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Car Forum / Australian Car Forums / General Car Topics (Australian group) / February 2005A few car air conditioning questions / thoughts
Hi guys, I've got a few questions about car air conditioners - I don't know where to look to find out this info, so any help would be much appreciated. 1. How much power does the typical car A/C unit consume? I've heard figures of 5-10 hp plus. Anyone know specifically the capacity of a a Holden Commodore unit? In ours (VYII N/A Calais), there seems to be a sudden surge of power from 3,000 rpm, especially noticeable in first gear, but not necessarily with full throttle (probably half or so). We've had a few of these cars and I don't recall any previous ones doing this. Is this because of some sort of change to the ECU, or is it just the A/C disengaging to give extra power? 2. When the capacity of a car A/C unit is specified, is that at a particular engine rpm, like an "average" rpm that the designer feels the users will be using most of the time, or is it specified for maximum engine rpm? I assume that, as the A/C unit is directly connected to the engine via an electromechanical clutch, it cannot operate independent of engine speed, therefore doing 6,000 rpm would make the A/C operate extremely fast (assume you're in a manual, not operating at full throttle so that the A/C clutch is engaged). In other words, when an A/C unit is specified as 10hp, is that at max. engine rpm, or is that at some set "average" rpm, say 1,500-2,000? If it is the latter, say this average value is set at 2,000 rpm and you have a 10hp unit, does that mean it will be drawing 30hp @ 6,000 rpm? Also, at idle, this will mean that you're only getting 5hp, and alternatively, only ~1.5hp if it's geared to consume 10hp @ 6,000rpm, which surely isn't enough to cool the car effectively. Are there any units around that can change compressor speed independently of engine speed? 3. At times I have also driven '89 Ford Telstar Ghia (2.2L), and cruising down a large hill at ~100km/h, I notice that when the A/C compressor turns on, the car increases speed faster than when the compressor is off. I am 99% sure this is when the compressor is *on*, not off, as I would have thought that the compressor would put a load onto the engine, thus effectively acting as an engine brake (ie: the other way around to what I am noticing). The only thing that I can think of is that the engine is receiving extra idle fuel (as the rpm increases when the compressor turns on at idle), and it is receiving too much. The car is doing around 2,000 rpm with my foot off the accelerator, cruising down this hill, and does around 2,300 rpm at this speed on the flat. Any idea why this could be happening? I just want to know purely out of interest :) Well, that's about all I can think of (for now :) ). Sorry about the length of the questions, but because of my limited technical knowledge, I don't know how to explain myself in less words :) I've just been pondering these questions for a while and decided to find out the answers off people who actually know what they're talking about :P Thanks for your help, Ben > Hi guys, > [quoted text clipped - 18 lines] > operate independent of engine speed, therefore doing 6,000 rpm would > make the A/C operate extremely fast (assume you're in a manual, not Curious, why would it being a manual make any impact on engine speed vs aircon speed? > operating at full throttle so that the A/C clutch is engaged). In > other words, when an A/C unit is specified as 10hp, is that at max. [quoted text clipped - 31 lines] > > Ben It wouldn't, I was just saying to imagine using a manual as I don't think it would be possible to hold 6,000rpm at anything but (close to) full throttle in an auto, unless you're able to manually hold it in gear. That's all :) > Hi guys, > [quoted text clipped - 10 lines] > doing this. Is this because of some sort of change to the ECU, or is > it just the A/C disengaging to give extra power? The A/C compressor on the VY (or VT onwards for that matter) does not engage or disengage, it is always on when the A/C is on. It is a variable stroke compressor, meaning that as load increases, so does the internal stroke of the compressor and pressure increases. As such, there is no surge. What you are feeling is likely to be just the powerband of the engine. > 2. When the capacity of a car A/C unit is specified, is that at a > particular engine rpm, like an "average" rpm that the designer feels [quoted text clipped - 13 lines] > units around that can change compressor speed independently of engine > speed? The load varies with the demand, as I explained above with the compressor used in the VY. At maximum load (vent not recirc setting, a very hot day with high humidity for example) you can expect the compressor to be leeching a few kw's maximum, but it's not really very significant. The only time that the compressor will be cut out by the ECU is under very high load and full throttle (such as when pulling up a steep hill towing a caravan and at full throttle) to give a maximum engine power, or when the engine is overheating to ease the load. The compressor will also cut out if the system is overcharged when the high pressure switch trips the system (independant of the ECU) or similarly when the A/C is very low on gas, to protect the compressor. > 3. At times I have also driven '89 Ford Telstar Ghia (2.2L), and > cruising down a large hill at ~100km/h, I notice that when the A/C [quoted text clipped - 6 lines] > increases when the compressor turns on at idle), and it is receiving > too much. Yep, the A/C idle speed is increased too much to compensate for engine load and that is causing the acceleration. This is adjustable. Typically, idle speed with the A/C on should be the same as with the A/C on so adjust for that. The car is doing around 2,000 rpm with my foot off the > accelerator, cruising down this hill, and does around 2,300 rpm at > this speed on the flat. Any idea why this could be happening? I just > want to know purely out of interest :) Man or auto? > Well, that's about all I can think of (for now :) ). Sorry about the > length of the questions, but because of my limited technical [quoted text clipped - 4 lines] > > Thanks for your help, If you don't ask you don't find out ;-) Regards, Clockmeister. Interesting, I had no idea that variable stroke compressor even existed (not that that should be suprising anyway ;) ). Is this the norm for most new cars now, or just larger/more expensive cars? Would the Telstar be of this type? Or, is it what all cars use (ie: they've always been like that and I just don't know what I'm talking about)? That's funny you should mention that the surge of power is from the powerband of the engine, because I could have sworn that it wasn't there before. Of course they could have just remapped the ECU or something, but it seems weird to make the engine surge rather than deliver a smooth power band. When I say "before", I mean the previous model, and most previous models before that (they're all company cars, kept for about one year each), except our last model was supercharged, so it felt totally different to this one. Oh well, we can only speculate... When would the system have a high pressure situation? Is it just when the load is too high for it to cope with (I've yet to experience this - the Calais' A/C is amazing)? The Telstar's an auto, and I was noting the rpm today @ 100km/h - it was more like 2,200rpm, which is quite low for a 2.2L 4-cylinder, although the engine is quite torquey (but at the same time breathless above 4,000rpm). > Interesting, I had no idea that variable stroke compressor even existed > (not that that should be suprising anyway ;) ). Is this the norm for > most new cars now, or just larger/more expensive cars? Would the > Telstar be of this type? Or, is it what all cars use (ie: they've > always been like that and I just don't know what I'm talking about)? Only the larger GM cars I know for sure use variable stroke compressors, I don't know if anyone else uses them. Not likely in the Telstar though. > That's funny you should mention that the surge of power is from the > powerband of the engine, because I could have sworn that it wasn't [quoted text clipped - 5 lines] > so it felt totally different to this one. Oh well, we can only > speculate... The supercharged V6 has an exceptionally smooth and even powerband, so that is perhaps why it feels different now. > When would the system have a high pressure situation? Is it just when > the load is too high for it to cope with (I've yet to experience this - > the Calais' A/C is amazing)? Generally under very high load. That can happen on a hot day if the A/C has been overcharged, or when the condensor fan has stopped working or the condensor is blocked with crap. > The Telstar's an auto, and I was noting the rpm today @ 100km/h - it > was more like 2,200rpm, which is quite low for a 2.2L 4-cylinder, > although the engine is quite torquey (but at the same time breathless > above 4,000rpm). Hence your rpm/speed will differ because of the slip in the auto transmission. Only in overdrive with the torque convertor locked will you see the actual rpm/kmh ratio. Regards, Clockmeister. "Ben R" > Hi guys, > [quoted text clipped - 5 lines] > figures of 5-10 hp plus. Anyone know specifically the capacity of a a > Holden Commodore unit? > In ours (VYII N/A Calais), there seems to be a > sudden surge of power from 3,000 rpm, especially noticeable in first > gear, but not necessarily with full throttle (probably half or so). The VL has a similar power surge around 3000rpm. Noticeable in the auto - Huge in the manual. I assume this has to do with power/economy curves in the ECU, and cam design etc - ie, it has nothing to do with the a/c >2. When the capacity of a car A/C unit is specified, is that at a >particular engine rpm, like an "average" rpm that the designer feels [quoted text clipped - 13 lines] >units around that can change compressor speed independently of engine >speed? The load isn't dependent on speed as you're assuming. Think of a refrigerator as a heat pump -- the power required to drive it is proportional to the heat load. Same way as the power required to drive the alternator depends on the electrical load rather than speed. There's a correlation between refrigeration capacity and power, ie BTU and HP (for those who like to think in obsolete units), which I've long since forgotten. I would've thought that typical car a/c would consume considerably less than 10hp (7.5kW) under any circumstances... to do otherwise would see bits of shredded "A section" drive belt spewing out in all directions. -- John H I get what you're saying, but still, is there a sort of limit to how much "power" the compressor can draw at any given rpm, and this limit increases with engine rpm? So it's like I was saying before, if it's quoted as being a 10hp compressor, is that @ maximum engine rpm, so half max rpm will give a 5hp limit? As you know, when the car's at idle, the A/C doesn't work all that well, but as soon as you start driving it cools right down. Surely this isn't attributed to insufficient airflow around the A/C heat exchanger, as it's got two large fans blowing/sucking over it? Another thing that happens in the Telstar that I just thought of which seems to be a weird phenomenon (to me, at least) is that when I go up a large hill at 90-100km/h the A/C seems to get colder, even when I'm still in top gear. There's more load on the engine of course, but prior to this hill I was just travelling at 110km/h on the flat (ie: more engine rpm), and on a really hot day, I'm almost 100% sure the air gets colder as I slow down a bit and engine load increases. Surely I'm not dreaming? :) I have no idea how much a car A/C draws, although I did see that an old 80's BMW 5 series had an 8 hp compressor quoted in the user manual. I read somewhere that the sun puts out 1,000 W of power (I'm assuming they mean heat only) per square metre, so assuming the roof of a large car is 3-4 square metres, that's 3-4kW of heat load in itself, and then you've got to add windows. Of course the roof would insulate slightly, but the windows could be letting in another 3-4kW of heat. Then you've got other sources of heat, like through the firewall and transmission, etc. so you could be approaching a 10kW heat load. The reason why I said 10hp is because I assumed that the Holden would have a bigger compressor than a BMW, and it seems to work exceptionally well under most circumstances, so I assumed it therefore must be larger than 8hp :) >I get what you're saying, but still, is there a sort of limit to how >much "power" the compressor can draw at any given rpm, and this limit [quoted text clipped - 5 lines] >insufficient airflow around the A/C heat exchanger, as it's got two >large fans blowing/sucking over it? The capacity of the entire system is determined by factors other than the compressor's theoretical capacity, nor is it determined solely by the amount of refrigerant moved by the compressor (speed). The amount of heat dissipated by the condenser would be the most relevant measure, and is what will ultimately determine how much power the compressor requires. Odd that you'd say a/c's don't work very well at idle. Almost any cycling clutch system in good condition should cycle at idle under all but the most extreme conditions -- if it's cycling it's achieving maximum refrigeration capacity for the prevailing conditions. Increasing the engine revs (and the compressor's) won't normally result in any decrease in evaporator temp (the source of the air that's blown into the cab) although it should reduce the compressor's duty cycle (the percentage of total time it spends running). When the car is stationary the heat load is almost certainly at it's maximum as a moving car will also be cooled by external air movement. Condenser temperature (what you refer to as the heat exchanger) is also relevant as it will almost certainly be higher when stationary with a resulting decrease in its ability to dissipate heat. >Another thing that happens in the Telstar that I just thought of which >seems to be a weird phenomenon (to me, at least) is that when I go up a [quoted text clipped - 4 lines] >gets colder as I slow down a bit and engine load increases. Surely I'm >not dreaming? :) The a/c has no way of knowing the total engine load and isn't influenced by the engine load other than at full throttle, when the a/c compresssor is switched off by the engine management sytem. Any differences you're noticing will almost certainly be due to external factors, such as ambient temperature, solar radiation and wind direction. >I have no idea how much a car A/C draws, although I did see that an old >80's BMW 5 series had an 8 hp compressor quoted in the user manual. I [quoted text clipped - 9 lines] >most circumstances, so I assumed it therefore must be larger than 8hp >:) I'd hazard a guess that most a/c's are designed to run at around 50% duty cycle under "normal" conditions, which suggests to me that only half of the compressor's theoretical capacity is being utilised. 8-10hp sounds awful high by comparison with refrigeration systems whose capacities are known -- for example a 3kW domestic a/c, where the power rating is obtained by converting heat units to power, is a relatively large system by car standards. -- John H > I'd hazard a guess that most a/c's are designed to run at around 50% > duty cycle under "normal" conditions, which suggests to me that only [quoted text clipped - 6 lines] > -- > John H A/C systems are genrally quoted with 2 power figures. The hp figure - eg 10hp relates to the cooling power, the kW figure relates to the compressor power consumption which is about a third of the cooling power depending on compressor/condenser/evaporator efficiencies. A 10hp A/C system will draw about 2.5-3kW of power to run Ah, that makes more sense - I mean, it was hard to imagine a car requiring several times the cooling power of a large house! Although, to me it seems a bit strange to rate it like that when cooling/heating power in houses is rated in BTU or kW, not hp. I just thought they'd transfer the same units to cars. > Ah, that makes more sense - I mean, it was hard to imagine a car > requiring several times the cooling power of a large house! Although, > to me it seems a bit strange to rate it like that when cooling/heating > power in houses is rated in BTU or kW, not hp. I just thought they'd > transfer the same units to cars. House ones are generally R22. I'm *not* switching away from R290 for my car... R290 really is ferpect. > House ones are generally R22. I'm *not* switching away from R290 for > my car... R290 really is ferpect. Lyk ewe. (-:  SignatureTake care, Feral >> House ones are generally R22. I'm *not* switching away from R290 for >> my car... R290 really is ferpect. > > Lyk ewe. (-: Dunno. Ask Athol or Damo, they probably knows more about me than I do :-) > House ones are generally R22. I'm *not* switching away from R290 for > my car... R290 really is ferpect. House ones are going to be, and quite a few already *are* running R410a. Which is shite, and no-one knows how to use it. Of course it's supposed to be ozone friendly, and will stay that way until independent research catches up to it, at which time it'll be banned. An example - so no-one reading this will be suckered into new machines for a while..... So, with brand new machines. R22 (old bad bastard gas)gets 6.5 degrees C at the split unit in the room. R410a (new stupid but not yet proven bad bastard gas) gets 16.5 degrees, or worse, when installed exactly according to specs. As a special bonus higher head pressures, special piping requirements, far more critical gas charging - so is harder to install, costs more to run, and of course simply doesn't work as well in any subjective sense - including NOISE from outdoor units. I might also add that one well-known company is selling units pre-gassed for 20 metres. So, not a hope that they'll work properly in the usual domestic installations. It gets better, though, in a batch of three I heard about, only one had the full charge. Which indicates totally f.cked quality control - hell, all they have to do to check is to weigh the things at the factory - couldn't even manage that.. SignatureToby quidquid latine dictum sit, altum viditur >>House ones are generally R22. I'm *not* switching away from R290 for >>my car... R290 really is ferpect. [quoted text clipped - 22 lines] > totally f.cked quality control - hell, all they have to do to check is > to weigh the things at the factory - couldn't even manage that.. Mate of mine is certified in that rap ( wot he calls it) only about 6 or so in QLD so far done the course... will be interesting if it gets dumped as its really is piss poor gas.. >House ones are generally R22. I'm *not* switching away from R290 for >my car... R290 really is ferpect. R290 is actually a near equivalent of R22, which is in the process of being phased out, but still available AFAIK. There's no reason why you couldn't use it as an alternative to R290 except the commercial refrigeration suppliers won't sell it to non approved customers (ie they voluntarily control the supply so it doesn't find its way into automotive a/c's). IIRC the near R12 equivalent is a mixture of R290 and iso butane (R600a?). -- John H > IIRC the near R12 equivalent is a mixture of R290 and iso butane > (R600a?). No need, I've been using R290 straight for some years now :-) > IIRC the near R12 equivalent is a mixture of R290 and iso butane > (R600a?). Yep. IIRC, you gave to add 40% R600a to the R290 to reduce its performance down to that of R12. :-) SignatureAthol <http://cust.idl.com.au/athol> Linux Registered User # 254000 I'm a Libran Engineer. I don't argue, I discuss. OK, here's what I've gathered so far, which has (of course) created even more questions: - A car A/C has two ratings, one in hp and the other in kW. The hp rating is the cooling capacity of the unit, and the kW is the amount of power it will draw to reach this capacity. - The compressor is able to consume its entire kW rating when the car is at idle, and hence provide its full cooling capacity (?). However, if the engine speed is increased, the duty cycle (percentage of time running) of the compressor will decrease. Does this mean that the compressor is consuming more power though (in kW, but the same rough percentage of engine power)? You said that "the power required to drive it [a refrigerator] is proportional to the heat load", which I understand to mean that it only draws as much power as it needs, regardless of engine speed. Without using a variable stroke compressor, how would this work? And why would the compressor need to cut in and out if it were able to vary its capacity independent of engine speed? - Does the cooling capacity specified on the compressor unit assume a certain amount of airflow over the condenser? So when you're stopped, it can't provide this cooling capacity because insufficient airflow is provided? I'd assume this specification would also assume a certain ambient temp? I'm not trying to disagree with you in any way here, I'm merely trying to understand how this all fits together. Thanks again for all your help, Ben PS: one more thing, totally unrelated...a friend has an old Ford Fairmont Ghia (98 model perhaps) and on the climate control it says "ambient temp", but it appears to be showing the temperature inside the cabin. We started a bit argument over the definition of "ambient" :) Ambient means the temp *outside* the car, not the "ambient" temp of the cabin, right? > - The compressor is able to consume its entire kW rating when the car > is at idle, and hence provide its full cooling capacity (?). However, > if the engine speed is increased, the duty cycle (percentage of time > running) of the compressor will decrease. Does this mean that the I'm not going into variable stroke. In a traditional system such as I have, the compressor will cut in on the advice of an amplifier circuit, the sensor being in the evaporator box. This probably needs some changes when you use R290 instead of R12. The system cycles in and out quite quickly while being *very* cold. > - Does the cooling capacity specified on the compressor unit assume a > certain amount of airflow over the condenser? So when you're stopped, > it can't provide this cooling capacity because insufficient airflow is > provided? I'd assume this specification would also assume a certain > ambient temp? Air flow over the condensor is critical, no real reason why being stopped is going to make a difference given working fan, etc. The older systems provide an "Idle up" feature. With R290, on a humid day I can achieve ice on the intake (suction side) of the compressor. |
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