Know Your Car AC System Part 2 - Automotive HVAC Systems

Preamble
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 - Does the AC Compressor run continuously or keeps cutting in and out - loading the Engine accordingly

OR

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
1) 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-1 and 2 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!!