AC factoid

[joshua43214]

I am writing this since we are hitting AC season, and I have some to kill at the moment.

---components of the AC system
Compressor; mounted to the engine,
Evaporator; under the dash, basically a long tube coiled back and forth with fins on the outside, looks a bit like a heater core.
Condenser; in front of the radiator, same basic design as the evaporator, but much larger, looks like a radiator.
Receiver/dryer; under the hood near the condenser.
Expansion valve; mounted at the evaporator inlet.
Auxiliary fan; mounted in front of the condensor.
High/low pressure switch; on the receiver dryer.

---Basics of operation
For purposes of simplicity, I will use the word "r12" for all refrigerants and treat it like it is always a gas so I won't have to get into discussions of superheating and the like.

The r12 flows through the components like this; compressor > condenser > receiver/dryer > expansion valve > evaporator > compressor.

The low side of the system encompasses the following parts; compressor, evaporator, expansion valve. Low pressure (suction) hose.

The high side encompasses the following parts; compressor,condensor,dryer,expansion valve. High-pressure hose.

AC works on the principle of an expanding gas freezes. When the system is off, the r12 has a uniform pressure through out the entire system. When the compressor starts, it draws the r12 from the low side and compresses it into the high side. The expansion valve creates a restriction that the r12 has to pass through in order to enter the evaporator. Since the compressor is attempting to create a vacuum on the low side the r12 will "spray" into the evaporator and rapidly expand to fill the area cooling dramatically in the process. As outside air passes through the fins on the evaporator, it absorbs the heat and transfers it to the r12.

This r12 that has absorbed the heat is then drawn into the compressor and highly compressed and passed through the condenser, where its heat is then transferred to the outside air with the assistance of the auxiliary fan. The r12 then passes through the receiver/dryer where it is filtered of debris and any moisture is absorbed by the desiccant inside it.

The expansion valve has a plunger that varies the amount of r12 entering the evaporator.

On most BMW's the compressor is operated by the DME, this is done so that the compressor will not operate when the car is overheated, and so the compressor can be shut off at wide open throttle. The fan speed and vent positions are operated by the panel and climate control module.

---
In order for the system to operate, every thing must work together and be self-correcting to a degree. If too much r12 is allowed to pass through the evaporator, the fins can freeze solid from the condensed moisture in the air. If there is no air movement through the condenser, the highly pressurized r12 can make hoses explode and even make weak engines stall from too much load.

To prevent damage to the system, a high and low-pressure switch is installed that will only allow the compressor to run if the pressures are within a certain range.

---Diagnosing AC malfunction

This can be divided into two categories; compressor runs but does not cool enough, and compressor does not run.

If the compressor runs, but you are not getting enough cold air, most likely the car is low on r12. If the dryer is equipped with a sight glass, you can attempt to recharge it yourself. Be warned, more is not better when it comes to r12, it is easy to over charge a system trying to get all the bubbles out, this will at the least reduce performance and at the worst, destroy the compressor. With out hooking up a set of gauges, it is only guess work as to what is really happening. Also, r12 leaks often leak oil as well, large leaks always do. If you lose too much oil, the compressor will fail. If you add too much oil, the performance will be greatly reduced and you can also destroy the compressor. Any time a part is replaced, the proper amount of oil needs to be added as well(no oil for expansion valve replacement). Expansion valve failure is a very common cause of reduced AC performance, and can make using a sight glass inaccurate, easily over charging the system. Also AC dye's use oil as a carrier, so repeated charging with dye will reduce performance or damage the system.

If you are experiencing AC performance or other troubles and the compressor turns on, then the problem is almost for sure related to the systems r12 charge and is not electrical. Compressor short cycling is almost always from undercharge. A performance check should be run before any electronic diagnosis is performed.


If the compressor does not run, either the system is too low on charge to activate, or there is an electrical problem. To verify proper charge, one needs hook up a gauge(in the USA, its the law). Using a torx driver, briefly depress the shraeder valve, You should get a big blast of r12 in your face if it is properly charged, go to the doctor and get treated for frost bite, then return to diagnosis. Don't do this where it is illegal.

There is a llot of variation in the AC wiring across the e34 line. So some quality time with a wiring diagram for your specific model may be required. The components listed above are common to all e34's. Some of the older models had 3 pressure switches on the dryer rather than a single high/low switch.

Simple things to check;
Check all of the fuses with a test light on the back of the fuse.
Check for corrosion at all the connectors.
Make sure the connector at the compressor is tight; it can rattle off, causing intermittent connection.
Using a wiring diagram, bypass the pressure switch and see if the compressor runs.
Verify you have voltage to the various connectors, do this with the ac on and the engine running.
Tap the top of the compressor relay with a screwdriver handler, if the compressor comes on replace the relay.
Check ac belt tension, it can slip and not make any noise. If it slips and the tension is good, check the aux fan for proper operation.
Make sure the compressor clutch is not slipping.
Verify the heater water valve is functioning properly.

Things to remember;
The blower is on a separate circuit, the blower can run with no compressor and vice versa.
The compressor will shut off after a short while if the blower is not working.
The compressor will shut off after a short while in the auxiliary fan is not working.
The auxiliary fan is pressure operated on some models, so it will not come on until the pressure is high enough.
Reduced air volume is usually caused by a clogged cabin filter.
Reduced airflow through the condenser will cause the system to leak faster.
Do not operate the system with components bypassed for any longer than absolutely necessary for diagnosis.
R134a and r12 use different types of oils.
If the system is empty, it's a very good idea to replace all the o-rings. Lube them with a small amount of r12 oil before installation, pag oil is not recommended for this.
Carefully clean any corrosion around the o-rings, use a wooden or plastic scraper if needed.
Slightly undercharged is better than slightly overcharged.
The under hood sticker will have all information pertaining to proper refrigerant and the amount to be used, ALWAYS use this reference first before all others.
Submerging cans of r12 in hot water will aid charging a great deal. I don't recommend a torch, no matter how tempting.
The system must be under vaccum before it is charged if it has been open to the air. If it has no pressure at all, it is best to take it to a shop for charging.
Make sure the service caps have the o-rings installed; the cap is the primary seal, not the schraeder valve. Get new caps if the old ones are missing.
When using a sight glass, check for bubbles with the idle at about 1500rpm. It is normal for few small bubbles to pass through when letting off the throttle, or even at idle in the right ambient temperatures.
When converting to r134a, it is best to replace the hoses since most r12 hoses will leak r134a. Drain as much of the old oil from the system(but not the compressor) as you can. And always replace the receiver dryer.
Never use AC sealer on systems with expansion valves or any other system with a capillary tube, it can clog the tube or cause the plunger to stick. This stuff is best used on orifice tube systems, and even there I don't recommend it.

Many AC problems can be fixed by the DIYer. Having someone walk you through the process of charging is a good idea if you have never done it before. Techs are often sympathetic to customers about AC since it is so expensive, and many techs prefer not to do AC service at all. They can be a wonderful source of tips and pointers, and can advise you on when its a good time to cut your loses and pay the shop for the service. Shops will sometimes even evacuate your refrigerant for free since its free parts for them, especially if you use r12, the stuff is quite valuable.

Having a professional service AC can get very expensive very fast. Many shops charge a premium partly because of the comparatively high comeback rate, and partly because both the equipment is extremely expensive and so are knowledgeable techs. It is common practice in many shops to simply service the AC, fix anything that shows up as bad, shoot in some dye, run it for a while and do a fast look over for leaks. Then they give the car back and tell you to bring it back if it stops working so they can check for dye traces. This method has proved to be more economical for the shop, but hurts the customer by having to pay for a service two times. Not to mention two visits to the shop for the same problem. Some shops will still use electronic leak detectors and actually have the patience to use them properly, these shops will charge a bit more for the service usually, but it is often money well spent if you are paying someone. Also, many shops will only cover warranty costs on hard parts and their labor, not the r12 itself or the labor of the evacuate and recharge.

Any corrections, feedback or flames welcome.

[Dave M]

Holy flying $hitballs. A 'bit of time to kill'?

I don't get that much work done in a day.

Since I removed my AC system, I didn't feel obliged to read it all, but i'm sure it was captivating.

Nice work ;)

Dave M

Edit: Isn't the aux fan mounted in front of the condensor (rather than the evaporator)?

[Blitzkrieg Bob]

Expensive if you stick to r134, and aren't an environmentalist.

a manifold, thermometer, vacuum pump.

You'll end up fixing a bunch of A/C s for all you friends and family

[SRR2]

Heh heh. No explanation of what the expansion valve does. You chicken! 8-)

Your writeup would benefit, IMO, by including the notion of liquid refrigerant, the evaporation and condensation processes, and at least a mention of latent heat of vaporization. After all, latent heat is how liquid refrigerant at 110F can cool incoming air down to 40F.

[Blitzkrieg Bob]

That reminds of a Physics professor that could drone on for hours about refrigeration and how efficient the thermal transfer is, measuring the BTUs of each side, put hot food in, measure the BTU rise on the condenser…Etc. Etc…Etc.. and the only loss was running the pump. Or if you got him going on an L.P. gas refrig, then the whole lecture was shot.

[zygoteer]

Thank you .. a cogent explanation. :)

[joshua43214]

Heh heh. No explanation of what the expansion valve does. You chicken! 8-)

Your writeup would benefit, IMO, by including the notion of liquid refrigerant, the evaporation and condensation processes, and at least a mention of latent heat of vaporization. After all, latent heat is how liquid refrigerant at 110F can cool incoming air down to 40F.

LOL every attempt I made to write an acurate description of the expansion valve got really technical. I had started with a nice desription of the entire liquid to gas to liquid process, and how superheated refrigerant behaves. But it got all boged down with definitions and further explanations and the like. Since I was aiming this at the layman and the DIYer, I decided that it was beyond the scope of the article. The professionals already have been to the workshops and don't need my help.

I could picture peoples eyes glazing over and then skipping to the next article. It is interesting stuff, and I think most of the forum members here would enjoy learning it, but the article would have been 1/2 again as long.

[GoldenOne]

could you post expansion valve explanation anyway, im curious to see what it does...

[SRR2]

True, true.

However, you have those knuckleheads that tell noobs that "all you need to do is go get a set of gauges, and that will tell you how much refrigerant you have in the system" You must have seen posts like that. At least one pass through the expansion valve might convince both of these parties that they just don't know enough about what they're doing to attack a refrigeration system, much less one that's as touchy as recent R134a systems. Example: the charge in my E39's system is 750g +-10g. Yep, TEN. Now how is Clyde Clueless gonna manage to achieve THAT, with his set of gauges from Harbor Freight?

So that's what I'm suggesting: scare them off with a pithy paragraph on the expansion valve.

Related story: Due to a poor installation, I've had quite a bit of trouble with the Trane heat pumps in my home. I've going through three "authorized" dealer service departments in trying to get them working right. "Professional" #2 was here a few years ago and overcharged the systems so far that the compressors slugged under some conditions (When the reversing valve switched off defrost. It was an awful sound.) and eventually killed the valves. I should have known because I found him using charging curves (my system has expansion valves in both heating and cooling modes) for systems with capillaries. I kid you not. After "Professional" #3 came in and changed out the compressors I took nothing for granted. *HE* left both systems overcharged by almost 1.5 lbs. *sigh*.

[joshua43214]

could you post expansion valve explanation anyway, im curious to see what it does...

OK, the simplified explanation of what is really happening.

---Basic theory

When you raise the temperature of a liquid to its boiling point, you are adding energy in the form of heat from an

outside source in the form of heat. The point at which it changes state from a liquid to a gas is called a

transition point. A liquid has its molecules loosely bonded to each other, it requires energy to seperate thse bonds

allowing it to become a gas. So as you add energy to a liquid at its transition point, its temperature will not

rise. Rather that energy is used to seperate the bonds in the liquid allowing it to become a gas. Once the substance

has become a gas, you can continue to add energy and its temperature will contine to rise as normal. This bit of

energy that was used to loosen the molecular bond allowing the liquid to become a gas is called "latent heat". Any

time any substance changes state, this energy cost of loosening the bonds of the molecules to each other must be

paid.

Now take the same liquid and place it in a vacuum chamber. As you lower the pressure (raise the vacuum), the

molecules of the liquid will be mechanicaly forced appart into a gas. The energy required to change state is taken

from the liquid itself. This will dramaticaly lower the temperature of the liquid as it gives up its engergy. If you

take this same gas and compress it, the energy used to change state back into a liquid be taken from the gas's own

latent heat, leaving you with a liquid of the same temperature as what you started with. In theory, this works out

as a zero sum for energy gain/loss, since all the energy was provided from out an outside source.

---Operation

The r12 is compressed into a liquid on the high side of the ac system. As the pressure on the low side is decreased,

the liquid passes into the evaporator and begins to change state into a gas. The liquid r12 becomes very cold. Once

the temperature of the liquid r12 falls below the ambient temperature, it will begin to absorb ambient heat, or

energy. This energy is then used to to pay the energy cost of changing state from a liquid to a gas and is "carried"

along the r12 molecule as latent heat.

If the system is properly balanced, almost all of the liquid r12 will have become gas as it reaches the end of the

evaporator. this is important since the gas will not carry oil with it, The compressor depends on the small amount

of liquid r12 to carry oil into its suction side. When raw liquid enters the compressor, it is called slugging and

can destroy it. Car compressors can tolerate a bit of abuse in this department, but they are really only designed to

handle the relatively small amount of liquid in the low side when you first turn it on.

What happens of course, is that on hot days the r12 becomes a gas before it reaches the end of the evaporator and on

cool days, there is still alot of liquid at the end.

Normaly with any gas, you would cool it to make it into a liquid. and heat a liquid to make it gas. The AC system

can compress a gas into a liquid that is far above its boiling point, or convert a liquid into a gas far below its

boiling point. Regardless of how the change in state was achieved, energy must be provided from an outside source.

---Enter the expansion valve

Since the energy absorbtion of latent heat is so efficient, idealy you want any heat absorbtion of the r12 to be

used in converting liquid to gas. In practice of course, the r12 gas is absorbing energy at the same time the liquid

is as it is changing state. Once the r12 has boiled off into a gas, any energy it absorbs is called "super heat",

since there is always a certain amount of energy absorbtion by the gas, there will always be a certain amount of

superheat. The expansion valve is calibrated to react to this superheat.

The thermostatic expansion valve is an alluminum block mounted on the inlet and outlet of the evaporator. it has

what looks like a small flying saucer mounted on its end. this saucer is a thermal diaphram. At rest, the expansion

valve is open to a set point that is calibrated to the system size and the refrigerant used. when the system

operates, liquid r12 on the high side forces the valve open, this is countered by the thermal sensor in the flying

saucer. When there is too much r12 passing into the evaporator, liquid will exit the suction side. This low

temperature liquid will cause the diaphram to pull down lowering the flow rate on the pressure side via a plunger.

If there is not enough r12 entering the evaporator, the high temperature gas will cause the diaphram to move up,

allowing more r12 to flow on the pressure side.

To a limited degree, one can test the reaction of an expansion valve by heating or cooling it with a hot gun or a

cold gun (yes, there is such a thing, the old timers who worked on carbs have them rotting in their tools box's).

These valve are sensitive to contamination from debris and commonly get clogged by deteriorating hoses or

reciever/dryers. It is also important to keep them insulated from ambiant air, this is why they are covered in

dumdum. Since they are calibrated for the refrigerant used, converting a r12 system to r134a does not always have

good results since r134a has different thermal saturation characteristics. r12 expansion vlaves can be used on r134a

system, but often require a fair amount of "tweaking" the charge for best operation.

---The other end

After the r12 is drawn from the evaporator, it is forced into the condensor where the thermal exchange is reversed

and the latent heat that was absorbed from the evaporator is transfered to the condensor and radiated to the

atmosphere. The liquid r12 is far above its boiling point even after leaving the condensor, the only thing

preventing it from cavitating is pressure, this is why it is normal to see a small amuont of bubbles in the sight

glass at idle.

Hope all this makes sense, I just fired this off since it was requested and didnt proof read it or anything. For

those physics nuts out there, I appologize for my over simplification of thermodynamics, it should be correct on all

the important parts. Any corrections are greatly appreciated, I only know this stuff accidentaly since I never took

physics.

[Blitzkrieg Bob]

the lectures full of BTUs, Jewels, efficiency ratios and black lab tables.

[SchnellE34]

Joshua,

This is great thank you very much. I am planning on fixing the A/C system myself but knew nothing about it before I read this thread. After a dye test I was told that the condensor is leaking (hopefully from a seal). Right now I am taking notes on an enlarged copy of the A/C system from Bruno's website, haha. No questions so far. But after I get a look at my own system I am sure I will have a few. Again thanks, and keep up the good work.

-cP

[myles]

Great write up - I'm keeping that on record for when I need it

[genphreak]

I just love this kinda thing- good practical explanation from someone that knows, applied to a specific application.

Don't worry one bit if it gets too complex on the expansion valve, people have to learn something every now and then or they will keep going to bed just as stupid every night...

Thank you Joshua, for demistifying a vital part of our cars for us- a part that many mistreat through ignorance and/or disrespect. :) Nick

[genphreak]

What an awesome contribution Joshua, thank you.

You got me thinking tho (a dangerous thing!), so if I may I'd like to ask your thoughts on an idea I had some time ago, but have been unable to validify so far...

When A/C is on the compressor works to maintain constant pressure in the system to enable cooling, if needed by the cabin enviro system which in our case is thermostatically controlled :). To do this the compressor is turned on and off as you have said by the pressure switch, which is over-riden by a Wide Open Throttle (WOT) condition is noticed by the Engine Control Unit (ECU).

It seems to me (living in a hot country and using AC regularly), especially with us all facing rising oil prices, that it would be smart to try to use as much as possible of the the kinetic (or rolling) energy the car has attained rather than only ever loading the engine when AC compression is required.

Many times I notice the compressor load the engine on acceleration and cut off when I start to coast or brake... and indeed the same can be said for encouraging alternators to charge batteries when the car is engine braking instead of when they are under acceleration.

However in respect to the AC; if the ECU can disable the compressor to limit power loss when the driver has requested all that is available (ie under WOT), would it not be smart to have this occur whenever the engine is accelerating in the interests of saving gas?

Although it is not completely effective, AFAIC tell this should have the effect that;

When driving in traffic(as most drivers do most of the time);
it would in effect power the AC compressor primarily from the energy required to slow the car down, energy that otherwise burns up brake pads or is taken the engine (or fuel powering it) directly. Of course if one were to spend 20 minutes climbing a mountain it would suck as the AC would not come on at all. Maybe it could be disabled by by the driver, a timer or an inclination switch in this instance. If a driver-controlled switch were in place we'd have more than AC on and AC off; we'd also have an AC 'Traffic mode'.

Really this could actually be a 'Hill mode' if the economy mode I describe could work as the default... and such smarts were not able to be built in automatically in the first place.

I would have thought that manufacturers would be employing this sort of thing already... at least to reduce AC load on the engine when it is running under inefficient speeds or when braking... maybe ECU's do more than I know (especially on new cars) to limit compressor operation... do you know?

For example, I know on many Toyotas there is an 'AC' on, off and 'Econ' mode, however the 'Econ' mode does not limit compressor operation under acceleration in any way... (its easy to tell in a shitty 4-cylinder car). Maybe it does it some other way? I can tell you that Econ mode does not cool the car as well.

Anyway, if this idea has legs, it seems to me that this could be a mod we could all do that might save fuel...

Many thanks for your thoughts... :) Nick

[joshua43214]

the expansion valve does much of what you describe, except it is not "smart" about when to cycle the system. Older style ac system used a fixed orifice tube. These systems controlled the r12 flow into the evaporator purely on pressure by cycling the compressor on and off. This causes an irritating surging of power that was very noticeable on low powered cars. The compressor also ran for longer periods. Once a car with an expansion valve reaches its set temperature, the compressor will normaly run for a brief period then shut back off unless the ambient temp is very high.

The introduction of OBDII required that manufactorers add alot of diagnostic power and feedback systems into our cars. These same systems allow the manufacturer to have better control over what is happening. Any OBDII car with a MAP sensor and a TPS, could in theory be reprogramed exactly as you speak. It would be a bit more difficult with a MAf sensor car, since the ECU bases load on a larger set of parameters. Most modern cars actualy turn off the compressor closer to 2/3 throttle rather than WOT, you can sometimes feel a sudden surge of power at high throttle when the ecm decides to turn off the compressor. Or the ecm will shut off the compressor any time a rapid movement in the TPS is seen. Some cars use electronicly controlled expansion valves as well. Some cars actualy have a tilt sensor that tells the ecm when the car is going uphill or actualy towing something.

The reciever/dryer does act as a storage device for unused liquid r12, and allows a more constant flow of r12 into the evaporator, but that is only a temporary fix, the heat needs to be removed by compressing the r12 into a liquid, this is where the most of the engine load goes. It requires alot less power to pull a vacuum, than to compress a gas into a liquid. So the real trick is dealing with engine load as it relates to the high side rather than the low side. The logical conclusion of this mental exercise would be to use an electric motor to power the compressor and allow a battery to store the engergy required to run it, this battery could then be recharged while the car is on decell and and actualy aid in brakeing the car. At this point, it makes sense to make the electric motor big enough to help power the car, thus reducing the size of the engine required to operate it. Thus is born a Hybrid. I am surprised that we never saw electric powered compressors in the '90, I can only assume that the combnation of added weight, increased complexity, and higher demand on the generator made it the systems either cost prohibitive or returned a net loss in overall performance.

Fun mind game, btw "econ" is usualy a fancy way of saying recirc.

[Bill R.]

early nineties were trying to come up with a suitable electic compressor for auto use... Toyota was the leader in this research. But not for the reasons that you think. They were trying to come up with a sealed hermetic compressor for auto use like you find in a home air conditioner. It was to get rid of the belt driven compressor and its potential for leaks around the shaft seals. This was a big issue when r12 was on its way out, the manufacturers were trying to come up with systems that leaked less gases to the atmosphere, my day job for 13 years until 91 was rebuilding commercial a/c and refrigeration compressors 1/2 ton to 500 ton and we were following this with much interest at the time. Toyota wasn't able to come up with a practical electrically driven compressor at the time. But now with the latest 2004 prius hybrid they are able to use their electric compressor for the a/c.. Its a 201.6 volt AC powered compressor and its also a variable scroll compressor. Strictly electric and sealed so less chance of refrigerant leaks. Also since its 201volts it requires a special insulating refrigerant oil that must be used... i can see problems in the future when idiots put regular pag oil in one and get electrocuted







the expansion valve does much of what you describe, except it is not "smart" about when to cycle the system. Older style ac system used a fixed orifice tube. These systems controlled the r12 flow into the evaporator purely on pressure by cycling the compressor on and off. This causes an irritating surging of power that was very noticeable on low powered cars. The compressor also ran for longer periods. Once a car with an expansion valve reaches its set temperature, the compressor will normaly run for a brief period then shut back off unless the ambient temp is very high.

The introduction of OBDII required that manufactorers add alot of diagnostic power and feedback systems into our cars. These same systems allow the manufacturer to have better control over what is happening. Any OBDII car with a MAP sensor and a TPS, could in theory be reprogramed exactly as you speak. It would be a bit more difficult with a MAf sensor car, since the ECU bases load on a larger set of parameters. Most modern cars actualy turn off the compressor closer to 2/3 throttle rather than WOT, you can sometimes feel a sudden surge of power at high throttle when the ecm decides to turn off the compressor. Or the ecm will shut off the compressor any time a rapid movement in the TPS is seen. Some cars use electronicly controlled expansion valves as well. Some cars actualy have a tilt sensor that tells the ecm when the car is going uphill or actualy towing something.

The reciever/dryer does act as a storage device for unused liquid r12, and allows a more constant flow of r12 into the evaporator, but that is only a temporary fix, the heat needs to be removed by compressing the r12 into a liquid, this is where the most of the engine load goes. It requires alot less power to pull a vacuum, than to compress a gas into a liquid. So the real trick is dealing with engine load as it relates to the high side rather than the low side. The logical conclusion of this mental exercise would be to use an electric motor to power the compressor and allow a battery to store the engergy required to run it, this battery could then be recharged while the car is on decell and and actualy aid in brakeing the car. At this point, it makes sense to make the electric motor big enough to help power the car, thus reducing the size of the engine required to operate it. Thus is born a Hybrid. I am surprised that we never saw electric powered compressors in the '90, I can only assume that the combnation of added weight, increased complexity, and higher demand on the generator made it the systems either cost prohibitive or returned a net loss in overall performance.

Fun mind game, btw "econ" is usualy a fancy way of saying recirc.

[genphreak]

... also since its 201volts it requires a special insulating refrigerant oil that must be used... i can see problems in the future when idiots put regular pag oil in one and get electrocuted:D Evolution takes many forms Bill, many forms! Some of us will even die driving our e34s- and some of us perhaps under them!

Thanks for filling me in fellows- I was also just thinking that a potential problem with the Economy mode approach might be that the system is obviously not always going to maintain it's effective operational pressure - as obviously the compressor is sometimes going to be delayed from turning on. Maybe this would negate the fuel saved by biasing the compression cycle to use power that is normally dissipated by the brake pads.

Have always wondered why something wasn't in place to bias the alternator to charge the battery on braking more than acceleration too- as you say Joshua, add more battery and thus use it for things the petrol could end up powering (such as AC, power steering (http://www.antonov-transmission.com/alternatordrive.htm), cup-warmers, In-traffic entertainment systems, massage seating and other parts of the house people like to take with them in their cars these days... 'cos being in traffic is heaps more fun than the house (as peeps in LA and major cities worldwide know so well. A great way to calm the masses- cacoon them in a cell, reduce oxygen, increase carbon monoxide, introduce parroted concepts they must conform with (ie shock jocks on the radio) and watch your votes climb each time you blame the minorities and evil foreigners). It is a bit like automotive hypnotism- heh, combined with 3 hours recuperation time in front of the toob every night it's probably more powerful stuff than any of us realise!

Anyway, ... back to improving an e34's economy!
An e34 could fit a lot of batteries in it- more than the average car: Perhaps 4 under the rear seat alone- a decent flat-pack of cells could go in the 'AK-47 compartment' and a few more in the trunk if load space was not to be conserved. But these would be costly and heavy and would thus reduce performance and efficiency.

However surely making an alternator that helps drive the car (how many hp could be added at the serpentine pulley I wonder) could be a good way of bringing older cars a tad closer to the hybrid style. Maybe having more battery available would allow one to accelerate to a say 5mph without opening the throttle on the gas engine (ie it's running at idle only) as the generator is assisting the engine when running in its less efficient rev-ranges. I am sure the petrol engine would be at its most inefficient at low speeds (plus heats up a lot) so the savings could be quite good...

Now apart from serpentine belts wearing out quicker, this would surely shave a fair amount off fuel bills- especially when driving moderately in traffic.

However, back to the real world:

I don't know why manufacturers never went for the few percent of gains they could have got by cleaning up their battery and AC compression systems, especially with smarter ECM systems as you describe, but obviously they have thought about it and some have done some of these sorts of things.

I'm still thinking of a cheap and simple way to save a few dollars a week with Frieda (as she does love to drink you know).

So I' might start work on a circuit and some wires to smarten the system up; as you say it could be a good idea on this car as it is not OBD-II. Tho from what you say Joshua, perhaps there are many OBD-II cars that would benefit too. I bet (as a starting point) if my compressor was only allowed to come on at zero throttle by the TPS it would screw up my cabin temperatures really badly. However if the cabin thermostats were used to overide this to keep one comfortable, perhaps the compressor would use less power under throttle and bias the AC loading towards the car under braking conditions. As Frieda has an automatic transmission it is simpler to test the outcomes of all this. There's a few options here- I will do some testing. I'll need to find some of those sensors you describe modern cars using :p... perhaps not easy, but it could be result in a good thing to have available to us.

[Blitzkrieg Bob]

I'm going out to have a few pints, with the hope, that maybe when I come back this will as make sense.

Naw, but the pints will.....

Cheers

[BillionPa]

Auxiliary fan; mounted in front of the evaporator.

... you mean the condenser?

[BigKriss]

subscribed

[joshua43214]

... you mean the evaporator?

Doh, that was obiviosly a silly mistake, I fixed it.

Thanks

[joshua43214]

Sounds interesting Genphreak, its even possible that there is something from the performance world that could be modified for this. Maybe some kind of control system for NoS exists that turns it off on no/low load situations.

Keep us posted.

[genphreak]

... maybe some kind of control system for NoS exists that turns it off on no/low load situations. Keep us posted.Good thought Joshua, I'll check NoS vendors out. In the meantime I have to work out a way to feed a 12V signal form the thermostats- they drive the flaps in the climate systems, not the fans :( but maybe I can fit a microswitch or find some form of activated feed in the circuit I haven't though of yet... thanks for getting me thinking on this.

Let you know more I will. :) Nick