i've been wondering...when i turn on a/c.. should i start with highest power 4 then decrease to 3 then 2 then 1...

or

Should i start from 1 then increase to 2 then 3 then 4...

My friend and i were discuessing about how a/c will affect MPG...
my friend thinks... when use a/c... always starts with lower power(1) then increase to higher (2 to 4)..

but what i think is.. either you are in 1 or 4.. a/c compressor is "on" .. so it is just fan speed that varies...

does a/c power(fan)..affect MPG?

 

i really want to know...

Thanks..in advance...

 

You are right. The fan speed does not affect the compressor.
If there is any difference in MPG between low fan and high fan, it should be miniscule.

The decision to use low or high fan speed could be based on this:
Say the compressor is running (A/C on) and the temperature knob is set to the lowest setting (blue).

If the fan is on "High", it is blowing all the air that's cooled, plus some of the surrounding warm air. Therefore, the breeze coming out of the vents is faster but not as cold.

Same settings except "Low" fan: slower breeze, colder air.

Based on this oversimplified generalization, here's what I usually do.

If I am getting in the car in the middle of a hot day, after first driving for a short distance with the windows open to let the severe heat out, I will turn the A/C on full blast with the fan on "High" to get some cooler, drier air circulating. After it starts to get more comfortable, I will turn the fan down to speed # 1 or 2.

If I am already driving and it's just starting to get a little too warm, I will just turn the fan to #1 to begin with, and then gradually make slight adjustments to the temperature knob until it feels like I want it to feel.

Hope this makes sense.

 

I'm not an expert or anything, and I've never looked at how car ACs work, but I used to be an electrition, so I'm just going to speculate.

The fan(s) for the car's ventilation use more electrical power the higher you set the speed. But that should all be taken care of by the car's alternator (really a small generator these days) which always provides the same amount of potential power. So that uses no extra gas at all.

I think the gas consumption comes in on however the car heats/cools the air.

Heating the air is probably done with an electrical heating element, or by running it past the engine. So that probably don't use any gas eaither.

So the problem must lie soley on the A/C method (air cooling). I really don't know how our Fits cool the air, let alone how any cars do it. But from Steeldog's description makes it looks like the A/C always cools the air the same amount, and the fan speed doesn't really increase the cooling.

So the only way to save gas with A/C is to simply not use more than your willing to save on gas. I guess, if it isn't too hot where you live yet, try driving on a tank without using the A/C and compare it to one of your last tanks when you used A/C. See if the savings are even worth your heat exhaustion.

Maybe get one of those insulated bottles and keep ice water in it so you can cool yourself down with water instead of air.

 

The fan does draw more current at higher speeds (so the alternator is putting out more power and the engine is doing more work), but this is absolutely miniscule...

When the AC is on (the switch), the compressor is not neccesarily always on... you can hear and feel it swithing on and off. The temperature of the coils (cooling element) controls that. The more air (faster) you blow across the coils, the faster they will warm up and the more the compressor will need to run to maintain their temp.

So... having the fan on a higher setting will technically require more gas... but I seriously doubt that the difference is enough to notice.

 

The Alternator(Generator) always turns at the same speed but has a maximum potential of power. I used to work on large 750KW generators, and I'm sure it works the same way, just on a smaller scale. Modern car alternators are supposed to be actual generators these days, I think.

What you said about the cooling coil needing to work harder due to more warm air hitting them is probably what causes the tiny shift in gas milage from fan speed.

I should read up on this. I'm wondering if the whole A/C reducing gas mileage is just a myth in modern cars.

Hard to test by just not using your A/C unless you drive exactly the same in the exact same conditions on every tank of gas. Can't use scientific method when you commute all the time.

 

The compressor definately does cycle on and off as needed.
It's very difficult to measure exactly how much the AC impacts MPG... it's probably very small
But basic physics says that it MUST impact MPG... the engine IS doing more work, so the same engine will use more fuel (assuming you use the same fuel)

 

I know dat ko-reck.

The A/C does cause the car to get significantly lower fuel economy, but it is a very reasonable trade-off as far as I'm concerned. Why not be comfortable, since you are getting pretty good MPG with this car anyway?

I personally like to keep the fan on a lower speed to keep from hearing it so much.

My only gripe with compromising the MPG is that the freakin fuel tank is so small, so you just have to fill up that much more often.

 

thanks for your reples...

well here is what i noticed since it got bit Hotter in the area where i live (mid 80)

i've been using a/c most of time.. and i "always" starts with highest fan speed. which is "4". then decrease it as it needed.

comparing to last week's mpg.. i don't see much difference... i mean may be just a little but not much ...

i know in my Honda minivan.. a/c does makes difference in MPG for sure.. but not much in My Fit !!!

Honda may have wired a/c system so "efficently" that it minimized fuel comsumption.. i can related this to automatic a/c on for front defrost mode..

 

In Dubai we use,constantly, A/C for 8-9 months per year. Difference in fuel consumption is appx. 20% . This is empirical findings on different cars that I had for the past 23 years in Dubai.

 

1. A/C compressor, as stated previously, cycles. With the fan on high, it runs continuously. With the fan on low, you can hear it kick in and out. Lower fan setting means the compressor runs less. Less compressor running = less engine load to run the compressor.

2. The alternator, like the A/C compressor, cycles. It engages to keep the voltage constant in the electrical system. I have a ScanGauge and at idle, with nothing on the engine load is about 30%. When I turn on the headlights, the engine load increases to about 36%. This is from the alternator supplying electricity, by way of the voltage regulator, to meet the electical demands of the vehicle. Turn on more accessories (rear window defroster, windows up and down, etc.) and the load increases to create the electrical supply that is needed.

 

Thanks David, finally an accurate reply.

The "cycle" of the alternator is completely load based and while not a true cut in/out like the compressor (the alternator turns constantly). The simple test for this is to listen to how the car idles in a SILENT area and turn on the accessories. At idle at neutral, the load can be noticed.

Why some guys in the race world will neuter these (a friend did with an Alfa Giulia Ti Berlina that he drove off the battery) with just a shaft and a pulley, but no guts (and no load as David points out).

But David I will argue: the A/C will also cycle on high blower speed. It does it to prevent the system for freezing. This can be noticed on a HOT day by a slight drop in temp out the vents, and occasionally by a split second fluctuation of engine speed.

Fit A/C on the highway has little effect on FE. The best results I have had were both identical, once with AC and once without. Around town it may be more due to the increased load. Having the windows down will increase drag and in theory, would increase FE in most cases.

....and to the other posters, heat (which employs a heater core and coolant from the motor for an exchanger, nothing electrical) and outside air/recirculation should have no effect on FE. Heating and cooling performance however....

 

While the electrical lode may increase it does not mean more engine power is being used to create that power. The voltage regulator regulates the voltage by keeping the alternator turning at regular RPMs. Voltage is *not* power. Watts are power, and every generator/alternator has a maxium amount of watts it can provide.

If you want an example, the generators I used to work on were turned by a steam turbine. The voltage regulator controls gears that keep the generator moving at a constant rate, even though the steam turbine can change speeds. Just replace the words steam turbine with gasoline engine. The alternator does not control the RPMs at which your engine turns, you do.

 

Hazard, while this may be the case in power generation, it is not exactly how it works in cars tho.

First, the alternator turns off the engine and its actual rotational speed is based off of the engine speed, not the regulator.

As the electrical demand is increased, voltage drops. The voltage regulator senses this voltage drop, sends more voltage to the alternator's field. Thus, the alternator creates more current (amps) to keep the voltage up and therefore power (watts) to the system.

W = VA

Now, since energy cannot be made out of thin air, there is an increased car ENGINE load to allow for the production of power (watts) to the car's system.

This is what David's Scangauge is showing, the increased engine load from the current generation (and power production) of the alternator.

 

I'm sorry, but it is very much the same.

The alternator does not turn directly off the engine, and if you constantly change an alternators rotational speed the voltage and frequency would change constantly and none of your car electronics would function properly. Most would seem to not work or just plain burn out or explode.

Voltage does *not* drop with power demand. I worked for four years as an electrition and the ohms law is not different in a car. Your working off of logic but it is faulty logic.

Current is directly proportional to voltage, not power. Please don't mistake current for power.

 

What?

Open the hood and look, there is a belt that turns the alternator right off of the motor. While it is not 1:1, it is still motor speed dependent, and yes, the speed changes. This, and the fact that the power (watts) load changes is why the regulator is required to maintain the constant voltage.

Every car I have worked on in the past 20 years both shade tree and in shops when working as a mechanic worked this way.

Voltage DOES in fact drop with increased demand, watch the voltmeter in the car (and your lights dim at idle as you increase the load, turn on the rear defogger, power windows, etc). And as load demands increase, so does current (amps), and hence watch an ammeter on cars such equipped. The drop that triggers generation is around 13V. Normal operation should be over JUST 14V in the car, even tho it is rated at 12V.

You are right, current and power are not the same, not sure where you are guessing that I think they are, the formula is posted. Alternators (and generators back in the day, I have worked on cars with both) are rated on their current generation (amps) and also voltage. Watts is the end demand of the power needed.

Here google turned this up, it should clear this up better than I can:

Understanding Alternators, How alternators work

 

That link explains the magnetic induction of the generator/alternator but it doesn't show or explain how it regulates the voltage. Which is really what we are arguing about.

You must be thinking about DC, and not AC which is what alternators produce.

 

There is no argument here?

Also, the regulator in a car works the same either in AC (alternator) or DC (generator), I have worked on both systems.

As for the regulator, it is in the link above:

REGULATOR
The regulator has two inputs and one output. The inputs are the field current supply and the control voltage input, and the output is the field current to the rotor. The regulator uses the control voltage input to control the amount of field current input that is allow to pass through to the rotor winding. If the battery voltage drops, the regulator senses this, by means of the connection to the battery, and allows more of the field current input to reach the rotor, which increases the magnetic field strength, which ultimately increases the voltage output of the alternator. Conversely, if the battery voltage goes up, less field current goes through the rotor windings, and the output voltage is reduced.

One thing that is VERY different from power generation in a power plant, you have a battery in a car system that the regulator wants to keep charged to its optimal voltage. So that is why charging occurrs (the alternator "charges") when there is a voltage drop sensed by the regulator.

If you have ever worked on a car with a BAD regulator (especially back in the days of the pre-solid state ones):

Volkswagen Voltage Regulators - JUST FOR EXPERTS

....that either sticks on (overcharge) or does the opposite (no charge, car runs soley off the battery). The same effect can occurr in external (common in the old days) or internal (modern):

Performance Cafe - VW Parts Aftermarket & OEM

...regulators, have replaced many of both types (well, not as many internal ones) you will have experienced readings with a volt meter that are either well above 14V or well below 12. The best of these was when a mechanical regulator in a 1968 Volvo I had shut down at night and I had to drive home on just the battery, and quckly dimming headlights. The regulator would allow charging once in a blue moon (everything would be OK for a minute) then limp mode again.

Here is an experiment that should make it clear:

Alternator Antics

While I sense you do have an electrical backgroud, I am sensing that automtove repair is new to you?

 

UHG!
The alternator turns constantly.
Voltage is constant (more or less).
Current (Amps) varies with demand (lights, stereo, rear defrost, etc)
Power = Volts * Amps, so power output is directly related to current draw.
The alternator is AC, but since we convert it to DC, the freq does not matter.
Since power output of the alternator varies, power output of the engine (and fuel consumption of the engine) must vary proportionaly.

The same is true in a steam turbine power plant too (i was a reactor operator in a former life...and nuclear plants are steam plants). If you draw more current from the generator, voltage will drop (momentarily). The voltage regulator compensates; that requires more tourque to turn the generator; the steam throttle valve (just like the throttle in our car) must open to supply more steam so the generator can maintain speed; so the steam plant must supply more steam and the power source must supply more power (burn more coal, dump more water, split more atoms, etc) to make that steam.

simply, power must come from somewhere... a voltage regulator does just that... regulates voltage (voltage is not power), it does not make or store power. There is only 1 power source in our cars (the engine). ANYTHING in the car that consumes power WILL effect fuel consumption (but probably not enough to notice without more precise equipemnt that our fuel guage).

 

That was my point... if you do more work (ie running the fan faster, cooling more air, whatever...) you must consume more power to do it and since gasoline is the only power source in the car, you must consume more gas.
The alternator is completely irrelevant. It is converting mechanical energy into electical energy; then the blower motor converts it back to mechanical... You could drive the blower with a pulley (take the alternator out of the calculation) and nothing will change. If you do more work, you must use more power to do it.