This has been an interesting and helpful debate, many thanks. I am shutting it down now, I think I have learned as much as I can. Deep respect. K

On 18/02/2017 at 19:36, MoggyTech said:

The older aircon systems used a magnetic clutch to engage the compressor, when these systems were off, no aircon fluid flowed and seals would dry out and eventually fail. The system in the Octy is the newer constant flow type, and the level of aircon flow is controlled by a valve. On my 1.4TSi I can set the aircon to light, normal or heavy. I leave it on all the time set to light and do not see any noticeable MPG hit.

This setting is mainly to control the fan speed, between slow and quiet, and fast and noisy. The eco driving mode should have a much greater effect on how hard the compressor works and thus fuel consumption.

On 18/02/2017 at 13:22, Trebor77 said:

This is interesting.   In what ways have modern a/c systems (with climate control) become so much more efficient than older versions?  They both use  a volatile refrigerant fluid, a compressor driven by the engine, a condenser, a throttle valve to drop the pressure, and an evaporator? Which of these has become so much better? I'm very interested in these things

To answer this question specifically.....

You are correct that modern a/c systems still comprise of the same basic components as in the past and the basic operating principles still apply. What has changed is the ability to better control what is happening in the system thus making it more efficient.

 

In older systems the refrigerant was static until you turned on the a/c system, when you did, a magnetic clutch joined the compressor to the accessories being driven by the auxiliary belt. The compressor was called a fixed displacement type compressor, meaning the amount of refrigerant it moves is always the same and so the power it requires from the engine to drive it remains constant. Pressure in the system is controlled by hi/low pressure switches and flow is controlled by either a fixed orifice tube or capillary tube thermal expansion valve. You control the temp in the car by selecting hot or cold. So can you see how this would effect mpg? The system is always work hard.  System off normal mpg, system on 5-10mpg (or worse!) less. 

 

Modern Climate control systems are more efficient because they are able to vary how much power they take from the engine. The compressor is always driven by the auxiliary belt there is no clutch it's called a variable displacement type compressor, meaning it can alter its displacement and therefore drain on the engine from between 5-100%. Refridgerant flow to the evaporator is accurately controlled by a block type thermal expansion valve depending on demand and core temperature is monitored as well as cabin temperature.  

If the user demands a cabin temp of 22c in a hot car the ecu demands max cooling the compressor goes 100% displacement causing maximum demand and mpg. as the desired temp is reached the compressor displacement is reduced to as little as possible to maintain the desired temperature which in turn makes the mpg better due to less demand. If you then open a window and let in a big slug of hot air in, cooling/demand is again increased to regain the set temperature, mpg goes down abit then back up.

When you select eco mode the a/c is still active but works less hard to achieve and maintain your desired temperature giving better mpg.

If you turn off the a/c, the compressor goes to its minimum displacement setting about 5% or less. the system is still circulating but causing no noticeable drain on engine power or mpg.  

 

Hope that helps! 

 

Edited 20 February, 20179 yr by MrAdamwood

On 2/20/2017 at 23:14, MrAdamwood said:

To answer this question specifically.....

You are correct that modern a/c systems still comprise of the same basic components as in the past and the basic operating principles still apply. What has changed is the ability to better control what is happening in the system thus making it more efficient.

 

In older systems the refrigerant was static until you turned on the a/c system, when you did, a magnetic clutch joined the compressor to the accessories being driven by the auxiliary belt. The compressor was called a fixed displacement type compressor, meaning the amount of refrigerant it moves is always the same and so the power it requires from the engine to drive it remains constant. Pressure in the system is controlled by hi/low pressure switches and flow is controlled by either a fixed orifice tube or capillary tube thermal expansion valve. You control the temp in the car by selecting hot or cold. So can you see how this would effect mpg? The system is always work hard.  System off normal mpg, system on 5-10mpg (or worse!) less. 

 

Modern Climate control systems are more efficient because they are able to vary how much power they take from the engine. The compressor is always driven by the auxiliary belt there is no clutch it's called a variable displacement type compressor, meaning it can alter its displacement and therefore drain on the engine from between 5-100%. Refridgerant flow to the evaporator is accurately controlled by a block type thermal expansion valve depending on demand and core temperature is monitored as well as cabin temperature.  

If the user demands a cabin temp of 22c in a hot car the ecu demands max cooling the compressor goes 100% displacement causing maximum demand and mpg. as the desired temp is reached the compressor displacement is reduced to as little as possible to maintain the desired temperature which in turn makes the mpg better due to less demand. If you then open a window and let in a big slug of hot air in, cooling/demand is again increased to regain the set temperature, mpg goes down abit then back up.

When you select eco mode the a/c is still active but works less hard to achieve and maintain your desired temperature giving better mpg.

If you turn off the a/c, the compressor goes to its minimum displacement setting about 5% or less. the system is still circulating but causing no noticeable drain on engine power or mpg.  

 

Hope that helps! 

 

Hi Adam, that's one of the clearest explanations I've seen yet for the system operation, thanks. Given that you say that there is always a minimum of about 5% circulation from the AC compressor, apart from dehumidifying, there would be absolutely no need to run the compressor at regular intervals during the winter- have I got that right?    

14 hours ago, Warrior193 said:

Hi Adam, that's one of the clearest explanations I've seen yet for the system operation, thanks. Given that you say that there is always a minimum of about 5% circulation from the AC compressor, apart from dehumidifying, there would be absolutely no need to run the compressor at regular intervals during the winter- have I got that right?    

Thank you!

i would take the 5% an absolute max, actual value probably less depending on compressor and vehicle manufacturers. But what I was trying to highlight was how the compressor load is variable. There is definitely some 'bleed' circulation so that can't be a bad thing for lubing seals etc but I'm not gonna stick my neck out and say you don't need to run the a/c in the winter! 

Considering a car a/c system is a big dehumidifier, and its more damp in a car in the winter, it makes sense to use our a/c systems more in the winter now we know they don't hurt economy as much. 

 

2 hours ago, MrAdamwood said:

Thank you!

i would take the 5% an absolute max, actual value probably less depending on compressor and vehicle manufacturers. But what I was trying to highlight was how the compressor load is variable. There is definitely some 'bleed' circulation so that can't be a bad thing for lubing seals etc but I'm not gonna stick my neck out and say you don't need to run the a/c in the winter! 

Considering a car a/c system is a big dehumidifier, and its more damp in a car in the winter, it makes sense to use our a/c systems more in the winter now we know they don't hurt economy as much. 

 

I shall have to try my Climate Control on a longer trip while the temperatures are still low - but I am currently seeing an increase in consumption of 10-15% when I run my AC at a cabin setting of 18-20 deg.C  with OAT of -3 to +8.  

Is the A/C actually running as A/C when the Ambient temperatures are 2*oC and below ?

4 hours ago, Awayoffski said:

Is the A/C actually running as A/C when the Ambient temperatures are 2*oC and below ?

Well something obviously is with mine, as per my previous posting. Even if it is only acting as a dehumidifier, it's making a measurable increase in fuel consumption when switched on. 

22 hours ago, Warrior193 said:

Well something obviously is with mine, as per my previous posting. Even if it is only acting as a dehumidifier, it's making a measurable increase in fuel consumption when switched on. 

Actually that all makes sense.

Early in February I did a trip to the South Australian Murray Riverland and on the return trip we experienced very unusual weather for us ....rain.

So we had overcast conditions, light rain, warm temperatures of around 24 degrees C and humidity around 98 to 100% from the tropical air directed over us.

My Octavia Mk3 does not have climate control just a simple air-conditioner so this was turned on to reduce the uncomfortable humidity with the fan set at 'one', the lowest setting and directed at the windscreen because it was too cold for comfort out of the vents.

The problem was it chilled the windscreen so much that condensation formed on the outside which even the wiper could not clear.     

The obvious solution was to turn up the heater and recirculation on.

Consumption was unusually heavy for the trip which I attributed to running through standing water as it was more than just running the a/c in hot (dry) weather.

So the dehumidifying effect of having both heater, a/c and recirc on would have all the elements 'fighting' against each other and increasing fuel consumption.