Air Conditioner Effect on MPG (tested at 65 MPH)

[Ares]

3) Always set the temperature control to full cold. Setting it a bit warmer with the temperature adjustment just mixes in warm air which you just spent power to cool down.

I agree with most, but the one quoted above. If the "recirculate inside air" is turned on, then there isnt much warm air to mix in. The benefit with having the thermostat NOT at full cold is that the compressor does not "kick in" as often as if the thermostat was in full cold.

Does that make sense?

It seems that the lowest setting is 61 or 62. I leave mine at 67. Maybe even 70 during the night.

[91cavgt]

I agree with most, but the one quoted above. If the "recirculate inside air" is turned on, then there isnt much warm air to mix in. The benefit with having the thermostat NOT at full cold is that the compressor does not "kick in" as often as if the thermostat was in full cold.

Does that make sense?

It seems that the lowest setting is 61 or 62. I leave mine at 67. Maybe even 70 during the night.

When you set the thermostat for anything above the absolute coldest setting, the blend door opens slightly, which mixes in hot air from the heater core along with the chilled air from the A/C evaporator. So, you are in effect flushing efficiency down the toilet so to speak with anything other than the coldest setting.

But as he said, when you adjust the temp of the interior with the fan speed, you will actually cause the compressor to not run as often. Here is why.


In our A/C system, you have a compressor that compresses the refrigerant to VERY high levels (150-250 psi on average). The compressor will continue to run until one of a few variables occurs. Either the high side pressure gets too high (which means something is wrong and the condenser fan is not running) or the low side pressure gets pretty low which is typically why the compressor shuts off. This occurs when the freon is still very cold when it comes into the compressor. This is caused from very little air flow going across the evaporator. So what causes little air flow going over the evaporator? Either a low fan speed or a slightly opened blend door which decreases air flow across the evaporator.

So, since the compressor is going to run about the same if the blend door is open or not, why not use all of that cooling that is available, thus raising the efficiency of the HVAC system by varying the cooling with the fan speed?



With that being said though, sometimes in the south, it can get so hot and so humid that your A/C compressor may not shut off at all regardless of where the thermostat and fan speed are set to.

[Ares]

When you set the thermostat for anything above the absolute coldest setting, the blend door opens slightly, which mixes in hot air from the heater core along with the chilled air from the A/C evaporator.

Then you're helping the car by cooling water temps! :-D

Both you and doax are implying that the vents are pre-programmed, which is a huge possibility because of how cheap the car is. If the vents are, in fact, preprogrammed, I would want to know, from the horse's mouth, when the heater vents are opened.

The image below is how I feel like the heater vents are opened. No heater vents at blue setting, very little, if any, at mid, and small to large opening in red.

To control the cool in the blue setting, I figure that they would just change the time/amount the compressor works.

[91cavgt]

Then you're helping the car by cooling water temps! :-D

Both you and doax are implying that the vents are pre-programmed, which is a huge possibility because of how cheap the car is. If the vents are, in fact, preprogrammed, I would want to know, from the horse's mouth, when the heater vents are opened.

The image below is how I feel like the heater vents are opened. No heater vents at blue setting, very little, if any, at mid, and small to large opening in red.

To control the cool in the blue setting, I figure that they would just change the time/amount the compressor works.

The only time I have seen the computer control how long the compressor stays on is with uber expensive luxury cars.

There are only 2 blend doors in cars. One separates the heater core from the evaporator, and the other separates the outside air from the recirculated air.



So yes, unless you have the knob on full cold, the car is letting some air from the heater core into the car. The amount of cooling of the motor you get from not having the selector all the way on cold is miniscule. But, a warmer motor does translate to a more efficient running motor. Since the Mirage has electric cooling fans that cool off the condenser and engine, there isn't anything to gain, efficiency wise, from not having the A/C set to full cold.

[Ares]

One can feel a difference just by holding the window motor up or down (when it's already up or down), I'm sure there is some drain on the engine when the fans turn on.

However, the fans arent the big power drainer - it's the compressor. The fact that the compressor removes 10-20whp is a big MPG deal. Don't turn on the compressor, the better MPG. How does one do that? Lower the AC temp - or manually keep turning on/off AC (not fan blower).

[91cavgt]

One can feel a difference just by holding the window motor up or down (when it's already up or down), I'm sure there is some drain on the engine when the fans turn on.

However, the fans arent the big power drainer - it's the compressor. The fact that the compressor removes 10-20whp is a big MPG deal. Don't turn on the compressor, the better MPG. How does one do that? Lower the AC temp - or manually keep turning on/off AC (not fan blower).

The only way to increase MPG with the A/C on is to use the fan to control the temp, not the temp knob. Basically, keep the temp knob as cold as it will go and just lower the fan speed when it starts to cool down in the car. Otherwise, keep the A/C turned off as much as possible.

[Alpha_One]

My Mirage does shut off the compressor when the cabin gets cold. You can hear the compressor relay click on and off and the engine will increase the idle to compensate. In fact when idling in 1st gear, the car will surge when the compressor kicks in automatically. I normally set my temperature setting at 26C.

And yes, I suppose that means increasing your temp setting should increase your MPGs, but I haven't quantified it.

The only time I have seen the computer control how long the compressor stays on is with uber expensive luxury cars.

There are only 2 blend doors in cars. One separates the heater core from the evaporator, and the other separates the outside air from the recirculated air.



So yes, unless you have the knob on full cold, the car is letting some air from the heater core into the car. The amount of cooling of the motor you get from not having the selector all the way on cold is miniscule. But, a warmer motor does translate to a more efficient running motor. Since the Mirage has electric cooling fans that cool off the condenser and engine, there isn't anything to gain, efficiency wise, from not having the A/C set to full cold.

The only way to increase MPG with the A/C on is to use the fan to control the temp, not the temp knob. Basically, keep the temp knob as cold as it will go and just lower the fan speed when it starts to cool down in the car. Otherwise, keep the A/C turned off as much as possible.

[Yttrium]

The TV show MythBusters tested this. On the SUV that they used to test, they found that 45 mph was the break even point. Above 45 mph the SUV used less fuel with the A/C on and windows up, and below 45 mph the SUV used less fuel with the A/C off and windows down. However, our Mirages have different aerodynamics and different power curves of the engine...The only real way to prove one way or other is to do some real world testing.

Since I have a long, fairly flat and boring highway commute I decided to do some testing. After getting onto the highway I reset my manual Mirage DE's fuel tracker and changed the car into one of the following configurations for ten minutes at roughly 65 MPH:

1. All windows up, air conditioning off (56.3 and 55.9 MPG)
2. All windows up, Air conditioning at max (52.4 MPG)
3. Front two windows down/back windows up, air conditioning off (54.7 MPG)

The windows up, no AC test was done twice with numbers of 56.3 and 55.9 MPG. The windows up, AC at full blast had 52.4 MPG while having the front two windows down came in at 54.7 MPG.

I had expected the windows down to be the worst, but it did better. Since these numbers are so close to each other I would image that slight differences in speed may account be adding a good deal of the variation.

[Donut]

thx for the tests.

I like the cross breeze from the windows best. Great to know that it has also a better fuel economy than AC on.

[Alpha_One]

@Yttrium, thanks for the tests! It verifies my own (non-quantified) experience that when traffic is clear, it's better (fuel economy wise) to have my aircon off and windows down.

The Mirage is just a very aerodynamically efficient vehicle.