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Any MVAC Automotive Air Conditioning Engineers?

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CyFi6

CyFi6

Mechanical
Aug 20, 2021
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I've long been extremely interested in automotive AC system/design but have repeatedly failed to find any substantial information on the topic in books or other online research. I have talked to other engineers that worked for automotive companies and worked on AC systems but to my knowledge everything they did is held tightly by the manufacturers.

I live in the hottest climate in north America and I have long dreamed of modifying my cars AC system to pump out cold air consistently and quickly even in the sweltering heat. I don't care if I have to sacrifice fuel consumption, what I really want is extremely FAST cold air after starting up my car that's been sitting in the sun.

My Lexus I have now does an excellent jot after about 15 minutes of driving, but in the first 15 minutes it slowly pumps down the vent outlet temperature from a starting 150 degrees F down to 38 degrees F. I want it to reach 38 degrees F at the vent within minutes of starting the AC. I know its possible because I have been in other vehicles that could do it (2017 Ford Edge) extremely cold and fast. So my big question is - what's different? What makes one system able to do this and not the other? Both are in spec and operating at factory refrigerant levels, all components working properly, radiator fan working etc. Even the compressors and condensers don't appear to be much different in size at all. It just takes more time than I want in my Lexus to get the vent temp cold. Once it is cold it is very cold and maintains it even in sweltering heat.

My trips are typically less than 15 minutes so its annoying to not have ice cold air blowing when I jump in and out of the car for short trips.

I cant find any literature that goes into component sizing or what things affect pump down time. Do I need to increase the mass flow rate of the refrigerant? Do I need to calibrate the TXV differently? I just need help finding access to the technical information.
 
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I can pretend to be an HVAC engineer on the internet. The problem you've got is that the A/C is running flat out anyway. The whole system is specced for a certain performance, there isn't one magic setting that can be changed. We have the same problem in Australia, cars specced for Europe just can't pull the inside temperature down in any acceptable timeframe. The reason they fit an undersized a/c system is cost, fuel consumption and I suppose weight and packaging.

Bad analogy time -it's like walking into a cold house and setting the thermostat to 80 instead of 72, the house doesn't heat up any faster initially, the system just runs for longer.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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Still - it is puzzling that two vehicles with similar sized compressor and condenser have vastly different cool-down times.

If you badly need to investigate this, borrow a 2017 Ford Edge and do a back-to-back test on the same day after substantial heat soak for both vehicles. Have an expert AC tech connect a set of gauges to each vehicle in turn and monitor (and log) system pressures during the cool-down process. The results will indicate which elements of the system (compressor, condenser/fan, TX valve, evaporator/fan) are underperforming in your car compared to the Edge.

je suis charlie
 
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If you increase the mass flow rate, say by using a more powerful fan, don't you have a problem with the air not getting as cold?
 
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If it were me , the first thing to examine would be the diameter of the lines to / from the compressors. The larger the diameter the more refridgerant can be moved with equal sized HVAC systems in the two vehicles.
 
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ICE vehicles have had that ability for decades, its called a remote starter.

As you have discovered, cooling performance requirements vary by manufacturer as its not a matter of safety driving regulation. 15 mins however seems like an awfully long time to cool down a vehicle and isnt something that I would see any OE accepting in 2021 due to customer dissatisfaction. I would complain to the dealer and if they tell you that lousy performance is common, I would call the OE service hotline and elevate the issue with them. JMO tho, but this strikes me as more likely an issue with your specific vehicle rather than a model and could be something as simple as a fresh air vent sticking open.
 
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I had similar symptoms on one of my vehicles, air exiting the vents wasn't as cool as I'd expected, and it generally took a long time to cool down, this being in a tropical area with higher temperature and humidity. Turns out the compressor was dying. Replaced the compressor and performance was much better.

As per what CWB1 stated, this sounds more like an issue with your vehicle.

EDMS Australia
 
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Hi thanks for the information. A litte bit more info:

The compressor and condenser appear from the outside to be similar, though there are no specs listed regarding compressor displacement at various RPMs. Some aftermarket compressors like the ones from Sanden have a very basic RPM/displacement curve included but even those are not very detailed.

Increasing the mass flow rate of the AIR over the evaporator coil would just cause the air temperature out of the vents to rise (though the car as a whole would cool faster). But keep in mind that in a car the air is blowing directly on you from the vent so the colder the air the better even if its volume is slightly less. I am limited by the amount of heat the evaporator can pull out. At max load the system is running full tilt but the evaporator cant pull out enough heat to get the vent temperature down quickly.

Increasing the mass flow rate of the refrigerant through the system - I don't know what that effect has. I would assume a higher displacement compressor would allow the TXV to open more (flood the evaporator) while also drawing the low side pressure down further (lowering the saturation temperature of the refrigerant enough to make the evaporator colder)and reducing the vent temperature. But the resultant increase in heat absorbed into the refrigerant would need to be balanced by a greater ability to reject heat at the condenser. So lets say I want to increase the compressor displacement by going with a larger compressor, how do I determine how much more condenser I need to balance this out? How do I determine how big the TXV orifice needs to be to balance this out? These are the things that MVAC engineers must do daily but I cant find any text that goes over these sorts of calculations, so I would just be guessing.

Right now when I log my pressures and temperatures, starting from a fully heatsoaked vehicle it takes 10-15 minutes before the low side pressure can drop low enough to produce 40 degree air out of the vents. With R134a, the evaporator at 40 degrees F the pressure in the low side needs to be 35PSIG. The TXV is there to maintain superheat so if it were to close down the pressure would go down but the superheat would go up (refrigerant would all boil off before reaching the exit of the evaporator and the evaporator outlet would be higher than saturation temperature/superheated). If the TXV were to open more the low side pressure would rise and saturation temperature would go up causing the evaporator to run warmer. What I *THINK* I need to do is increase the TXV size to allow more refrigerant through AND use a larger pump to move enough refrigerant that it can still drive the low side pressure down to 35psi even with the increased flow from the larger TXV orifice.

Once the low pressure drops low enough while maintaining the correct superheat it can produce nice cold air. If I leave the air door on fresh instead of recirc, I will never get 40 degree air from the vent since outside air temp is 100+ degrees. I know for sure the fresh air door is functioning properly. I also know that the heater is not introducing heat. The vent temperature correlates to the low side system pressure as I would expect it to, but it takes too long to pump the low side down. Any time spent at idle just increases that time further since low side pressure rises at idle (low compressor speed, less airflow over condenser).

I know the compressor is in good working order because I can block airflow to the condenser and I can watch as high side pressure goes up to 300+psi. A weak compressor wouldn't be able to pump up this high.

It seems I am just exceeding the design capacity of the system, I just don't know how to increase system capacity. I have already upgraded the condenser fan to a hydraulic type which pulls a lot of air through the condenser.

I already have a remote starter on the vehicle but when the cabin is 150+ degrees and the AC is running at idle it really takes a long time to pull down the temperature. You really need to be driving around to get good cooling.
 
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GregLocock (Automotive)

I agree with you totally. What I want to do is increase the capacity of my system since it seems I am exceeding what it was designed to do. Unfortunately they have to make cars that work all over the world for all different climates so they are not going to optimize them for either of the extremes.

My question is if I were to build a whole new system myself how do I pick each component to know that it will have the capacity to achieve what I want?
 
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miningman (Mining)21 Aug 21 19:52
If it were me , the first thing to examine would be the diameter of the lines to / from the compressors. The larger the diameter the more refridgerant can be moved with equal sized HVAC systems in the two vehicles.

If the line diameter were too small as you imply then I would expect there would be a pressure drop across the length of the line. A pressure drop would correlate to a temperature drop assuming its full of refrigerant. I dont have any noticeable temperature drop from one side of the line to the other which tells me there is no significant restriction due to line size.
 
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It's possible that the compressor has an unloader to prevent it from cutting out completely from high pressure when condenser pressure is already high and evaporator load is high.

Does the low side pressure fall slowly slowly slowly? Or at some point does it make a big sudden drop?

A factory service manual might have enough information to give some insight about how the system is supposed to work.

Free and easy things to try: Start with the evaporator fan on LOW. Drive in a low gear.
 
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No sudden changes in low side pressure. I have a datalog that I will attach to this post. This was about 15 minutes worth of driving and you can see the vent temp never actually went below 45 degrees. The high side pressure correlates mildly to driving conditions. Goes up when vehicle slows/stops. This is city type driving where the vehicle is stopping and starting repeatedly.
Capture_osd8yl.jpg
 
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As you say the heat and airflow capacity of the heat exchangers looks OK.

Next step - repeat your test with the engine free-reving at about 50% of redline. If there is a big improvement in cool down time and high-side pressure is still reasonable, you can fit a bigger compressor or simply spin up the one you've got. Check the max speed for this model and see if its safe to change the pulley ratio.

je suis charlie
 
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I don't have my pressure sensors hooked up anymore, but running in low gear (cruising about 3000 RPM instead of usual 1500-2000RPM) doesn't seem to make any noticeable difference in cool down. I also evacuated and recharged the system by weight just to be sure the charge level is correct and the performance is the same as before.

This system does have an EPR (evaporator pressure regulator) which I want to make sure is working properly. The EPR is a mechanical valve located between the outlet of the evaporator and the inlet of the compressor on the suction line. Its job is to maintain a minimum pressure in the evaporator to reduce compressor cycling and have a constant temperature from the vents (instead of cold warm cold warm). There is a capillary tube that goes all the way from the TXV under the dash out to the engine bay at the EPR. I believe that the EPR is spring-closed and pressure-opened. As evap pressure falls the valve will close off to increase the pressure in the evap to keep it from freezing and prevent the compressor from having to cycle to keep the evaporator temperature above freezing.

I have the factory service manual for the vehicle but I don't see anything about testing the EPR. My assumption would be that if the EPR was closing when it wasnt supposed to I would see a much colder temperature after the EPR than before which I am not seeing, it is about the same temperature before and after the EPR (by feeling with my hand).

Any ideas? I have considered bypassing the EPR entirely (gutting it) but I am worried that could cause liquid refrigerant to enter the compressor and damage it. If this valve was stuck (perhaps in the middle of its range) I could definitely see it causing the evaporator pressure to be too high when under max heat load.
 
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