In the summer I have a habit of switching off the air conditioner every time I pull away from a stop, then turn it back on after I get up to speed again. Because the engine needs the most power when pulling away from stops, I have the impression that this saves fuel. Is this necessary? What is the Car Talk opinion about this practice?
Engines are so powerful today (overpowered in my opinion) that the few HP that the AC uses are irrelevant, about 2-6 HP depending on size.
And even if that were not true, it still would not save fuel.
And turning it off and on can cause wear in the AC clutch.
b
Unless you’re driving an underpowered 89 Hyundai, your car knows when it needs power and when to turn off the A/C because of the load on the engine. I can’t see any fuel savings or increased power from doing this.
At the end of a year you might be able to buy a stick of gum with the fuel savings. OK, half a stick.
You’ll spend more money replacing the switch you’re wearing out than the gas you save. Turning off the a/c while accelerating was needed when motors were weak, not to save gas. You could save the same amount of gas turning off the a/c any time you want.
Not worth it to me.
The OP can count this as one more vote for NOT doing what he has been doing.
However, the OP did fail to identify the make, model, model year, and odometer mileage of the car in question. If the car in question is an older economy car, then it is possible that there might be a slightly perceptible difference in the amount of power available for acceleration when the A/C is switched off. However, the difference in fuel economy would be so meager as to be almost un-measureable, IMHO.
And, if the car in question has been made in the past 8 years or so, the A/C compressor’s “drag” on the engine isn’t really a factor.
Additionally, as was stated, the additional wear and tear on the compressor’s clutch would FAR outweigh any possible fuel savings.
Completely unnecessary. Even if you have to floor it, some cars will turn off the ac compressor under full throttle conditions. So my turning it off manually, you’re not accomplishing anything as the car will do it automatically if needed.
My wife (and I) used to push the A/C compressor button off when getting up to speed in her 1995 Mercury Tracer wagon because it was painfully underpowered. After that it would maintain speed no problem. Just donated it to Goodwill after many years of mostly trouble-free service, and she how has a Prius v wagon and the A/C has nothing to do with the engine.
Unless you have a difficult time accelerating to the same speed as other traffic from a stop, I wouldn’t do it. The AC compressor – when it is on – does draw considerable power from the engine; I think I have heard recently modern AC compressors takes close to 40 HP. But most engines these days are in the 150 to 200 HP range, so they have enough to accelerate rapidly enough to keep up with traffic even with the AC compressor on.
Doing what you are doing probably does save a little fuel. Anytime the AC compressor is working the engine is using more fuel. But the fuel savings wouldn’t amount to much I expect. But why speculate? Do the experiment. For one month drive without turning off the AC when accelerating, and the next month do what you are now doing, turning it off. Compare the measured mpg.
If there is any difference, it wouldn’t make up for the cost of replacing the switch that will fail prematurely due to being turned on and off all the time. However, if your engine won’t accelerate the vehicle to speed fast enough that it becomes a safety concern (as evidenced by cars behind you honking to get out of their way, etc) , then it would make sense to turn it off.
This all reminds me of the time back in the 1960’s my dad purchased a new Ford Galaxy 500, our first family car w/AC. He was afraid to turn the AC on for fear the mpg would decrease. So I showed him there’s two setting, one “fresh air” and on “recirc”. I claimed when it is on “fresh air” it doesn’t turn on the AC. For a while there I was riding along in air conditioned comfort. Then one day he brought out a thermometer and discovered even on “fresh” the air was colder than the outside air. No more AC comfort for me! Well, one exception, he’d allow me to turn on the AC if we were going down a long steep hill … lol …
A question for the HVAC-minded: I was told that cycling an AC unit on/off/on quickly was a good way to shut it down due to overpressure–that once you shut it off, you need to wait a while before turning it back on.
Is this true for AC in general, or auto AC in particular? If so, is a modern car smart enough not to overpressure the system?
In the early years, 80’s maybye earlier a friend with an under powered corolla, joke was if you wanted to slow down, turn on the ac, I do not believe it a factor in modern cars.
The first car we had with a/c was a 1962 Ford Ranch Wagon with a 352 V8 and two-speed automatic, bought used in 1965. That thing was a tank and took forever to get up to speed. Whether the a/c was on or off didn’t make much difference. My dad sure liked the a/c, though. Every car or truck he bought after that had a/c.
On a modern vehicle, I think it makes very little difference if the a/c is on or off when you accelerate.
Cliff
Home AC has a 3 to 5 minute delay to restart the compressor after stopping. That’s to let the high pressure drop so the electric motor can easily restart. Car AC can cycle pretty frequently; on my old Ford F250 it cycled about every 30 seconds, but my current Insight is much less frequent. Short cycling might be harder on the compressor, but I don’t see why that would overpressure the high side.
George who ever told you a car air conditioner requires 40 HP is just plain wrong.
Yeah, about 8X too high. More like 5 hp or so.
I think turning almost ANYTHING on and off repeatedly is almost always an awful idea.
The first air conditioned car I had was my 74 Olds Cutlass with 350 Rocket V8 with 4 barrel carb. If you hit the gas too hard from a stop you’d squeal the tires. It was on if it was hot and off if it was cold.
Turning the A/C on and off is a waste of time, energy, and if anything could lead to premature replacement of the switch, relays, or possibly premature failure of the compressor clutch.
As to horsepower consumption, the compressor won’t draw 40 but I think it might be more than 5.
In one of the old car mags somewhere in my stash in the attic I vaguely remember a dyno test being done which involved testing certain accessories.
I seem to remember that a water pump driving a mechanical fan was eating up 7 horsepower and a Ford AOD transmission chewed up 15% of the horsepower when transferring it to the rear wheels.
Modern compressors use a lot less power than those old days A6 GM compressors. One of those compressors on a small displacement engine back in the day would have stalled it at idle and the car would likely not make it up even the slightest rise with the compressor engaged.
There were reliable and would freeze a side of beef though…
Manufactures change the air conditioning performance and output on vehicles when the “Economy” mode is selected. This on the Lexus IS250 for example;
ECO Drive Mode Control
(1) Under the control of the ECO drive mode, the air conditioning amplifier assembly restricts the air conditioning system performance under specified conditions, thus improving fuel economy.
(2) The ECO drive mode control is activated when the combination switch assembly (drive mode select) is operated, and then restricts the air conditioning system performance as described below:
Inside/outside Air Switch Control
Automatically switches the air inlet port to the internal air circulation mode when the outside air temperature is equal to or higher than a predetermined temperature and reduces the power consumption.
Blower Level Control
Sets the blower level in AUTO mode lower than normal, and suppresses the power consumption.
Seat Heater Control
When the ECO drive mode is selected, the air conditioning amplifier assembly turns on the front seat heater system in accordance with the air outlet mode (front seat control mode or driver seat control mode), outside temperature and room temperature. For details, refer to the seat heater system.
There is no mention of changing the compressor clutch cycling because it doesn’t cycle on this model.
Other manufactures reduce the compressor and alternator outputs during acceleration on special Eco models.
You actually have a choice here: Turn off the AC for a year or just bend over and pick up the next dime you see on the parking lot. The dime will put you in a better financial position than turning off the AC as you pull away from a stop.
Ah, the good old A6
It was physically exhausting to replace one, where it was hanging low, rather than up top
And if that low hanging application used spacers between the compressor and the bracket . . . your arms and wrists were in sad shape by the time you were done
But the darn thing would blow ice cubes in your face . . . !
