vs. tailwind, battery starting charge vs. ending charge, etc.), and we assume that gasoline prices near you are $3.80 per gallon.

You used 0.85 X 0.116 = 0.099 gallons for A/C costing you 0.099 X $3.80 = $0.38 to keep cool for the duration of the ride.

If your ride took you about 40 minutes that works out to about $0.01 per minute for the A/C. ;-)

 

Thanks for the mathematical perspective, Danny!! I've been starting the AC at 88 degrees or higher (99 here today) but may back down on that at a penny a minute!

I find it amusing that I am now disappointed when I see 54 MPG on the display rather than 58 MPG!

 

How much elevation difference exists between the endpoints?

If the engine did not cool significantly before the return trip, the pre-warmed engine could have accounted for as much as a third of the difference. Without AC, an elevation change of about 400 feet could account for the rest.

With a warm engine, good conditions, and no AC, this is not ridiculous. It is well within the capability of this machine.

 

My commute is 50 miles one way. Chicago area. Per my prius, outside temp was 99 today and yes it looks like high outside temp gives better mileage. I have always been getting ~55MPG but my current tank so far is 65MPG this week. I keep my AC on all the time set at 78.

 

Last year when we were in Reno Nevada temps were in the 90's and
even with ac our Prius was mid to high 50's MPG's.

Thats even with a trip up to Lake Tahoe.

The big hit in MPG's is winter here in the northwest when temps are in the mid 30's to 40's, rain soaked roads, and constant rain for days on end.

MPG's than tank into the mid to high 40's MPG.

alfon

 

Let's assume the one-way trip took about 30 minutes and the A/C was using 1500 watts from the battery (which is reasonable if you are maxing out the A/C the entire time), then you used .75 kWh of energy from the battery.

Let's assume an overall thermal efficiency of about 20% to convert energy in gasoline to energy actually used by the A/C. This is not unreasonable since max thermal efficiency of the ICE is 38%, and we have conversion losses to charge the battery, then discharge the battery. Remember the A/C runs off the battery, primarily.

So to get 0.75 kWh to the A/C, we required 3.75 kWh of gasoline, which is 0.111 gallons of gasoline. (1 gallon of gasoline has 33.7 kWh of energy)

The 0.111 gallons of gasoline compares favorably to the differential of 0.116 in the actual case.

Of course I am guesstimating many things, but since the order of magnitude is the same, it is not unreasonable with level terrain that you could have seen a 20+ mpg drop due to the A/C alone, if you are getting 76 mpg to begin with.

 

Also in Chicago here. The last few weeks have been great for our hybrids, too.

 

Sounds like you had a tailwind. a 10mpg jump is not consistent with mild temperature variation. From dead cold and wet winters to spring weather maybe but not from day to day fluctuations.

 

I also suspect the state of charge of the traction battery. If it wasn't full outbound the ICE ran longer to charge it. The inbound trip reaped the benefit. For short segments the reported FE is extremely volatile. I can knock it off the scale for 2 miles but it averages out.

On a given tank, my average fluctuates about .5mpg every day across 2 trips. Sometimes I recover more than I lose, sometimes I don't. Higher numbers are addictive.

When the MID drops below 60mpg I know I'm looking at an average tank.

 

So you started with a cold engine on the first trip and an already warmed up engine on the return trip? Then you're comparing not so much AC vs. no AC but starting with cold engine vs. warmed up engine.

 

As a side note, for the last few weeks in Chicago, I've found the "break even point" to be around 2-3 miles. Once I've established a typical good fuel economy on a tank, that's about how far I need to drive to maintain it. For the first mile or so it drops then it starts to pick up again. It can be double that distance when it's below freezing. I'd say about half my trips are below 3 miles, which is why my winter fuel economy drops so much compared to those with mostly longer trips.