How much does your ac affect your mileage?

[Hersbird]

Side note I just read somewhere that a normal car AC system is a 5 ton 60,000 BTU system. So even if everything was 100% efficient in getting 120,000 BTUs out of a gallon of gas that would be 1/2 gallon per hour at full load. That takes a 30 mpg car at 65 down to 23 mpg. Luckily I don't think it needs 60,000 BTUs continuously. I wonder if better insulation would do more to limit heat gain/cooling loss would help much.

[mpg_numbers_guy]

Pretty sure my AC needs a recharge, but I don't really know because I haven't tried using it since last year when I test drove the car before buying. So my AC loss is 0 MPG. But I presume I would see a significant loss were I to do the crazy thing and refill the AC and actually use it...something I can't fathom doing. Windows cracked at <40 mph speeds on hot days keeps the interior cool enough, and just the fan on low at higher speeds if it gets hot enough, which it hasn't yet. There's also a slight forced airflow through the vents on my car with the climate control off that keeps the car cool (and warm during the winter) as well. The gray-ish beige interior also seems to stay a lot cooler than the gray cloth in my Civic.

I also like the heat and rarely have a passenger in my car.

There was a MPG loss with the AC running on the Civic I used to have, but I have no idea how much it was due to no actual testing and too much variance and inaccuracy with Torque's fuel economy instrumentation. Plus, I never ran the AC in that car even though it worked fine. I even removed the blower motor for weight reduction...

[mort]

Originally Posted by Hersbird

Side note I just read somewhere that a normal car AC system is a 5 ton 60,000 BTU system. So even if everything was 100% efficient in getting 120,000 BTUs out of a gallon of gas that would be 1/2 gallon per hour at full load. That takes a 30 mpg car at 65 down to 23 mpg. Luckily I don't think it needs 60,000 BTUs continuously. I wonder if better insulation would do more to limit heat gain/cooling loss would help much.

That's not how an A/C works. The 60,000 BTU is the amount of heat it can remove from the cabin, but the amount of power the compressor needs is some smaller number. The ratio of heat moved to power input is called the coefficient of performance. For a car A/C the COP is between 2 and 6, probably. Depending on operating conditions.
For example, Toyota says their Prius A/C consumes 4500 watts (about 15000 BTU/H) and I guess it is capable of about 60,000 BTU. So the COP is about 4.
15000 BTU/H is about 1/8 gallon of gas.
-mort

[MeteorGray]

I make no effort to measure the loss of fuel economy when running my Mazda3's air conditioner. When the weather is 90 degrees plus and the humidity close behind, who cares? Not me.

Because I live in a hot and humid state, my air conditioner is on almost all the time. If I removed the air conditioner, I doubt my fuel cost per mile would rise much above the 4.6 cents my car has achieved since it was bought new in 2015.

[jcp123]

I might do a dedicated a/c summer, but I already decided that I’d rather do a faster transit home from Dallas. Where I might get some data is doing the first few miles on the freeway using a/c to cool the interior plastics and such.

I’m still of a mind that Japanese a/c kinda sucks, but I should use the dang thing now that I recently had to go through the effort of dropping a motor mount to replace my drive belts.

[MeteorGray]

I drive my Mazda3 mostly on long commute trips on the highway, and as already noted, I can't tell the difference in fuel mileage with and without the air conditioner. I try to see evidence of it on the Scangauge via its instantaneous mileage readout, but the variations of highway, wind, etc tend to mask any contrast between "on" and "off" mileage differences for me. I know the air conditioner takes energy to run, but it's small enough that I can't "see" it, either on the Scangauge or at the fuel pump.

One technique I use when I don't have a passenger is to push the button that controls the air conditioner's compressor on/off activation. I leave the thermostat/temperature setting on "maximum cold," and when I get cold, I push the button to turn off the compressor and let the temperature gradually rise until I'm too hot, then repeat the cycle. When I push the compressor button, I can't tell any difference except for the temperature of the air; ie, I can't "see" whether the compressor is on-or-off on the Scangauge or "feel" it via the ambience of the car or in any other way to indicate the compressor is running.

One reason I use the compressor's on/off button in this way is the fact that at one time in automotive history (and maybe today in some cars?), some genius figured it was a good idea to let the compressor run all the time and use the engine's hot water to moderate the cabin temperature. It works, but at a higher fuel price. A workaround for me was the compressor on/off technique. I don't know if it's better for fuel economy on my Mazda or not, since I suspect the Mazda's compressor cycles as needed and doesn't use the "hot water" idea. But, is it better for FE to use the on/off technique or just set the temperature/thermostat at the desired temperature level and let the compressor cycle? Don't know.

In any case, I'm too old to drive around in a tropical climate while being hot and sweaty with the roar of hot-and-humid-and-dirty air blasting through the car for several hundred miles at a stint. I just can't take that anymore. Nosir.

[jcp123]

Originally Posted by MeteorGray

I drive my Mazda3 mostly on long commute trips on the highway, and as already noted, I can't tell the difference in fuel mileage with and without the air conditioner. I try to see evidence of it on the Scangauge via its instantaneous mileage readout, but the variations of highway, wind, etc tend to mask any contrast between "on" and "off" mileage differences for me. I know the air conditioner takes energy to run, but it's small enough that I can't "see" it, either on the Scangauge or at the fuel pump.

One technique I use when I don't have a passenger is to push the button that controls the air conditioner's compressor on/off activation. I leave the thermostat/temperature setting on "maximum cold," and when I get cold, I push the button to turn off the compressor and let the temperature gradually rise until I'm too hot, then repeat the cycle. When I push the compressor button, I can't tell any difference except for the temperature of the air; ie, I can't "see" whether the compressor is on-or-off on the Scangauge or "feel" it via the ambience of the car or in any other way to indicate the compressor is running.

One reason I use the compressor's on/off button in this way is the fact that at one time in automotive history (and maybe today in some cars?), some genius figured it was a good idea to let the compressor run all the time and use the engine's hot water to moderate the cabin temperature. It works, but at a higher fuel price. A workaround for me was the compressor on/off technique. I don't know if it's better for fuel economy on my Mazda or not, since I suspect the Mazda's compressor cycles as needed and doesn't use the "hot water" idea. But, is it better for FE to use the on/off technique or just set the temperature/thermostat at the desired temperature level and let the compressor cycle? Don't know.

In any case, I'm too old to drive around in a tropical climate while being hot and sweaty with the roar of hot-and-humid-and-dirty air blasting through the car for several hundred miles at a stint. I just can't take that anymore. Nosir.

Your Mazda is a lot newer than my old beater. I would expect better a/c abilities. My Ma’s ‘06 Mazda 3 is a mystery, I don’t think I have ever driven it. But my Civic AC sucks, my Ma’s former ‘04 Sienna a/c sucked, and the worst of all was my old Toyota Echo, the only one I checked with an actual manifold setup.

So, given that my dataset is old, i’ll say that 90s/early 2000s a/c royally sucked.

[teoman]

I did some data logging which I will try to post once I get back.

My cars AC causes about 10 newton meters of drag torque to the engine when on coldest. (I will compare it to the engine torque produced later)

In car display says 0.7-0.8 liters of diesel per hour.

It reports this value even when the car is in dfco mode. And it appears to be a pre estimated value.


Car is audi a3 1.6 tdi 2012 with 200k km.

[slowmover]

Originally Posted by MeteorGray

I drive my Mazda3 mostly on long commute trips on the highway, and as already noted, I can't tell the difference in fuel mileage with and without the air conditioner. I try to see evidence of it on the Scangauge via its instantaneous mileage readout, but the variations of highway, wind, etc tend to mask any contrast between "on" and "off" mileage differences for me. I know the air conditioner takes energy to run, but it's small enough that I can't "see" it, either on the Scangauge or at the fuel pump.

One technique I use when I don't have a passenger is to push the button that controls the air conditioner's compressor on/off activation. I leave the thermostat/temperature setting on "maximum cold," and when I get cold, I push the button to turn off the compressor and let the temperature gradually rise until I'm too hot, then repeat the cycle. When I push the compressor button, I can't tell any difference except for the temperature of the air; ie, I can't "see" whether the compressor is on-or-off on the Scangauge or "feel" it via the ambience of the car or in any other way to indicate the compressor is running.

One reason I use the compressor's on/off button in this way is the fact that at one time in automotive history (and maybe today in some cars?), some genius figured it was a good idea to let the compressor run all the time and use the engine's hot water to moderate the cabin temperature. It works, but at a higher fuel price. A workaround for me was the compressor on/off technique. I don't know if it's better for fuel economy on my Mazda or not, since I suspect the Mazda's compressor cycles as needed and doesn't use the "hot water" idea. But, is it better for FE to use the on/off technique or just set the temperature/thermostat at the desired temperature level and let the compressor cycle? Don't know.

In any case, I'm too old to drive around in a tropical climate while being hot and sweaty with the roar of hot-and-humid-and-dirty air blasting through the car for several hundred miles at a stint. I just can't take that anymore. Nosir.

It’s not temp modification so much as it is humidity control. It’s also an extra level of dust removal. Start with air on Recirc, move to Norm, and then to Bi-level. Crack a rear window slightly until that last change.

Control over humidity while parked is a goal for you. Given shade (no direct sun) the A/C load is then minimized.

Plenty of legal window tint the other. 3M makes a huge variety ($ to $$$$) and it’s likely you’ll need to head to Houston or Dallas for the right installer for the good stuff. Quality matters. (I use REFLECTIX press-fit interior shades cut from the four foot tall rolls for my pickup & big truck. Use the windshield one EVERY time you park while on errands).

Inspect the door & window seals. R & R as needed.

And, yes, a sheepskin seat cover (medical grade) helps.

Every vehicle takes 1.5-hrs to warm-up (tire pressure equalization). 45-miles just to get oil to operating temp. Coolant temp means little. Its burning off the accumulated crankcase acids that counts, thus trips under three hours are to be avoided.

As a member of EM it (should) be fair to assume you’ve eliminated the multiple short trips you may have once made as a matter of course.

Thus long trips are highlighted for highest average mph as a greater percentage of use.

HVAC use is BEST for highway as airborne pollutants are reduced. Fatigue via wind noise is reduced.

HVAC is thus a fuel savings as the INEVITABLE onset of operator fatigue recedes farther into the day.

The “penalty” for A/C use correlates with full warm-up and overcoming short trip acids, etc. At three-hours plus, it’s using more than would the heater (per se), but today’s cars aren’t built with dash-controlled kickpanel vents and front vent windows. There’s no “Intermediate” adjustment of exterior-admitted airflow without the HVAC system. (No spring or fall, as one guy said).

As I my post above, what’s the actual ANNUAL CPM penalty? 3/100’s of a cent? A half-cent? Two cents?

Solve for that number.

.

[JSH]

According to my Spark EV A/C uses 5% of the total energy consumed on my commute. Heat consumes about 33% but the Spark EV has an inefficient resistance heater.