Pulse and Glide in an auto while using A/C!

[sully06]

The weather is warming up down here in the South! We've already had a few warm days. In the past I haven't used my A/C much to really get some good mpg in the summer. I just sweat some. I tried an experiment. I P&G about 95% of the time.

The experiment!
Cycle the A/C on only when the car is in gear and I'm accelerating( I usually speed up to 60 and coast down to 50) turn A/C off then put the car in neutral coast down then repeat.

My results!
Day 1 fairly warm say high 80s. 15 miles to work I averaged 43 mpg and I didn't sweat!
Day 2 not as warm say low 80s. Same drive I averaged 40 mpg

Results per SGII

[oldtamiyaphile]

Originally Posted by sully06

Cycle the A/C on only when the car is in gear and I'm accelerating( I usually speed up to 60 and coast down to 50) turn A/C off then put the car in neutral coast down then repeat.

Turning the A/C on while the engine is already loaded is the worst time to run it. Normally with a manual if you EOC and you want A/C you've got no choice, but if you're EOnCing an auto, you can run the A/C during the idle phase, much more efficient.

I have found that with this method the A/C doesn't get super cold as the compressor is only ever spinning slowly.

[user removed]

Put the AC on recirculate, max cool, high fan, any time you are forced to slow down. Keep the engine speed above 2k rpm and get free AC when you are in DFCO.

Engine is most INEFFICIENT when idling. If you can't force DFCO to get free AC, then the OP has it right, under load at peak efficiency, adding ac means the lowest consumption increase in fuel.

What is possibly more important is to make sure you get the most out of that cold air, which will come out for some time after the compressor is disengaged. High fan when running the compressor, low fan when not.

Always recirculate when running ac.

regards
mech

[oldtamiyaphile]

Agreed the most efficient time to run the A/C is during DFCO.

However, when you're accelerating at BSFC, you want to covert as much energy as possible into momentum.

The engine is inefficient at idle, which is why that's the best time to add some load to it (to make it works harder and therefore more efficiently). This also has the effect of spinning the compressor slower, so it's like an underdrive pulley.

Many modern cars with variable displacement compressors will turn the A/C off when accelerating and ramp it up when idling. Same thing with alternators.

[user removed]

Since a V8 engine burns .5 gph idling, adding ac to that waste only makes the waste becomes worse.

I would rather add that to best bsfc load and take the slight load penalty becoming longer duration of the acceleration event. Remember my car will go 35mph on .5gph, while the V8 will not move.

Stating that adding ac when idling means that V8 is now using .75gph, not moving at all. When it could easily (over powered as it is) run the compressor without a significant change in acceleration load and a minuscule additional amount of fuel.

AC on idling would be my LAST resort.

regards
mech

[oldtamiyaphile]

This thread is about an automatic where the engine is idling anyway during coasts in neutral. It's not worth running an engine at idle just to get A/C, but if the engine has to idle anyway, that's the time to use it.

[roosterk0031]

How much does you GPM go up from having it on at idle speed. Then compare that to your GPM during your pulse with and without AC. That's going to be tough to see, but maybe compare to constant speed at 60. If GPM goes up less at speed, its more efficient to use it accelerating than idling.

You still can't have the throttle as open at constant speed as your pulse but I think the trend could continue. Now if you could find a big hill to go up with cruise set at 55 and see the change. That would be a good test.

[Daox]

I'm going to have to agree with Old Mech here. Do as much as possible with the fuel you're burning when the engine is at peak efficiency. Do as little as possible when the engine is running at worst efficiency.

[aardvarcus]

My experiences have been slightly different than what some have posted, so whether or not it is better to run the AC under load or at idle is probably vehicle specific. I have found at least for my Tacoma it is better to run the AC when coasting in neutral. Granted it is a reasonably heavy vehicle with a relatively small engine that I like to lug around at low RPM, sometimes running the AC is the difference between staying in 5th gear or downshifting on a hill. Alternatively, my big block Chevy really doesn't care if or when you turn the AC on, it isn't even noticeable.

Also remember BSFC is only part of the story; benefit provided per fuel consumed also plays into the equation. It doesn’t really matter what your BSFC is in neutral, you are getting zero forward propulsion out of the fuel being consumed because you can’t utilize the horsepower to go forward, so it might as well be zero. Assuming engine off coasting is not an option, my thought would be to use these horsepower you are generating anyway for another purpose, keeping the cab cool. I realize this isn't 100% true, since you use more fuel to maintain idle when the compressor is kicked on, but from my non-ideal measurements of playing with the button the AC penalty is higher when you were already utilizing all the horsepower you were making for other reasons, like forward propulsion. Once again that is for my Tacoma, other vehicles (like my Chevy) with more HP to spare may not react the same.

I have noticed that with a base idle fuel consumption of about 0.28 GPH, kicking on the AC moves the consumption to 0.38 GPH. So for 0.28 GPH I get no benefit, but for 0.38GPH I get cold air, basically a 0.1 GPH penalty when you are in neutral. I also can typically get away with running the AC in neutral only part of the time to keep the truck cool (and dehumidified). Example: If my average speed is 50 MPH and average FE is 26.5 MPG it would take 1.88679 gallons to drive for an hour. Running the AC while idling 10% of total travel time would use an extra 0.01 Gallon every hour, or 1.89679 gallons which equates to a 26.36 MPG, or a 0.14 MPG penalty at 50MPH average. This works fine on moderate days when the truck is traveling at highway speeds, on scorching hot days or at slow speed I have to run the AC more all to keep the truck cool, which has a significantly higher MPG penalty.

I have found the absolute best time to use the AC is when slowing down in gear, I would guess this is probably universal with few exceptions. Without DFCO there is still a MPG penalty versus being in neutral but it is small but you can downshift and spin your AC compressor at higher RPMs to produce more cold.

[sully06]

Thanks for all the replies.

Per the scangauage accelerating without A/C 23 mpg with A/C 21.5 mpg. Coasting in neutral without A/C 190-210 mpg with A/C 125 mpg. So you can see why it makes more sense to have it on only when accelerating because I'm only losing 1.5 mph and gaining 75 mpg on the coast.