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Any MVAC Automotive Air Conditioning Engineers?

Any MVAC Automotive Air Conditioning Engineers?

Any MVAC Automotive Air Conditioning Engineers?

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.

RE: Any MVAC Automotive Air Conditioning Engineers?

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.


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Any MVAC Automotive Air Conditioning Engineers?

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

RE: Any MVAC Automotive Air Conditioning Engineers?

Q ∝ ∂m/∂t * t

There are few choices to reducing t
> pre-cool the space to reduce the amount of Q needed to achieve the desired temperature
> increase the mass flow rate

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Any MVAC Automotive Air Conditioning Engineers?

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?

RE: Any MVAC Automotive Air Conditioning Engineers?

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.

RE: Any MVAC Automotive Air Conditioning Engineers?

In the world of electric cars you could just start the air conditioning a few minutes before entering the vehicle.

RE: Any MVAC Automotive Air Conditioning Engineers?

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.

RE: Any MVAC Automotive Air Conditioning Engineers?

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

RE: Any MVAC Automotive Air Conditioning Engineers?

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.

RE: Any MVAC Automotive Air Conditioning Engineers?

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?

RE: Any MVAC Automotive Air Conditioning Engineers?

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.

RE: Any MVAC Automotive Air Conditioning Engineers?

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.

RE: Any MVAC Automotive Air Conditioning Engineers?

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.

RE: Any MVAC Automotive Air Conditioning Engineers?

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

RE: Any MVAC Automotive Air Conditioning Engineers?

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.

RE: Any MVAC Automotive Air Conditioning Engineers?

3DDave thanks for those resources I am going to read through them now.

RE: Any MVAC Automotive Air Conditioning Engineers?

What was the outdoor temperature when that chart was recorded?

In an earlier post CyF16 stated


With R134a, the evaporator at 40 degrees F the pressure in the low side needs to be 35PSIG.

No. You need to factor in the superheat.

RE: Any MVAC Automotive Air Conditioning Engineers?

The bulk of the evaporator will be operating with a saturated mixture. Only the last small percentage will be in superheat.

CyFi6. Your low-side pressure compared to evaporator temperature should indicate pressure-drop across the EPR.

je suis charlie

RE: Any MVAC Automotive Air Conditioning Engineers?

Outdoor temperature was approximately 105 deg F. From the part you quoted me - what I was saying was that in order for a fully flooded evaporator (0 deg superheat) to be 40 degrees the pressure inside it would need to be 35PSIG with R134A.

Gruntguru - Why assume that the bulk of the evap is flooded with saturated mix? How do you know the evap isnt only 5% saturated and the rest superheated or vise versa? Also, the low side pressure port for my gauge is located between the evap and the EPR. I have no way to know the pressure in the suction line between the EPR and the compressor.

RE: Any MVAC Automotive Air Conditioning Engineers?

Evaporators are designed to operate predominantly flooded. Convective transfer to the coils is great from liquid refrigerant - pathetic from gas.

It would be good to know what is happening in that EPR - or bypass it somehow. Is there any metal surface that might indicate the temperature at the compressor suction? Perhaps use an IR thermometer to compare temp before and after the EPR.

je suis charlie

RE: Any MVAC Automotive Air Conditioning Engineers?

Quote (gruntguru)

Evaporators are designed to operate predominantly flooded.

Not for comfort cooling applications.

In comfort cooling, the evaporator coil is predominately saturated liquid-gas, transitioning to super-heated gas at the exit.

This direct expansion operation allows the coil to take advantage of the coil-to-liquid heat transfer and the latent heat of evaporation of the transport fluid.

The thermal expansion valve, or capillary tube, or orifice is selected to maintain super-heat at the coil exit to reduce the possibility of liquid entering the compressor. Because compressors do not like liquid.

RE: Any MVAC Automotive Air Conditioning Engineers?

What would be great to know is:

What model Lexus?
Is the compressor constant volume, cycled by a clutch, or variable volume?
How is the compressor cycling or flow rate controlled?
Is that control working properly?
How does is the system designed to protect itself from high pressure, and is that control working correctly?

The averagish high side pressures of 230 - 240 psig suggest a condensing temperature in the range of 140 - 145 F. Pretty high for the reported 105 F ambient.

As a rule of thumb the condenser coil could be expected to operate about 20 F above ambient.

So the system would seem to be condenser capacity constrained.

Or the condenser inlet air temperature is much hotter than the reported ambient.

Or the condenser fan is running backwards, blowing hot engine compartment air over the condenser.

RE: Any MVAC Automotive Air Conditioning Engineers?

The TXV is designed to maintain a certain level of superheat but we don't know that the TXV is operating properly necessarily and we don't know what the design capacity of the system is. Its possible that the heat load could get so great that even with the TXV wide open there is not enough refrigerant flow to maintain the desired superheat - if the system was designed for a certain max heat load.

I can measure the suction line temperature before and after the EPR but there is only going to be superheated vapor going through it, so I don't think you'd see any temperature change even if it was restricted.

This is a 1999 Lexus LS400. It has 24oz of R134a in it that was weighed in.
Constant displacement compressor with a clutch on it. The EPR valve is designed to prevent the need for cycling but it does have an evap thermistor so that the clutch will cycle if the evap did get too cold under extremely low heat loads.

There is a high pressure cutout switch that will release the compressor clutch if high side pressure gets too high.

When I mist the condenser down with water from the hose the high side pressure does drop but it doesn't make the vent temperature come down any faster. Driving on the freeway at high speed when the fan isn't needed anymore doesn't make it cool down faster either. There does seem to be some subcooling because the sight glass at the drier shows no or very few bubbles. So it seems the TXV is getting a good supply of liquid. Fan is definitely operating well and if I turn it off at low speeds the high side pressure will spike.

Since I am seeing a lot of superheat I am wondering if the TXV is malfunctioning, undersized for the heat load or maybe the EPR is choking of the suction line too much.

RE: Any MVAC Automotive Air Conditioning Engineers?


It's an aluminum micro-channel evaporator coil.

Exceed the design capacity and they stop working like cooling coils and instead start behaving like capillary tubes.

No cooling because all you have inside is bubbly froth.

Replace the evaporator with a fin-tube coil.

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