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Poor gas mileage in 100F weather

heat texas climate

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#1 OFFLINE   jackalopetx

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Posted 24 July 2017 - 09:33 AM

It's been in the 100s every day in Austin, and my gas mileage is suffering. I'm down to 32mpg average, and on some trips I only get 25mpg! I know part of it is due to the AC having to work so hard, but is it also related to the temperature of the battery? Is available battery capacity reduced when it's hot? The temperature inside the car reaches 140F even with windows cracked, so even after being parked all day the battery is already hot. Maybe these cars would have benefited from automatically running the fan when parked like the solar Prius models do









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#2 OFFLINE   Plus 3 Golfer

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Posted 24 July 2017 - 10:38 AM

My experience from 3 yrs in Phoenix with my C-MAX at 100F+ is that AC will hit FE hard on very short trips. This is because the AC compresser load will be between 4+ and 2 kW for 2-4 miles for the initial cool down of the cabin.   What is your normal FE compared to the 32 mpg?  Also on short trips, if your state of charge of the high voltage battery is low to start and higher when the trip ends, FE will take a big hit.  What are the length of the trips that give you 32 and 25 mpg? 



#3 OFFLINE   SnowStorm

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Posted 24 July 2017 - 11:07 AM

Of course AC is a big culprit but it seems I have a drop in mileage at higher ambient temperatures even when not using AC at all and windows up (just the fan).  Is it possible that tires soften enough to increase rolling resistance or that the road surface gets sticky due to hot tar in the pavement?  From air density stand point, mileage should just keep going up but I don't think it does.  It would be nice to get some data on cold versus hot roads (although concrete shouldn't have the sticky tar problem).



#4 OFFLINE   jestevens

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Posted 24 July 2017 - 12:28 PM

Car definitely loves slightly cool weather.

 

The average mileage I used to get on my HHR was 25MPG, the best I -ever- got was 30MPG for half a mile on a very straight, long, stretch of highway.

 

I average 40MPG in the CMAX, for short hot, high humidity trips in the summer it might be 23MPG, in the winter @ 4F it might drop to 15-32 MPG but most trips 35MPG or higher .. still better than most gasoline cars.

 

As far as A/C vs. no A/C - If you are comfortable chances are your battery pack will be comfortable too.


Edited by jestevens, 24 July 2017 - 12:32 PM.


#5 OFFLINE   Plus 3 Golfer

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Posted 24 July 2017 - 12:56 PM

Of course AC is a big culprit but it seems I have a drop in mileage at higher ambient temperatures even when not using AC at all and windows up (just the fan).  Is it possible that tires soften enough to increase rolling resistance or that the road surface gets sticky due to hot tar in the pavement?  From air density stand point, mileage should just keep going up but I don't think it does.  It would be nice to get some data on cold versus hot roads (although concrete shouldn't have the sticky tar problem).

High humidity also helps mileage (water vapor molecules weigh less than N2 and O2).  So, dry air is more dense than humid air and cold air is more dense than hot air.  So, I don't know off hand where or if the crossover point is reached.  It certainly can be calculated over a range of temperatures and humidity.  And of course, higher temperature generally results in lower humidity.  In PHX you can start out in early morning at 90F+ and 35% humidity and by afternoon the temp is 110F with single digit humidity.  So, it's possible under the right conditions that FE might not continue to go up with temp. because humidity is falling.

 

On road surfaces, I believe the alphalt softens and thus, the coefficient of friction increases.  I don't believe it's the tire softening or "tar" per se. I believe a while back I linked to a study / paper on tire rolling resistance on various surfaces.  I'll see if I can find it.



#6 OFFLINE   jackalopetx

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Posted 24 July 2017 - 01:35 PM

My experience from 3 yrs in Phoenix with my C-MAX at 100F+ is that AC will hit FE hard on very short trips. This is because the AC compresser load will be between 4+ and 2 kW for 2-4 miles for the initial cool down of the cabin.   What is your normal FE compared to the 32 mpg?  Also on short trips, if your state of charge of the high voltage battery is low to start and higher when the trip ends, FE will take a big hit.  What are the length of the trips that give you 32 and 25 mpg? 

 

Normally i get 40mpg both in city and highway. My commute is 4 miles all city. The drive home is when I get 25mpg, which to be fair is slightly uphill. I might try turning off EV+ so the battery has more charge in the afternoon when I drive home


Edited by jackalopetx, 24 July 2017 - 01:36 PM.


#7 OFFLINE   Plus 3 Golfer

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Posted 24 July 2017 - 03:49 PM

Normally i get 40mpg both in city and highway. My commute is 4 miles all city. The drive home is when I get 25mpg, which to be fair is slightly uphill. I might try turning off EV+ so the battery has more charge in the afternoon when I drive home

 

In 100F temps, your FE will take a big hit from using AC for your 4 mile compute.   

 

Let's assume your AC demand averages 2.0 kW for the 4 miles and it takes you 10 minutes.   The AC used 0.333 kWh.  Let's assume that the overall efficiency to produce that 0.333 kWh is 33.3%.  So, the fuel used is (0.333/0.333) = 1.0 kWh.  There's 3412 Btu in one kWh.  So, the Btus of gas used would be about 3412 Btu.  One gallon of gas has about 115,000 Btus.  So, about 0.03 gallons of gas was burned for the AC.   If you normally get 40 mpg for the 4 miles, you would uses about 0.1 gallons of gas.  So, using AC you use 0.13 gallons of fuel to cover 4 miles.  Your FE would then be 30.8 mpg not 40 mpg.  Now, the uphill home requires more fuel and hence your 30.8 mpg drops to 25 mpg.

 

Once the cabin cools down, the AC demand will likely drop below 0.4 kW.  So, your gas consumption to run the AC would fall considerable and the hit to FE would be significantly less.  

 

Instead of turning off EV+ which won't help, turn off your AC for the 4 mile trip. :)


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#8 OFFLINE   raadsel

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Posted 24 July 2017 - 05:45 PM

High humidity also helps mileage (water vapor molecules weigh less than N2 and O2).  So, dry air is more dense than humid air and cold air is more dense than hot air.  So, I don't know off hand where or if the crossover point is reached.  It certainly can be calculated over a range of temperatures and humidity.  And of course, higher temperature generally results in lower humidity.  In PHX you can start out in early morning at 90F+ and 35% humidity and by afternoon the temp is 110F with single digit humidity.  So, it's possible under the right conditions that FE might not continue to go up with temp. because humidity is falling.

 

On road surfaces, I believe the alphalt softens and thus, the coefficient of friction increases.  I don't believe it's the tire softening or "tar" per se. I believe a while back I linked to a study / paper on tire rolling resistance on various surfaces.  I'll see if I can find it.

 

I don't think it is quite that simple. First, while the air might be less dense (at similar barometric pressures), you then run into the issue that there are less O2 molecules for the engine, reducing engine power. Additionally, the car is harder to cool as the humidity makes it feel hotter -- and the A/C is limited in how much water it can remove from the air. Basically, you'll need the temperature set about 70 to get the car to feel as cool as 75 at low humidity. Granted, over a long enough period the A/C can get most of the humidity out of the air when on recirculate -- though it is hurt by any fresh air it brings in the cabin and periods when the compressor shuts off.

 

Also, though I admit I may misunderstand, but I believe the engine is less efficient. Not only do you have less pressure but, I believe, the engine is still trying to use the same amount of gas; unlike at altitude where it senses there is less air and adjusts the fuel used. The engine still senses the same amount of air, by pressure, just there is less O2. Worse, the water in the air is actually retarding the gasoline burning, making the ICE less efficient.

 

It is an interesting question. I'll be interested if others have more insight.



#9 OFFLINE   Plus 3 Golfer

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Posted 24 July 2017 - 06:54 PM

I don't think it is quite that simple. First, while the air might be less dense (at similar barometric pressures), you then run into the issue that there are less O2 molecules for the engine, reducing engine power. Additionally, the car is harder to cool as the humidity makes it feel hotter -- and the A/C is limited in how much water it can remove from the air. Basically, you'll need the temperature set about 70 to get the car to feel as cool as 75 at low humidity. Granted, over a long enough period the A/C can get most of the humidity out of the air when on recirculate -- though it is hurt by any fresh air it brings in the cabin and periods when the compressor shuts off.

 

Also, though I admit I may misunderstand, but I believe the engine is less efficient. Not only do you have less pressure but, I believe, the engine is still trying to use the same amount of gas; unlike at altitude where it senses there is less air and adjusts the fuel used. The engine still senses the same amount of air, by pressure, just there is less O2. Worse, the water in the air is actually retarding the gasoline burning, making the ICE less efficient.

 

It is an interesting question. I'll be interested if others have more insight.

Forget about engine power.  What you want to run is the correct mixture of gas and O2. This goal is to burn all the gas. You may loose power on hard acceleration with less dense air but generally the intake can handle the additional flow (to get more O2 to mix with the gas so power is not lost). The goal is always a stoichiometric mixture. Hence, the various sensors like mass flow sensor and the O2 sensor, barometric pressure sensor and so forth.

 

Yes, you can calcalute the enthalphy of the air at a given pressure, humidity and temperature and see how much energy is required to change enthalpy (lower temp. and reduce humidity).  I have also done this to get a handle on AC use in AZ. Yes, more humid air requires more energy to cool (heat of condensation same as heat of vaporization except opposite sign).  Again it's not a hard calculation to make but we don't know the exact conditions that we are comparing.  All you can do is make assumptions based on ones conditions.  Of course, you will get different results depending on the assumptions made.   

 

Engines are generally more efficient at higher operating temperatures.


Edited by Plus 3 Golfer, 24 July 2017 - 07:01 PM.


#10 OFFLINE   raadsel

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Posted 24 July 2017 - 07:38 PM

Forget about engine power.  What you want to run is the correct mixture of gas and O2. This goal is to burn all the gas. You may loose power on hard acceleration with less dense air but generally the intake can handle the additional flow (to get more O2 to mix with the gas so power is not lost). The goal is always a stoichiometric mixture. Hence, the various sensors like mass flow sensor and the O2 sensor, barometric pressure sensor and so forth.

 

Yes, you can calcalute the enthalphy of the air at a given pressure, humidity and temperature and see how much energy is required to change enthalpy (lower temp. and reduce humidity).  I have also done this to get a handle on AC use in AZ. Yes, more humid air requires more energy to cool (heat of condensation same as heat of vaporization except opposite sign).  Again it's not a hard calculation to make but we don't know the exact conditions that we are comparing.  All you can do is make assumptions based on ones conditions.  Of course, you will get different results depending on the assumptions made.   

 

Engines are generally more efficient at higher operating temperatures.

 

Yes, my point wasn't so much about power; it was about how you have less O2 at similar air pressures, which would seem to cause the engine to run rich -- giving too much fuel because of the air pressure. Though I'm not sure how much difference that really makes.

 

I can't speak for Austin, but Houston frequently has 80%+ humidity at 80 degrees at night and in the morning, meaning it feels like it is almost 90. As it heats up, the humidity will drop to 60%+ with mid-90s temperatures. I lived in Chandler for a summer and found that I prefer summer in the Phoenix area than in Houston.



#11 OFFLINE   Plus 3 Golfer

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Posted 24 July 2017 - 08:15 PM

Yes, my point wasn't so much about power; it was about how you have less O2 at similar air pressures, which would seem to cause the engine to run rich -- giving too much fuel because of the air pressure. Though I'm not sure how much difference that really makes.

 

I can't speak for Austin, but Houston frequently has 80%+ humidity at 80 degrees at night and in the morning, meaning it feels like it is almost 90. As it heats up, the humidity will drop to 60%+ with mid-90s temperatures. I lived in Chandler for a summer and found that I prefer summer in the Phoenix area than in Houston.

A properly maintained modern vehicle will always run with a proper fuel / air mixture not lean or rich because the PCM is monitoring the variables and continually adjusting the mixture to a stoichio mixture.  

 

Humidity will affect the AC demand.  That's why it's hard to compare AC usage / affect on FE in Houston vs Phoenix vs Austin even though the ambient temp. may be the same.  What's common though is that short commutes affect FE more than long commutes when using AC.  



#12 OFFLINE   obob

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Posted 24 July 2017 - 08:25 PM

Just a reminder, just in the off chance there happens to be someone who at some time might forget

 

"Recirculate" makes a huge difference.


Edited by obob, 24 July 2017 - 08:26 PM.

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#13 OFFLINE   jestevens

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Posted 25 July 2017 - 05:36 AM

..and until heat wave last week I forgot all about the "MAX A/C" button on the far right side - which isn't that wonderful for fuel economy but works great in 90F+ weather.  It's a little quirky but I like that our cars have actual buttons and controls.



#14 OFFLINE   Plus 3 Golfer

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Posted 25 July 2017 - 06:06 AM

..and until heat wave last week I forgot all about the "MAX A/C" button on the far right side - which isn't that wonderful for fuel economy but works great in 90F+ weather.  It's a little quirky but I like that our cars have actual buttons and controls.

Also, the MAX A/C button initiates recirculation automatically.  I don't use MAX A/C as I find normal A/C cools down the car quick enough for me and I can't stand the air and blower noise at MAX A/C.  

 

Since I'm moving back to AZ in a few months, I'll test MAX A/C to see what the power demand is compared to normal A/C at high ambient temps. Although the demand is likely significantly higher at MAX A/C than normal, the total energy requirements may not be that much higher assuming efficiency stays about the same at normal and MAX A/C. 


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#15 OFFLINE   Plus 3 Golfer

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Posted 25 July 2017 - 08:59 AM

... Is it possible that tires soften enough to increase rolling resistance or that the road surface gets sticky due to hot tar in the pavement?  From air density stand point, mileage should just keep going up but I don't think it does.  It would be nice to get some data on cold versus hot roads (although concrete shouldn't have the sticky tar problem).

I couldn't find the study I was refering to but found this info below.  It appears the higher temperature up to a point makes the tire easier to flex and thus reduces the energy loss associated with tire flexing when rolling.   Also, there appears to be virtually no difference between asphalt and concrete in rolling resistance if the surface smootness is the same.  The one study refered to IIRC suggested less than 1% increase in rolling resistance due to pavement deformation (asphalt vs concrete). 

 

Bottom line is that rolling resistance due to ambient temperature increase should likely not go up especially since the 4 mile commute is city traffic and likely goes down.

 

1) Temperature: with both solid and pneumatic tires, rolling resistance has been found to decrease as temperature increases (within a range of temperatures: i.e. there is an upper limit to this effect). For a rise in temperature from 30°C to 70°C the rolling resistance decreased by 20-25%.  http://www.ajer.org/.../R037141148.pdf

 

2)  Overall, the literature review indicated that a smoother road can decrease fuel consumption by decreasing the vibrations of the tire and suspension. However, pavement deformation and energy loss will vary based on the scale of roughness, vehicle speed and vehicle type. Pavement deforms when it comes in contact with the tire and, therefore, dissipates some energy. Figure 1 Schematic of the effect of aggregate on different scales of texture. While one study suggested that a stiffer pavement may be best for this reason, other references concluded that most deformation and energy loss is associated with tire rather than with pavement deflection. ...

 

> Pavement stiffness does not appear to have a significant effect on the rolling resistance or fuel economy.

 

> Rolling resistance on concrete or asphalt pavements with the same surface texture should be almost identical.

https://www.eng.aubu...-resistance.pdf


Edited by Plus 3 Golfer, 25 July 2017 - 09:26 AM.

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#16 OFFLINE   raadsel

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Posted 25 July 2017 - 11:08 AM

I couldn't find the study I was refering to but found this info below.  It appears the higher temperature up to a point makes the tire easier to flex and thus reduces the energy loss associated with tire flexing when rolling.   Also, there appears to be virtually no difference between asphalt and concrete in rolling resistance if the surface smootness is the same.  The one study refered to IIRC suggested less than 1% increase in rolling resistance due to pavement deformation (asphalt vs concrete). 

 

Bottom line is that rolling resistance due to ambient temperature increase should likely not go up especially since the 4 mile commute is city traffic and likely goes down.

 

1) Temperature: with both solid and pneumatic tires, rolling resistance has been found to decrease as temperature increases (within a range of temperatures: i.e. there is an upper limit to this effect). For a rise in temperature from 30°C to 70°C the rolling resistance decreased by 20-25%.  http://www.ajer.org/.../R037141148.pdf

 

2)  Overall, the literature review indicated that a smoother road can decrease fuel consumption by decreasing the vibrations of the tire and suspension. However, pavement deformation and energy loss will vary based on the scale of roughness, vehicle speed and vehicle type. Pavement deforms when it comes in contact with the tire and, therefore, dissipates some energy. Figure 1 Schematic of the effect of aggregate on different scales of texture. While one study suggested that a stiffer pavement may be best for this reason, other references concluded that most deformation and energy loss is associated with tire rather than with pavement deflection. ...

 

> Pavement stiffness does not appear to have a significant effect on the rolling resistance or fuel economy.

 

> Rolling resistance on concrete or asphalt pavements with the same surface texture should be almost identical.

https://www.eng.aubu...-resistance.pdf

 

Unfortunately, it does leave the question about asphalt at higher temperatures. The concern, as I read it, is that a lot of asphalt in bright daylight with temperatures above about 90 degrees starts to get "sticky" from melting, when the temperature of asphalt can be 120 degrees or higher. Of course, this also varies by the type of asphalt, as there are some compositions that have higher melting points.



#17 OFFLINE   Plus 3 Golfer

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Posted 25 July 2017 - 01:18 PM

Unfortunately, it does leave the question about asphalt at higher temperatures. The concern, as I read it, is that a lot of asphalt in bright daylight with temperatures above about 90 degrees starts to get "sticky" from melting, when the temperature of asphalt can be 120 degrees or higher. Of course, this also varies by the type of asphalt, as there are some compositions that have higher melting points.

I think it's more deformation / deflection of the asphalt rather than sticking to the tire. Like the truck ruts you see sometimes in asphalt.  I think it was in the same study that I can't find, where the author believes the deformation causes contact patch of the tire to increase when the asphalt deforms from weight and heat and hence rolling resistance increases.  

 

Anyways, I don't think this is contributing much to why the OP is getting significantly less FE at 100F+ than at a lower temp.  

 

When I lived in Phoenix I found that when I wanted to get the highest FE, I tested in very high heat 100F with no AC, windows cracked, and  took side streets which have at minimum a top layer of asphalt.  Here's the link where ambient was approaching 100F and it was early afternoon with full sun.  Over 67 mpg.  I don't think my tires were sticking to the asphalt or deforming the alphalt payment much. ;)   Phoenix, Austin, Houston would likely have asphalt specs for high ambient temps.  Otherwise, the roads would continually form ruts from vehiclular traffic. 

 

I think maybe we are thinking of the old days ('60s) when many roads were tar and chips.  When you walked across one of these roads on a hot day, the tar would stick to ones shoes, you'd track it into the house, and get yelled at by your mother. :)


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#18 OFFLINE   jackalopetx

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Posted 25 July 2017 - 05:44 PM

In 100F temps, your FE will take a big hit from using AC for your 4 mile compute.   

 

Let's assume your AC demand averages 2.0 kW for the 4 miles and it takes you 10 minutes.   The AC used 0.333 kWh.  Let's assume that the overall efficiency to produce that 0.333 kWh is 33.3%.  So, the fuel used is (0.333/0.333) = 1.0 kWh.  There's 3412 Btu in one kWh.  So, the Btus of gas used would be about 3412 Btu.  One gallon of gas has about 115,000 Btus.  So, about 0.03 gallons of gas was burned for the AC.   If you normally get 40 mpg for the 4 miles, you would uses about 0.1 gallons of gas.  So, using AC you use 0.13 gallons of fuel to cover 4 miles.  Your FE would then be 30.8 mpg not 40 mpg.  Now, the uphill home requires more fuel and hence your 30.8 mpg drops to 25 mpg.

 

Once the cabin cools down, the AC demand will likely drop below 0.4 kW.  So, your gas consumption to run the AC would fall considerable and the hit to FE would be significantly less.  

 

Instead of turning off EV+ which won't help, turn off your AC for the 4 mile trip. :)

Wow, that's amazing that the AC uses so much power

 

Today I got 18mpg driving home with a short detour, the lowest I've gotten with this car. Today was even hotter than the previous couple weeks so I think the hot battery combined with the AC power usage really brought mileage down.

 

I wonder what kind of gas mileage trucks and normal cars are getting in this heat... 



#19 OFFLINE   obob

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Posted 25 July 2017 - 08:26 PM

As I see it, the first few minutes of running the ICE is the least fuel efficient by far, so adding a substantial air conditioner load just makes it way worst.  So waiting a few minutes might be worth the wait, though sometimes it might not be worth the wait.


Edited by obob, 26 July 2017 - 09:32 AM.


#20 OFFLINE   jackalopetx

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Posted 26 July 2017 - 08:27 AM

I wish there was a way to run the fans periodically so the interior wouldn't reach 140F. If the battery fans could be programmed to run when the temperature reaches a certain point, that could get the temperature way down 


Edited by jackalopetx, 26 July 2017 - 08:29 AM.







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