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-   -   Running the AC during P&G - An observation (https://ecomodder.com/forum/showthread.php/running-ac-during-p-g-observation-1447.html)

Harpo 03-17-2008 08:50 PM

Running the AC during P&G - An observation
 
The Scangauge is waaayy cooool.

P&Ging around town, I noticed that my Civic reports zero fuel flow coasting in gear from 55mph to 35mph, then under 35mph, it's show's .32 gph. Dropping it in neutral from 30mph down gets it around .28 gph. If I keep it in OD, it feels as if around 30-35mph, the Torque Converter goes into a no load condition, about the same time the fuel flow returns.

I started thinking cool thoughts on the ride home, as our FL weather has started to all ready require some periodic AC usage. So, I used my AC in tandem with my P&G. I turned the compressor on only on decel (55mph to 35mph) which appeared to impose virtually no load. And during that time, the fuel flow still reported as zero! I kept it on RECIRC to retain dehumidified air as much as possible. Regeneration AC!

Level road conditions, fixed 30% TPS, shows about 4.5-5mpg less with AC operation. These little cabins can be cooled quickly, but even then, it's nice to do it without using fuel when possible.

MetroMPG 03-18-2008 11:09 AM

This is the second report of A/C on decel only operation I've seen in the past few days. Interesting.... and not applicable to my situation (no A/C!).

tasdrouille 03-18-2008 12:12 PM

Nothing comes free, chances are you are merely defeering the fuel use related to the A/C to your next pulse. You should do some tests, but I'm pretty sure you won't coast as far with A/C on than off.

boxchain 03-18-2008 06:54 PM

I often turn my AC off when accelerating...don't know if it helps FE any, but it alleviates the double load whammy, so it theoretically should. Now that I've got a manual, I'm gonna try turning the AC on during engine braking.

Got numbers on idle fuel flow with AC on/off?

Always use recirc with AC or def...no sense bringing in more hot humid air.

tasdrouille 03-18-2008 07:25 PM

I would definately use the AC during engine braking, as it's energy I'm gonna waste anyway. So instead of totally wasting it in friction and heat on the brake pads, I'd use some to cool the cabin.

During P&G though, your goal is to carry your momentum as much as you can during a coast.

Harpo 03-18-2008 08:22 PM

Quote:

Originally Posted by boxchain (Post 14894)

Got numbers on idle fuel flow with AC on/off?

.

Yep. At steady state 35-55mph w/ AC fuel Flow is .20-.24 gph higher. On decel, from 55-30mph, with or without AC on Fuel flow is zero. It makes sense that I won't glide as far because there is additional engine load, albeit without fuel flow. At idle, in NEUTRAL, AC Fuel flow is .16gph higher (in my case - .25gph No AC to .41 gph W/ AC)

Today was a banner day, even with AC on decel this afternoon. 49.7 miles round trip. 40.7mpg or $4.11 for the trip.

boxchain 04-12-2008 12:33 AM

Quote:

Originally Posted by Harpo (Post 14916)
Yep. At steady state 35-55mph w/ AC fuel Flow is .20-.24 gph higher. On decel, from 55-30mph, with or without AC on Fuel flow is zero. It makes sense that I won't glide as far because there is additional engine load, albeit without fuel flow. At idle, in NEUTRAL, AC Fuel flow is .16gph higher (in my case - .25gph No AC to .41 gph W/ AC)

Thanks for the info Harpo. What's your rpm at DFCO trigger?

I'm going to assume that the same applies to my ride :D And I think I'm not far off, been trying DFCO for decel this tank and my odo at 1/2 tank (accurate I know) jumped from 225 to 280 this tank!

I also tried an AC/DFCO experiment. Coming down from a bridge (since we don't have hills :p) I noticed that in gear there is definitely added drag when the AC is on. AC off, the car seems to slow down less quickly. I pushed the button several times going down and I could feel it. No good data, but I did A-B-A-B-A :p

RH77 04-12-2008 12:45 AM

Quote:

Originally Posted by MetroMPG (Post 14834)
This is the second report of A/C on decel only operation I've seen in the past few days. Interesting.... and not applicable to my situation (no A/C!).

It's supposed to return to the low 30's again, so no worries yet on the A/C -- but I use the same kinda technique in the warmer months: manual cycling of the A/C on-off with recirc and med-hi fan depending on decel or highway downhill.

For me, last summer was the most operation without A/C since I had a car without it! Sweatin' for FE...

RH77

Elixer 04-12-2008 04:33 AM

It's called DFCO or Deceleration Fuel Cutoff, when your instantaneous FE goes to zero. . What your fuel injectors do is when your engine is above some threshold rpm (~1500rpm) and you are applying no gas is it completely cuts the gas to the engine and just lets it turn from the car's momentum. However as the engine is still spinning your AC still works. However the AC is still causing drag, so it will shorten your "glide". I use quotes there because if you're P&Ging you should be gliding in neutral. If you're gliding with the engine on in neutral you'll see a significant decrease in FE as a idling engine with AC on uses a lot more gas than without.

Chris D. 04-12-2008 04:45 AM

the SG shows fuel flow?!?

just got in on a group buy just in time!
cant wait for all this new instrumentation..
I love gadgets.. :)

I still can't quite grasp this pulse and glide technique..


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