Honda works a lot of miracles, but even they can't violate the first law of thermodynamics.
To do more work, you must consume more energy. Turning a fan faster, or running the compressor is more work.
The alternator is irrelevent. It does not "produce" power; it only converts it from one form to another. This is true of the engine as well... it converts chemical energy to mechanical energy (which the alternator converts to electrical...). The only actual source of energy in the car is gasoline, so if you do more work, you must, without any doubt, use more gasoline.
But like i said in my first post, the amount is so small that i doubt you could measure it with our fuel guage and odometer. Hauling around a 2000lb car consumes so much that the AC is just not worth worrying about.

 

The alternator will never output more power than the elec system is using at any given time. If it did, it would break itself very fast.

 

it's not a bottomless pit (although I'm afraid this thread might be)... You may be right... I'm not 100% sure.
If you are right, then if i go start my car and leave it running at idle, the alternator will eventually damage itself (after it destroys the battery). I don't know what the rating is for the alternator... probably 50 or 60 watts, but it would easily overcharge the battery in minutes.

 

Mostly, alternators are 750-800 W

 

Try over 50 amps, a single low beam headlight is 55 watts!

Claymore, you know my thoughts on this issue from before on the rotors don't warp thread.....yes, I know your answer (and feel your pain).

 

err... listen to what i mean, not what i say...


claymore... I agree with your entire last post, OTHER than the comment about the regulator taking care of overproduction

 

What, that watts and amps are interchangeable? And yes, the regulator does prevent OVERCHARGING. I have replaced many over the years for this very reason (mostly pre solid state ones with the twin coils, Bosch and SEV-Marchal units).

 

Very good points Claymore, and here is my take:

1. True, most start this way, full cold, full blower, reycle air.....and move back as the car (or your passenger) cools off.

2. Assuming that there is increased cut-out at the lower temps? AH! But, since the system works "on or off" and temperature is set by a mix of warm and cooled air via the mix switch which either controls a flap or a valve to the heater core in the big box behind the dashboard. ACC cars are different (atleast Mercedes) as far as compressor use. Otherwise, it is on or off (excluding automatic cut-out to prevent overpressure or freeze up).

3. True, the only slight negative is it can give fogging problems in VERY humid environments.

The other factor: Max AC use is most common in heavy city driving, sitting at the light in the sun, once the car is moving....and how is that for FE......

Thoughts by anyone else?

 

Hi freaks,

I've had my Fit for less than 2 months, this looks like a great board. I agree with the following:

1. Set the blower to your comfort level, even on high, the effect on your mileage won't be noticed. (see explanation below) My wife seems to think that car ventilation systems are an all or nothing affair, all cold or all hot, max blower or off. I keep trying to explain that all the marks inbetween are there for a reason.
2. For max efficiency, use A/C at full cold setting, only add warm air mix if full cold at lowest fan setting is too cold, if so, consider cycling the A/C manually in 5 minute cycles.
3. Once the inside cabin temp falls below outside air temp, go to recirc to conserve previous cooling effort. Basically, use the coolest air available for input into the vent system.

I was worried about the first half of this thread, but clear heads have appeared to prevail. I hope that everyone can agree that the use of ANY powered accessory affects your mileage, but most things in your car have an imperceptable effect.

To elaborate on the alternator discussion, lets keep it simple by leaving wiring, friction and conversion losses out of the conversation. If your engine, and therefore your alternator were spinning, and you had no electrical power needs; then the regulator would operate the alternator at 0 amps and would result in no additional engine load. Any electrical drain will put a load on the engine via the alternator. A general conversion factor that you can use is 750 watts = 1 hp. For example, 2 x 55w headlamps = 110w = about .15 hp. I can't imagine that the cabin fan blower ever pulls more than about 3A @ 12v = 36w = 1/20hp. The one standard electric accessory that can be felt in most cars is the rear defrost, I don't know what they pull, but at 20A, they would add a 1/3hp load.

Now on to the A/C Compressor. A/C units are rated by Btu/hour ratings. A one ton unit is rated as 12000 Btu's (288000/24) The average factory installed auto A/C unit is rated at 1-3/4 ton. A BTU is 0.3 watts. So the average auto A/C compressor is 12000 * 1.75 = 21000btu * 0.3 = 6300w = 8.4hp Even if the Fit A/C system is only rated to 1 ton, that's almost a 5 hp load all the time that the compressor is running! Our compressors do cycle, mostly to keep from freezing up. If we go with the 1 ton rating, the compressor produces 200 BTU of cooling per minute. If the fan is on low and we're only drawing 100btu/min from the condensor (edit: actually evaporator), then the compressor will cycle itself to 50% on, 50% off. At that setting, you would be using an average 2.5hp for cabin cooling during your drive.

With all that said, I owned a 1990 Geo Metro 5M with a 1.0L 3 cylinder, bought new, that delivered just over 51mpg averaged over 130,000 miles. In that car, I found that driving with the driver's window open and A/C off was less efficient than windows up and A/C on in temperate weather (80-90 degrees). Even though it wasn't a Fit, I think that the original question was something like that.

And completely off topic, I found that an empty 2 bar Yakima round bar roof rack with no fairing up front cost me 10%, 4mpg with my 1996 Mazda Protege in wind drag alone.

Whew! That was way too long for a first post.

See you guys around,
Eric

 

Yes, very true.

One thing, the modern rotary compressers are MUCH more efficient than the old piston type (imagine opeing the hood and seeing what would look like a single cylinder go cart motor under the hood):

http://www.firstfives.org/faq/AC/York%20compressor.jpg

These old beasts were hell to pull.....

Now, one other thing that some cars had, (especially with the systems above) was a cut out to the compressor during hard driving of the car. Some systems had specific settings for City Vs Highway and would cut out to not rob so much HP.

 

The effort that I quoted above did not factor in the inefficiencies of compressor design, those numbers were calculated by the definition on the energy units themselves, hence my conservative rating for the Fit system. If the compressor was only 75% efficient, the effective load on the engine would go up 33% to achieve the same cooling. Of course. this means that you can get the same amount of cooling out of a lower rated modern compressor than an early piston design. Does anyone know the actual cooling rating of the Fit A/C compressor? But even without this specific info, we can see that the compressor will probably effect fuel mileage by an order of magnitude (10x) over any electrical accessory in our cars.

Eric

 

hmm ok how i think it works is if there is a greater resistance in the electrical circuit then there would be a greater load on the alternator right ??

all else equal when you introduce more resistance in the electrical cirquit the alternator must work harder to compensate for that resistance right ??

forgive me if im totally off base since i havnt done any electrical stuff since physics in highschool and im kinda tired right now since it's 2AM

 

Correct. Any/all electrical load in your car is covered by alternator effort (drag on your engine). Your battery operates as a reserviour of "banked" alternator effort, if your load exceeds the alternator's ability, the battery will cover the difference, until it is spent. But even if the battery fills in for alternator shortcomings, the alternator will just have to over produce until the reserviour is re-filled. Common reasons for dipping into the battery are; the engine is not running (every time you start the car, or listening to the radio while you're parked, etc.) or accessories that exceed your alternator's capacity (off road lighting, or winches, not really issues for a Fit). I'll guess that the Fit alternator is rated around 60A, which translates into 720w @ 12v, about 1 hp at max ouptut. That means that it would take 2 Fit alternators to power your wife's hair dryer (it's actually worse than that due to efficiency losses inherent in the whole process).

The A/C compressor (when running) is not an electrical load; it is mechanical, like the alternator itself, and therefore can operate well in excess of the alternator's capacity.

Eric

 

BINGO!!
that's the point i've been trying to make here.
If there is no load on the elec system and the battery is charged(so you can't use it as a sink), then the alternator is not putting out any power, and therefore, not putting any load on the engine (well, very little load)

 

yes, that's the point... if the regulator prevents the battery from overcharging, and the elec system is drawing essentially no power, what is the alternator doing with all that power?
Nothing... because there is no power. it can't produce more than the system is consuming...

 

There always is some load on the alternator though because it has to produce power for the spark plugs as long as the engine is running right? I don't know how much that is but it is something.

Also, the alternator will be turning as long as the engine is running, so it will always be taking some power just from the physical effort required to turn the thing, load or not.

Maybe I'm wrong (I don't really know anything about this subject) but I would think that the alternator "keeping up" with engine rotation would provide some small amount of "free" power? I guess I could be wrong about that because with no load the engine could use less fuel to get that given rotation. I bet even if there was sort of a buffer there it would be smaller than the amount of power required to create spark, and that is why there is no overcharging!

Sorry guys, just thinkin’ out loud.

 

This is wrong, LOWER resistance causes higher electrical load (i.e. current flow or "amperage") not higher resistance. More current will flow through a low resistance device (an example would be a starter motor, which is close to being zero resistance) than through a high resistance device (such as a dome light bulb). LOW resistance leads to high current flow, not the other way around.

 

It might be easier if you think of all electical loads; ignition, instruments, lights, etc. running off of the battery. Then the alternator doesn't directly deal with the rest of the car. The battery is a reserviour of electrical energy, and the regulator operates like the float valve in your toilet tank, when the level drops, it tries to keep the tank (battery) full with the alternator. That's why you can run your car off of the battery when either the alternator or regulator have failed.

Where a generator works with permanent magnets, producing power with any rotation, an alternator uses electromagnets and only makes power when the alternator field is energized. Without a request for power from the regulator, the alternator acts like a spinning hunk of metal. Of course there is some load from friction and mass, but that is negligable.

As stated before; any/all electricity used in your car is covered by alternator effort (drag on your engine). A 6A load is about 1/10hp, so regular operating requirements have very little impact on the engine power available to drive the car.

If you wanted to, you could remove the insides from your alternator leaving just a shaft, like some racers do, and rely only on the battery handle everything electrical. Your car would have at least a theoretical improvement to your gas mileage and could be the worlds cheapest plug-in hybrid.

Eric

 

Witout battery, alternator equiped car, will NEVER start the engine - by push/roll start.

 

I've never had a manual car, but I've heard that if your battery is dead, you can put one into second, push it down a hill, pop the clutch and start it, with a dead battery.

I did have an experience once when I had left my lights on all day at work, and my battery died. Then I made the horrible mistake of putting jumper cables on my battery backwards, and blew it out. I was able to jump the car only after disconnecting the battery, and was then able to drive it, as long as I didn't use any of the electrical systems. Out of habit, I put my turn signal on at a left turn, and the car died. Luckily, there happened to be a service center right there, so I was able to coast in. Also luckily, the guy there was working late and was able to get me a new battery. If he hadn't stayed late, I would have been stuck!