I like your thinking. Reuced RPM with no change in throttle position would in fact increase manifold pressure and lengthen the intake valve duration, allowing the cylinder to fill more completely. However reduced velocity through the venturi would also increase lateral wall pressure and reduce the amount of fuel drawn in, the float bowl pressure being stable.
I’m still pondering this one. Were your theory totally accurate the idle would never nee dto be bumped up to compensate for added loads at idle.
People having been idling cars whilst stationary for years and people aren’t keeling over and dying of CO2 poisoning.
True, but there are other problems and cost of CO? Those who say we will all die next year because of CO? and those who say we can ignore the whole thing are both partly right and partly wrong.
I don't think there are many people who are suggesting that we need to go to either extreme.
Personally I believe it is time to start moving in the right direction or reduced CO? I believe that reasonable adjustments will be best for all of us.
I believe you will find there is a wide range depending on the car and a number of variables.
If the question is which one uses more, you are right. That question is answered. Unfortunately, the OP asked, “How much gasoline is used if I idle my car with the air-conditioning on vs. air-conditioning off?” and that question hasn’t yet been answered.
lbeckman asked “how much?” and your answer is “more.”
Since this is “junior high school stuff” can you give a direct answer to the direct question posed?
The idea of 8 pounds of gasoline producing 20 pounds of Carbon isn’t that extraordinary if you include the mass of the Oxygen burned in your calculations. Air does have mass, and it is the air/fuel mixture that gets burned, not just gasoline.
I’m just saying it won’t kill them to stop the engine and open the windows. Or, better yet, park and get out. They don’t have to sit in the car and roast.
Yes, I can give a direct answer, Whitie, when the OP explains how 8 pounds of gasoline produces 20 pounds of carbon dioxide (not “Carbon”) when it is burned. Elements are properly not capitalized. A little effort goes a long way.
Here’s an exercise for you: what’s the weight of a mole of carbon, and the weight of a mole of carbon dioxide? This really is junior high school stuff, and I had to do it without the internet. Further research confirms that the 20 pound figure is not far off. Nontetheless, let us not forget about the water product of combustion. I however am done doing other people’s research.
It’s still a good way to hook up with college chicks at anti-globalization protests.
Check back when you ask the OP for “Make, Model, Year, Automatic or Manual”.
It will use more with the A/C on, probably not by a whole lot though. Most cars increase the idle slightly to compensate for the load with the A/C on. More engine speed=more fuel burned. Perhaps this would be a good (albeit boring) one to submit to the Mythbusters…
Not even kidding here, but aren’t herds of farting cattle supposed to be one of the highest producers of greenhouse gases?
All your replies to this board have a similar message, which is to tell everyone “the answer is simple elementary school math or physics”. You’re quite good at doing it in a condescending way.
Actually, the engine doesn’t have to speed up to burn more fuel. More engine load = more fuel burnt.
Example: You will burn more fuel going up a hill at 1000 RPM than you would in neutral at 2000 RPM.
The inefficiency manifests itself as being unable to keep running more than using more gas. If you load an idling engine enough, the engine stalls and then it burns zero gas until it’s restarted.
An unloaded idling engine is running at zero percent efficiency, it is burning fuel but it is making zero torque and thus zero horsepower.
Force the idling engine to turn some kind of load and now it is making some torque and horsepower and thus it has an efficiency above zero.
My 2009 Saturn Aura with a 4 cylinder engine uses 0.36 gallons per hour with the air conditioning on at idle and in drive. With the air off, the consumption rate is 0.29 gallons per hours in drive. In neutral (or park) it uses 0.26 gallons per hour. So, the AC uses about 0.07 gallons per hour - at $3.00 per gallon, it costs 21 cents more per hour worth of gasoline when you are stopped as opposed to just letting the engine idle without AC. Naturally, the total cost as opposed to having the engine off is about 78 cents per hour.
Yeah, my Fleetwood that had a 472 did not increase the fuel mixture a bit, it did not kick the idle a bit, with the AC on. You actually have it backwards, these big engines were burning a fair bit of gasoline just idling, but had so much momentum that adding AC load probably didn't increase the idle burn much if at all. On the other hand, the vehicles I've been in with smaller engines (including a few civics), the idle air (and fuel) are increases considerably when the A/C is on to prevent stalling. Don't get me wrong, this is just because the Civic engine is light and uses so *little* power to idle with the A/C off, not some indication that it's somehow less efficient than the 440! 8-)
Anyway, I like boogie1468's answer best since he has actual numbers. I have heard a typical AC will use 5 horsepower (probably more for the ice-cold old school GMs and less for the Hondas), in a 100% ideal system then, 5HP comes out to 124238 BTUs, and 5 HP for an hour is 12725 BTU, so for 1 hour it's about 0.10 gallons.
However a real engine is far from ideal, there’s already fuel burned to turn the engine, some of which can turn the compressor, and so on. I prefer boogie’s actual measurement of 0.07 gallons an hour over my calculation.
That’s only true because the throttle is open more when you are going up a hill at 1000 rpm than in neutral at 2000 rpm.
At a constant throttle opening, the slower the engine turns, the less air it sucks past the throttle and the less air it sucks past the throttle, the less gas needs to be added to the airflow to maintain a stoichiometric air/fuel mixture.
Modern EFI systems automatically either open the throttle or the idle air bypass a little to compensate for the additional load put on the engine by putting the transmission in drive or turning on the AC or even the headlights and the increased load is reflected in the idle fuel consumption. In my car, turning on the AC actually increases the idle rpm by a hundred rpm or so.
Old low tech carburetted V8 engines used a constant throttle opening at idle and the engine just slowed down a hundred rpm or so when you turned on the AC and I’m pretty certain that this resulted in a slightly lower idle fuel consumption. Ditto for keeping it in drive.
However, these old low tech very large V-8 engines probably burned nearly a gallon of fuel per hour just idling with no load.
A modern Toyota Yaris engine burns about 0.16 gallons per hour idling no load once the engine is fully warmed up.
Just to set the record straight, petroleum dealers do not chant “a pint’s a pound the world around.” A gallon of gasoline weights only 6.3 pounds, a dramatic 24% less than water (at 8.3 pounds, close to a pint a pound). When fire departments convert old fuel oil trucks into tankers, they have to be careful not to overload the suspension.
There’s an excellent US Government website that answers these questions about fuel and carbon dioxide. That figure of 20 lbs. of carbon dioxide per gallon of fuel is correct. See the fueleconomy.gov website at http://www.fueleconomy.gov/Feg/co2.shtml.