AndyRKett said:

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.

Click to expand...

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.

jrkitching said:

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.

Click to expand...

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.

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...

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!

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!

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.

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/presentations/2004-hill.pdf

Society of Automotive Engineers, what would they know?

UFI said:

Society of Automotive Engineers, what would they know?

Click to expand...

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?

Regards,
Mick.

UFI said:

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%...

Click to expand...

:yeahthat:

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.

UFI said:

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

Society of Automotive Engineers, what would they know?

Click to expand...

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..... [emoji849]

I LOL too !!

AndyRKett said:

An 8.1 litre V8 SUV and a 4.6 litre V8 sedan..... definitely a direct comparison to a 1.2 litre 2015ish fiat 500..... [emoji849]

I LOL too !!

Click to expand...

In percentage terms, the 500 will likely be worse then the SUV.

AndyRKett said:

in 2004

Click to expand...

The sun's power hasn't changed much since 2004. The amount of heat you need to remove to keep the car cool in summer hasn't changed much either.

jrkitching said:

In percentage terms, the 500 will likely be worse then the SUV.



The sun's power hasn't changed much since 2004. The amount of heat you need to remove to keep the car cool in summer hasn't changed much either.

Click to expand...

Back in 2004 and not forgetting we are talking about V8 engines at 8.1 litres and 4.6 litres, people buying these cars and the time period, where not concerned about gas mileage. They are big vehicles requiring a lot of energy to cool, they are likely built with all the aerodynamic properties of a barn and all the technological prowess of a coal fired power station.

Verses a new little car where fuel economy is a high priority, technology has moved on with more advanced technology. Smaller cabin and much more aerodynamic little car, yes there will be a huge difference between the cars in this very old study and a current day little fiat.

No need to carefully engineer and match components for best all round usability, we'll just fit the biggest aircon compressor we can find to cool down our road barge and the engine will have plenty of power to turn it over.

AndyRKett said:

Smaller cabin and much more aerodynamic little car, yes there will be a huge difference between the cars.

Click to expand...

Are cars AS aerodynamic now, as say the early 90's ? Where everything was totally flush with the body ( like no door handles sticking out or wipers that sat below the bonnet line) headlights that were sealed to the bonnet edge raked screens and rounded corners, and 0.3 drag coefficient was a selling point? Can't help feel modern cars are not as slippery as older ones?

John202020 said:

Are cars AS aerodynamic now, as say the early 90's ? Where everything was totally flush with the body ( like no door handles sticking out or wipers that sat below the bonnet line) headlights that were sealed to the bonnet edge raked screens and rounded corners, and 0.3 drag coefficient was a selling point? Can't help feel modern cars are not as slippery as older ones?

Click to expand...

The 500 isn't particularly aerodynamic; IIRC the drag coefficient is about 0.36, which is why fuel economy drops substantially once you go over about 55mph. It was designed as a city car, after all.

The best of the modern motorway cruisers will have a Cd of around 0.25. This is a few years old now, but still relevant.

John202020 said:

Are cars AS aerodynamic now, as say the early 90's ? Where everything was totally flush with the body ( like no door handles sticking out or wipers that sat below the bonnet line) headlights that were sealed to the bonnet edge raked screens and rounded corners, and 0.3 drag coefficient was a selling point? Can't help feel modern cars are not as slippery as older ones?

Click to expand...

We'll early 1990s cars we're still left over bricks from the 1980s, the likes of the panda, uno and tipo are prime examples of that, later cars like the punto where more aerodynamic but these were not really 'early 90s' cars and were more late 90s,

However what we are talking about In relation to the above article is a early 2000s car, more specifically an 8.1 litre SUV which looks something like this




It's probably worth noting that the design of these cars also relates back to the 80s and even 70s without any major update to the shape and aerodynamics. Just for comparison here is a 70s version



This is not an aerodynamic car as cars go, parachutes have a slightly better aerodynamic coefficient. Opening the window on a car like this is never going to improve or worsen its aerodynamic properties and therefore you'll be unlikely to see any significant deterioration in fuel economy windows up or down.

It is however so large that it will require an industrial air conditioning unit to make even the slightest dent on the ambient interior temperature.

Basically the one study people are quoting to try and prove their rule, has no more basis in this discussion than the colour of the sky or the latest pop group to come out of South Korea. A huge 2004 America SUV is not a small efficient European hatchback made much later.

The 500 has a very primitive A/C, it's 90's tech - doubly so for the manual system, the 500 is based on a 2003 Panda, not that different to the tech you'd find in a mid 00's American.

The 500 has a CD of 0.36, pretty much line ball with the 0.38 of a Tahoe or Suburban. Even on a CdA basis, it's clear that the 8.2 will have power to spare over that poor little 1.2. You can't tell aerodynamics by looking, a Jaguar E-Type is as aerodynamic as a Jeep Wrangler. Today's mid-large vans are in the 0.31 range - just a box on wheels.

The interior volume of a 'full size' US sedan is not that dis-similar to a 500.




The main difference is the 4.6 will barely feel the drain of an A/C compressor. My 4.0 sure doesn't.


If you've got a 1.2 (not a MJ or TA) or possibly a 1.4, try this, turn the A/C on in say 3rd at 1500rpm and floor the throttle. Slow down and try again in with the A/C off. That's how much power an A/C compressor saps when it's working.

It's like night and day. Some cars (and the TA is one of them) will compensate for A/C usage by changing the throttle mapping, so you can't tell if the A/C is on or off, but the extra drain is still there.

A/C automatically cycles on and off so no idea how that's different for growing bugs vs turning it off yourself? My Renault completely ignores the A/C button and won't run the A/C if the selected temp is above ambient - to the point where on a wet day I have to turn the temp way down and freeze if I want it for demisting.

I suspect that you guys think you have the A/C on when in reality all you've got is that light on the dash.

Also, we all know petrol cars are more efficient in hot weather. Diesel engines might like cold air, but even then you're loosing out on longer warm ups, thicker fluids, denser air molecules, generally increased electrical loads and drag from wet roads.

Here's more from SAE:

http://articles.sae.org/12053/

Strangely, I can't find a single piece of evidence that A/C makes no measurable difference as is often suggested on various forums.

If you can drive consistently, you can pick up pretty small things:



The slight dip (from a linear progression) in May is because I tried running without the DPF cleaner - looks like I lost 1mpg from that.

UFI said:

The 500 has a very primitive A/C, it's 90's tech - doubly so for the manual system.

The 500 has a CD of 0.36, pretty much line ball with the 0.38 of a Tahoe or Suburban. Even on a CdA basis, it's clear that the 8.2 will have power to spare over that poor little 1.2.

The interior volume of a 'full size' US sedan is not that dis-similar to a 500.

http://4.bp.blogspot.com/-BsIMeGPnaSQ/T5hc0c_gqwI/AAAAAAAHeKg/oIGtVJDe_5A/s800/Spark-o-Graphoic-2%2560.jpg

If you've got a 1.2 (not a MJ or TA) or possibly a 1.4, try this, turn the A/C on in say 3rd at 1500rpm and floor the throttle. Slow down and try again in with the A/C off. That's how much power an A/C compressor saps when it's working.

It's like night and day. Some cars (and the TA is one of them) will compensate for A/C usage by changing the throttle mapping, so you can't tell if the A/C is on or off, but the extra drain is still there.

A/C automatically cycles on and off so no idea how that's different for growing bugs vs turning it off yourself? My Renault completely ignores the A/C button and won't run the A/C if the selected temp is above ambient - to the point where on a wet day I have to turn the temp way down and freeze if I want it for demisting.

I suspect that you guys think you have the A/C on when in reality all you've got is that light on the dash.

Also, we all know petrol cars are more efficient in hot weather. Diesel engines might like cold air, but even then you're loosing out on longer warm ups, thicker fluids, denser air molecules, generally increased electrical loads and drag from wet roads.

Here's more from SAE:

http://articles.sae.org/12053/

Strangely, I can't find a single piece of evidence that A/C makes no measurable difference as is often suggested on various forums.

Click to expand...

With the 1.4, if you turn off ac and let the car go down the hill at 50 kmh in 3rd or 4th, the car slows down much quicker with AC on , it's like you hardly need to press the brake to slow down!

Consequently, with the increased load you have to use the throttle much more than with it off, i estimate 0.5l/100 km easily with the AC at power setting 2, and a good 1.0l/100 km with the AC on full blast

AndyRKett said:

<SNIP>

You can't compare an electric car air con compressor, it requires a dedicated motor just to spin the compressor and keep it going without any other outside forces acting upon it. So all the time the aircon is on, the motor needs to be running and burning fuel, it's not practical to keep switching a motor on and off so the system will be balanced to operate constantly with the motor giving a set speed.

<SNIP>

Click to expand...

This is not correct. Modern motor driven AC compressors (not just car ones) use brushless DC motors with electronic control. True they run all the time AC is required, but they run just fast enough to provide the cooling required. Much more efficient than turning a big compressor (motor or engine driven) on and off system losses are lower at lower speeds. Some electrics have heat pumps that give you 3kW of heat for less than 1kW of electricity in winter and cooling in summer.


Note that climate control is normally more efficient than plain AC, the system has more control and does a better job than most humans can.
I never notice the climate control coming on in terms of performance, driving a Croma with a 150hp mJTD helps and overall consumption is in the ballpark of what some 500 and Panda owners are reporting. big engine driven gently can have advantages over a small engine being driven hard to keep up with real life traffic conditions.
Apart from aerodynamics, open windows cause other issues with noise (bad low frequency resonance on a lot of cars) distraction, pollution and safety.

Robert G8RPI.

UFI said:

The 500 has a very primitive A/C, it's 90's tech - doubly so for the manual system, the 500 is based on a 2003 Panda, not that different to the tech you'd find in a mid 00's American.

The 500 has a CD of 0.36, pretty much line ball with the 0.38 of a Tahoe or Suburban. Even on a CdA basis,


If you've got a 1.2 (not a MJ or TA) or possibly a 1.4, try this, turn the A/C on in say 3rd at 1500rpm and floor the throttle. Slow down and try again in with the A/C off. That's how much power an A/C compressor saps when it's working.

Click to expand...

Just to address these three points.

The basics of air conditioning means that climate control or not, it will generally use the same compressor, evaporator, condenser and expansion valve, these are the main components that decide how efficiently the system will work, I know from direct experience that older cars have more Heath Robinson set ups with oversized compressors, more flexible rubber hosing and huge evaporators or undersized condensers. So no matter how primitive you think the systems in modern cars, are they are much more efficient these days as the components are designed to all work together.

The surburban above has a Cd of 0.45 so a long way off what you're suggesting, further more the drag coefficient does not directly relate to what we are talking about here nor the energy needed to move through the air. A Boing 747 has a much lower Cd than a fiat 500 but I can guarantee that when moving at 30mph the 747 will have a lot more surface area and therefore air, pushing back against it.

The Cd is not a simple way of comparing one vehicle to another, unless you've got a degree in fluid dynamics, and you want to factor in all the other variables.

Finally if you put any car in any gear and floor it with the aircon off, and with the air con on, there should be no noticeable difference between the two as cars are designed to kill the AC at wide open throttle to give maximum power to the engine, as long as your foot is buried in the carpet, you won't have aircon.

This argument is getting silly now, trying to compare a massive V8 to a small fiat 500 is like chalk and cheese. Basically the research done above is nonsense in modern, normal European cars.

jrkitching said:

Some folks just don't want to hear the truth.

Click to expand...

I agree.

I agree that the more the load on it, the more it effects the fuel consumption. It's obvious, isn't it. I don't think anyone is arguing with this fact.

The experiment linked up thread, had the ambient temp at 24degC .......... a rather warm and rare summer's day here in Cornwall.

I said up thread, that we rarely ever have the AirCon set at the extreme ends. Generally, it's set in the 500TA at 20degC ................. so it's hardly going to affect the fuel consumption at all.

Mick.