Declining Mileage

[asb]

When I first started driving the C-Max, I was getting between 47 and 50 mpg.  But now I'm  getting a steady 43.  Right now I'm running the air conditioning, but that wasn't the case a few weeks ago.  

 

My miles are mostly highway.  Sometimes in traffic, sometimes not.  I've tried P & G and I've been working hard at finding that "sweet spot"  when there is no traffic on the highway.

 

I'm about to hit 5,000 miles.  

 

Thoughts?

[catsailor]

set the a/c temp higher than 68F , To maximize my mileage I set my A/C at this early time of year to 73F so the compressor does not run as long.

[fotomoto]

I'm experiencing the opposite.   For this time of year, we just had a strong cool front so I haven't been using the a/c and my mileage has shot up.  I only have had it for a month and 2,000 miles so not a lot of experience but I'm thinking at least 3-4mpg hit for a/c use.  My morning commute leg usually averages 44ish but this a.m. it shot up to 48 without a/c even though I had to wait on a long train.  Glides last longer, etc.

 

 

If using auto temp, be careful about setting the temp too high as that can trigger the ICE to run sooner and/or longer to provide heat even though you have the a/c on.

[DR61]

AC obviously takes power and reduces fuel economy in all vehicles.  Once we are used to summer temperatures, we set our AC to 76-80 F for longer trips and find it comfortable. If it is somewhat cloudy with less direct sun through the windows then higher setting is OK.  Also one can turn on recirculation for shorter trips with AC and that reduces power use (not advised for longer trips).

 

Window tinting with a good quality material reduces solar heating of the interior and reduces power draw from the AC.

[Jus-A-CMax]

Tinting does help somewhat but when you have a dark interior as I do, it still gets warm. I've been using the AC on and off again and like I have posted before, it will drain the main batts a lil faster than usual and naturally, your regen is not as fast either - gotta give something up for the comfort of the coolness. With that in mind, my thoughts is that going from 47MPG to 43MPG, it ain't too bad. With the extra drain and less regen in mind, using P&G helps but I suspect you're using the EV at the tail end of the glide to push the extra miles and probably draining the batts to the low point. I could be wrong but hey, I can only work with what was posted so I am making some assumptions. The better strategy is to keep the battery working on the "high side", use P&G to build up to 75% and work it down to no less than 50%, rise and repeat. Really feather the EV (1/2 or less) to maintain the speed and work this region. You will tire of it so just throw it on ICE High MPG to help with the regen but be smart with it as the downgrades, turn it off and do some P&G so you save the gas. Thats the way I play so it may or may not work for you and your CMax.

 

Use AC and enjoy the trip, no sense in sweating for a few bucks of gas. Can't fight physics nor mother nature. Hope this helps  :)

Edited April 26, 2013 by Jus-A-CMax

[asb]

To respond to all of your comments:

 

set the a/c temp higher than 68F , To maximize my mileage I set my A/C at this early time of year to 73F so the compressor does not run as long.

 

I gave that a try on the way home today.  89 degrees outside this afternoon.  42.9 mpg.  This morning - no air conditioning and 41.9 mpg.

 

 

 

 

AC obviously takes power and reduces fuel economy in all vehicles.  Once we are used to summer temperatures, we set our AC to 76-80 F for longer trips and find it comfortable. If it is somewhat cloudy with less direct sun through the windows then higher setting is OK.  Also one can turn on recirculation for shorter trips with AC and that reduces power use (not advised for longer trips).

 

Window tinting with a good quality material reduces solar heating of the interior and reduces power draw from the AC.

 

My windows are tinted.

I generally always have the air on recirculation.  Why is it not advised for longer trips?  I remember being confused about this before.

 

 

Tinting does help somewhat but when you have a dark interior as I do, it still gets warm. I've been using the AC on and off again and like I have posted before, it will drain the main batts a lil faster than usual and naturally, your regen is not as fast either - gotta give something up for the comfort of the coolness. With that in mind, my thoughts is that going from 47MPG to 43MPG, it ain't too bad. With the extra drain and less regen in mind, using P&G helps but I suspect you're using the EV at the tail end of the glide to push the extra miles and probably draining the batts to the low point. I could be wrong but hey, I can only work with what was posted so I am making some assumptions. The better strategy is to keep the battery working on the "high side", use P&G to build up to 75% and work it down to no less than 50%, rise and repeat. Really feather the EV (1/2 or less) to maintain the speed and work this region. You will tire of it so just throw it on ICE High MPG to help with the regen but be smart with it as the downgrades, turn it off and do some P&G so you save the gas. Thats the way I play so it may or may not work for you and your CMax.

 

Use AC and enjoy the trip, no sense in sweating for a few bucks of gas. Can't fight physics nor mother nature. Hope this helps  :)

 

 

That sounds way too complicated!  :)

 

I'm mainly worried about failing batteries, etc.  But if that doesn't seem like the cause, then I'm fine with the lowered mileage.  Though, I would prefer to be hitting 50 again.

 

Thanks everyone.

[JAZ]

Tom and Ray on the recirculate/fresh air thing:

 

http://www.cartalk.com/content/recirculate-versus-fresh-air-tom-and-ray-settle-one-familys-bet-line-steak-dinner

 

I think once upon a time the idea was that recirculated air would become stale and more noxious than fresh air.

[DR61]

Tom and Ray on the recirculate/fresh air thing:

 

http://www.cartalk.com/content/recirculate-versus-fresh-air-tom-and-ray-settle-one-familys-bet-line-steak-dinner

 

I think once upon a time the idea was that recirculated air would become stale and more noxious than fresh air.

Interesting, but bottom line is recirculation uses less energy if only because the difference between air being cooled and desired inside temperature is smaller.  Their initial statement ("On most cars, the compressor does the same amount of work on both settings") is just wrong, especially so for a modern AC system with an electric variable speed compressor like the C-Max has.

 

Most likely it is correct that recirculation still allows some fresh air in, so staleness probably isn't a problem with only 1 or 2 passengers.

[CassidyB]

Have you changed where you purchase gas? I had a few weeks where my MPG dipped.. Nothing environmentally changed, but I did fill up at a different gas station. Just my two cents...

[asb]

Have you changed where you purchase gas? I had a few weeks where my MPG dipped.. Nothing environmentally changed, but I did fill up at a different gas station. Just my two cents...

 

I go to a variety of places.  I'll see if I can figure out a pattern.

[DR61]

Reading the manual today, it mentions in the AC section that the MAX AC button is the most efficient way to cool down a hot car, and it automatically uses Recirc for a time.  It also says that for normal AC operation, setting the fan on low speed and setting a desired temperature is most efficient (presumably more efficient than Auto AC, but maybe not as effective depending on how hot it is outside).

[Wingrider01]

AC obviously takes power and reduces fuel economy in all vehicles.  Once we are used to summer temperatures, we set our AC to 76-80 F for longer trips and find it comfortable. If it is somewhat cloudy with less direct sun through the windows then higher setting is OK.  Also one can turn on recirculation for shorter trips with AC and that reduces power use (not advised for longer trips).

 

Window tinting with a good quality material reduces solar heating of the interior and reduces power draw from the AC.

 

Confused or missing something, according to this thread AC temp and compressor load the compressor is run by a electric motor, can't verify by looking at the moment because mine is back at the dealer again for issues.

[plus 3 golfer]

Confused or missing something, according to this thread AC temp and compressor load the compressor is run by a electric motor, can't verify by looking at the moment because mine is back at the dealer again for issues.

 

Yes, the compressor is electrically driven with speed determined by maintaining the evaporator core temperature at a certain level.  

 

So during re-circulation of air, as the input air to the evaporator core becomes cooler, the evaporator core cools, and the speed of the compressor can be reduced to maintain the evaporator core temperature at the proper level.  If one would switch the re-circulation off and the ambient temperature were higher than the cabin air temperature, the compressor would have to speed up to maintain the evaporator core temperature.  So, running with the re-circulation on for longer periods should result in lower energy use.

 

Also in auto-mode, the blower speed is used to maintain cabin temperature.  If more cooling is needed (like when one lowers the set point cabin temperature), the blower speed will be increased.  More air will now be blowing across the evaporator core and increasing the core temperature.   The electric motor will then speeds up to lower the core temperature to the appropriate level. 

[fotomoto]

Also in auto-mode, the blower speed is used to maintain cabin temperature.  If more cooling is needed (like when one lowers the set point cabin temperature), the blower speed will be increased.  More air will now be blowing across the evaporator core and increasing the core temperature.   The electric motor will then speeds up to lower the core temperature to the appropriate level. 

 

I don't find this to be the case when lowering the temp a few degrees on one controller in DUAL auto mode.  

[plus 3 golfer]

I don't find this to be the case when lowering the temp a few degrees on one controller in DUAL auto mode.  

You can't cover every situation but generally if one lowers the overall temperature a few degrees blower speed.will increase (assuming nothing else has changed).  But I'm going out now and will test and also I will monitor energy usage in My View.

[Wingrider01]

Yes, the compressor is electrically driven with speed determined by maintaining the evaporator core temperature at a certain level.  

 

So during re-circulation of air, as the input air to the evaporator core becomes cooler, the evaporator core cools, and the speed of the compressor can be reduced to maintain the evaporator core temperature at the proper level.  If one would switch the re-circulation off and the ambient temperature were higher than the cabin air temperature, the compressor would have to speed up to maintain the evaporator core temperature.  So, running with the re-circulation on for longer periods should result in lower energy use.

 

Also in auto-mode, the blower speed is used to maintain cabin temperature.  If more cooling is needed (like when one lowers the set point cabin temperature), the blower speed will be increased.  More air will now be blowing across the evaporator core and increasing the core temperature.   The electric motor will then speeds up to lower the core temperature to the appropriate level. 

 

then unlike regular vehicles where the compressor for the A/C is run on the engine which puts a higher load and lower gas mileage it has minimal affect on the C-Max MPG or am I missing something?

Edited April 29, 2013 by Wingrider01

[ceemax71]

The A/C compressor electric motor is powered by the high voltage battery.  This adds load to the engine to keep the high voltage battery charged.

This should be more efficient than being directly driven by the engine, since the compressor motor speed can be optimized for the actual cooling load.

[plus 3 golfer]

I monitored a/c load and other load through MyView.  Ambient temperatures were between 92 - 96 F. Humidity is very low about 10%.  No re-circulation.  Here's my observations:

 

1) After car has been sitting in full sun, the compressor load appears to be about 2 1/4 - 2 1/2 kW upon initial start up. Fan is on a high speed about 0.8 kW (my best guess as there is other load being metered on the "Other" load besides the fan).

 

2) After a few minutes of driving (cabin temperature has dropped significantly) the fan speed drops to around 4, the compressor runs about 0.9 - 1 kW.

 

3) After cabin temperature has stabilized the compressor runs at about 0.5 kW and the blower is running around the 3rd indicator (more on this later) and on average the other load is  0.4 - 0.5 kW.  So, it looks like the total a/c load is around 1 kW.  The other load does jump around especially when accelerating maybe 0.3 kW which may be the radiator fans coming on. I wish there was a way to know when the radiator cooling fans come on.

 

4) Speed affects compressor load as it seems to vary between maybe 0.4 - 0.8 kW.  I attribute this to condenser load.  Higher speed means more air flow through condenser, more heat removed from the refrigerant circuit and the less the compressor has to work to maintain the appropriate evaporator temperature.

 

5) The blower speed when in manual mode shows 7 manually selected speeds.  But it appears when in auto mode the speed adjust is infinite or at least significantly more speeds than 7.  There seems to be at least one other speed between the manual indicated speeds.  The way one can tell this is if one changes back and forth from manual to auto.  It appears the manual speed indicator will select the nearest manual speed to the current blower speed.  You can hear a change in sound (when stopped) and feel a change in air flow when flipping back and forth yet the indicator stays the same.  It's hard to detect while moving.  While stopped and the temp is stable, check the manual fan speed and return to auto. If no change in blower speed is detected, then lower or raise the temperature one or two degrees and wait a few seconds.  Then, repeat the check of the manual fan speed.  The indicator will likely be the same as before but you should notice a change in blower speed either up or down depending on whether the temp was increased or decreased.  It appears to work the same on dual by changing only one temperature dial but I had to change the temperature at least 2 degrees or more.  Of course, if you change the temperature too much under either mono or dual, such that the speed changes to the next manual blower speed, the blower speed manual indicator will also change.

 

So, lets assume an average compressor plus radiator cooling fan load of 0.7 kW or for ease of calculation 2400 Btu.  Now assume the engine efficiency is 30%. So, the amount of gasoline need would be 8000 Btus worth of gas or about 0.07 gallons for each hour under the conditions I describe about.  Assume my overall fuel economy was 40 mpg and my average speed was 40 mph. So if I traveled for one hour and burned one gallon of fuel with 0.07 used for AC,  I lost 7% on FE due to the use of AC.  I'm assuming that I would have run the blower the same with / without a/c.  If I assume I would shut down the HVAC system, I probably would have saved another 2-3% or 10% total.  I can't wait until it gets to triple digits and the monsoon season with higher humidity (maybe 30-40%).   :) 

 

Here's a few things from the shop manual:

 

The PCM monitors the evaporator temperature sensor thermistor as air is passed through the evaporator core and controls the A/C electric compressor speed. If the temperature of the evaporator core discharge air is low enough to cause the condensed water vapor to freeze, the A/C electric compressor reduces it speed.

 

The ACCM engages when it receives the A/C demand message and target Revolutions Per Minute (RPM) request. Instead of cycling on and off like a traditional A/C electric compressor, the PCM monitors the evaporator discharge temperature from the HVAC control module to raise or lower the A/C electric compressor speed as required.

 

The blower motor speed control uses a PWM signal from the HVAC control module to determine the desired blower speed and varies the ground feed for the blower motor to control the speed. (This implies that there are likely more speed control points than the 7 manual ones.)

[Riddley]

Excellent Analysis Plus 3!!

 

It is great to have engineers in the forum!!