We've mentioned medium - larger cars here, what sort of drop in mpg do the super minis show, though?
AND how about drop in performance?Bought a Ka for a customer, only 2000 miles on it, still quite tight, but reasonable go, but turn on the air & caravans could pass you!!
AND, bet there will be pots of money,or not, for air con service centres in the not to distant!Can't see private owners finding £80-£100 for checks & recharges every year or so, let alone penny pinching fleet managers!!
(When did you last change that pollen filter, even??
Mark
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Wow - thanks for all your interesting replies.
Another thought... Is it really neccessary to change/recharge the refrigerant on a routine basis as some garages/manufacturers would like us to? I spoke to an industrial refrigeration guy at work who said no. So long as there are no leaks or other problems you don't need to change the gas, like your domestic fridge at home the same gas goes round and round for years on end without being changed.
BTW, I know of a local garage that recommends you let them change the anti-freeze every year 'ready for the winter' and the aircon gas every year 'ready for the summer'!!!
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A typical car compressor has an output of about 30,000 BTU, which is a hell of a lot. A home unit that you can buy from B&Q for a room has a compressor output of 6000 BTU, so from those figures you can see that it will consume considerable power. Somewhere in the region of 12KW or 9HP. However, that is only whilst the compressor is actually engaged, as most cycle on and off according to demand. Therefore in cold weather, it will not cycle much, so it sort of sorts itself out, much like an alternator that reacts to demand. As to driving with the windows open, who knows. All I can say to those who are concerned about consumption, is turn it off!
All cars use similar output compressors, therefore the damand is the same if car is small or large. As a rule, large cars don't have larger systems, just longer pipes that hold more gas. And around the world, most have similar systems, it just seems to work better in hot countries due to the difference in temperature when you get out of the car. The system can only go down to zero degrees at the evaporator, and as long as it has the capacity to do this, then they will all have the same ultimate temperature.
Now for recharging. Fridges don't have flexible hoses, rubber seals, and have hermeticaly sealed compressors, therefore the chance of leaks is very small. Cars do, with a resulant increase in gas leakage. It is the leakage of gas over a period of time that affects performance and lubrication. So you pays your money and takes your choice. If you're keeping the car for a long time, pay a bit of time and attention to making sure it always has the correct amount of gas and oil in it. If not, don't worry about it. Same with engine oil changes. You could probably run a car for 70K miles without changing the oil and no obvious problems, same with a/c. It's an insurance policy, not a guarantee, so only you can decide if it's money well spent. I personally recommend a recharging after something like 3 years, then maybe every couple of years after that. But if people don't wish to heed that advice, I don't lose much sleep over it.
All this 'having to turn off a/c to get up hills' is a load of tosh, as the ecu reads the throttle position sensor, and if it sees a big movement it will cut the compressor, but it's fair to say that the drag of a 10hp compressor on a small engine of say 60hp, is quite a lot. Especially as that 60hp is only developed at 5000rpm, and the engine may only be developing 30hp at 2000rpm. But you don't get something for nothing in this world.
And finally, beware advice from some refrigeration engineers. Whilst the physical process of refrigeration or a/c is the same, ie. converting a refrigerant from liquid to gaseous phase, and back again, the actual mechanical process of achieving, and more importantly, controlling this, isn't always the same. Cars are subject to a much harsher enviroment, are much less tolerant to problems with gas charge, use special lubricants, critical airflow requirements, and use many inter-related components, the failure of which can cause huge problems elsewhere in the system. Take a simple device like a timing belt, and the consequences of it's malfunction.
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Dave,
Thanks for a most informative post. I hadn't realised that a compressor was as large as 30,000 BTU.
I understand your point about the compressor cutting in more often in higher temperatures, also that bigger cars have a similar size compressor.
However do you know if, say, a Jaguar made to export specification for Kuwait would have a larger compressor(50,000 BTU?) than a UK spec car? Or would the UK spec compressor cope with Middle East temperatures, albeit it would be engaged more.
C
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I think you'll find it's the same compressor, just stays on longer, as you say. You may find they fit a more efficient condenser or larger fans, to try and shift a bit more heat, but the limiting factor isn't the compressor. They are so oversized already, as they have to get the car down to temp in a reasonable time, that the limiting factors tend to be other items, like the condenser and evaporator. In this country at 30 deg, the compressor will probably only run about 50% of the time once it's got going (at a lowish fan setting), so 40deg isn't that much more load. You will also find that cars with front and rear a/c only have a larger condenser, and everything else is the same.
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One of the reasons aircon systems need recharging regularly is the the gas molecules are smaller than rubber molecules, so the gas actually escapes through the rubber hoses. About 10% per year is reckoned to be average - so after 5yrs you're down to 50% gas and should really be having it recharged.
It's exactly the same reason why you change brake fluid every two years - water vapour and air molecules pass through the rubber hoses INTO the fluid. Of course the rubber hoses are at the business end of the brake system so as soon as the brakes get hot....... you know the rest of the song.
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The compressor pump on a small car is not much smaller than that on a large car so turning on the a/c will have a greater effect on performance-having said that,the module will turn off the pump if you put the throttle to the floor and come back on when you ease off.
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