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Old 10-07-2017   #16
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Re: Aircon light

Quote Originally Posted by AndyRKett View Post
Yeah I'll concede the insurance point, but stand by the rest
............ and I agree with you on the rest too.
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Old 10-07-2017   #17
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Re: Aircon light

Our Fiat does not have ac but the Mazda has, I'm more that happy to spend that little extra for a cool car, given its not that great on economy especially the way it's driven I don't notice any increased fuel consumption 30-32, about 42 MPG on the motorway
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Old 10-07-2017   #18
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Re: Aircon light

The basic calculation is quite simple.

Petrol has an energy density of about 9.6kWh per litre; a modern engine like the 500 will likely be about 25% efficient, so a 2.4kW A/C compressor running at full power will burn about a litre of fuel each hour.

Now most of the time the compressor will be cycling on & off; if it's working 50% of the time (and 1.2kWh is a reasonable figure for the energy needed to keep a small car cool on a summer's day) then you're buning an extra half litre of fuel an hour. Cruising at 55mph, I'm using a little under 4.5l/hr for propulsion, so that extra half litre an hour broadly agrees with the 10% mpg reduction I've measured over a number of different trips.
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Old 10-07-2017   #19
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Re: Aircon light

For those thinking about the air con light . Isn't very cold air coming out of the vents enough indication that the AC on?
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Old 10-07-2017   #20
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Aircon light

Quote Originally Posted by jrkitching View Post
The basic calculation is quite simple.

Petrol has an energy density of about 9.6kWh per litre; a modern engine like the 500 will likely be about 25% efficient, so a 2.4kW A/C compressor running at full power will burn about a litre of fuel each hour.

Now most of the time the compressor will be cycling on & off; if it's working 50% of the time (and 1.2kWh is a reasonable figure for the energy needed to keep a small car cool on a summer's day) then you're buning an extra half litre of fuel an hour. Cruising at 55mph, I'm using a little under 4.5l/hr for propulsion, so that extra half litre an hour broadly agrees with the 10% mpg reduction I've measured over a number of different trips.


Energy for heating or cooling is measured in BTUs or British thermal unit. 1 BTU there are several explanations of this but it's basically 1 BTU is equal to about the energy released by a match.

To cool a 100 sq:ft room you need something like a 5000btu air conditioner. A fiat 500 is considerably smaller than this but cars are more complex as they are generally badly insulated and act like mini moving green houses car air con units are capable of very high levels of thermal cooling, but an aircon system like a car getting up to speed, will use more energy to get the temperature down but once the temp is down to what's require it can cruise, using very little energy. Further points you should note l, that mechanical power is not directly comparable to electrical power. The amount of power the system uses varies massively depending on the conditions inside and outside the car and within the refrigerant within the system.

You can't simply pluck some figures out of the air, make some assumptions and come up with some numbers that suite you. As simple as it seems, squeeze expand and cool, aircon is amazingly complex and the maths involved in working out these complexities goes well beyond some simple number crunching on a forum.

One degree in ambient temperature change either inside or outside the car will totally change the equations and the amount of energy needed to keep the car cool.
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Last edited by AndyRKett; 10-07-2017 at 22:30.
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Old 11-07-2017   #21
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Re: Aircon light

Quote Originally Posted by AndyRKett View Post
Energy for heating or cooling is measured in BTUs or British thermal unit. 1 BTU there are several explanations of this but it's basically 1 BTU is equal to about the energy released by a match.
BTU's are one of those wonderfully quirky Imperial units, originally defined as the amount of heat required to raise the temperature of 1 pound of liquid water by 1 degree Fahrenheit at a constant pressure of 1 atmosphere*. They've not been used by the scientific community since the 1960's but are still sometimes used in HVAC circles as some older folks have an intuitive feel for them, in the same way some of us can better estimate length in feet and inches and weight in pounds. 5000 BTU is equivalent to 1.465 kWh, or a shade under 2HP.

I have a vague memory of having been told the match equivalent somewhere in the dim and distant past of my youth.

*not a precise definition as this also depends slightly on the initial temperature of the water.
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Old 11-07-2017   #22
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Re: Aircon light

Quote Originally Posted by jrkitching View Post
Now most of the time the compressor will be cycling on & off; if it's working 50% of the time (and 1.2kWh is a reasonable figure for the energy needed to keep a small car cool on a summer's day) then you're buning an extra half litre of fuel an hour.
This is the issue here.

How cool?
How about keeping warm?
What temperature do you want?
All these things are variable.

Rarely, do we ever have the aircon in either car set to minimum. Mostly, it's set to warm, and only set to cool on a sunny summer day.

Today for instance. It's cloudy cool and damp out there. The temp outside now at going on 07:30 it is 16degC. If I were to drive away now, the temp control in our 500TA with automatic climate control would be set to maybe 20degC. The manual system in the Clio would be set to mid-way.

Yes, it's a complicated subject, and cannot be put down to absolute numbers.

Hope the weather cheers up!
Mick.
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Last edited by Mick F; 11-07-2017 at 08:10. Reason: Typo
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Old 11-07-2017   #23
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Re: Aircon light

People, its simple! If you have air con installed in your car, you exercise your right to creature comforts (or not) as you see fit- its your car, your money, your hot/cold self in that seat! But its an interesting debate.... Go on...
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Old 11-07-2017   #24
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Re: Aircon light

Like Mick I've left the aircon permanantly on in all my cars for years. I was advised to do it by a couple of aircon specialists as it reduces the chances of leaks and smells. By coincidence Mick I ran a 2003 Clio for 9 years (with my daughter) and when she sold it the aircon was still blowing cold and was never re-gassed. Don't know how we'll all get on with the new R1234 refrigerant, which is more prone to leak, is flammable and costs four times as much to re-gas. My own experiments could not detect a difference with aircon on or off.
To go further off-topic, the 500x I have now has a PDC heater as part of the climate control, which gives warm air within about a minute of start-up in the winter. Excellent feature, but probably another thing to hurt mpg!
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Old 11-07-2017   #25
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Re: Aircon light

It takes energy to cool down a hot car or heat up a cold one for that matter, that energy has to come from somewhere and that somewhere is your petrol tank. Obviously prevailing local conditions along with the setting selected on your AC will dictate the load. I would imagine if I was Ahmett in Athens my mpg would be affected considerably. I've never done any 'scientific' tests but from experience leaving my ACC on and set to 21 degrees seems to reduce my mpg by about 2mpg over a tank- I clearly always get better mpg with it off without doubt and the conditions here are certainly not extreme. Mine is left off but I run it fairly regularly to keep lubed up etc. Personally if the car is warm I prefer to open the sunroof!
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Old 11-07-2017   #26
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Re: Aircon light

One of the main bonus of aircon is to remove moisture from the air, so you get cool dry air out the vents, it's well known that when you stop this moisture which collects in the fins of the evaporator and usually drains harmlessly out the bottom of the car, will sit and grow mold when the car is off, it's nice and close to the heater Matrix which is nice and warm long after the aircon is switched off.

Turning on and off the aircon means you're even more likely to grow mold and nasty bugs in this moisture. Keeping the aircon switched on, however keeps the moisture in the evaporator and flowing out the bottom of the car.

If you drive a long switching on and off the aircon, each time the air con is switched off, the collected moisture will renter the dry cabin air, this means with the vent blowing warm air you will be spraying yourself liberally with a fine mist of bugs and mold.

And that is the most important reason for keeping the air conditioning on all the time.

As stated over and over now by numerous people, you can't in any scientific way get any sort of accurate reading on how much increased fuel use is caused by having the aircon switched on, definitely not, to the extent of accurately saying it's 1p per mile. As stated before, if it was 3-4p a mile difference then you could probably attribute the bigger difference to being in relation to the aircon, but 1p could just be extra fuel use because ok hot days the engine uses a bit more fuel aircon or not and it's a coincidence you use the aircon on those days.
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Old 17-07-2017   #27
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Re: Aircon light

Always a good LOL when the topics of A/C or fuel use comes up.

Let's just say there's a reason why some of us can beat the NEDC by 50% and other's can't even get within 50%...

Here's the SAE paper referenced in JR's post:

http://www.sae.org/events/aars/prese.../2004-hill.pdf

Society of Automotive Engineers, what would they know?
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Old 17-07-2017   #28
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Re: Aircon light

Quote Originally Posted by UFI View Post
Society of Automotive Engineers, what would they know?
They may "know" but they didn't state the temperature of the interior.

That is the very point of AirCon. It's variable. The greater the load on it, the more power it consumes. Maybe their cars had an on/off switch rather than a temperature control?

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Old 17-07-2017   #29
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Re: Aircon light

Quote Originally Posted by UFI View Post
Always a good LOL when the topics of A/C or fuel use comes up.

Let's just say there's a reason why some of us can beat the NEDC by 50% and other's can't even get within 50%...


It takes a lot of power to maintain a comfortable temperature inside a car on a hot sunny day. With the windows shut on a 30C day, the interior temperature will exceed 45C within half an hour. Essentially it's the same as running an air conditioner in a greenhouse.

Some folks just don't want to hear the truth. You might as well try to convince a creationist that the planet we live on is more than 6000 years old.
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Old 17-07-2017   #30
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Re: Aircon light

Quote Originally Posted by UFI View Post

http://www.sae.org/events/aars/prese.../2004-hill.pdf

Society of Automotive Engineers, what would they know?

An 8.1 litre V8 SUV and a 4.6 litre V8 sedan..... in 2004, definitely a direct comparison to a 1.2 litre 2015ish fiat 500.....

I LOL too !!
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