Lousy mileage due to heat wave?

Increased A/C use due to high temperatures takes a lot of energy. That either comes from the engine (when it's running) or from the battery. In my own low-speed commute, 100F gets me ~55mpg at most whereas I can consistently break 65mpg when temps are a bit cooler. And I have noticed the hesitation to use the battery, though I don't know exactly why. It could be programming to keep SoC high enough, or it could be that all that load plus the heat puts the battery at the high end of its temperature comfort zone. If it were to get too warm the controller would limit current draw, meaning more work for the ICE than normal. (The same thing happens when it's below freezing.)

Long story short, normal. All cars take a hit when the A/C gets cranking, we just get testy about it because the baseline is higher.

 

If the temps inside the car get too hot the Prius limits battery use to keep it from over heating. I always drive with all the windows down for the first 3 minutes or so to vent out all the heat, then I turn on the A/C when I roll them up. Some suggest opening the right rear window for a full 5 minutes to let the air flow into the battery vent on the right side of the rear seat. Give it a try and see if it helps. Also you don't want to have a cover or cloth covering the vent.

Its usually best to use the A/C on 90+ days especially if its sunny. I think 75 on the Auto AC feels quite comfortable.

 

I was reading another thread about how the batteries work best at room temp (~77deg), so I have been doing an experiement:

We are in a heat wave here in the piedmont of North Carolina (Raleigh)
10 straight days of upper 90's and several over 100deg!!

It has taken a few days to get used to it, but I have been driving with the windows down and leaving the A/C off to see if I noticed any difference.

The MDS says it all! I'm now at 65.4 mpg for 375mi. on this tank. This 30 minute screen was missing the first five minutes which has been consistently between 50-60mpg! (I've even had a couple 70+ readings)

There is no loss of milage for me as the temp increases inside the vehicle. In fact, I am having a better tank now than during the regular warm weather before that was close to ideal 77deg.

Thanks for listening...

Jim

 

My typical trips are in town. This trip was running around town at 25-45mph. I use the "B" mode a lot and make people crazy behind me as I coast up to red lights, but I figure I'm doing us both a favor that way

The truth is "Yes" ...aerodynamics are probably lost, but I'm not up for the "Keep the windows closed at 100deg." experiment!

Jim

 

If anything my mileage has improved in the heat. I drove 128 miles yesterday at 55.4 mpg. The first leg of the trip was 57.5 mpg. I probably could have maintained 57+ mpg but rush hour traffic kind of screwed me up. This was on a trip that was about 80% highway @ 55 - 63 mph. I've been using SHM (Super Highway Mode) lately and it definitely makes a difference.

AC will hurt your mpg. You can gain some mpg by working the AC button, or you can roll down the windows. I crack all 4 windows which provides a nice breeze with minimal wind drag.

If you are getting bad mpg look at the gas you are using. Cheap gas costs mpg. Fuel with ethanol will rob you of mpg. Stay away from it if you can (it's getting hard to do this.). You might save a few cents at the pump but you really save nothing because your mpg will drop. Read the labels on the pump to find out what is in the fuel you put in your chariot.

Cheers,

 

Heat wave?? ound:
In Florida we call that summer.
My mileage does not drop like that in the summer so it must be something else causing your drop in mileage.

 

In AZ we call that early spring.

 

Haha lived there last year, no kidding...out here its only 80 but with the 60% humidity sure feels hotter and my MPG last takn was 48, now its 43-44 for this one....maybe its my driving style

 

60% huidity? Is that even possible. I thought 80% was the minimum required by law. Hmmmm.... Maybe that's just a Florida law.

Bob

 

I got 55.9 MPG on the tank just before the heat wave (I'm in the Central Atlantic Coast). Now that the temp and humidity have dropped (about four days in) I'm slowly recovering through 49 MPG.

A/C definitely can slam your MPG.

 

Has anyone noticed if humidity alone, with temps being constant will affect mileage much ?

I understand there is a humidity sensor for the climate control. If the AC runs longer with high humidity I presume the MPG hit is greater.

I tried the AC in my new 2008 and saw my 20 mile commute drop from about 60-63 US MPG to about 53 MPG. This was a 35 minute commute max 37-43 MPH with AC set to 75, outside temp 85 and outside humidex about 104 F.

With AC time dependent, I'm wondering if I should speed up a bit; say 6 MPH more which is over speed limits on my route, but reasonably cop safe.

This could save me about 5 minutes of AC and commute time.

In any case, I'm seeing now that AC use adds another factor to the equation of optimum speed for MPG.

 

I have seen the same. Weather was in the 90s a few weeks ago, and my AC was out. Mileage went down considerably once the AC was fixed

I think the problem is even in 90-100 weather, the air in the car can reach 150 or so sitting in the sun. Thats the kind of range where the battery controller will be limiting discharge, so you've got to get the temps down (or park in the shade so it never gets up there to begin with). I keep my auto on 80-82, and park in the shade whenever and wherever possible and have gotten most of the mpgs back. Using auto over blasting on high does seem to make a big difference

Rob

 

A few thoughts on summer heat and AC ....

All things being equal, the warmer the outside air temperature the better the fuel economy. But we humans have certain wants -- like not staying in a closed box in the sun with no air circulating. So we run the AC or open windows. Either can have an adverse effect on FE.

AC use will affect Prius FE more in driving conditions where the ICE is not needed for propulsion: stop & crawl, pulse & glide driving, and other low speed conditions. The AC runs off the battery, but eventually the ICE will have to run to recharge it. Opening the windows affects it more at higher speeds where aerodynamics become more important.

The Prius MPG Simulator performs modeling using a variety of user-specified criteria, including speed, outside air temperature (OAT), automatic climate control (ACC), and the following combinations of window positions: all up, all down, two halfway down, and two diagonally opposed 1" down. This chart shows simulator results with 90F OAT, with and without ACC set to 75F (the temperature I find most comfortable in the summer), and all window position models.



The models assume the following:

• Steady speeds
• Level terrain
• No wind
• 190' elevation
• 30.00 barometric pressure
• Smooth dry pavement
• Michelin Hydroedges inflated to 50/48
• E10 fuel
• 300# load, driver + cargo

The conclusions:

• All windows up and no ACC gives the best results across all speed ranges -- for the brave soul who can tolerate it.
• Two diagonally opposed windows 1" down is next best, and might be tolerated by some at high speeds.
• Two windows halfway down is superior to ACC at speeds below the low 50s.
• All windows down is superior to ACC below 40 MPH.

Specific values of course will change somewhat with different combinations of typical summertime OAT and ACC thermostat settings, but I doubt the overall relationships change much.

A hot hybrid battery muddies the waters. As OAT approaches 100F, battery temp can do the same. I agree with Rob about keeping the car in the shade. In the peak of summer heat I always look for a shaded spot when I expect to be parked for an extended period, even if it means a longer walk. (The exercise sure won't hurt! ). I had an episode last summer of sluggish performance and a spontaneously aborted glide that was likely caused by high battery temperatures. The battery cools very slowly, whether with AC or windows open, and can heat up even more with operation. Better is to prevent it from getting hot in the first place.

Someone asked about the effect of humidity. All things equal, the effect of outside air humidity is pretty insignificant. The simulator shows an increase of 0.3 MPG with an increase from 10% to 90% relative humidity at 55 MPH. (I know you Floridians think 10% is physically impossible. Traveling in Las Vegas at the moment I can tell you it happens. ) However, it seems intuitive that the ACC might work harder in humid conditions. I'll leave it to others more familiar with the technical side of ACC to comment on that.

 

One thing that seems different from me to what I am reading is the way we park the cars in the sun. My car has the darkest tint possible (which in FL is darker than most other states can get). I have those panels in the back seat windows you can get to block the sun from kids. Finally I have a really good sun reflector I put in the front windshield. Doing all these things allows me to come back to my car even during the hotest times of the day and get into a reasonably comfortable car. I am able to set the AC to 80. It blasts at medium speed for a minute or two and then settles right down. Maybe other people are getting bigger hits in MPG because of the time it take to cool your cars down and to keep them cool? I have been getting 52-54 MPG pretty consistent.

 

I've had to change my driving style to compensate for the heat, including different routes which previously allowed for more pulse and glide or low-mph driving. While the engine takes about 20 seconds to reach operating temp, the A/C (I use a windshield block) takes a while to cool off the cabin even after cracking the windows during parking and assuming I'm using recirc. I now choose routes that may include more ICE useage, but also reduce dependance on the battery to run the A/C.

I am averaging 48.56 (MFD) mpg lifetime since October of last year and I travel roughly 420+ miles per week, 1/2 rural and 1/2 city.

STILL beats the SNOT out of any ICE out there except mopeds... I think...

 

LOL!

I thought we just called it "Hot". You know as in:

Northerner: Don't you miss having seasons living in Florida?

Floridian: What do you mean? We've got seasons!

Northerner: You do?

Floridian: Yeah, there's three of 'em: Tourist, Hurricane, and just plain Hot.

 

You forgot "Snow Bird" season.

Bob