Any MVAC Automotive Air Conditioning Engineers?

[CyFi6]

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.

 

[MintJulep]

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.

 

[gruntguru]

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

 

[CyFi6]

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.

 

[gruntguru]

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

 

[MintJulep]

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.

 

[MintJulep]

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.

 

[CyFi6]

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.

 

[MintJulep]

Oh!

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.