I'll leave this here, I think. There's no point in presenting good science to someone who isn't listening with an open mind.
It’s not good science, it’s an open access journal, and a paper written by someone with a gmail account based in India... and it’s one paper with one case study that you’re using to try and prove a rule across all cars of all makes and models, the Wagon R isn’t modern by any stretch of the word.
When I look at my fuel consumption figures they are about 3-4 mpg lower than average so about 10% higher than expected. In reality as the engine is never running at full pelt it will be producing less output, let's say its average is 20kw to allow for hills and acceleration full loads carried etc, now I'm using 10% more fuel so a really rough calc means I'm using an extra 2kw somewhere, so using my aircon most of the time is perhaps the culprit.
There is no way on Gods earth an aircon system only uses 200 watts, why do you think the ECU compensates so much when it engages?
Im in total agreement with JRK.
Ok so an airconditioning system uses most power when it is under the highest load, when a car has an interior temperature if say 40’c, the outside temp is 30’C and the desired inside temperature is 20’C, then the system does this by compressing and expanding gas, the more heat the gas takes out of the car the more it wants to expand as it is carrying more energy and therefore the more power is needed to compress it it again, that extra energy is wasted as it’s radiated off by the condenser. As the temperature of the interior of the car reaches the desired temp, and you switch it to recycle the air, the target of 20’C is still the same but the amount of effort the air condiontion is having to put in would be minimal, reducing the ambient air temp from 21’C to 20’C the gas does not expand as much the demand on the aircon system is much lighter and therefore hardly any energy is needed to keep it ticking over.
So reducing the air temperature by 1’C obviously uses considerably less energy than dropping the temperature by 20’C over the same time period.
Stir a big pot of water with a spoon you’ll need energy to get the water moving, stop stirring and the water will then push the spoon round, same thing happens in aircon if the demand is low then next to no energy is needed to compress the gas.
The lower the output of the engine or driving economically then the difference will be greater. Before I got the aircon regassed on my 177bhp Rover 75 my average fuel consumption was 20mpg after it was sorted and the following year it dropped to 19 mpg. The lower power engine will quite clearly be working harder.
Which is it 3-4mpg or 1mpg
1mpg over the course of a week doing the same trips is not really of any concern using the aircon over the course of a year versus not using it and only getting a 1mpg change in economy isn’t even worth mentioning. The rover 75 (I assume it’s the v6 you) was a horribly dated and inefficient engine by modern standards and had an old style aircon system, and a relatively large cabin. So A ~5% increase over the course of year, not taking any other factor into to account such as wear and tear on the engine, tyres, tyre pressures, wind speed, air pressure and temperature, the amount the aircon contributed to the change may be considerably less than 5% (or more I concede) it’s not exactly however, a yard stick by which you can measure all cars. Especially not ones made 15 years later. These days the system is matched to the engine it’s used with, in the past they would put essentially the same aircon compressor into every car, with only differences made for mounting.
I'm confused - am I the only one. What I know is that my previous 100HP had climate control on permanently, except when I turned it off to see if the mpg changed. It didn't by a noticeable amount. Likewise the aircon in my current 4x4 TA - no significant difference.
The key word you mention is “significant” you can drive the same route to work every day for a month at the same time at the same temperature and still see a difference of 1-2mpg because of wind speed, things like rain or air pressure lots of things can make little difference to economy. Modern air conditioning does not make a “significant” difference. I notice no significant effect on fuel economy with any of my cars while using air con.
A modern ECU can compensate for the additional load by injecting more fuel when the compressor kicks in; this takes away the noticeable power loss you could feel with first generation systems (unless you're already demanding full power from the engine). You don't notice the additional power drain so much, (because the engine automatically compensates), but it's still there, and the impact on fuel economy is much the same.
The additional fuel still has to be accounted for; intelligent sensing can maximise the power when you most need it, but the total energy needed (which directly equates to fuel use) has to be made up later. For example, you can reduce the instant consumption by temporarily disengaging the alternator, but it'll have to work harder when you reengage it to recharge the battery, and that will burn the fuel you saved in the first place.
What will save fuel are systems which make use of energy which would otherwise be wasted; regenerative braking being one obvious example.
Modern systems can turn off and on the alternator as and when it will gather power most efficiently, for example when coasting in gear using no fuel at all the alternator can gather free power, but then switch off for the next up hill stretch. Old cars the alternator would be running near constantly trying to put charge into a full battery and generating heat that would be waste.
My VW has regenerative braking and so when driving normall there is little power going to charging the battery but when breaking the power is shunted into the battery to charge it rapidly to minimise power loss, not something fiat are doing but things like a freewheeling alternator do a basic version of this.
The car can be run leaner or richer by the ecu with no decernable increase or decrease in power but can have a significant affect on the fuel economy. Timing can be adjusted to change the power characteristics needed for when the aircon is on versus off. It’s a bit more complex than just increasing fueling to meet demand.
Newer aircon systems are more and more being run electronically, this means they can run at a constant speed without switching in and out, the motor can vary the speed of the pump to meet the environmental conditions. They also have variable compressors which can compress more or less at the same engine speed so that switching in and out is not needed again saving fuel and matching the air conditioning perfectly to the conditions to allow maximum economy, again this can be match so that when getting a move on the aircon is demanded less and when rolling in gear more power can be sent to the pump to gather a little, what would be otherwise lost energy. Also the engine can switch a mechanical aircon compressor in and out to match the torque characteristics of the engine, making use of wasted torque, or switching out when you are accelerating and need the extra boost, in the past the aircon would only switch on and off with wide open throttle, but you can still accelerate hard without fully pressing the pedal into the carpet, and while you do this the aircon saps power and makes the car less efficient, take you foot off the throttle and the aircon comes in and makes use of overrun energy. Also new compressor designs with better tolerances all reduce the amount of fuel and drag on the engine, needed to power them.