Sunday, April 7, 2013

Know Your Car A/C System : : Must Read for Best Results from your Car AC

Author: S K Gupta
Source :

Know Your Car A/C System Part - I
It’s that time of the year when Car AC’s are just about being switched in – even in North India. They perhaps never got switched off ‘down south’ of Nagpur!

Considering the fact that most motorists face problems with their Car AC’s sooner than later, it’s worthwhile to get the hang of it all – coz ignorance in such matters can cost one dear – in more ways than one. 

The System

All Car A/C Systems are basically similar, with a cooling capacity of almost
1-1.5 tons (!), in the sense that they essentially comprise:
1) An Engine Driven ‘Compressor’ with an ‘Electro-magnetic’ Clutch. The Compressor Capacity is designated in ‘CCs’, like the Engine. 80 to 120 CCs being the most popular ratings for passenger Cars of the types on our roads. The ‘Refrigerant’ used so far was ‘R-12’ of the ‘CFC’ family of gases but is now progressively replaced by ‘R-134A’, a ‘NON-CFC’, for environmental considerations.

2) A Grill-front mounted ‘Condenser’, cooled either by a common (Radiator) fan or a ’Dedicated’ fan of its own. For example Maruti 800 and Zen/Esteem - respectively. The latter is a superior and hence a more efficient but naturally a more expensive system.

3) This is followed by a ‘Receiver-Drier’, mounted somewhere in the Engine Compartment. Its purpose is to control and ‘Purify’ flow of the Refrigerant to the cooling coil, under various operating conditions.

4) A ‘Cooling Coil’ with a multi-speed Blower mounted in the Passenger Compartment, generally inside the Dashboard. This unit has two additional and vital parts viz. (a) An ‘Expansion Valve’ and (b) a ‘Thermostat’. The latter can be a ‘Bellows’ type like in the pre/EU-I M-800s or ‘Electronic’, as in the Zen/Esteem and some others. All Mpfi Cars today have the Electronic one only. Larger vehicles like the Tata Safari have two such units, one up front and the other at the rear, to cool the entire passenger area effectively.

5) The ‘Expansion Valve’ regulates the quantity of gas flow to the Cooling Coil (also called the ‘Evaporator’), depending upon the ‘Heat Load’ on it and the ‘Thermostat’ prevents ice formation on the cooling coil, which not only affects the cooling efficiency but if allowed to happen, can also damage the system.

 How to get the best out of your Car A/C.

Let us consider the M-800 System, being most omni-present and ‘Edgy’ by design, by virtue of the Engine itself being < 50Bhp. This exercise can be divided into two parts:

(I) Your Garage:

(i) It should check whether the System is Healthy, by way of Suction/Discharge Pressures, Internal or External Component Blockages, ‘Pressure equalisation’ times on Switching Off and Cabin Grill Temperature.

(ii) On M800 ‘Retrofits’, check whether a 120Watts Radiator Fan is provided, by replacing the ‘Standard’ 80-Watt one.

(iii) Ensure proper Foam sealing on all the four sides between the Radiator and the Condenser, between the Exhaust Manifold and the Condenser (behind the Bumper). This is the most neglected area.

(iv) Existence of a ‘Heat Shield’ around the Compressor discharge pipe and the Condenser in an M800, as provided in an ‘OE’ fitment. This again is generally thrown away by mechanics right during the first service of a new car, thinking that it serves no useful purpose. If it were so, it won’t be provided by the Manufacturer in the first place and be subsequently priced as an MGP/SGP Spare costing over Rs: 350/-!

(v) Whether the Radiator Fan comes on and stays on even when the Radiator Thermo Switch cuts in. Approach in reverse sequence especially for the Retrofits.

(vi) Check all parameters of the Engine Tune, such as Idle RPM/CO, FICD RPM (1050), Ignition Timing including satisfactory working of Centrifugal and Vacuum Advance features, Dwell angle and its stability up to 4000RPM, Spark Plug condition and Gap, Air Cleaner/Petrol Filter Cleanliness, Carburetor II Butterfly in good working order, Engine Valve Clearances and Timing (if suspect).

(vii) Wheel alignment, Engine Compression/Power balance (if suspect), Radiator/Condenser Cleanliness (inside/out), condition of Radiator Cap/Thermo valve (Change if suspect). In our Dusty conditions, these two have to be pressure washed every 2-months from both sides.

(viii) Correctness of Dash Board Temp Gauge vis-a-vis digital thermometer in the Radiator neck, Radiator Fan coming on without A/C around 85*C to 90*C, full closing of Fresh Air Damper in ‘Recirc’ mode and proper alignment in ‘Full Forward’ Mode. 

 (IV) Yourself:
(i) Do read and follow the Owner’s Manual on the Car A/C usage.

(ii) Maintain the recommended Tyre pressures.

(iii) Always drive in a gear one step lower with A/C on, than what is recommended for normal driving.

(iv) Ensure that the ‘Recirc’ flap is always closed and avoid driving in our dusty conditions with Fresh Air flap open, as it deposits dirt on the cooling coil, thus making it less and less efficient.

(v) Always get your A/C checked out at a competent and reliable Garage at the beginning of a season.

(vi) Check frequently the ‘free-rolling’ of your car, to guard against ‘sticky brakes’. This can be very easily done when coming to a stop, say at traffic lights. Just let go of the brakes when the car is about to stop and shift to neutral. The car should continue to move forward without any noticeable feeling of a ‘drag’. Don’t forget to engage the hand brake when you come to a stop!

(vii) Last but not the least, it’s of utmost importance to switch on a Car AC atleast once a week even in Wintertimes and let it cut-off on its Thermostat once or twice – to keep the System internals well lubricated. This is coz the lube oil of an AC System moves along with the AC Gas!

 (V) What can go wrong!: 

Perhaps the most common ‘complaint’ of most Car Owners is that either the AC is not cooling well enough OR when they use it, the engine over-heats.

Here are some tips to equip you against being taken for a ride by who so ever you choose to have it fixed by:

i) Not Cooling enough:
The main reasons for this, assuming that your System is physically in good shape, can be – a) Under OR over Gas Charge, b) Dirt-clogged front-end AC Condenser, c) Choked ‘Receiver-Drier’, d) Faulty ‘Expansion-Valve’, e) A dirt-clogged ‘Cooling-Coil’, f) Faulty ‘Anti-Frost’ Thermostat on the Cooling Coil, g) Loose AC Compressor Drive Belt OR its faulty ‘Clutch’.

ii) Engine Over-heating:
Likewise - a) Dirt-Clogged Engine-Radiator, b) Engine ‘Out of Tune’, c) Jammed II-Butterfly - especially in Carb type M800’s, d) Driving in a gear higher than what the engine demands, e) ‘FICD’ engine speed being much higher than recommended, f) Car not ‘free rolling’ enough for whatever reason – as covered above.

Once you have got the hang of it as to what makes it tick and keeps it ticking, there is no reason why it should not deliver satisfactory performance even through the peak of our North Indian summer. I have personally driven an M800 through Rajasthan in the month of May with outside temperature being 48*C, Cruising speeds up to 100kph, Cabin Temp. around 25*C, and the Engine Temp. not crossing the halfway mark!

In Part–II of this Article to follow, we will see how the next/present generation Car ‘HVAC’ Systems work.

Know Your Car A/C System Part - II Automotive ‘HVAC’ Systems
In Part-I of the above Article, we tried to understand how the ‘Cooling’ or the ‘Refrigerating’ part of the System works.

From April 2000 onwards, most Manufacturers have switched over to ‘R134A’ as the ‘Mandatory eco-friendly’ Refrigerant - as opposed to the earlier Ozone-depleting ‘R12’.

To an average Car Owner, it should suffice to bear in mind which System his Car has. This is clearly stated in his Owners Manual + suitable ‘stickers’ in the Engine Compartment – such that while ‘topping-up’ no mistakes are made.

Suffice to say that ‘R12’ in a ‘R134A’ System can be tolerated to some extent but the other way round is a no-no. This is because the ‘latent heat of evaporation’ of R134A is ‘lower’ than R12 and therefore, R134A systems call for larger front-end Condensers and in-cabin ‘Cooling-Coils’.

To make things a little more complicated, most of the present generation Cars nowadays come OE with an ‘HVAC’ – Heating Ventilation and Air Conditioning - System. The major advantage of such a System is that it allows one to choose the most comfortable in-cabin Temperature without having to suffer the suffocating thermostatic ‘dead-band’ of an ‘AC System Only’ in the earlier Cars.

Now let’s see how this is achieved and what are its plus/minus points – as far as load on the engine and hence the fuel consumption is concerned.

A Typical HVAC FAQ

People often wonder or wish to know if there is going to be a difference in Load on the engine/fuel consumption when one keeps the ‘Red to Blue’ Thermostat at max blue or less. For example, what happens when one moves the Thermostat from the coldest to an intermediate position -
1) Does the AC Compressor run continuously or keeps cutting in and out – loading the Engine accordingly.


2) Does it run continuously and thus load on engine more?

One would ask such a question when failing to notice any difference on the Load on the engine when the AC thermostat is set to less than ‘max-cool’.

Or in other words, the AC Comp doesn’t seem to cutout even when one desired less than max cooling from it. So - where does the extra cooling go when the Thermostat is set to low!

 Fundamentals of ‘HVAC’ Systems

The present day HVAC Systems work this way:

The Cooling Coil of the A/C has a Thermostat of a 'fixed' setting of (+) 2-4*C, to prevent its 'frosting'. In the mpfi Cars, it's an Electronic 'Thermistor' type. Therefore, in the 'max cool' setting, it cuts off the Comp whenever the Cooling Coil approaches frosting, depending upon the 'heat load' on the System.

2) In the earlier A/C cars like the M800 which didn't have a Heater also, the Thermostat used to cut off the Comp, resulting in a 'dead band' of about 2*C before it could cut-in again. During this dead band, like in Room A/Cs, there comes in a feeling of suffocation.

3) Therefore in the present generation Cars with HVACs, instead of cutting off the AC Comp altogether and thus get that suffocating feeling, there is a ‘progressive inflow’ of Hot air let in from the Car Heater and mixing it with the Cold air from the A/C Cooling Coil, as you slide the so called the ‘Thermostat’ to 'less cool' positions.

A simple diagram below illustrates how this is achieved –

4) Consequently, depending upon the in-cabin thermostat setting below ‘max-cool’, the AC Comp is on most of the time. And whenever the Comp (fixed displacement type) is on, it will lead to the same 'drag' on the Engine.

5) However, with the advancement of Technology, fixed displacement type AC Comps are increasingly getting replaced by 'variable displacement types' on the more expensive Cars (Cielo was the first one to have it), which are able to 'adjust' their 'output' depending upon the total 'system requirements'. Consequently, this results in lower drag on the Engine even when the Comp is on, depending upon the Heat Load demand on it.
To sum-up, particularly in the present "B" gr Cars' HVAC Systems having ‘fixed-displacement’ type AC Comps, it's advisable to keep the in-cabin Thermostat on ‘max cool’ setting and bear some of the suffocating feeling if one wishes to get the best possible FE with A/C on - rather than to let the AC Comp run all the time by selecting a lower setting.

However, during certain times of the year when the Summer is just setting in or exiting – coupled with the Windscreens’ ‘defrosting’ needs depending on the atmospheric humidity at the same time - it may become inevitable to select an intermediate setting for the best/most comfortable results. In such a situation, the HVAC System would decide for itself as to how much the AC Comp has to work. Afterall, life is meant to be enjoyed and not spent in counting pennies all the time!

And before concluding Parts-I and II of these Articles, some FAQs:

Q1) What is the best way of putting on the A/C when the car cabin is hot (i.e when the car is under the sun for a long time)? I have heard people say that the windows should be down for sometime with the A/C on? Is this correct? Should the A/C be on or is it just the blower that should be on?
A1) Most Owners Handbooks cover this aspect. The idea is to keep the heat load on the Car AC system as low as possible to begin with for 'perceived' faster cooling - by driving out the 'trapped' hot air which can at times be + 20 to 30*C above the ambient temps ! I have personally 'clocked' +60*C inside in N/I peak Summers with +40*C ambient!!
The best way to do it is to atleast roll down the 2-front windows, switch on the blower only to max/fresh air mode and drive off for ~ 2-3 mins. Thereafter, roll up the windows, turn the FAD to 'recirc' and sw on the Comp - leaving the blower to max - to be progressively lowered to comfortable speeds - usually 1 or 2 after a while. 

Q2) I know of people who put the A/C off and on at intermittent intervals to save on fuel when the car is running/at signals. Is this a good practice?
A2) It's out of ignorance. The more you manually sw on/off the Comp - the more you're cutting into its Clutch life. Above all, why sweat at the traffic lights. AC is meant to keep you cool and not sweat. If you choose to have it, use it ! If it has to cut-off per System needs, it'd do on its own!! 

1 comment:

  1. Dear Sir,
    Many thanks for this informative post on how car AC works. Particularly the two FAQs as they are very much in everyone's mind about car ACs.