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How come A/C compressors aren't driven by electric motors?
Seems like you ought to be able to get enough power to drive the pump, and you wouldn't lose any power or drivability when the AC's on.
Would add alittle weight though.
Waddya think?
Would add alittle weight though.
Waddya think?
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My guess is that, for one, you'd need a larger alternator to generate the power, which is going to put more strain on the engine at all times, not just when the A/C is running. Or maybe you could get around it somehow by using a couple of deep cycle batteries that would take longer to deplete, but that would add weight.
Or maybe A/C technology has improved enough that it's just not feasible to go with an electric one. I read somewhere awhile back that air conditioning adds about 150-175 lb to the weight of an average car, but maybe it's less nowadays.
On another note, does anybody know how much the air conditioner belt and pulleys saps the power of a car when the A/C is turned off? After all, the A/C compressor's not running, but the belt and pulleys are. Maybe it'd be worthwhile to make a system that could totally disengage the belt from the crankshaft pulley, so it's not sapping any power at all. Y'know, kind of like the pulley that engages and disengages the blade deck on a riding lawn mower.
I'd like to hear some thoughts on the more auto-educated out there!
Then they would have to figure out where to put it, since that size of motor would also create some pretty good heat, it would have to be somewhere that there is a fair amount of air flow.
When the new higher voltage vehicles come out, we may see it yet.
the first law of thermodynamics is that there is no free lunch... every time you change energy forms, you lose some. the engine turns chemical energy into rotational force, and is maybe 10 percent efficient. the a/c compressor turns rotation into pressure, and loses energy.
now, if the engine turned the alternator for 20 more HP, and you lose 90% of your input energy in generating electricity according to the steam power plant surveys quoted by the power companies, you are wasting 90 percent of 10 percent of applied energy. electric motors are supposedly 15-20% efficient once you get up into the big devils that power elevators and worse, and are much more inefficient in smaller sizes. a 20 HP electric motor is the size and weight of a big-block, all-iron car engine. you couple that to the a/c compressor, and how much power is left?
plus you taxed the engine even more by adding half again the weight of your car for the electric motor, and have cut the airflow seriously by doubling-plus the engine compartment.
we now have Yugo performance with a V-10 engine that gets 3 miles to the gallon when the a/c is on, and 3-1/2 mpg when it is off, and it takes two parking spaces.
the physics condemns the idea.
if you have a smaller passenger compartment, or a much more efficient refrigerant, or both, you can improve the odds slightly.
rough numbers, but there's the facts... the more energy conversions you have, the worse off you are.
oh, forgot something... starter motors for the diesels can run up to 40 amps. if you crank 'em for over a minute, they overheat and short out the coils. I'm talking about equipment built to last on your long drive to Gramma's house, not for short peak period use.
Basically, right off the drive belt you get the kinetic energy to drive the compressor. No need to convert to electric energy and back, taking a loss at each conversion.
Well, those 20hp figures aren't very accurate.
While there is no denying that it would require a healthy sized motor, especially being a DC motor, but if what your saying is true, then it would require at least 20 hp to run a household AC unit or an air compressor, yet in some instances, a 3 hp motor works just fine for them and at lengths of time.
the engine turns chemical energy into rotational force, and is maybe 10 percent efficient.
Uh, since when??
If the engines were only 10% efficient, DEQ and EPA would have an absolute fit.
Granted, they are much more than 40%, but that is quite a bit more than 10%.
Also,
a 20 HP electric motor is the size and weight of a big-block, all-iron car engine.
Sorry, but that isn't entirely true either.
The 24 volt DC motors we run on trailor units for hydraulic units that are 10-20hp are a whole lot smaller than an engine.
The last few years, the efficiencies of the AC units has jumped considerably (had to with the advent of R138).
With alot of newer electric hybrid vehicles being made, using electrics to operate accesory drives, such as the AC are being looked at more and more.
Now, these aren't any hard numbers or facts, just real world experience.
What do you think about harnessing wind power to cool the interior? That's the way they used to do it.
Just kidding.
No matter what the efficiency is for combustion engines, you'd end up with a loss going to electric energy first.
Ok, then why do modern freight train engines use a diesel engine or gas turbine to drive a generator, then use electric motors to drive the wheels?
Hmmmm
TB
1st is that the electric motors can provide more power.
Now, someone can argue that, but we have large machines that lift log loads off of log trucks, they run a diesel engine for the electrics and everything is 480v electric. I talked to one of the engineers of the machines and he said they tried to rung a drivetrain on a few of them, but with a 40 ton load on them, they hardly move.
2nd reason, you don't have to have a drivertrain from the engine to the drive.
The electric motors can be placed in any position, as long as you can get the wiring to them.
When saying that an electric engine provides more power (relative to its size), it is interesting to see pure electric train engines on electrified tracks. They are much smaller, but are far more powerful.
Also, electric engines provide a relatively flat torque-curve and have torque at the very low end of their rpm range, which is key when actually trying to get a train into motion. The stress on a clutch for a diesel engine/drivetrain application would also be enormous.
With the electric traction motors, you get torque with 0 rpm
You can run the diesels at a nearly constant rpm (peak power mode) which means the size and weight of the diesels are minimized
You don't need a transmission
But first, the battery systems on automobiles will be a much higher voltage. Something in the region of 48 volts as opposed to 12 volts. All motors used on cars will be driven by alternating current obtained from an invertor which will convert the 48 volts direct current to a higher alternating voltage. This allows for motors of lower amperage and higher efficiency. Losses are lower because less current is drawn. That means voltage drop across the wiring is lower. The old 6 volt systems were very inefficient.
Because the aircon will be driven by an electric motor it will be possible to mount the unit in a more convenient position. Also, being a sealed unit, the refrigerant should be less likely to leak out. Some domestic refrigerators run for 30 years without losing any refrigerant. The major source of leaks on car aircons at the moment is where the drive shaft passes into the compressor. The seals more often than not leak.
If I read your post correctly, you're saying that we'll start with a 48V battery, chop the DC to AC in the inverter and have 48V electric applications throughout the vehicle ?
I do realize that an A/C electric motor is much simpler to build, more efficient and more reliable. I also realize that resistance (or impedance as we're talking A/C here) in wiring is reduced as we increase voltage.
My concern would only be the complexity of having different voltage levels in the vehicle as most electronics do require DC.
I'm also not 100% sold on the electric A/C compressor in combustion engines yet. But then, if we move to hybrid or fuel cell based vehicles soon, that might not be an issue at all.
Many systems in a car will will be more efficient. The lighting system can also be high voltage and run off the same voltage as the motors. Even if the starter motor is still left a direct current type, running it off 48 volts direct current from the battery will still be more effecient. Remember those old 6 volt starter motors?
Ironically, I drive a car with two waterpumps, one is electric and the other one is driven by a serpentine belt off the crankshaft. The car also uses a viscosity clutch for one radiator fan in addition to two smaller electric ones. The combination does indeed provide very good cooling during engine idle.
The thermostat is actually still needed because you might want to divide the coolant flow differently depending on the operating conditions. (The heater circuit opens before the radiator circuit in cold weather.)
I see your analogy with the computer, but most electric/electronic components (and the wiring in particular) are far less accessible. From my personal experience it is the electronics that often provide the greatest challenge to automotive mechanics.
What I'm mostly afraid of is a push towards a proprietary standard. One might compare it to early fuel injected engines that were very proprietary. It took a while until a standard was developed.
Because of all of the electronics in vehicels to day, add to that, the power window, power seats, power this and that and it is seriously putting a strain on the electrical system.
"42-volt automotive electrical systems are soon to replace the existing 14-volt charging systems. Car makers can power electric steering, shifting, suspension and braking, etc. Electromagnetic valve actuators will replace the camshaft ... belt drives will disappear. The starter/alternator will be flywheel mounted. The water pump, PS, A/C compressor will be electrically powered and operated on demand only, saving power and fuel otherwise lost to belt drives. All car makers have agreed to 42 volts and the first systems will appear in next year in Europe."
I am doubtful that the solenoid-actuated valves or all-electric brakes will be developed very soon but the electric power steering development is well under way by TRW. If inverters are used as you guys indicate, then sealed (and "submerged") induction motors can be used for the A/C compressor and water pump. This should allow the refrigeration circuitry of the A/C system to be optimized for greater efficiency but, at this point, don't you agree that it's doubtful the refrigeration improvements will offset the losses associated with the two energy conversions (mech/elec and then elec/mech)....at any voltage?
Can you imagine an engine compartment with no drive belts ..each item driven by an electric motor located at the point of use?
And I think that this is what most people on the board are talking about.
Unfortunately, I don't have any good data on how much more efficient a more compact (electric) A/C unit would be, and if it would be enough to offset the initial energy loss in a combustion engine.
I also don't think that we'll see a 100% electric braking system. I'm pretty sure we'll have an electronic brake linkage (like in the upcoming E-Class) in most vehicles in the near future, as it great simplifies ABS and electronic stability systems. But the hydraulic system still offers the great advantage of adding very little unsprung weight to the suspension.
I'm further pretty sure that belt drives won't disappear completely. While they aren't the most reliable way of transferring mechanical energy, there is hardly anything more efficient.
I'm pretty positive they're around to stay for applications as fan belts or alternator belts.
And don't forget regenerative electrical braking. That is, if the car is electric motor powered the motor can be used to brake the car and feed energy back into the system as it becomes a generator. In fact, with computer control the motors can come to a standstill and lock. If each wheel is driven by its own motor, the computer can brake each wheel independantly and prevent skids and loss of control. I believe the time will come when a hybrid car with an internal combustion engine will do just that. That is, drive 4 lightweight flat motors built into the hubs of the wheels.
These ideas are as old as motor vehicles. It's just that we now have the technology to put these ideas into practice.
The problem however is weight and complexity. To brake a car safely in such a fashion you'd need to either have generators mounted in each wheel hub or drive shafts to a more centrally located generator.
The first approach would impact the handling of the vehicle (increase in unsprung weight), the second makes it more tricky to brake each wheel independently.
Why look as far back as the DKU ? The Honda Insight has a starter motor that acts as a generator for its batteries. (I'm not sure if the main batteries also drives the 12V systems in that vehicle.)
I just mentioned the DKW car because they were using that type of starter back then already. Finally, If anybody had told me back in the 1950's when I was playing with gas powered model aeroplanes that an electric motor would be able to power a model plane one day, I would laughed at them. Electric motors have also come a long way and today you do get model planes that fly using batteries and electric motors.
I feel sure the overall vehicle energy efficiencies will ultimately improve as indicated in several above points. However, unless the inverter can be used to greatly increase refrigeration thermal efficiency by optimizing the compressor speed for the cooling load, I believe the overall A/C energy requirement will increase. Based on industrial drive data, power transmission efficiency of the existing belt drives is probably ~92%. For the electric drive (assuming 100% inverter efficiency to give the inverter benefit of any doubt) the mech-elec conversion efficiency of the generator will be ~88% and the subsequent elec-mech efficiency of the compressor motor will also be about 88%. The product of the two figures indicates efficiency in the range of 75% to 80% to the compressor. This is the power-conversion efficiency range that must be compared with the above ~92%. If anyone has more accurate 42V motor/generator efficiency data, I am certainly willing to "stand corrected".
The only snag when using that in a car is the time lag involved in getting the system working from cold. However, if we use it as a back-up system for long trips, we only need to use a conventional system when staring up a cold engine. So, here we have a system of A/C that does not take any extra energy at all once it is up and running. No doubt that if the concept is followed through the A/C system can become a sort of hybrid system whereby elements of both systems are combined into one unit for economy of manufacture.
Systems based on thermal gas expansion also need excellent ventilation to work, so you'd probably end up with another radiator system. Since it's based on differences in thermal energy, it would also work the least when the ambient temperature is the highest. I really don't think you wanna go there.
thank you
seaspy
I don't think you necessarily want to combine the two A/C systems. The conventional one works best when combined into a small unit to minimize heat losses.
The thermal A/C needs to be stretched out to tap into different 'heat pools' to utilize the temperature gradient for refrigeration.
This is also why a small camping refrigerator works well. It has a small (contained) heat source and a much cooler ambient temperature. The larger these units become the less efficient they get. (The problem is keeping the heat pools separated. This is mostly addressed through added ventilation, etc.)
If you have your car retrofitted with this kind of A/C, I'll gladly pay you a visit in Belmont to find out for myself.
http://www.auto.com/travcity99/wpowers_aug5.htm
Please explain it.
Although I have seen no such report, I would guess that one reason for considering the electric-drive compressor has to do with "packaging" in the engine compartment. If all belt-driven accessories can be electrically driven, less space is needed at that end of the engine. With transverse engines, lateral space is precious and the new flywheel-mounted generator/starter will need some room. I focus on the A/C system in this regard because it is surely the biggest challenge to convert to electric drive. Other thoughts?
By having the A/C electric motor driven, the unit can be mounted quite remotely from the engine compartment. The engine compartment is the worst environment for the A/C system. If all belt driven accessories such as water pumps, power steering etc. are motor driven, design restraints as to engine layout are simplified. The pump for the power steering could be mounted on the steering rack for example. The water pump can also be moved to a more convenient position for engine designers. In fact, you could end up with more engine space.
As to the efficiency of such an all electric system - the answer lies in the design of the motors. These will be of the alternating current type with variable frequency speed control. I myself have experimented with a standard 10 horsepower 3 phase electric motor using an off-the-shelf ac speed control unit that used variable frequency from zero Hz to 100Hz to control speed. I was impressed by the torque at all speed ranges. Start-up torque was impressive. Conversion losses by the speed control unit were minimal. This was about 25 years ago and my facts and figures for my experiments are not at hand at the moment. No doubt more effecient units are available today.
Briefly, what I imagine might happen in the future is this - the internal combustion engine will still be with us for many more years to come. But, the way energy is coupled to the drive wheels will undergo a radical change. Simply put, there will not be a mechanical transmission anymore. There are losses in a mechanical transmission. An electrical coupling has less. The internal combustion engine will drive an alternator. The output of the alternator will be rectified and converted to direct current. This will charge a 42 volt battery. The 42 volts in turn will be inverted to an alternating voltage which can then be any voltage the automaker chooses and will be much higher than 42 volts. It could be 120 volts ac for example. This is the voltage that can be used to drive all motors in the vehicle including the main drive motor or motors. Although the conversions seem many, just bear in mind what is happening in a conventional automatic transmission.
In a conventional automatic we first have the torque convertor. A source of energy loss. In fact, if we follow through with all the energy wasting components in such a transmission we find out that a lot of the energy produced by the engine is lost in its journey to the drive wheels. The differential gears needed for turning corners can be eliminated by having a drive motor for each wheel. The computer can handle drive requirements as the vehicle turns.
It is cheaper and easier to manufacture electric motors than what it is to produce automatic transmissions and differentials. Also, electric motors are easier to repair. This is a very brief overveiw.
Jim
I agree that inverter drives offer big benefits -- for example the A/C compressor speed could "ramp up" at starting over a period of 2-sec or so, thus making the start very smooth and quiet. However, I haven't seen any clear reporting from the industry that inverters are a part of the 42V plan. 7937, have you seen specifics on this?
So instead of an inverter, I believe a small portion of electricity will be converted to DC, while AC (naturally produced by a generator) could be used to drive the wheels, a/c compressor, etc.
You would then have the engine run at a constant speed so the "Alternating" part of AC would be a consistent frequency.
I wonder what sort of break in challenges that provides?
TB
I myself am not involved in these developments but the company I work for supplies research equipment for laboratories which are involved in such research. I also meet researchers in the course of my duties and try and pry as much as my inquisitive self can out of these people. You must understand that a lot of the information is also (whisper) secret! I am deeply interested in power electronics and have always worked in such invironments.