Higher MPG? Turn HVAC Off.

[mtberman]

I read somewhere here that HVAC is a big drag on mileage. It seems whoever said that was right. Makes me wonder if they have the HVAC system off when they do EPA certification.

 

My car lives in an underground garage in the city, so fortunately it's always at least 55° when I start it. Like other hybrids I've owned, the car does an engine-on + rich-mixture + heavy-generator-load thing for about three minutes after cold start. I understand this makes for super hot exhaust to light up the catalyst. For me, this phase gets me out of my garage, down an alley, and out onto the street for about two blocks. It also produces noticeable exhaust steam if it's cold outside, and adds quite a bit of charge to the battery.

 

What's surprised me most is that my car then goes into EV mode. This is after perhaps 3 blocks of city driving. Neither of my previous hybrids were anywhere near as aggrssive with EV mode. However, and this is the kicker, it does this only if I make sure the HVAC system is off. Otherwise the engine doesn't start cycling on/off for a couple of miles, similar to other hybrids I've owned.

 

Needless to say, the difference in fuel mileage is significant. We're talking 45 MPG vs. 30 MPG for a 3 mile journey from a cold start.

 

So, does anyone know if the EPA leaves the heating system on or off when they test? It sure makes a big MPG difference in my experience.

[F8L]

I've argued this with Prius owners for years. The HVAC system most definitely has a big impact on fuel economy. You can mitigate the losses by turn the system on low or if using the auto feature by setting the temp to as close as the outside temp as is reasonable. With the heater or AC on high, the engine will run more often than if you had the AC turned off.

 

I've monitored a 7-10mpg (instantaneous) drop when using the AC in my Prius on a hot day. After the cabin reaches the desired temp the load goes down and mpg comes back up but this takes quite awhile. If you do short trips and constantly run the AC your average mpg is going to plummet!

 

If you are in truly cold areas then the grille block will help get things warmed up faster and reduce the mpg impact of the heater. As will parking in a garage and using an engine block heater.

 

I have a bunch of tips for summer driving if anyone is interested.

[Bob999]

I read somewhere here that HVAC is a big drag on mileage. It seems whoever said that was right. Makes me wonder if they have the HVAC system off when they do EPA certification.

 

....

 

What's surprised me most is that my car then goes into EV mode. This is after perhaps 3 blocks of city driving. Neither of my previous hybrids were anywhere near as aggrssive with EV mode. However, and this is the kicker, it does this only if I make sure the HVAC system is off. Otherwise the engine doesn't start cycling on/off for a couple of miles, similar to other hybrids I've owned.

 

.....

 

So, does anyone know if the EPA leaves the heating system on or off when they test? It sure makes a big MPG difference in my experience.

EPA mileage testing is done with both heat and A/C OFF.  It is  done at ~70 F ambient air temperature.

 

The CMAX will run the engine to provide heat when the HVAC is turned on and heat is demanded.  If you turn the HVAC off then the engine only runs as necessary to provide power to move the car and/or to charge the battery.

[mtberman]

All hybrids do this, but the surprising thing is how aggressive this behavior is in the C Max.

 

I had a 2004 Prius for two years and a 2008 Escape Hybrid for two years. Also, we have newer Priuses and several 2010 Camry Hybrids at my work, which I drive from time to time. None of these other cars hybrids ever go/went into engine-off mode as quickly as the C Max.

Edited January 18, 2013 by mtberman

[F8L]

Watch the bottom left corner of the Scangauge.

 

Edited January 18, 2013 by F8l

[jmckinley]

OK, I'm puzzled.  Why should my car use more gas with the heater on?

 

With a gasoline engine, heat is free.  In fact it is something to get rid of thru the radiator.  Why would I use more gas using the engine's waste heat to heat the interior instead of heating the air passing through the radiator?

[shaners]

OK, I'm puzzled.  Why should my car use more gas with the heater on?

 

With a gasoline engine, heat is free.  In fact it is something to get rid of thru the radiator.  Why would I use more gas using the engine's waste heat to heat the interior instead of heating the air passing through the radiator?

 

 

If the heater is on and it it is cold outside, the engine will run if for no other reason, just to make heat for the cabin.

[mtberman]

This video is interesting but deceptive in my opinion.

 

When the driver turns on the A/C, he simultaneously increases engine speed from about 1600 RPM to just over 1700 RPM, and then says basically "Look how the MPG dropped". Well of course the car uses more fuel when you speed up. He holds this higher engine speed, even creeping up to nearly 1800 RPM a few times, while he's remarking about how the gas mileage dropped. He claims he's holding the pedal steady but he really isn't.

 

As soon as he turns the A/C off, he lifts the pedal (whether intentional or not, I don't know) and slows the engine speed back to about 1550 RPM. The gas mileage reading goes back up, naturally. But he attributes thisto turning off the A/C, not to the fact that he slowed then engine down.

 

It's pretty obvious. I assume he was assailed for this deception over on Priuschat.

 

Watch the bottom left corner of the Scangauge.

[Tree63]

What I like about the C-Max Climate Control is that I can choose where the CC will work - defrost, vents or feet - independently.

 

In previous car, I tried to minimize use of defrost because AC was coupled to automatically work with it, even in cold weather, whichh of course affected gas mileage. 

 

When I told him another brand didn't work that way, dealer told me all cars were now coupling AC and defrost, to make sure AC condenser/system was used periodically. 

 

Anyone know if C-Max defroster is coupled to work the AC automatically?  I don't mind running it if AC is smart enough to stay off unless it is hot or I put AC on.

Edited February 7, 2013 by Tree63

[plus 3 golfer]

This video is interesting but deceptive in my opinion.

 

When the driver turns on the A/C, he simultaneously increases engine speed from about 1600 RPM to just over 1700 RPM, and then says basically "Look how the MPG dropped". Well of course the car uses more fuel when you speed up. He holds this higher engine speed, even creeping up to nearly 1800 RPM a few times, while he's remarking about how the gas mileage dropped. He claims he's holding the pedal steady but he really isn't.

 

As soon as he turns the A/C off, he lifts the pedal (whether intentional or not, I don't know) and slows the engine speed back to about 1550 RPM. The gas mileage reading goes back up, naturally. But he attributes thisto turning off the A/C, not to the fact that he slowed then engine down.

 

It's pretty obvious. I assume he was assailed for this deception over on Priuschat.

His test is not meant to be precise but does represent that use of A/C on high mpg vehicles has a noticeable (larger number) effect on FE.  

 

If you assume cooling load is identical in two vehicles (requires same amount of energy to cool car) but one gets 25 mpg without A/C and the other gets 50 mpg without A/C, then running the A/C would reduce FE more on the higher mpg vehicle.  So, a 5% hit on the 25 mpg vehicle would be  about 1.25 mpg (small number) but about a 10% hit or 5 mpg (larger number) on a 50 mpg vehicle. This is because the cooling energy (same on both cars) is a larger % of the total energy requirements of the 50 mpg vehicle than of the 25 mpg vehicle.

 

So, if one sees a 1-2 mpg hit on average FE using A/C on their less fuel efficient vehicle, you may be disappointed when seeing the effect of using A/C on the C-Max.

 

I have checked my A/C drain on FE many times in my 2009 Jetta TDI  with cruise on (no rpm variation) and very flat ground while cycling the A/C.  The instantaneous mpg would change as much as 20% depending on speed (about a 10 mpg drop at 50 mpg).  I estimated my average FE drop in July / August in Phoenix to be about 8% over shoulder months like Oct, Nov, Mar, and Apr where A/C is not used very often but temps are still fairly warm.  I would expect the C-Max to be similar. 

Edited February 7, 2013 by Plus 3 Golfer

[plus 3 golfer]

EPA mileage testing is done with both heat and A/C OFF.  It is  done at ~70 F ambient air temperature....

 

I believe EPA does a test with A/C at 95 F and sun load:

 

Beginning with 2008 models, three additional tests will be used to adjust the city and highway estimates to account for higher speeds, air conditioning use, and colder temperatures.

[dalebunker]

I posted this in another forum sometime ago.  I hear lots of debates about the EPA testing.  If you really take the time to study the tests, and more importantly the environment, conditions and duration of the test, it is very (very) feasible to achieve the EPA numbers.  The graphs are very telling....

 

Curious to see what the team here thinks......

 

Note: look at the "Highway" test speeds; they never exceed 60 MPH, with an average of 48 MPH.  However, they have added the 3 new tests.  My guess is, if some geeky soul (such as I, but I'm patiently waiting for mine) followed to the letter of these tests with their car --after it's break in period and with the same conditions as their lab.............

 

Note2: they also mention: "The EPA has established testing criteria for electric vehicles and plug-in hybrids that are slightly different than those for conventional vehicles."  I'm wondering if this implies regular hybrids follow the convention test cases?

 

http://www.fuelecono...schedules.shtml

I've read with great interest the fuel mileage debate regarding C-max actual economy vs EPA Ratings. Here are the tests the EPA uses to determine the ratings. 

Based on what I've read across the forums, people who drive within the same parameters of the EPA tests, are achieving (or very closely) the EPA estimates. Also --based on forum comments-- temperature has an impact on economy. Give the car has been released late in the year, and nationally we've had a pretty cold year, I will not be surprised as weather warms up and people have a few thousand miles on the car, they're going to be much closer to estimates; of course this will be highly dependent on your driving style and conditions. 

P.S. The Detailed Comparison Tab is the most interesting (IMHO)

Edited February 8, 2013 by dalebunker

[plus 3 golfer]

It's my understanding that hybrids basically follow the tests in the link. The issue with plug-ins is that if the HV battery has too much charge, operation from the HV batteries will affect the results. So the charge on the HV batteries has to be reduced so the results on plug-in are comparable after EV operation is used up to non-plugin hybrids. I can't find a link now but that's what I recall.

 

Hence, the Energi is rated 44 highway and 41 city vs. 47/47 for the C-Max hybrid.

 

Edit: here's the link. http://www.smidgeindustriesltd.com/leaf/EPA/EPA_test_procedure_for_EVs-PHEVs-1-13-2011.pdf

Edited February 8, 2013 by Plus 3 Golfer

[asb]

 I have a bunch of tips for summer driving if anyone is interested.

 

I live in Florida, I'd love to hear your thoughts on summer driving.  I try my best to monitor the temperature in my cars and keep the AC fan on low or a higher temperature.

[mtberman]

The video is at least partly, if not fully, bogus. First, running electric AC won't directly affect MPG as depicted in the video. The only way to get the MPG to fluctuate instantly as shown was for the driver to increase and decrease power demand. That's done using the pedal on the floor. The one on the right.

 

The AC compressor in both C Max and Prius is driven by the high voltage battery and is a variable type. Its power draw depends on the cooling needs of the car. When you turn it on, it doesn't kick immediately on at full load. It spins up relatively slowly. Even if it were an "instant on" type, changing the load on the HV battery while underway wouldn't manifest as an instantly detectable increase or decrease in metered fuel usage. The number would change gradually, especially when driving on a flat road as depicted in the video.

 

The compressor in both the Prius and the C Max spins up slowly with power draw increasing similarly slowly. In the C Max, we can see this draw in the info display if we've set it to show that.

 

There is of course a drain on the HV battery when the compressor is on. That energy has to come from somewhere, which is of course the gas tank. It comes in the form of lower overall fuel mileage when the compressor is running than when it isn't. That much is true. But the video is BS in my opinion. I would be more inclined to believe this if there was another one out there in which the engine speed is steady.

[plus 3 golfer]

You can't keep the engine rpm steady only your ground speed.

 

On the C-MAX, It appears to take very little HV battery discharge from a target level to trigger a change in RPM to maintain the charge level.  Here's a graph of data I logged while testing grill covers at 70 mph (GPS set) and cruise, 27 F ambient, no HVAC, no radio, just head lights on.  From about the time stamp 710 - 1213, it appears the CUs are trying to keep the charge level within a narrow band and increase engine rpm rather significantly to maintain the charge level while speed is virtually constant. 

 

The A/C (with the flip of a switch) is going to be a sensed by CUs and I see no reason to believe that the CUs won't increase rpm to maintain charge level as the compressor ramps up.  I'll be able to test / monitor / log the effect of A/C when it gets hot it Phoenix. :)

 

[F8L]

This video is interesting but deceptive in my opinion.

 

When the driver turns on the A/C, he simultaneously increases engine speed from about 1600 RPM to just over 1700 RPM, and then says basically "Look how the MPG dropped". Well of course the car uses more fuel when you speed up. He holds this higher engine speed, even creeping up to nearly 1800 RPM a few times, while he's remarking about how the gas mileage dropped. He claims he's holding the pedal steady but he really isn't.

 

As soon as he turns the A/C off, he lifts the pedal (whether intentional or not, I don't know) and slows the engine speed back to about 1550 RPM. The gas mileage reading goes back up, naturally. But he attributes thisto turning off the A/C, not to the fact that he slowed then engine down.

 

It's pretty obvious. I assume he was assailed for this deception over on Priuschat.

As the driver in the video above I can say with certainty that I did not adjust pedal pressure during the video. I've done this test dozens of times to ensure I wasn't somehow causing the decsease in mpg by pedal pressure or changes in surface grade. You have to understand how the electric AC and hybrid system works before you can make claims of deception. My standing at PriusChat and other forums is too important to me to try and pass falsehoods. Besides, what did I have to gain by lying? I'm just trying to add to the hybrid knowledgebase. :)

 

Plus 3 Golfer pretty much covered the rest.

[F8L]

You can't keep the engine rpm steady only your ground speed.

 

On the C-MAX, It appears to take very little HV battery discharge from a target level to trigger a change in RPM to maintain the charge level.  Here's a graph of data I logged while testing grill covers at 70 mph (GPS set) and cruise, 27 F ambient, no HVAC, no radio, just head lights on.  From about the time stamp 710 - 1213, it appears the CUs are trying to keep the charge level within a narrow band and increase engine rpm rather significantly to maintain the charge level while speed is virtually constant. 

 

The A/C (with the flip of a switch) is going to be a sensed by CUs and I see no reason to believe that the CUs won't increase rpm to maintain charge level as the compressor ramps up.  I'll be able to test / monitor / log the effect of A/C when it gets hot it Phoenix. :)

 

 

Agreed.

 

In testing with the Torque App, myself as well as three other PriusChat members, recorded near instant draws of 1500watts-1800watts when outside temps were over 90F and the auto AC temperature was set to 78F or lower. This draw stayed at 1500w to 1800w for 5-10min at least before the cabin temp started coming down and got close enough to the set temperature to ramp down the compressor speed. Even after 30min on the freeway I was observing a 600w draw.

 

Now you can imagine what such a daw would do to fuel economy if one made only short trips in which the AC was working at full capacity the entire driving cycles. eek!

[Bob999]

Thanks to all who have posted specifics/data.

 

I think it is intuitive that running the A/C puts an additional load on the engine/battery and, all other things equal, would have a negative effect on MPG.

 

What is less clear to me is whether "all other things equal" is the correct construct for most people.  In the old days, prior to A/C in cars we drove with the windows open and my understanding is that there is a significant drag penalty compared to driving with all windows closed with a resulting hit on MPG.  I think, but don't have data to support this, that driving with open windows in a hybrid like the Prius or CMAX at highway speeds would  negatively impact MPG.

 

Only controlled experiments could determine the relative impact of running the A/C vs driving with the windows open but I think this would be the relevant comparison for many people. 

[F8L]

Thanks to all who have posted specifics/data.

 

I think it is intuitive that running the A/C puts an additional load on the engine/battery and, all other things equal, would have a negative effect on MPG.

 

What is less clear to me is whether "all other things equal" is the correct construct for most people.  In the old days, prior to A/C in cars we drove with the windows open and my understanding is that there is a significant drag penalty compared to driving with all windows closed with a resulting hit on MPG.  I think, but don't have data to support this, that driving with open windows in a hybrid like the Prius or CMAX at highway speeds would  negatively impact MPG.

 

Only controlled experiments could determine the relative impact of running the A/C vs driving with the windows open but I think this would be the relevant comparison for many people. 

Hi Bob,

 

While my tests on the above subject were not very scientific, I did try and minimized other variables as much as possible including elevation, surface type, wind speed and direction, weather, etc..

 

I found that having the driver's side front window and the passenger side rear windows completely down had a very minimal affect on mpg. I could not dectect a notiecable difference amongst the "noise" from variables like road surface undulations (small dips in the road). On those same exact stretches of road I would observe the 7-10mpg drop when switching the AC on.

 

I found that cracking the driver's side front window about 3-4" and the passeger's side rear window about 3-4" was enough to create airflow through the cabin. When temps are less than 100F I found this to be sufficient for cooling purposes although it is still quite hot inside the car. For hypermiling reasons this is just something you deal with. The other option is to roll the driver's window down completely and stick your arm outside the window and let the airflow go through the arm hole in your shirt. :)

 

I created a thread on minimizing the impact of AC on mpg. I will compy and paste it here so I don't have to link you off site.

 

Ok since the loss of MPG through the heavy use of the Prius A/C system has become such a big subject I thought it would be nice to have a thread that focuses on mitigating or reducing those losses. I have posted some basic ways in which you can reduce the losses but I am hoping those smarter and more creative than I can help me expand the list so that other Prius owners are able to benefit from our knowledgebase here at PC.

 

Problem:

 

Heavy A/C usage in hot climates and significantly reduce your average MPG. Why?

 

• The A/C system is electric and therefor requires energy. The energy essentially comes from your HV battery which then requires recharging. The energy for recharging is supplied by your ICE which runs on gasoline.
• A/C systems inherently run less efficiently in hot temperatures. The hotter the ambient temperature the less efficient the system is and the harder it has to work to reduce temperature inside your car (or house).
• The temperature inside your car is often much warmer than the outside ambient temperature by 30 degrees or more depending on many factors like interior color, number of windows and their construction, etc.. The A/C system has to do a lot of work to reduce those temperatures to your desired temperature. Imagine trying to reduce the temperature inside your car from 130 degrees to 70 degrees. That is a lot of energy! Where does that energy come from? Ohh yeah, gasoline! The lower your temperature setting, the higher the energy required.
• I have measured an 1800watt continuous draw for over 10min while the A/C tried to cool the car! Even after 30min the draw was over 600w. This was measured by Torque app in my GenIII.

Methods for controlling heat buildup in your car:

 

 

 

 

1. Park in a shaded area. Under a tree may help but a better tactic is to park on the eastern side of a large building. As the sun moves to the west in the later portion of the day, the building will block the sun from hitting your car. With your windows cracked, the heat from the earlier part of the day will dissipate faster than if you parked your car in the full sun the entire day.
2. Crack your windows a few inches to allow warmer interior air to vent outside the vehicle. Assumes you are parked in a safe area
3. Tint your windows
4. Use a high quality windshield sun shade. Priuschat shop has a very nice one!
5. If you cannot park on the east side of a building try parking your car facing the west and use a quality windshield sun shade. This will reduce the number of windows directly facing the sun and thus not allowing as much of the suns radiation to enter the car.
6. Drive the car for the first few minutes with all the windows down to circulate the hot air and direct it outside.

Best Practices for A/C usage and maintenance:

 

 

 

 

1. Use the A/C sparingly or use the crossflow venting technique (see below)
2. Crossflow Venting - Roll the driver side window down approx. 3" then do the same for the passenger side rear window. This will create a nice cross breeze and will not adversely affect aerodynamics in a noticeable way. In the event this is still not enough, roll the driver's window completely down and stick your arm outside the window. This allows wind to flow through the arm hole in my shirt and flow out through the other arm hole and the bottom of my shirt. This can dramatically add to the the cooling effect and is still more efficient than running the A/C , even on the freeway at 60mph.
3. Use a setting that is high enough for maximum efficiency but makes you comfortable. 78 degrees seems to be sufficient for most drivers
4. Ensure your A/C system is charged to spec levels and operating efficiently. i.e. condenser is clear of obstructions, bugs, etc.
5. Use Eco Mode if your car is equipped with the feature

These tips will not only help reduce the fuel economy hit of the A/C system but it may also extend the life of your HV battery by reducing interior temperatures while the car is sitting in the parking lot. If your interior is 130F then your battery may be too!

 

When outside temps are over 100F it is recommended to run your A/C on longer trips to try and cool down the HV battery. I've measured a 15F difference between ambient temps and battery temps when the A/C was running. After turning off the A/C the battery quickly returns to near ambient temperatures. Fuel economy is important but your bodily well being and HV battery life are more important.

Read more: http://priuschat.com/threads/how-to-reduce-mpg-loss-because-of-a-c-use.97814/#ixzz2KbpPLITy

 

[Bob999]

I found that having the driver's side front window and the passenger side rear windows completely down had a very minimal affect on mpg. I could not dectect a notiecable difference amongst the "noise" from variables like road surface undulations (small dips in the road). On those same exact stretches of road I would observe the 7-10mpg drop when switching the AC on.

 

Thanks.

 

Not what I expected but great information!

[F8L]

Thanks.

 

Not what I expected but great information!

Keep in mind that this was for the GenII and GenIII Prius. The CMAX could be different because of aerodynamic differences. :)

[StoBro2]

I've found with the last 3 or 4 hatchback and wagon-style cars I've owned that I get a nice refreshing breeze when I have the front and rear passenger side windows opened an inch or two. The low pressure area near the leading edge of the front window and the high pressure area at the back window work together to create a nice swirl through the cabin. The wind is at my back when I'm driving, and the wind noise is on the far side of the car. It hasn't been warm enough (It hasn't been warm AT ALL, actually.) since I got my C-Max, so I haven't had a reason to try this window opening arrangement yet, but I'm sure It'll work.

[valkraider]

Jebus.

 

My C-Max with AC at full blast will still do better than my truck with AC at full blast.

 

Why the hell would I want to make myself miserable all the time?

 

There is a fine line between genius and insanity...