2002 Civic Si – Evaporator temperature sensor

Friendly format provided to inquire about automotive a/c systems.
Archived Forum

Moderators: bohica2xo, Tim, JohnHere, Dougflas, HECAT

Post Reply
Charles5654
Posts: 14
Read the full article
Joined: Sun Oct 18, 2020 5:13 pm

2002 Civic Si – Evaporator temperature sensor

Post by Charles5654 »

My understanding is that the TXV is controlled by a sensing bulb that sits at the exit of the evaporator to sense the superheat.
What is the purpose of the evaporator temperature sensor that connects into the electrical system of my Civic?
Thanks
User avatar
JohnHere
Posts: 329
Joined: Sun May 13, 2018 10:20 am
Location: South Carolina Upstate - USA

Re: 2002 Civic Si – Evaporator temperature sensor

Post by JohnHere »

Charles5654 wrote: Tue Mar 23, 2021 5:15 pm My understanding is that the TXV is controlled by a sensing bulb that sits at the exit of the evaporator to sense the superheat.
That's true with a TXV that uses a sensing bulb. Self-contained H-block TXV's do essentially the same thing but have internal sensing. However, superheat (and subcooling) are not typically used in mobile vehicle A/C systems due to their constantly changing conditions, as pointed out in other posts. The refrigerant amount in a MVAC system is charged exclusively by weight.
Charles5654 wrote: Tue Mar 23, 2021 5:15 pmWhat is the purpose of the evaporator temperature sensor that connects into the electrical system of my Civic?
Unlike a (mechanical) TXV that regulates refrigerant flow, the (electrical) evaporator sensor or thermistor monitors evaporator temperature, turning the compressor clutch on and off at certain points so that the evaporator doesn't freeze up and block airflow.
Charles5654
Posts: 14
Joined: Sun Oct 18, 2020 5:13 pm

Re: 2002 Civic Si – Evaporator temperature sensor

Post by Charles5654 »

Thanks for explaining the function of the electrical evaporator sensor.
As far as the type of TXV in my 2002 Civic SI, I am not too sure at this time. I plan on taking a picture of the TXV the next time I have the panels that cover the TXV off.
From the attached 3 photos of a 2000 Civic (that I would expect to be similar to my car with regard to the AC system) it appears that the TXV has a sensing bulb. So my car set-up should be similar to the set-up of a residential unit.
Could you please clarify your statement ‘The refrigerant amount in a MVAC system is charged exclusively by weight’? Could you please explain the function of the equalizer (?) line between the valve and the evaporator discharge line in Photo3?
Many thanks
Attachments
Photo 3 - Line between TXV and evaporator outlet.jpg
Photo 3 - Line between TXV and evaporator outlet.jpg (254.68 KiB) Viewed 867 times
Photo 2 - TXV from 2000 Civic.jpg
Photo 2 - TXV from 2000 Civic.jpg (236.81 KiB) Viewed 867 times
Photo 1 - Evaporators and TXV from 2000 Civic_edited-2.jpg
Photo 1 - Evaporators and TXV from 2000 Civic_edited-2.jpg (228.84 KiB) Viewed 867 times
User avatar
JohnHere
Posts: 329
Joined: Sun May 13, 2018 10:20 am
Location: South Carolina Upstate - USA

Re: 2002 Civic Si – Evaporator temperature sensor

Post by JohnHere »

Charles5654 wrote: Wed Mar 24, 2021 11:37 am Could you please clarify your statement ‘The refrigerant amount in a MVAC system is charged exclusively by weight’?
I don't work on residential or commercial A/C systems. But the way I understand it, refrigerant is charged into such systems until the optimum superheat and subcooling temperatures are reached. In MVAC, refrigerant is introduced into a system until the specified weight is reached using an accurate refrigerant scale.
Charles5654 wrote: Wed Mar 24, 2021 11:37 amCould you please explain the function of the equalizer (?) line between the valve and the evaporator discharge line in Photo3?
Ahh...now you're entering into the realm of refrigeration theory.

Essentially, a TXV, which meters the flow of liquid refrigerant into the evaporator, is all about pressure relationships at three different points internally. The three pressure points interact with one another to achieve a correct balance which, as you'll see, is very important.

Without getting into a detailed discussion of superheat, the TXV needs to meter the flow of liquid refrigerant into the evaporator at the same rate as the refrigerant is changing state from a liquid to a gas caused by the heat load. The three aforementioned pressure points, working together, cause an internal needle valve to allow more or less refrigerant into the evaporator so that the refrigerant "evaporates" within the evaporator, thereby creating the desired refrigeration effect and cooling the cabin when air is blown across it.

For the externally equalized TXV in your pictures, the equalization tube compensates for a slight pressure drop across the evaporator by providing "equalization" pressure from the evaporator suction line to the bottom of the TXV diaphragm. An internal superheat spring also acts on the bottom of the diaphragm. Both of these forces coordinate with the pressure at the top of the diaphragm generated by the sensing-bulb tube, thus controlling the needle-valve's throttling action.

This rather complex pressure relationship ensures that the correct amount of liquid refrigerant always enters the evaporator via the internal needle valve at any given time.
Charles5654
Posts: 14
Joined: Sun Oct 18, 2020 5:13 pm

Re: 2002 Civic Si – Evaporator temperature sensor

Post by Charles5654 »

JohnHere,

Many thanks for your detailed answers to my questions!
User avatar
Tim
Site Admin
Posts: 560
Joined: Tue Apr 12, 2016 2:19 pm
Location: Scottsdale, Arizona
Contact:

Re: 2002 Civic Si – Evaporator temperature sensor

Post by Tim »

Charles,

Thank you for the site donation.
------------------------------
Please support ACKITS.com for your Auto A/C Parts and Tool needs.
P:602-233-0090
Al9
Posts: 257
Joined: Tue Mar 10, 2020 3:26 am
Location: Southern Europe

Re: 2002 Civic Si – Evaporator temperature sensor

Post by Al9 »

Your 2002 Civic should have the block type expansion valve. In that case, there's no equalizer line, yet the valve is still, technically speaking, externally equalized, since the refrigerant is directly able to exert force upon the diaphragm as it leaves the evaporator. Depending upon the valve's design, the refrigerant exchanges heat with the sensing charge through a bulb that protrudes inside the low pressure pathway from the disc or a thermal interface located upon the diaphragm, refrigerant side of course.

The TXV, as JohnHere said, is only able to regulate refrigerant flow.
But, the fact that superheat is hard to measure and actually quantify in MVAC won't necessarily imply that it won't matter.

Therefore, i'd add that, roughly speaking, the TXV is only able to regulate the extent to which the whole evaporator coil is filled with liquid refrigerant or two-phase refrigerant (a mix of liquid and vapor). That is to avoid using the term superheat, even though the extent i'm talking about is directly linked to the actual evaporator's superheat. It does so by running the evaporator's pressure through a system of forces and determining a final opening (or closing) force, as JohnHere described.

Having a "liquid head" near the evaporator's outlet is very different than having it right after its inlet.
In the former condition, the evaporator is running near 0 superheat. It's a "flooded" evaporator. Refrigerant has very little space left to pick additional heat after it managed to entirely turn into a vapour, hence the very little (if not absent) superheat. Practically speaking, the whole of the evaporator coil is filled with two-phase refrigerant, and therefore nearly the whole evaporator's surface is being used to exchange heat (the evaporator is being used to maximum capacity). The evaporator's pressure will be decreasing at a gentle rate as the compressor does its work. The oil return will also be optimal, since compressor oil dissolves in liquid refrigerant way easier than in vapor. However, losing control of this liquid flow can end up damaging the compressor (liquid refrigerant is obviously incompressible and, as i've said, it washes any oil away), so in this condition a properly functioning TXV still controls the flow so that very little (ideally, none) two-phase refrigerant makes its way to the compressor. This is the perfect condition in a low heat load scenario (a cold interior, or a rainy day, for example).

In the latter condition, the evaporator coil is severely deprived of refrigerant. It's a "starved" evaporator. That little amount of liquid refrigerant that made its way inside the evaporator coil will be able to pick up an immense amount of heat even right after turning to 100% vapour. But, very little of the evaporator's surface will be used. Evaporator pressure will be decreasing at a way faster rate as the compressor pulls refrigerant, but that doesn't necessarily imply cold air blowing from the vent since so little of the evaporator coil is exchanging a sizable amount of heat by evaporating liquid refrigerant. Also, oil has a hard time making its way back to the compressor, especially if the refrigerant flow is very low (a fully destroked variable displacement compressor, or a severe undercharge, for example). However, that's an interesting condition in a high heat load scenario (AC just turned on for example). Due to the reduced refrigerant flow, the compressor isn't being overloaded, and the evaporator pressure is dropping fast. Aids transitioning to a lower heat load condition as a result.

The TXV is what controls all of this. Sometimes, it just opens and closes in order to control to a fixed "evaporator fill" (what i think the proper MVAC equivalent of superheat is) and that's it, but that's rare occurrence since it's an inefficient approach.

A lot of times, the TXV's sensing charge is deliberately designed so that the evaporator runs in a starved condition whenever the evaporator pressure is high enough (high heat loads, such as right after AC has been turned on in a hot sunny day) and then progressively switches to a "flooded" operating condition as the evaporator pressure (and temperature) drops. It's a matter of how much force the sensing charge is able to exert against the superheat spring (and the system's refrigerant) at a precise refrigerant temperature and it's achieved by using a specific mixture of gases as the sensing charge.
Even more, each TXV is calibrated to a maximum evaporator pressure above of which it will basically stop controlling superheat and act as a fixed restriction instead, preventing excessive compressor load (the MOP value the TXV is set to seems to depend upon the engine's displacement and whether the compressor is of the variable displacement type or not; remember that enough liquid refrigerant has to flow across the evaporator for cooling to happen, so even too low of a MOP value can be detrimental to cooling performance).
On the other hand, a compressor capacity control system is always needed, even if the TXV is equipped with a sensing charge that is able to keep the evaporator's pressure from falling down too sharply by keeping the evaporator well fed even at low heat loads, as the TXV isn't able to act as an evaporator pressure regulator (aside from the maximum operating pressure feature). Hence the need for an evaporator temperature sensor, which cuts the compressor clutch off whenever the evaporator surface gets too cold and re-engages it once it's warmed up enough (whenever it's lacking, it's because the system features an evaporator pressure control system such as a mechanical compressor displacement control valve or an evaporator pressure regulator valve; also, electronic compressor displacement control systems need an evaporator temperature sensor too). Not the finest way of controlling compressor capacity, but it gets the job done nonetheless.

Obviously enough, a TXV can do all of this only as long as it's receiving a liquid head as its feed. Flash gas at its inlet seriously hampers its operation.

The AC system has to be charged by weight, or else the TXV will malfunction and be unable to accomplish everything i've just explained. Basically speaking, too little refrigerant, and the evaporator will be starved when it's supposed to be flooded. Cooling capacity and compressor oil return will suffer accordingly.

TL:DR The TXV controls how much of the evaporator coil is evaporating liquid refrigerant (rather than further heating vapor refrigerant up) in a precise moment. It ain't able to regulate evaporator pressure (which is related to evaporator temperature) to a precise value, so something else has to adjust evaporator pressure. Compressor clutch cutoff through an evaporator temperature switch/sensor is one possible way of adjusting evaporator pressure, though it ain't the finest available.
Last edited by Al9 on Sat Mar 27, 2021 4:28 am, edited 2 times in total.
Charles5654
Posts: 14
Joined: Sun Oct 18, 2020 5:13 pm

Re: 2002 Civic Si – Evaporator temperature sensor

Post by Charles5654 »

AI9,

Thank you for the clarification on the type of valve installed in my car and all the additional interesting information provided in your post.
Post Reply