h a l f b a k e r yYou gonna finish that?
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There's no free lunch. The power to run the 12 volt system would have to come from the engine/belt driven alternator which would have to work harder to supply the needed electrical power. The additional step of coverting mechanical to electrical and then back to mechanical energy would introduce its own set of inefficiencies. |
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However, if the compressor didn't need to run at full speed all of the time, you might be able to gain some efficiency in the same manner as residential A/C units with variable speed compressors. |
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Though I haven't looked specifically, I assume the A/C compressor on electric vehicles works the way you propose. Many of the parts you need to implement such a system in your vehicle might be available "off the shelf". I don't know if scroll compressors are used in automotive applications. |
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Obvious answer in that part of the world is solar powered A/C - but that's Baked, if not Widely Available. |
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I agree with the no free lunch concept, but where this system wins hands down against the conventional crankshaft driven AC is when the car is parked. |
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In the heat, most drivers will run the engine just to power the A/C. Running a 5-liter engine at idle is incredibly wasteful for the incremental power diverted to the compressor. |
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The optimal solution would be an A/C that runs on battery when the engine is stopped and on the crankshaft when the engine is running. The system should either run from an independent battery, or there should be a low battery cut-out so that the A/C doesn't fully drain the battery and there is always enough power to restart the engine. |
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Mr Burns, please direct me to where the roof sized array of solar cells can be had for free. I want some. |
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The number that sticks in my mind is 5. But, another number that sticks in my head is 12. I guess that's not much help. Even if it's only 2, that'd be a fairly hefty drain on an ordinary car battery. |
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I'm thinking it's higher. When it comes on, it's really noticeable. |
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I've now searched around a bit, 12 was the highest number I could find. Considering that as a percentage of even the peak horsepower of an engine, it's not too surprising that it's so noticeable. I'll have to dig out a book when I get home to see if I can find a real number. |
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Install a RV air conditioner on the roof of your car. Only draws about 8 amps. |
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I agree with [FM] - running the A/C from battery while at idle or stopped would be nice. Unfortunately, I suspect that it will take a LOT more battery than the average car has available. |
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Generating extra electrical power to run an electric A/C compressor doesn't make sense to me, I think you'll just lose energy to the conversion process. A better solution would be to improve the fuel efficiency of the engine - better fuel efficiency overall is key (and it would provide additional "excess" power for running the compressor)... |
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<aside>It seems to me that the automotive industry would have worked out alternative power sources for compressors already, as a means to improve hot weather performance of small cars. I had a car that would just about stall if the compressor came on while the car was idling. My memory of the numbers was that the compressor sucked 5-10 hp from the 100 hp engine. Keep in mind that's hp off the top -- a significant part of the "excess" power available for acceleration, cruise efficiency, etc. In that particular car, when the temperature went above 100 degrees the car never really cooled off - so I had reduced efficiency, and a hot car.</aside> |
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I like it. Yeah, there's some loss from converting from here to there, but you get that anyway with belt-driven models (just how efficient IS it to hook up a belt to the crankshaft, loop it *all the way around* those pulleys, and then turn the A/C from it?). And besides, even when yer not running it, the belt is turning something that's providing resistance, yet giving nothing. |
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What this would do, essentially, is make the belt system simpler, and make the engine's work in non-A/C mode a bit easier, without compromising anything significant (just how inefficient could it be to take a maximum of 12 more HP out of an alternator that's *already* powering the rest of the electrical junk?). It's not a world-peace maker, but it is an improvement, so +. |
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12 hp at 12 volts is close to 750 amps. Care to calculate the size cable that takes? I don't think I've seen any automotive alternators over 200 amps. |
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True. But that's for the big compressors. Maybe this would only work for small cars. |
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OK, going the other way. Geo Metro with a 55 amp alternator. You need 15 amps to just run the system. 40 amps * 12 volts = 480 watts = a little better than half a horsepower. Not much cooling, and don't turn on the fan or the battery goes flat. |
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Actually, the biggest problem in all this is the low voltage of the electrical system. Otherwise, it should be quite doable. |
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Just occurred to me, though, that (if my memory serves me correctly) the alternator itself doesn't run the car, it charges the battery which runs the car. Perhaps you could verify whether that's true or not, because I *know* I've seen batteries with capacities of over 600 amps. Of course, that completely sidesteps the possibility of using step-up transformers to crank out more power; but then, that'd just be another inefficiency. Dunno. |
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The alternator does charge the battery. Actually the alternator has to run the electrical system and charge the battery. In most (all?) cars you can start the car and then disconnect the battery and drive on. (which by the way is a quick and dirty way to test a suspect charging system) Otherwise, jumpstarting a car with a dead battery would serve no purpose, the car would die again as soon as the jumper cables were removed. |
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It's logical that the alternator has to be capable of supplying at the amount of power used by the electrical system (other than the starter) plus a little more or the battery would always be dead. |
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The cranking amp capacity of a battery is how much current a battery can deliver for a given time at a specific temperature (link). |
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Re: [lurch]'s comment about the low voltage issue - lots of info out about upcoming 42 volt systems just for that reason. |
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A free lunch would imply no cost. What if there was actually a net gain?
Present automotive A/C compressors are driven by belts to the engine. When the engine turns fast the compressor turns too fast. When the engine turns slower the compressor turns too slow. This is not very efficient. What we want is for the compressor to turn just the right speed. For the engine to have enough power to drive the A/C compressor, all of the other loads and still provide acceleration when required we need large engines. If the A/C compressor was separated from the engine then the various peak loads would not combine at the same times and we would need less engine for the same performance. The problem is the power for the A/C compressor. To run the compressor on 12 volts would require high current levels. But if we increase the voltage to 42 volts the required current level drops by 2/3's (What we refer to as a 12 volt system is actually a 14 volt system, 42 volts is then 3X the voltage) Aside from lowering the current level, higher voltage motors are smaller, lighter, more powerful and less expensive. This system will result in better performance and better fuel economy. |
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In due time...the auto industry is advancing right now. With more electrical power needs than ever its getting harder and harder to squeeze more power out of the alternator. As of right now your idea would not work. In the years to come I suspect the automobile industry to switch over to a 42 V system, and thus switch to an electrical AC compressor rather than a belt driven one. The advantage of this is that you can run the compressor at its most efficient speed, not the speed of the engine. This saves you gas and emissions. In the years to come more and more belt driven components will be eliminated, and eventually your car will even be able to "shut down" at stop lights instead of idling, as an improved electrical starter system will also come into effect |
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The issue has been covered/developed and is already a reliable capability. It has gone as far as the ability to provide bursts of extreme cooling of the intake to provide performance gains (cooler air) without the loss of power from the AC pulley. The system I have seen uses a low amp pull with great results. There is even use of co2 systems with a fair amount of success. |
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I am interested in as cheaply as possible putting an air conditiner in a 86 chev deisel truck. Like the idea of putting rv air conditioner on roof from cedar park but wondering how to do it. Any ideas will be greatly apprecited. |
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Did anyone think to ask the manufacturers whether they were already doing this? |
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I seem to recall Audi came out with a special A8, with optional sunroof solar panel, specifically for the purpose of running a cooling system within the car while parked. |
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If you check http://auto.consumerguide.com/auto/new/reviews/full/index.cfm/id/38064 you will see the mysterious optional solar package offered. |
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Maybe someone can either confirm or refute whether this is a true A/C unit, but in the meantime, the plot thickens further with this strange product offering:
http://www1.easyupdateweb.com/model/showroom/mode1/mode1/frame.asp?uniUser_ID={9475F564-0B85-4BF9-8BF4-A7D5DB693FB7}&intChEn=3&strChEn=En&intOrderby=1&intUserType=2&intSecretApplication=1&intPriview=0. |
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Has anyone solved this, because I'm considering working this one out on my Jeep. Is this something that battery/solar power, going through an inverter might be able to manage? |
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Or should I go to the "Popular Mechanics" website? ;-) |
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This isn't original- on a gas only car, it is. On hybrids and small electric cars, this was already created. But since none have been made electric on a gas powered truck, I'll stay at neutral. |
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