Recent Topics
Valvoline HD CK-4, CJ-4/SN Ford Approved Spec
by fantastic
8 minutes 35 seconds ago
Lucas Oil, is it any good?
by Bob570
23 minutes 3 seconds ago
Revived a old iMac, mate; now minty fresh
by simple_gifts
50 minutes 22 seconds ago
Why is the new Castrol GTX not dexos1 approved?
by wolf_06
Today at 08:01 PM
Uhauls meh - minivans are the way to move stuff
by madRiver
Today at 07:01 PM
Jeep 4.0 oil for summer and winter
by HoosierJeeper
Today at 06:28 PM
mPerks+QS MIR deal
by fdcg27
Today at 06:02 PM
Gas $1.81 per gallon
by gman2304
Today at 05:53 PM
sulfur free Oil
by Wick
Today at 05:50 PM
Echo chain saws
by Syntheticuser
Today at 05:27 PM
Using PP on-line
by sleddriver
Today at 05:12 PM
Pennzoil Euro L 5w-30, ~2000 Miles 2013 Camaro LS3
by Geoff
Today at 05:05 PM
Petya GoldenEye Ransomware Tips
by Y_K
Today at 05:02 PM
Pennzoil Platinum 5W-30 - 6.2 LS3 - 7500 Mile OCI
by JParrott
Today at 05:00 PM
Rock Auto Fram Filters - $1.10 Each
by Greg L
Today at 04:40 PM
Anyone here use oil filter crusher?
by car51
Today at 03:41 PM
200k mile Acura ISX
by LotI
Today at 02:42 PM
home deck issue
by CincyDavid
Today at 02:13 PM
The FBI is looking at Kaspersky Labs
by Skid
Today at 02:13 PM
Study defends green credentials of EV's
by supton
Today at 01:06 PM
Newest Members
fantastic, ACuna545, koryg0327, Fred_L_Johnson, turk182
62006 Registered Users
Who's Online
112 registered (04SE, amusing, A310, Al, AITG, 4ever4d, 13 invisible), 2325 Guests and 12 Spiders online.
Key: Admin, Global Mod, Mod
Forum Stats
62006 Members
66 Forums
267179 Topics
4419284 Posts

Max Online: 3590 @ 01/24/17 08:07 PM
Donate to BITOG
Page 2 of 2 < 1 2
Topic Options
#3371373 - 05/14/14 11:10 AM Re: AC Compressor Load on Engine [Re: A_Harman]
440Magnum Offline


Registered: 02/01/09
Posts: 8584
Loc: Texas
Originally Posted By: A_Harman
Setting a higher temperature on the A/C control would make the compressor clutch cycle off more often, which would save fuel.


Actually, almost NO automotive systems work that way. The HVAC maintains the temperature of the evaporator coil at about 35 degrees F (or as close as it can get). If you set the temperature to warmer, it may slow the fan down on fully automatic climate controlled systems and that does help. But on most systems, it just blends in some warm air from the heater core and does nothing to reduce the load on the compressor.

Fan speed always has more effect- slower fan speed means that the compressor doesn't have to run as much (or if its a variable-displacement compressor, it can run at a lower displacement) to keep the evaporator coil at the target temperature.

Even older systems (for example, Chrysler Airtemp with the old V2 and RV2 compressors) ran the compressor all the time, but used an evaporator-pressure regulator valve and variable thermal expansion valve to "throttle" the system. The EPR valve would begin to block the suction side of the compressor when the returning refrigerant temperature got down to about freezing, which reduced the compressor head pressure, and in turn reduced the load on the crank. But mainly, it made the system stupid-slow to respond to a high load. All my EPR systems got converted to cycling-clutch systems when they got converted to R-134a.
_________________________
'66 Dodge Polara & '69 Dodge Coronet R/T both 440/727
'08 Ram 1500 4.7/545RFE
'12 Challenger SRT8 392/6-speed
'99 XJ 4x4 4.0/AX15, '14 WK2 4x4 3.6/8HP

Top
#3371476 - 05/14/14 12:47 PM Re: AC Compressor Load on Engine [Re: Chris142]
zzyzzx Offline


Registered: 05/18/12
Posts: 3918
Loc: Baltimore, Maryland, USA
Originally Posted By: Chris142
now lets throw a variable displacement compressor into the mix


I suspect that not enough people have one of those to comment. I do, and when driving that car I just leave it on. On my other car witha normal compressor that turns on and off, I only have the A/C on when not accelerating, since turning the A/C on is roughly equivalent to a "drop anchor" button.

Top
#3371523 - 05/14/14 02:11 PM Re: AC Compressor Load on Engine [Re: actionstan]
accent2012 Offline


Registered: 02/08/12
Posts: 443
Loc: Orlando, FL
On my older car, turning on the AC noticeably bogged the engine making it run at higher RPMs in idle. Acceleration time would also grow by a second or two. IF I'm on the highway and wanted to pass someone in that car I had to turn it off so as not to take too long lol. It felt like the car lost some weight lol.

Would cycling the AC clutch all the time cause it to wear out faster? If that's the case, wouldn't it just be better to run the AC at max so the clutch doesn't engage?

I was told this bogging down the engine was much worse in the older days. I knew someone who drove a first-generation Ford Fiesta from the 80s with AC. The car would lose half of its power with it engaged!


Edited by accent2012 (05/14/14 02:13 PM)
_________________________
Hyundai Accent 2012

Top
#3371531 - 05/14/14 02:24 PM Re: AC Compressor Load on Engine [Re: accent2012]
zzyzzx Offline


Registered: 05/18/12
Posts: 3918
Loc: Baltimore, Maryland, USA
Originally Posted By: accent2012
I was told this bogging down the engine was much worse in the older days. I knew someone who drove a first-generation Ford Fiesta from the 80s with AC. The car would lose half of its power with it engaged!


That could be said for all or most 4 cylinder cars of the era yes.

Top
#3371539 - 05/14/14 02:34 PM Re: AC Compressor Load on Engine [Re: zzyzzx]
dishdude Offline


Registered: 11/14/08
Posts: 7390
Loc: Phoenix
Originally Posted By: zzyzzx
Originally Posted By: accent2012
I was told this bogging down the engine was much worse in the older days. I knew someone who drove a first-generation Ford Fiesta from the 80s with AC. The car would lose half of its power with it engaged!


That could be said for all or most 4 cylinder cars of the era yes.


I had a '95 Neon that when new, I couldn't tell when the compressor cycled. By around 150-160k, I'd lose a ton of power each time the compressor cycled.

I think smaller displacement engines that don't have a lot of low end torque are the ones that really get bogged down.
_________________________
2015 Grand Cherokee 3.6
M1 5w-30 Wix 10010

2017 124 Spider 1.4T
FF

Top
#3371546 - 05/14/14 02:44 PM Re: AC Compressor Load on Engine [Re: actionstan]
exranger06 Offline


Registered: 01/25/08
Posts: 3583
Loc: Guilford, CT
Most cars have a wide open throttle relay that will automatically turn the AC compressor off when you're accelerating hard.
_________________________
2006 Ford Ranger Sport
1992 Honda Accord EX
2004 Honda CR-V LX
1994 Ford Bronco XLT (project)
2014 Subaru XV Crosstrek (wife's)

Top
#3372471 - 05/15/14 12:36 PM Re: AC Compressor Load on Engine [Re: zzyzzx]
artificialist Offline


Registered: 09/23/07
Posts: 8440
Loc: Florida
Originally Posted By: zzyzzx
Originally Posted By: Chris142
now lets throw a variable displacement compressor into the mix


I suspect that not enough people have one of those to comment. I do, and when driving that car I just leave it on. On my other car witha normal compressor that turns on and off, I only have the A/C on when not accelerating, since turning the A/C on is roughly equivalent to a "drop anchor" button.

Really? I'm used to seeing them on the GM 2.2 OHV, 3100, 3400, and all Ecotec engines. They were so common, that it was the first time I saw a clone of a compressor made brand new in China.

Anyway, the burden the A/C put on the car varied by compressor design. My father installed retrofit kits in the late 1960s and early 1970s. The kits were not recommended for I6 and smaller engines, and that was because they had a huge 1-cylinder compressor.

Dad did have a car where the AC was an enormous burden. It was a 1987 Hyundai Excel. The A/C compressor disengaged at full throttle, and because of how weak that engine was, there were plenty of times when he had no A/C at all.
_________________________
2010 Lancer Ralliart Sportback

Top
#3372878 - 05/15/14 10:02 PM Re: AC Compressor Load on Engine [Re: actionstan]
1 FMF Offline


Registered: 08/12/02
Posts: 1680
Loc: CT
Originally Posted By: actionstan
Hello All,

I was wondering.. when you turn on the AC in a vehicle and the compressor starts putting load on the engine.. does it put the same load on the engine all of the time? Or does it depend on how fast the blower motor is running.

For example if you use the AC but keep the blower on low, will you save fuel/load on the engine, V.S. running the blower on high with the AC on?

Just wondering because it may make more sense to run it high.. then turn it off when I am comfortable enough, then run it high again. I have a 99 SL1 and the compressor basically completely kills its power, and in the 2013 it idles rougher with the AC on so I like to minimize use as much as possible.


Thanks,


there are two types of compressors, clutch cycling and variable displacement.

for either one, the load that the compressor puts on the engine is directly proportional to the heat the refrigerant is moving from the evaporator to the condenser and how well the condenser is shedding that heat. so on a hot day with your blower on high- the most hot air is flowing over the evaporator coil to be cooled, putting the most heat in the system. now if your parked and you notice your radiator fans kick on high speed, that's because of all the heat in the refrigerant and it's unable to be shed at the condenser easily- high side pressure of the AC system is very high and that pressure difference from what the compressor is pumping is the load placed on your engine.

on a variable displacement compressor which is what most cars have now, i don't know what model years the clutch cycling got mostly phased out, the compressor varies its output (pressure) based on internal valving to keep low side pressure above 30 psi for r134a refrigerant. so when heat load is low the compressor swash plate is more flat and it's pistons pump less, when heat load is high the swash plate is angled most and the pistons pump at max capacity, as heat builds in the system and high side pressure rises, that pressure between low side and high side of what the compressor is causing is the load placed on the engine. you will never hear a normally operating variable displacement compressor clutch cycle off unless it's turned off or you're low on refrigerant and the clutch is tripped by the low pressure sensor.

in a clutch cycling orifice tube system, when it does cycle off under normal conditions that's because the heat load on the system is very low causing that so just prior to it cycling off the compressor is placing the least amount of load on the engine it could be.

and in either case a blower setting on low vs high puts less mass heat flow over the evaporator coil = less heat put into refrigerant = lower system pressure = less load on engine regardless of outside temperature.

Top
#3373790 - 05/17/14 12:07 AM Re: AC Compressor Load on Engine [Re: actionstan]
Barkleymut Offline


Registered: 01/27/04
Posts: 2670
Loc: Richmond, VA
I learned to drive in a '82 Corolla station wagon with AT. With the AC on I often wondered if I could get up certain hills. At stoplights the AC quit running until you gave it some gas. That little car was tough, but very unpleasant to drive.
_________________________
09 Vue(95K) 2.4 4spd AT -PU 5w30 / 09 Flex(52K)3.5L PU / Moms car 04 Pilot(105K) 5w20 PP + Puro

Top
#3373849 - 05/17/14 05:17 AM Re: AC Compressor Load on Engine [Re: actionstan]
SteveSRT8 Offline


Registered: 10/10/08
Posts: 18344
Loc: Sunny Florida
Sounds like my Dad's old Corona. The AC clunked on and half your HP vanished!

At least it was cold. But the constant vibe and cutting in and out was annoying as the dickens...
_________________________
"In a democracy, dissent is an act of faith."
J. William Fulbright
Best ET-12.79 @ 111 mph
4340 pounds, Street tires
Just like we go to Publix

Top
#3373864 - 05/17/14 06:03 AM Re: AC Compressor Load on Engine [Re: actionstan]
Colt45ws Offline


Registered: 08/15/06
Posts: 9168
Loc: Central Washington
My old '98 Cavalier had a 2.2L OHV w/ 3-speed AT and the A/C button was a turbo button. I remember reaching over and hitting that button all the time when I was driving to school or work in Las Vegas. Also made it vibrate a lot at idle. I think it was literally a 20-21 second 0-60 vs. 14 or 15 seconds. It was horrible. I wished it had a WOT cutout.


Edited by Colt45ws (05/17/14 06:08 AM)
_________________________
-Colton
2002 Ford Crown Vic PI, 200k "Highwayman"
Ex-Kootenai County, ID Unit #42
Maxlife 5W20
Lubegard Bio/Tech oil treatment
Ultra XG2

Top
Page 2 of 2 < 1 2