[Jeroen]

Originally Posted by PistonCrown

Since the compressor is belt drive driven I assumed it would be moving most of the time and clutch would get activated due to rpm of the engine. Correct me if I am wrong in assumption.

If you have a clutch/variable compressor the pulley is turning as long as the engine is turning. Once the clutch is activated the compressor starts turning too. RPM is not relevant. An AC compressor will work across the normal RPM band of the engine. It needs to work when the engine is idling and when the engine is running at, or near maximum RPM.

At high engine RPM the compressor displaces more volume and cools quicker. Still, when a car is at stand still and the engine idling it must be able to provide sufficient cooling power as well.

On modern cars, the clutch will be disengaged at engine start, just to make starting of the engine a little easier. Only after a few seconds, the AC clutch will kick in.

Other than that, as I mentioned earlier, on most system the clutch tends to be switched on/off depending on pressure of the refrigerant.

Originally Posted by venkyhere

I think there is a typo in your text :

Yes, you are correct. My bad, a “senior" moment! It had already been mentioned by another member too

Originally Posted by venkyhere

One basic Q :

A) variable compressor with clutch
B) variable compressor without clutch

When A/C is OFF

- in A, the 'gas+oil' is not circulating through the system, since the clutch is not engaged.
- in B, the 'gas+oil' is circulating, without getting compressed (swatch blades are put to 'no compress' position), getting condensed into liquid by the condenser (assuming the main fan of the car gets switched on by the engine ECU), further expands into a gas through the expansion valve, passes through the evaporator without really getting heated (since A/C cabin blower is not running) and then gets back into the compressor; rinse and repeat the same cycle.
Doesn't B suffer more wear and tear, because moving parts of the A/C compressor are moving all the time, even when cooling is not called for ?

When A/C is ON

- in A, what will the ACC computer decide to do, when the cabin temp has just reached the target number, and the computer now wants to wait for the temp to rise above a target+delta threshold, to call back the compressor into action once again ? Does it : (i) keep the clutch engaged but put the swatch blades into 'no compression' position OR (ii) disengage the compressor clutch.
In other words, there will be a hysterisis window around the target temp, where the compressor doesn't need to work yet, how will the computer handle case A ?

It really depends on the system and ambient conditions, I would think. But yes there will be a certain hysteresis. From what I have observed on some cars is that once the AC clutch engages the control is done mostly with the variable part of the compressor. So the AC clutch does not, or hardly, disengages.

The clutch would only disengage if the compressor is running with its swathe plate in no-compression for an extended period of time.

Originally Posted by vivek_lo

Very illustrative explanation of fundamentals. Thanks for sharing. I face a unique problem in my 10year old Ecosport. The cooling is adequate while running in 2nd gear and above. But while in neutral and first gear, the compressor seems to be not working, the cooling drastically reduces and in case of extended duration in idling and first gear crawling, it starts blowing hot air. The Company service technical says the compressor needs to be changed. I am not convinced since the cooling is good while running. Can there be any other suspects to check? I do not want to change the compressor since I will be changing the car soon. Please give your expert opinion.

Check if the cooling works properly with the car standing still and just pressing the accelerator to rev up the engine to about 2500-3500 RPM. Keep it at constant RPM for about 20-30 seconds with the AC on. If you notice a considerable difference in cooling, my fist suspect would be your system is beginning to run a little low on refrigerant.


Jeroen

[Blooming Flower]

Originally Posted by venkyhere

One basic Q :

A) variable compressor with clutch
B) variable compressor without clutch

When A/C is OFF
....

Doesn't B suffer more wear and tear, because moving parts of the A/C compressor are moving all the time, even when cooling is not called for ?

First of all, a fantastic question by Venkyhere. Let’s look into case B. Clutchless variable compressor. What is described in detail by Jeroen Sir is essentially the teardown of a variable displacement compressor with a magnetic clutch, which is also known as an internally controlled ECV-type compressor.

So, the question of lubrication, wear and tear is more relevant to type B (aka externally controlled ECV type) as the compressor shaft is always rotating with the engine. Most of the latest Hyundai cars available in India are coming with this type of compressor nowadays.

Case 1: When the AC system is running: The swash plate is at an angle with the main drive shaft pushing out refrigerant as required which is controlled by an electronic feedback mechanism in conjunction with ECV (electronic control valve) getting signal combined from the BCM, ECM based on the output of APT (ac pressure transducer), thermistor, sun load sensor etc. The feedback loop and circuitry are really complex due to the plurality of various control parameters.

In some vehicles, the expansion valve is of cross-charge type (basically known as TXV) where a small bottle is attached to the expansion valve (superheat unit) which is connected in a loop with the condenser output. This superheat unit is a separate circuit and also carries a small amount of different refrigerant which never comes in contact with the main R134a for the cooling system. This superheat system also controls the amount of refrigerant passing through the expansion valve according to the instantaneous cooling demand.

A compressor can only compress refrigerant in gaseous state, and the refrigerant carries the lubricating oil in mist form. So when the AC is running, the lubrication part is taken care of well.

Now Case 2: When the AC system is not running but the compressor shaft is rotating with the engine.

When do we need lubrication? A dynamic system with moving components with some kind of sliding/frictional motion. When the AC is off the swash plate is operating at nearly zero degree angle (orthogonal with the compressor shaft) which is often termed as ‘no flow’ or ‘low flow’ condition.

So, which parts inside the compressor housing need lubrication?

The first one is the pistons. When the swash plate is orthogonal with the shaft the pistons are having NO to-and-fro motion (stroke) through the cylinders. Therefore, no friction or sliding. Also, most of these piston comes with a thin film of PTFE (commonly known as Teflon) coating. PTFE has very low mu (coefficient of friction) and itself acts as a primary lubricant. If these coatings are damaged somehow no oil can prevent the compressor from a seizure. Hence, with an orthogonal swash plate position pistons are safe from wear and tear.

Next, the swash plate itself. Generally, the angle is controlled by the mechanical link of two half ball bearings attached to the surface of the swash plate (one at the belt side and one at piston side for each piston). This has also been pictorially shown by Jeroen sir in the OP. While in orthogonal position the swash plate and the half ball bearings have ‘no relative motion’. The swash plate and the bearing slide with each other only when some angle is to be maintained. Now, the entire compressor housing stores some oil while the refrigerant's static pressure without any flow or compression is equal at low and high sides. In fact, each main component of an AC system viz. compressor, condenser, cooling coil etc. holds some specified oil amount as per OEM chart.

What about the drive shaft and pulley? They are in motion with some roller (needle) or ball bearings which are already lubricated (as per bearing spec.) for continuous operation.

Moreover, these clutchless, continuous-operation compressors have a built-in fail-safe mechanism. This is achieved by a ‘torque limiter’ or ‘shear plate’. The name itself indicates how it helps protect the drive belt and allied components in case of the onset of a seizure. This ‘shear plate’ consists of some high-grade elastomeric holders and an aluminium plate held to each other, which are designed to shear off and disconnect the drive shaft from the rotating pulley on meeting the condition of excess torque demand than the preset limit.

For obvious reasons, the basic visual diagnosis of a kaput clutchless compressor is whether the centre bolt is rotating along with the pulley. If the centre bolt is not moving the ‘torque limiter’ is broken and the compressor is damaged.

Hope, this answers your query.

[samitjain]

I have a few questions which I hope can be answered by the experts.
My corolla altis 2014 ac compressor was not working and showed to authorised Toyota workshop and they said, we don’t change the compressor only. The whole ac kits needs to be replaced which includes pipes, condnsor etc and estimate of Rs.1.5 lacs plus taxes.

Left with no option, i approached FNG and he replaced the compressor with some other spare ones, not brand new and ac is working fine. But he said that the sensor which is in the compressor has to be byepassed.

I want to know what is the purpose of this sensor and byepassing it could cause what kind of damage. FNG says it is not an issue and it’s completely safe.

Please advice, if any major issue, otherwise I will not keep the car for long and sell the car.

[Sumedik]

Originally Posted by Jeroen

An AC compressor will work across the normal RPM band of the engine. It needs to work when the engine is idling and when the engine is running at, or near maximum RPM.

At high engine RPM the compressor displaces more volume and cools quicker. Still, when a car is at stand still and the engine idling it must be able to provide sufficient cooling power as well.

Excellent thread !

Here is a basic question:

How does low pressure and high pressure behave as per engine RPM? In other words:

As RPM increases will low pressure decrease and high pressure increase?
Or will both(low pressure, high pressure) decrease with increase of engine RPM?
Or will both(low pressure, high pressure) increase with increase of engine RPM?

[Jeroen]

Originally Posted by Sumedik

How does low pressure and high pressure behave as per engine RPM? In other words:

As RPM increases will low pressure decrease and high pressure increase?

Correct, the general answer is increase of RPM will decrease low pressure and increase high pressure.

On systems with only a clutch this is very noticeable or rather measurable. On systems with a variable compressor, such as this, the various pressures are also determined by the volume being pumped around. To achieve the same cooling capacity at higher RPM less volume is required. And thus also affects the pressures a bit.

Jeroen

[deehunk]

Posting pictures of my AC compressor from Renault Scala, the clutch was removed for preventive maintenance today. The difference in cost is ₹3000 for clutch repair as against ₹20K for a new compressor.

[Jeroen]

Originally Posted by deehunk

Posting pictures of my AC compressor from Renault Scala, the clutch was removed for preventive maintenance today. The difference in cost is ₹3000 for clutch repair as against ₹20K for a new compressor.

Preventive maintenance of an AC clutch? Why would you that? These clutches tend to give clear indication when they are on the way out. When everything works and no noises, it seems as a waste of money to do preventive maintenance.

Only two things wear on the clutch. The clutch itself, but on most types I have worked you can adjust the airgap for wear. Is that what is being done?

The other part that really wears is the bearing. The more you use the AC the less wear on the bearing!

Jeroen

[deehunk]

Yes, Jeroen, the problem with the clutch was identified during the AC gas refilling. There was a slight, uneven movement accompanied by a typical whining sound when the AC was switched ON. Even after refilling and checking of other related parts, the cooling was not effective in the vehicle.

[Sumedik]

Originally Posted by Jeroen

Correct, the general answer is increase of RPM will decrease low pressure and increase high pressure.

Thanks.

So the AC system is mechanically most stressed when the car is driven spiritedly on hot days with AC on at full blast as reasoned below:

- AC on at full blast: compressor on for long time.
- Spirited drive: High RPM resulting fast spinning of compressor which results in high pressure refregerant flowing in the system. This puts lots of stress on mechanical parts, seals, pipes etc.
- Hot weather: Higher temperature makes gases expand and this contributes to HIGHER pressure of refregerant in the system.

[Jeroen]

Originally Posted by deehunk

Yes, Jeroen, the problem with the clutch was identified during the AC gas refilling. There was a slight, uneven movement accompanied by a typical whining sound when the AC was switched ON. Even after refilling and checking of other related parts, the cooling was not effective in the vehicle.

Well, I do hope it was related to the clutch. I doubt it somehow. A bad bearing would have no effect on the cooling. A bad clutch burns up very quickly, meaning virtually no cooling at all.

Would be interesting to hear what they actually did to your clutch and whether it solved the cooling issues.

Jeroen

[Jeroen]

Originally Posted by Sumedik

Thanks.

So the AC system is mechanically most stressed when the car is driven spiritedly on hot days with AC on at full blast as reasoned below:
.

You make it sound as if it is a bad thing. But an AC system is designed to deal with it. So it is not stressed as such, it is just loaded up a bit more.

About mechanical stress the same applies. All these parts are simply designed to handles these pressures, nothing is stressed.

When it comes to the compressor components and whether its stressed more due to high RPM, people always forget how the pressure on top of the piston is compensating for the centripetal forces due to the RPMs. So rods and bearings might actually be subjected to higher forces at low RPMs.

Jeroen

[deehunk]

Originally Posted by Jeroen

Would be interesting to hear what they actually did to your clutch and whether it solved the cooling issues.

Jeroen

Your diagnosis was right; it was the bad compressor components that were faulty. The piston and seal were replaced; the service comes with a 6-month warranty. The cost was ₹5900 plus AC gas filling ₹1800. The sound is gone, and there are no fluctuations when the AC is switched on.

[Jeroen]

Originally Posted by deehunk

it was the bad compressor components that were faulty. The piston and seal were replaced; the service comes with a 6-month warranty. The cost was ₹5900 plus AC gas filling ₹1800. The sound is gone, and there are no fluctuations when the AC is switched on.

Thanks,
good to hear.
Here in the west it would not be possible to have your AC compressor overhauled like that.

INR 5900 is roughly Euro 60. Any car mechanic will charge more just per hour! I doubt Inwould be able to get the parts for that sort of money either.

So apart from DIY enthousiast most AC compressor work is straight forward replacement. The cost of labour is simply too high here.

Jeroen