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.

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!).

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.

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.

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.

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.

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

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

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.

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..

Chris -

From what I have observed, if the SG instant MPG goes to 9999 (think of 9999 as divide by zero, i.e. "X MPH / 0 GPH" ) when you are decelerating, then you have DFCO. I don't, :( .

The P&G is definitely counter-intuitive, and car specific. It works best with cars with good aerodynamics. For boxy cars like a Scion Xb, it's not practical because the glide is not long enough to justify the pulse.

CarloSW2

I believe I have one of the most aero styled minitrucks out there..

I'm sure I could mod accordinly..

the numbers you threw out there dont make sense to me
(mathamatically disadvantaged)

Load and RPM dependent
 

As Elixer mentions, there are some hard factors like RPM. Anything above 35mph in OD seems to get DFCO to trigger. Below 35mph on OD DFCO does not trigger, implying the rpms are below the threshold. In the same scenario, with trans in 3rd, you can get lower DFCO mph, due to higher rpm, but glide is affected.

The manual says flat towing at ~35mph. Therefore, I won't go into neutral above 35mph. I've built several RWD tranmissions in my day, and some do not oil well without engine ouput and should not be flat towed at all. I try to get 'er in to N right at 35mph before DFCO releases resulting in a .05-.08gph reduction of fuel flow and longer glide if required. Timing with traffic is essential. Jumping back-forth to N in some traffic just isn't worth it to me.

I accept DFCO in OD on the long grade downhills where I can maintain 50mph+. Sure, I'd like to coast a bit farther, but no fuel flow and four 9's is where it's at! If it's rolling and I know I need energy to maintain speed without undue excessive load condition, I'll roll in 0.5gpm toward the bottom and accept a 10mph gain so I can sacrifice it at the top.

H.P. Long has a good grade! I grew up around your parts. I last lived on West End BLVD. Flood Town USA.

And that's why I'm trying to find out what the rpm trigger is. Might be different for an aut*m*tic, I know idle speed is and lots more complicated control algorithms. I've heard 1200-1500, but knowing where on my model could make a big difference for me.

I don't really P&G much on long steady stretches, since I don't have many. I mostly DWLBE (by ear :rolleyes:) and EOC, lots of traffic lights on my route, but not enough that they're easy to time, and the city of NO doesn't understand light timing one bit :mad: But I've been using DFCO more, in a lot of cases I'll come up to a stale red and try to engine brake from afar while trying to conserve momentum since I think it's turning green soon. The difference between shifting up at 1500 and 1200 seems significant from a speed rate perspective, but I'd hate to find that I'm burning fuel during some of this. :confused:

I guess you left before they put the Lake in Lakeview? They are widening the HPL, I wanna take my gf and some other new in town folks over it so they can experience 2-9' lanes and no shoulder, steep grade, with a curve near the bottom. :thumbup: A while back a truck tried to pass another truck (strictly forbidden) and they got stuck and had to close the bridge for an afternoon. :eek:

I drive to Michoud daily, so I can either take Paris road or the high rise, both very tall bridges over stupid canals.

Yeah our low was in the mid 20s last night...Celsius! :rolleyes: But we're getting part of the same cold snap, so we'll be in the 40s for a few nights.

Ok what do these abbreviation mean please? If we could cut down some of this it will help those new to the site until we get some kind of usable glossary.

DWL = driving with load.

HPL = Huey P Long, which is the second to last bridge over the Mississippi.

I love that Smart on your icon. Saw it on Jalopnik, but would really like to see it in action :D

Ah, this brings up an interesting point about recirc and Cd. In my VW Jetta, when I travel at highway speeds, when I turn on the recirculate for the AC, I begin to hear wind noise near the front of the windshield (passenger side). If I'm hearing it inside the cabin, I'm sure it must be hurting the Cd. I'm not hypermiling my Jetta, but its definitely an interesting thought.

The fresh air intake gets closed, which I'm assuming is why I hear the wind noise. I've planned to tape over the air vents in the front of my Aspire for this very reason.

Another test to do! Coool!

Yep. We left in 1995, back to Gainesville Area. I had my own business that was region agnostic, and the spousal unit wanted to go back to school. I was born/raised in Bossier and went to LSU through graduate school. One of my childhood nightmares was about that stinkin' bridge. I've made many a trip over HPL since. I miss the town and lifestyle, but not the traffic or routes.

Commentary and Questions...with Pix!
 

A couple of ideas here -- just humble speculation, if I may :) ...

The recirc intake is generally on the passenger side under the glove-box, which may explain the noise.

Second, I would tend to think that not having to force air into the cabin would decrease drag (like blowing air through a straw -- and the one-way flapper in the rear of the vehicle lets the fresh air flow out at its own rate). Avoiding this creates a cleaner flow?

Now for my situation: the cabin air intake is completely covered by the hood:

Hood Closed:

http://pic16.picturetrail.com/VOL698.../313045783.jpg
Note on the passenger side, the notch for the false panel...

Hood Open:

http://pic16.picturetrail.com/VOL698.../313045781.jpg
Oddly placed inlets...

The rubber seal should, well, seal -- but it doesn't. This makes Winter better and Summer a scorcher -- sucks engine bay air. You can smell what's going on under there (it's how an oozing valve cover and resultant oil burn-off was the tipping point for replacement). :p

I need to get a snorkel or something fabricated this Summer -- the windows rolled down should definitely increase Cd compared to a slight redirect (?). It may double as an ambient-air intake. There's a removable panel by the battery that's about 12" wide and a few inches deep. Not sure why this is here :confused:

http://pic16.picturetrail.com/VOL698.../313045780.jpg

The battery and wiring would need some genuine weather-protection. So there ya go...

Highway driving in the Summer with the windows up is a bake oven for this reason (even with the windows cracked). So the A/C comes in. I can't sweat too much for work purposes :o.

Constructive criticism/suggestions are welcome. :thumbup:

RH77

What we need is an A/C Auto setting. Only Run compressor on Decel/ super light load. With tat load point increasing as the interior temperature rises.

Sounds Like a plan?

I still need Tint :-(

A micro switch that when closed powers AC circuit. However, I'd still need manual override. There are a few months that AC on decel only won't cut it.

I need tint too. I just can't decide on product or installer I trust here yet. All my other vehicles have factory tint.

I got Sun-Gard Endurance film installed a couple weeks ago. Lifetime warranty. Looks good. My cousin owns 4 tint shops so the price was more than right.