Stupid A/C question... (any MPG effect of max. vs low setting?)
 

Hey guys,

[Keep in mind that I'm pretty ignorant when it comes to engines, accessories and how they all work together once the key is turned. So if you can type an answer to this dummy's question between falling off your chair and laughing hysterically, I'd appreciate it.]

I've been running without my A/C turned on since learning more about hypermiling. BUT, last week we had our first 95* days up here in NJ. Stuck in traffic heading over the George Washington Bridge into Manhattan, that A/C had to be turned on. So here's my question...

Once you turn the A/C on, does it matter whether you run it at a low level as opposed to MAX? In other words: As far as gas consumption goes, does setting the controls to a higher temperature and lower fan speed use less gas than full-on cold and blast the fan? Or is it that once you turn the A/C on, it uses the same amount of fuel no matter what the setting?

Thanks in advance,
John

Use recirculate and max cool until you get it comfortable, then turn the fan speed down, but keep it on recirculate and max cool. If you can after you get it cool enough inside then you can cycle the AC on and off but keep the fan blowing until the evaporator looses all of it's "coolness" then cycle the AC back on to get the evaporator cold again.

You can also turn the AC on when you are forced to slow down, and downshift to keep your RPM up and possibly get free AC with DFCO when decelerating.

Got stuck on that same bridge picking up an Eldorado for my 83 year old mother. God what a mess.

regards
Mech

On my truck, like most vehicles, the AC compressor is wired to a pressure switch in the Freon high pressure side. The faster you run the fan, and the hotter or more humid the air, the more the compressor runs.

I run the AC fan on low with the dash vents blowing cold air straight at me. I'll cycle the either the AC or the fan switch off if it gets cool enough. The compressor only runs when the fan is on and the Freon pressure switch is closed. The compressor is off much more than it is on when the fan is on low.

When I run it that way, the gas mileage hit is too small to measure.

First rule of thumb (imho) NEVER SUFFER!!!!!!!!!
as a general rule, when its's on low or high, it's on. I let mine get my car as cold as fast as it can. the I adjust the therosate (I have a digital screen) to 74 or what ever and forget it.

I have run tests on the highway at 60 mph. A/c on window up. then a/c off windows down. in a 99 vaden plaus jag and an 2002 infiniti Q45. both lost 3-4 mpgs with the windows down.

I'm no stranger to that sort of traffic jam and heat here in metro NYC. I usually drive a convertible in summer - but short of that, here's a similar strategy: Leave the fan running but switch the A/C off while accelerating or when it's adequately cool enough; turn it back on while decelerating, or at idle. An added benefit is that turning off the A/C while accelerating won't rob you of power when you need to move forward, to avoid having someone cut in front of you.

If I were to be snide I'd say that if you can afford the price of gas and the bridge tolls between NY and NJ, you can afford to leave the A/C on. (I'm half joking...) ;)

Hi John,
It depends somewhat on the car. There are 3 control systems used.
On smaller cars the A/C compressor cycles, only running when the cabin is warmer than the thermostat setting. Using the lowest fan and warmest setting that is comfortable saves fuel.
Some larger cars; when you call for A/C the compressor is engaged and cabin temperature is regulated by also running the heater.
On hybrids with high voltage systems the compressor is run by a variable speed electric motor. It runs just fast enough to give the desired cooling.
In all cases the A/C system uses less power and therefore less fuel if you stay on the edge of comfort. Running the A/C full blast uses more fuel.
An air condioner is a heat pump. It uses power, usually from the engine, to move heat out of the cabin air. The amount of power required from the engine is the amout of heat removed divided by the "coefficient of performance." The COP depends on system design and outside temperature.
Car A/Cs have COPs from 3 to 5 and usually about 50,000 BTUH capacity.
-mort

Not a stupid question at all. I was coming on this evening to ask it myself. I started getting into all this last fall, and this is my first tank that I've let myself use the AC. It's just gotten too hot here in OKC. Without the AC I'd walk into a client's office dripping sweat. Thanks for asking!

I recirculate the air as Old Mech suggests, but run the fan at full speed, cycling the AC compressor as Thymeclock suggests. Modern climate control fans draw nearly as much current running in the #1 position as they do running in the #4 position. They inefficiently wire in resistors to the #1, #2, and #3 fan switch positions to cut current to the fan, while drawing the same amount of power usage. Ergo, you're better off running the fan at full speed.

I switch the AC on when needed when gliding or holding speed on level ground, and turn it off as much as possible when pulsing or climbing hills. That way, I'm in control of when the AC compressor is working. The other recommendations let the car's computer control the AC compressor duty cycle.

But I have a dumb climate control system. If you have a luxury car, you're hosed, because it will run the AC all the time (within its duty cycle), mixing heat with the cold air in the Summer to regulate temperature, and running the AC to dehumidify the heater air during the Winter.

But you still had the AC belt on when you were not using the unit. If you take the whole unit out, the situation changes, since the parasitic draw on the engine from the off AC is gone. For me, I installed dark, metalized, heat reflective tint. My windows are never more than cracked on the freeway. I run the fan full blast.

Not all fan speed controls are inefficient like that. I've measured the voltage drop on mine. It draws proportionally more power with each fan speed setting.

Fan speed - volts
0 - 12.3
1 - 12.2
2 - 12.1
3 - 12.0
4 - 11.9

The big answer is, "it depends".

- Some cars mix in warm engine coolant when you set the temperature warmer. Run these on max cool and cycle on and off.
- Some cycle the compressor on and off to get the desired temperature. Run this one at the highest temp you can tolerate.

Of course, the most efficient answer is, "None". ;) Even in Texas.

You can also "up" the performance of an A/C system by bypassing the heater core, even if for just the summer months. Where I live an A/C system in a car is not optional, and it runs 9-10 months full-time (at least at mid-day). High humidity is a bear to condition.

Proper -- legal -- window tint is of benefit.

Use a windshield cover (interior; keep the vehicle windshield pointed away from the sun when parked year round; OEM-quality); crack the windows if security allows when parked (finger-tip width), and run MAX A/C after opening windows a bit as you get rolling to exhaust hot air, then closing them.

It will take a while to cool more than just the air as the A/C needs time to cool the "furniture" of the vehicle. I say "get it to that point" before switching to FRESH air. (The drier the climate the sooner this will be; humidity takes time to overcome). One can back off the TEMP setting a click or two to reduce compressor cycling, but maintain MAX until nearly blue-knuckled (your clothing is long past dried). This really is the earliest point to change over. Not chilled clothing, but dried & chilled.

FRESH plus an increased temp will take care of it the rest of the day (or until shutoff). It is also when I switch over to combined foot & head airflow (what we used to call the "highway" position). One can -- from hereon -- experiment with further reductions in, first, temp, and second, airflow.

I point most all vents as high as possible, and towards the back of the cabin. This produces the most rapid cooldown, and maintains even sorting (layering) prior to exhaust.

I use a thermometer to measure outlet temps, and this also informs me of how the system is doing as a check against the way I feel (a bit more consistency in operation).

All of this assumes: clean condenser (rare), clean evaporator (look into this, it is vital), proper refrigerant charge (rare), and proper weather seal function (as well as cabin exhaust function). The best long-term operation of the HVAC system trumps a tenth or two. Do some investigating. There are enthusiast sites with the occasionally well-informed membership (rare) and a couple of sites dedicated to mobile HVAC with service techs and others online (better). Some systems vent refrigerant more often than they should, and some leaks are harder to diagnose/find than others. Obviously, living in TX, AZ, and FL one finds the better, more experienced shops.

Another DIY is a separate condenser fan to "up" performance and to keep the compressor from cycling as often. Head pressure can be quite high at startup (hard on whole system).

Today's compressors are nothing in power draw compared to what we used to see on late 1950's thru mid-1970's cars (and why small cars simply didn't have A/C available). On the other hand you could hang meat in those cabins . . none of this bull about 20F below ambient stuff! 119F full sun Las Vegas heat and 62F in the car, ha!

A garaged car, or one under cover, always has better HVAC operation than one exposed to all elements. I pay for covered parking (not just for this benefit).

.

I used to do a lot of hitchiking. I hated seeing a late model luxury car pull over on hot or cold days. The worst hot day ride was 97 degrees and humid outside. The driver had the thermostat set to 55 degrees, and the air conditioner was holding to that temperature. BRRRR!

That was balanced by the driver who set his thermostat at 85 degrees when it was 20 below outside.

Thanks for the collection of cooling tips, Slowmover. I'll be taking many of them, but there's one I do a little differently: The air coming out of the vents when I first get into the car in the afternoon is hot. So I roll the windows down and driver for about 90 seconds, after which the incoming air cools to ambient temperatures and the interior air is flushed (even with tinted windows and reflective windshield and rear window covers, it gets hot). Then I roll up the windows and proceed along your guidelines.

You're welcome. My Dad started driving in 1939 and was one happy guy when he moved to Texas and could also afford a new car with a SEARS underdash A/C unit (a '54 Ford). From him I learned to "set" the vents . . and never move them again.

I agree with wanting to flush out the interior stale air, but have found that it can sometimes work and sometimes not depending on both local traffic/route, plus whether a dry or humid climate. What worked for me in 108F Dallas is a bit different than 94F Corpus Christi with high humidity (and, even among humid areas it can differ, ha!) I sometimes crack a rear window to accelerate airflow, but only if moving steadily.

It's worth a little trouble to find the cabin exhaust vents to be sure they're clean; no torn weatherstripping, etc.

If the system is a few years old, have a shop hold the system under vacuum (500-microns; or 28") ALL NIGHT. Sometimes a small leak will show up this way. A thirty minute check is only JUST adequate. There are more details to learn for a Florida boy on some of the mobile HVAC sites for best performance.

.

For those who are interested ... Mythbusters tested the windows down vs AC thing and found that running the AC with windows up is much better than no AC and windows down. While I don't agree with Mythbusters on many of their techniques (sometimes they don't use scientific method) I do agree on this one as their science was sound.

Another was with pickups, its best to have the tailgate closed and not use one of those nets or leave the gate open as the truck is designed to have better air flow when air cycles in the bed. I remember they used a bed cover as well and if memory serves even that hurt gas mileage. This does not include boat tails however.

just my 3 cents

Did their test run with the windows all the way down? Did they try the fan option with windows narrowly cracked? And most importantly did they remove the AC belt? Even turned-off, the AC's compressor belt is a parasitic draw on the engine. I run dark metalized heat reflective tint, sheep skin seat covers, the fan, and slightly cracked windows. It is July and humid and I'll be fine (but I have almost never used AC, even when I lived in Virginia's tidewater humid zone).

On the '96-2000 Civic it seems the compressor stays on as long as the "A/C" button is activated and the fan is running (it lights up green). Adjusting the temperature slider only seems to add heat from the engine coolant and leaves the compressor running. It's interesting to know that some other cars still use a thermostat to switch the compressor on and off like all our other older cars did.

When I do need the a/c either for cooling or for keeping the glass clear, I run the system on recirculate and cycle the compressor on and off via the "A/C" button. It's very rare indeed that I run the compressor without the "recirculate" setting switched on.

If only keeping the glass clear, I find it doesn't need to run much of the time, maybe 10-20%. If it's truly hot out the percent on time can definitely go higher. Mostly I can run the fan on speed "1" but sometimes I want more.

Like others wrote, I try to have the compressor on for downhills and off for uphills but that doesn't always work out. In those situations, I remind myself that I'm still getting probably 2x the mpg of most passenger cars and even better when compared to all the pickups and SUVs.

I also use a collapsible sun shield. If I need to get hot air out of the car after it's been sitting, I'll drive a couple minutes with windows open and compressor off

Agreed, the physiological stress on your body due to higher temperatures can effect your reaction time and alertness. I know that when I'm driving that I get sleepy when it warms up in the car, but when it's cooler I'm more alert. That is enough of a reason for me to use the A/C as much as I feel I need to.

They found this using an Expedition I believe, probably with a V8. For an engine of this size, yes, it is probably more efficient to use the A/C... the engine barely notices it at speed.

In a small, 4-cyl, A/C use puts a larger ratio of load onto the engine, so this curve of more efficient for windows vs. A/C likely changes. A/C takes a HUGE chunk of FE away at ANY speed, and is very noticeable on the highway, whereas windows down doesn't have as big an effect.

I also don't use MAX A/C because it causes the compressor to run constantly. If I have it to the "upper vent" position, you know, the picture of the seat with the arrow, and turn on the A/C, the compressor cycles itself at some point. I still self cycle at a higher temp though. Temperature dial is always at cold, because the compressor still runs, but heat is added from the coolant, as mentioned before, and is a waste.

yes high end cars will default to A\C on.

but ive yet to find one that has no loop hole back door,
set it to feet and turn the A\C butten off. but the moment you turn it back to defrost or turn the engine off/on the A\C resets to being on.



in short, any auto base model or high end, with working A\C set to defrost has it running, weather the A\C butten is pushed or not

thus why if you have it on the floor mode with A\C butten off and it starts raining. you flip it to defrost and it automatically turns the A\C on to deumidafy the window. weather you want it too or not this is happening
well ok ive yet to find one, so there may be one out there that does not. but im gessin theres very few of them, if any.

also the only difference in normal to MAX A\C mode is that normal mode is fresh air
and MAX mode is reticulate mode

MAX is not in any way a higher out put, other then cooling the same air in the cab.

My engine is so gutless that I can "feel" the load of the AC if it is on and from what I can tell it loads the engine just about as much no matter what setting I have it on. Turing it off while accelerating is like a mini-nitor boost! As a result, I cycle it on only going down hill or decelerating since I have a little energy to spare then. It blows cold for a good 20 seconds after I shut it off.

Also you should never come to a stop (arrive at your destination) with the AC switched on for two reason:
1. It's a waste of energy since it will blow cold for a while after you switch it off and number 2, even important to non-ecomodders, the wet cooling system will take much time time dry this way and smell of dank mildew over time... even in new cars but especially in older cars with some dust inside the vents. Do yourself a favor and switch it off (but leave the fan running) about 60 seconds before you park.

On a side note, it never occurred to me to run re-circulate in hot weather! lol. I used it all the time in the cold in Oregon but I an new to driving in the heat. Thanks all for the tip!

Do I ever know what you mean. When I turn on my A/C it feels like I just threw an anchor behind me. I can't wait to get my UG and see the difference in fuel economy when I turn that A/C on.

There are also cars that use an adjustable swash plate compressor. Basically it changes the volume of coolant being pumped, depending on the required pressure in the system.
That means it will draw less energy on low cooling demand than on high demand.
Don't know about yours, maybe you could ask Toyota.
This shows roughly how it works
One-Way Swash Plate Internally-Controlled Type | Compressors - TOYOTA INDUSTRIES CORPORATION

My truck's 360 cid V8 doesn't feel the compressor, I can run it all I want.
Trucks are not like cars, their engines are designed to do one thing: Pull load.
And that is where 90% of the engine's energy goes, into pulling the load.
Even the best aerodynamics make small effect on a truck's fuel economy.

Why those square yellow brick school buses wouldn't get better fuel mileage if their front ends were more aerodynamic, their engines are pulling so many thousands of pounds of load that even the aerodynamics of a billboard hardly affect its fuel economy.

The only thing that affects its fuel mileage are speed variations, and their amount and intensity... The few things that frustrate me the most, other than folks tailgating, are drivers cutting me off, getting in the space in front of the truck without thought as to why that space exists other than they're trying to get to the proverbial finish line.

When a truck is following the car in front with a 4-6 second distance...
DON'T cut into the empty space in front of it!
It is the truck driver's "safe following distance."
It exists as a buffer, to help in case of sudden stops.
Not because it saves fuel, but because it takes that long TO stop!

When a truck is SLOWLY coasting to a stop light ...
DON'T cut into the empty space in front of it!
Doing so may force that truck to stop completely.
A truck coming to a complete stop wastes a tremendous amount of fuel re-accelerating.

My truck's mpg hovers between 10 and 11 miles per gallon.