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-   -   Steady drive or P&G for best FE with AC? (

hummingbird 07-30-2010 06:02 AM

Steady drive or P&G for best FE with AC?
I had AC clutch coil failure some time ago, and didn't bother to get it fixed. Now with summers making me pant like a dog, I had to get the repairs done with no choice.

After the AC switch ON, I try to do Engine ON coasting - this had not been much of a problem earlier bug earlier I had not been observant enough. Now I feel that with my 67 BHP vehicle, I am not getting good FE with Engine ON coasting, with th Speed-up/coast-down approach The acceleration is taking too much fuel now, that the AC load is also always ON. If drive on with very light right foot, I can get better FE than I can by engine-on-coasting.

What is going wrong? Is what I see logical?

nayeliesuncle 07-30-2010 08:39 AM

Could you rephrase the post?

I'm having trouble trying to figure out your problem.

RobertSmalls 07-30-2010 09:13 AM

You won't see any fuel savings by alternating between heavily loaded and moderately loaded. You have to alternate between heavily loaded (at a minimum of RPM) and engine off, or nearly off.

Now that you've added a 4kW accessory load, I think you'll need to switch from P&G to DWL. You'll also want to drive slightly faster to get there sooner, because the sooner you get there, the sooner you can shut off the A/C.

hummingbird 07-31-2010 09:24 AM

Hi Uncle, I meant, I am not getting good FE by using the routine of speeding up to say 50 mph and then coasting down to 30/25 mph. I have to do this with engine ON because of the AC working continuously.

If I use a very light right foot (very light throttle setting) and drive in the top gear continuously, I get better FE. This contradicts the basic understanding that speed up / coast down will save more fuel than steady driving in gear/ DFCO use.

@Robert - Will there be any adverse effect on the AC clutch or AC compressor if I continue to use Engine OFF coasting like I did with AC ON? I see the blower load on battery as a potential downside, as it will continue to drain battery. My already weak battery is likely to give up its ghost if strained further. But apart from the battery, is there anything bad about using EOC coasting when AC is kept ON? The AC clutch disengages as soon as the engine stops, so the AC will cycle with the engine too, which will save further fuel than driving on continuously.

RobertSmalls 07-31-2010 12:28 PM

Yeah, it's just going to get warmer in your car while the engine is off. As long as you can stay in top gear during your pulses, it sounds good.

ShadeTreeMech 07-31-2010 12:53 PM

If you live in an area devoid of hills, I'd say driving with load is the best approach. Basically that is keep a moderate throttle, allow the car to speed up a bit on downhills and slow down as much as you can in top gear on the uphills, besides timing lights and watching traffic.

pulse and glide is only beneficial if you're gliding with engine off, or idling VERY slowly, and you can glide for a long time. But if your engine is barely turning in top gear at your desired speed, driving with load is better. Pulse and glide really only improves the mileage on cars with low top gear ratios, ie the engine is spinning very fast when in top gear.

gascort 08-01-2010 03:30 PM

In the wife's car I usually turn off the AC when accelerating or hill climbing and leave the car on always. not noticeable to passengers unless it's off for more than 45 seconds, and acceleration is much better with the compressor off, so I'm sure the mpgs are a bit better too.
I think this could be programmed into new cars with the auto AC systems in them; a throttle or load input from the ECU that shuts off the AC compressor for, say, a max of 30 seconds when encountering heavy load. This way it wouldn't shut off completely when towing a trailer or going up a long hill, but for small hills or stop and go traffic, the AC would be running at the most opportune times. Would be unnoticed by most drivers, except for seeming to have more power available from the same engine, and with the EPA's new "Air conditioning" and "highway acceleration" tests, it could net them an MPG or so in their ratings.

NiHaoMike 08-02-2010 12:52 AM

Ideally, you'll want to run the fan full blast before the A/C is used at all, but the controls are designed such that having the A/C on with the fan on full blast gives you a lot more A/C than you need and wastes a lot of energy. So how about modify the controls to allow less A/C. Maybe a PIC or Arduino sensing factors like supply register temperature, engine load, brake lights, etc. and controlling the compressor automatically.

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