Test on number 88
 

88) Cycle the A/C if you have to use it
If you have to use the air conditioner, set the air flow to recirculate and manually turn the A/C on and off as needed. For greater efficiency, switch it on when under light engine loads or deceleration fuel cut off and off when under moderate/heavy loads. (Note: some newer vehicles do this automatically.)

I was rereading the 100+ Hypermiling Tips to freshen up on my tactics and realized that I was doing it wrong! But I started to cycle the AC on when I'm on my Pulse phase and cycling off on my Glides. I tried it vice versa and realized that there was a substantial lose in instant MPG during my glides. So I decided that I'll use more gas during the pulses, although I do feel a distinct lack of power when doing this.

So I will be actually doing a trip log from home to work and vice versa alternating styles and seeing what works better.

let us know what happens. i haven't done a test on my car but it would only make sense (to me ) to use the a/c when the engine is under a small load. I actualy did a test today turning on my a/c at 60mph cuts my fuel economy but about 19%

Well, I am a dummy because I forgot to record my To-Work trip! So I ditched the To-Home info. But I cycled AC on today on the glides. I noticed that I only got 38MPG to work and 36 to home. Which is definitely lower than my 40MPG to work and 38 MPG to home norm.


So let's discus my trips since I can't do simple tasks!

To Work; First of, my work is at a lowering elevation than my house. So my P&Gs are over a 15MPH spread. So I leave the development and hit the first highway, about 1 mile; first segment is 30MPH and the second 45MPH. I hit Highway1, which is 65MPH, for about 2 miles. And I merge onto Highway2, which is 65MPH for about 8 miles and then lowers to 55MPH for the last 3 miles. I will go through the front gate, which is a dead stop and typically has a line of cars waiting to get through. Then I spend the balance of my 22.2 miles going through a 45MPH rural road and a 35MPH "city." There are a few lights, about 4, that are favored for the opposite street.

So my thinking for cycling on during glides is that the AC, mine is off the belt, is running at idle and won't be wasting as much energy as cycling during pulses.

But I switched the AC on and off during a few glides and noticed that the AC gobbles up about 10LOD on my scanguage. (went from about 20 to about 30) So I would pulse with 80LOD with AC on instead of 70LOD with AC off.

But, for the test, I am going to leave the truck at 70LOD with the AC on. The thought in favor of this method is that the glide phase is racheting up the trip MPG number and the pulse is when I am actually using gas. I will be accelerating for a longer duration.

Obviously, cycling the AC as needed is the most efficient way to use it, but I found the assumption (For greater efficiency, switch it on when under light engine loads or deceleration fuel cut off and off when under moderate/heavy loads.) to be dead wrong.

Using GPH as a measurement, I saw that it increased more while gliding than when the engine was already under load. This wasn't just by percentage but by actual increase amount. For example (and pulling numbers randomly out of my ass here) say I was gliding at 0.4 gph and hit the AC which brought me up to 0.8 gph. The compressor then added 0.4 gph to what I was already using anyway. Under load at 4.0 gph the AC only brought me up to 4.2 gph. So while gliding the AC costs me 0.4 gph but under load it only costs me 0.2 gph. Add that to the fact that I have to leave the engine running during the glide if I want to run the AC, then in practice it's costing me 0.8 gph instead of the 0.2 gph that it costs when I need to be running the engine anyway.

Finally plugged in some numbers to my "experiment." Sorry, but probably won't ever get A B A results for you guys. Let's call this a scientific survey instead.

Unfortunately, my results so far support the established rule. But I also recently removed my lower grill block and and changed my oil from 10w40 to 5w20 and only did one day of driving with that.

did you do this while maintaining the throttle position, or cruising at a steady speed?

just thinking, if you were holding the throttle steady while accelerating, when you turned on the a/c you may not be able to perceive the car accelerating at a slightly slower rate.

Very good point. In my old Suzuki Esteem, the AC was a major part of engine load- under hard acceleration I would get a noticeable boost by shutting off the AC. With plenty of power it's not noticeable at all, but it does still exist. I did it under acceleration at a steady throttle position and while I didn't notice any lag I wasn't measuring for it.

Very perfect your post is! I just read articles of other's before, and this time I am pondering your words seriously. It is my appreciation to learn from you something.

Adding a load to an engine in a high load state of operation will give you energy for about half the cost of the same load at idle when your efficiency is terrible. Best time to engage the compressor is when you would normally experience DFCO. Then your normally wasted inertia is doing something constructive.

If you only P&G without any constant load operation then on the pulse or DFCO are your only good options.

regards
Mech

FC -

Not sure I understand why you say (For greater efficiency, switch it on when under light engine loads or deceleration fuel cut off and off when under moderate/heavy loads.) is dead wrong, based on the example you provided... You compared AC use during idle (which is no load) while gliding to full load, whereas the recommendation was to use AC under light load (not no load). Did you also compare AC use during light load (e.g. cruising) to AC use under full load?

my scangauge reports a LOD of about 20 while I am gliding, with AC on or the alternator going, or the lights on, or the fan running it gets up to the low 30s. So is there a way to zero my scangauge to read idle as LOD0?

Anyways idle out of gear wouldn't the engine still be doing some work? Moving the pistons, valves, sucking in air, oil and fuel pumps, and all the parasites on the timing belt. And wouldn't that work be getting down at a very inefficient load? Explaining why it takes 30 LOD to idle the engine and use AC, but 70 to have the tranny in gear and accelerate to highway speed.

Is that what is meant by Tip #88? Light load = idle? I thought 'light load' meant something like driving in gear on a level road... in other words, running at the 'cruising' rpm, not at idle.

this might also vary from vehicle to vehicle. for underpowered vehicles, or vehicles where running the a/c makes a noticeable difference in performance, there could be an indirect affect. i have about a half mile long steep hill on my commute right now, with the a/c on i cant maintain 4th gear up the hill. even hitting the base of the hill at 65 i drop under 40, and out of 4th gear, by the top. with the a/c off i can maintain speed up the hill. same applies to a variety of situations for me, cutting the a/c can either allow the trans to shift earlier, or hold a higher gear.

There are other factors that might come into play to really skew these results.

For instance, in my car, when I have the A/C on it takes more pressure on the pedal to get up to the same speed as if the A/C were off.

With that said, the A/C almost forces me to drive slower and with less pressure on the throttle, thereby making it impossible to accurately judge exactly how much of a load it is putting on the engine.

Do you have a scangauge or ultragauge? I can sit at 70LOD all day, pedal pressure varies. Going uphill and downhill are two very big variables. Also AC, which is why I started the thread, hoping someone already thought of this.

NachtRitter- Sorry, I was reading "light/DFCO" to mean "idle/engine braking" and "moderate/heavy" as "cruising/accelerating."

I do as little constant load cruising as possible, and with my hills that means almost none. About the only time that I'm not actively P&Ging is when my wife calls me. As terrain dictates I'll have light pulses, but my goal is to either be on or off.

I'm still new at this (just over a year) and the roads I live on don't lend themselves to refining my technique away from hard P&G. With my P&G mindset if I'm not putting a real load on it then I shouldn't be putting any load on it at all, and with that kind of operation I'll try to pulse the compressor along with the rest of the car.

Not a problem... you thought it was one way & I thought it was another, but I don't think the correct answer has been provided yet (at least not in this thread).

In any case, whatever it is you actually are doing is certainly the right thing, based on the FE you are getting with the car you are driving, and that is really all that matters... :thumbup:

And that's why the "correct answer" is so vague! ;)

It all depends on the driving conditions. I rarely use AC, except on road trips through/to hotter places. Last year on my road trip to southern California, I tried "P&G" with my AC. Kept the "recirculating" switch on. I tried to engage the compressor only on level or downhill stretches. Another method I tried this year was engaging the compressor only when instantaneous MPG on my ScanGauge was higher than Tank Average MPG on my Mustang's information display. If it gets too hot for comfort, AC stays on.:cool: