Air Conditioning & Fuel Consumption

[Rabbers]

I keep the air conditioning in my car permanently on, and, depending on the time of year, I set the cabin temperatures somewhere between a minimum of 18°C (64°F) for short bursts and a maximum rarely much higher than a year-round 21°C (70°F).  With daytime temperatures these past few weeks in my area having averaged around 35°C (95°F) with midday highs of 39°C (102°F), I have kept the cabin, once cooled, at the customary 21°C, noticing a worsening of the displayed average fuel consumption from my customary 14.8-15.2 km/l (42-43mpg) to 14.3-14.5km/l (40.5-41mpg).  I have seen hot-weather increases in fuel consumption in the past but none quite so measurable.

Having read that temperatures in parts of the U.K. are heading in much the same direction, though perhaps not quite to the same levels, I thought these figures worth reporting.

 

[Anevb]

I think dropping only 1.5-2 mpg as a result of using the AC is quite a good achievement. I wouldn't call this a big drop in economy at all.
I am used to 5 mpg drop. Although, this was on low-powered 1.4 petrol car so the % of engine power that the AC used was more than my current 450h.

I don't know what effect AC has on my car but considering that we in UK get 3 days of sunshine a year, I just use the AC without thinking.

[NemesisUK]

TBH I think a change in wind direction is likely to affect mpg more. I guess it's because the a/c compressor is electrically driven so any effect on mpg is somewhat clouded by the hybrid interactions.

 

[Las Palmas]

I believe the AC is consuming much power and it is starting engine very often to charge hybrid Battery.

Have AC on 23° C all year so difference between driving the car and going out of it is not too extreme.

Often sit and listen to music while wife is shopping and engine start rather often. Never more than 10 minutes or so between starts.

This evening it was not hot and I was again waiting in the car listening to music and tried not to have AC on but a window and sun-roof open. 35 minutes listening to music and engine did not start once.

AC (with fan) is probably part of poor fuel efficiency.

[Rabbers]

There are lots of variables governing fuel consumption, including  the effect of air conditioning on it, and, obviously, the frequency and duration of fan activity is a major one.  And, be it automatic or manually requested, fan activity is directly related to outside temperatures.  Given that these have been substantially and consistently higher than normal so far this summer, it is not illogical to suppose that they are responsible for a noticeable - though by no means dramatic - worsening of fuel consumption in respect of the barely detectable ones usually experienced.  It could be added that any observed increase in summer fuel consumption for whatever reason would itself be net of the improvement expected in respect of winter.

[Linas.P]

14 hours ago, Las Palmas said:

I believe the AC is consuming much power and it is starting engine very often to charge hybrid battery.

This precisely. On straight-petrol car the difference is less noticeable, but that is because it is generally less efficient without hybrid system. The difference on hybrid is that without A/C your car is quite efficient with engine-start stop (much more efficient than petrol start-stop which always seems to engage in wrong time), but if you have A/C on, this means engine is kept running for A/C instead of starting and stopping like the hybrid should. 

So basically it is not like A/C is using 0.2 of extra fuel, but having it ON does not allow hybrid system to work optimally. But 5.47-5.6/100km drop to 5.7-5.8/100km between A/C ON and OFF is very good result i.e. having A/C on only uses between 0.1-0.33L for 100km of driving. Generally speaking, the larger is the engine on the car and the faster you driving (the more loaded it is), the less of an impact A/C will have i.e. small engine car in traffic could consume a lot more fuel because of A/C, but large engine car on motorway may see little to no difference. I always guesstimated A/C fuel consumption to be between 1-2% on engines above 2L in displacement, which seems to be supported by your figures. 

That said Lexus thought about it and in most (if not all) Lexus hybrids you have "eco mode" on A/C, which basically prioritises hybrid system efficiency over A/C performance. However, I would advise to avoid it - the eco mode on A/C allows engine to stop (and preserve fuel) instead of keeping it on for A/C, so yes it will save fuel, BUT if you in start stop traffic it will fog-up the car horribly, because A/C will stop every time engine stops, but ventilation continues. Then all the humidity accumulated in condenser, instead of being condensed and draining out will get's blown back into cabin. It is actually better to drive without A/C at all, than to use eco mode in my opinion, but obviously the best is to keep A/C on all the time, best for you, best for A/C and as it turns out not really that much of impact on fuel economy. 

[Las Palmas]

1 hour ago, Linas.P said:

obviously the best is to keep A/C on all the time, best for you, best for A/C and as it turns out not really that much of impact on fuel economy. 

Absolutely best for me. Here temperatures are normally above 30° C in the day and open windows will not cool down plus will fill the car with dust, sand from Sahara and that is not fun to clean. I think that AC in a CT is consuming more than you believe it does.

Fuel economy is so poor where we live that it is not much difference AC on or off. Today I visited friends only 500m above sea level and on the way back home hybrid Battery was full after 500 - 1000 m and was full all the way to garage. Close to 10km with hybrid Battery full so even though driving slow (due to traffic) it did gain nothing having a hybrid. Possibly better not to have the weight of the Battery as going up it is no use and going down it is - no use. On roads not going up or down it probably is fine, but how would I know? Still the car is great and will not be replaced till something better come. Hydrogen combustion is what I would like, but that seems to be mostly for ships and heavy trucks.

[Bluemarlin]

I leave my A/C on all the time, so have no comparison, but I'd guess that turning it off and having the windows open instead would result in me trying to drive faster to keep cool, rather than just cruising gently in comfort, so any gain from having no A/C would be lost by a heavier right foot.

I also make sure it's in recirculation mode so that it's not having to cool hot air from the outside.

[Linas.P]

11 hours ago, Bluemarlin said:

I also make sure it's in recirculation mode so that it's not having to cool hot air from the outside.

It always amazes me how many people don't even know what it is or simply do not use it. Likewise I always insist on all windows being closed in A/C is ON - "darling either you have your window open, or A/C" 🙂 

I use recirculation for two things 1. it is just more efficient 2. it prevents dirty air from traffic entering the cabin (or at least significantly reduces it). So generally if I can I leave window slightly open (or back of sunroof lifted) when car is parked, I start driving in quiet dead-end street to ventilate the car and then I turn on recirculation for the rest of the journey to make sure it is mostly the relatively clean air which is as well being filtered constantly. So it is in my opinion healthier as well, but I haven't seen many drivers doing it - they just sit in the traffic sucking-up the exhaust from the car ahead of them and it doesn't seem like it bother them at all!

And sure even outside air is being filtered, but filter can remove CO₂ nor NO_X, nor even smaller particles like soot. 

Other thing I noticed - most German cars has outside air by default and you can't force them to stay in recirculation, which it seems they consider like temporary function and then just turns it off right before some mo**** comes in front of you with his diesel and starts rolling coal in front of you. Either that or they have start-stop and every time engine starts again it goes back to outside air... which drives me nuts!

[Rabbers]

17 hours ago, Bluemarlin said:

…. I also make sure it's in recirculation mode so that it's not having to cool hot air from the outside.

I find the system’s switching between outside and recirculated air modes works well automatically, and I rarely find myself needing to manually override it.  An exception regards the “sudden” presence of exhaust gases from preceding vehicles, e.g. upon entering a poorly ventilated tunnel, when the switching is not sufficiently immediate to prevent the fumes getting into the cabin despite the sensitivity being set to maximum in the customization menu.

[DTR]

Hi

By chance this popped up this morning;

https://www.honestjohn.co.uk/askhj/answer/171873/how-much-fuel-does-air-conditioning-use-?utm_medium=email&utm_campaign=Newsletter 21 July 2022&utm_content=Newsletter 21 July 2022+CID_5700bbda13ffb88c6b22c0c981e8660e&utm_source=campaign monitor&utm_term=How much fuel does air conditioning use
 

Obviously not aimed specifically at hybrids, but may be worth taking into account. I was amazed with the suggestion to partially open the windows at start up. Certainly when it was recently very hot, the fan was going berserk.I find it easier generally to switch the system to Auto and leave it like this. I have noticed that if there is nobody in the passenger seat, the temp readout ion that side is extinguished. Does that mean it isn’t cooling that side of the car? Weird.

 

[NemesisUK]

52 minutes ago, DTR said:

I was amazed with the suggestion to partially open the windows at start up.

On very hot days I always open all windows and the sunroof for the first 5min or so, just dissipate the hot air in the cabin. One can feel when the cabin is cooling and then I close the windows. At this point I would switch on the ventilated seat, to draw the cooler air from the footwells. Sunroof stays open until speeds get above 30mph, hot air rises so helps remove the residual 'parked heated air' once above 30 mph one can feel the warmer outside air being drawn back down into the cabin. Climate control is always set to Auto everything... Must say the RC (OK it's a small cabin volume being a coupe) is one of the most effective climate systems I've experienced. Add to that there's no screaming protestation (cracking, clicking, creaking) from the ductwork either! 

[Linas.P]

5 hours ago, NemesisUK said:

On very hot days I always open all windows and the sunroof for the first 5min or so, just dissipate the hot air in the cabin. One can feel when the cabin is cooling and then I close the windows. At this point I would switch on the ventilated seat, to draw the cooler air from the footwells. Sunroof stays open until speeds get above 30mph, hot air rises so helps remove the residual 'parked heated air' once above 30 mph one can feel the warmer outside air being drawn back down into the cabin. Climate control is always set to Auto everything... Must say the RC (OK it's a small cabin volume being a coupe) is one of the most effective climate systems I've experienced. Add to that there's no screaming protestation (cracking, clicking, creaking) from the ductwork either! 

I think Lexus generally having high performance A/C is something they do because of american influence. In US A/C being ice cold is very important criteria and Lexus being US-centric product puts a lot of focus on that. 

The suggestion that car is using 5-10% on A/C is a bit wild, only in exceptional circumstances like having large car (like SUV) with tiny engine and driving in start stop traffic that could be true. Sadly such cars are more an more common nowadays - does anyone want Nissan X-trail with 1.4L or Qashqai with 1.1L engine? Yes on cars as such I would not be surprised that the fuel consumption to be 5% or higher with A/C. Now I am kind of biased, but I never had a car with engine smaller than 1.8L and I personally consider that engines smaller than that should not even exist in cars. So to be fair I can't speak from experience here, but with anything above 2L A/C fuel consumption is certainly much lower than 5% nevermind 10%.

The fuel consumption can be somewhat calculated based on HP of the car, as typical A/C compressor will use ~1.6-5HP... this becomes slightly harder to calculate impact because car does not make it's peak power until certain RPM... but just taking IS250 for example, it makes ~140hp at 2000rpm, the A/C once cabin is cooled and when it only needs to maintain the temperature (meaning no open windows and no outside air) will not work very hard so we can assume lower figure of 1.6HP to be true. This is exactly between 1-2% of fuel consumption which I assumed A/C is using. Now however if we change the car to Nissan Micra Convertible 🤮 with 1L engine and 88hp and driving with roof down and A/C on full blast all the time "cooling the planet", then we can quickly see how this could turn into 10% consumption, but that isn't exactly "normal". 

[NemesisUK]

41 minutes ago, Linas.P said:

I think Lexus generally having high performance A/C is something they do because of american influence. In US A/C being ice cold is very important criteria and Lexus being US-centric product puts a lot of focus on that. 

Let's not forget Japan also has a very humid and hot climate ..

[Linas.P]

1 hour ago, NemesisUK said:

Let's not forget Japan also has a very humid and hot climate ..

I am sure many places around the world does have the need for A/C, like middle east for example, but I heard americans can literally choose cars on A/C performance alone, I mean evidence is anecdotal at best, but I was told they rock-up at dealerships get in the car, crank A/C to the max and expect instant freezer inside the car, and that alone can impact buying decision - the car with best A/C gets bought. Now it is kind of funny and probably not even true for majority, but in other hand A/C you get with car most likely can't be upgraded, so if performance is inadequate one will be stuck with it. As well there is brand loyalty based on that like "I only buy Lincoln because they have good A/C and I don't buy Fords or Nissans" or something along that line. Going to middle east part - in my experience they are just used to heat and therefore they don't have same high expectations for A/C, on top it would be counter productive, because if you get car down to 16C in 44C weather then cold is guaranteed right away. So they use A/C all the time, but they don't go to certain part measuring contest whose A/C is coldest. 

In short what I am saying in US A/C performance is disproportionally important in car purchasing decision making process compared to rest of the world. Lexus is US centric product designed with US consumer and not Japanese consumer in mind, so it just makes sense that Lexus puts a lot of focus on A/C performance and design - quick example... how many cars do you know with cooled ventilated seats? Literally apart of Lexus these are only optional on likes of 7-Series and S-Class, these were not available in likes of BMW 3 or 5 Series whereas Lexus fitted them to entry level products like IS.

But... I am not saying it is bad thing, apparently not everything that comes from america is bad. 

[NemesisUK]

26 minutes ago, Linas.P said:

how many cars do you know with cooled ventilated seats? Literally apart of Lexus these are only optional on likes of 7-Series and S-Class, these were not available in likes of BMW 3 or 5 Series whereas Lexus fitted them to entry level products like IS.

AFAIK only the LS has cooled ventilated seats? The seats in my RC are ventilated but only using cabin air, which is the reason I don't switch them on until the cabin has chilled or you are just blowing hot air through them! It doesn't take long but is noticeable. 

I suppose my point was that the Japanese naturally expect a high level of climate performance, as they have such  hot, humid summer/autumn.

[Linas.P]

4 minutes ago, NemesisUK said:

AFAIK only the LS has cooled ventilated seats? The seats in my RC are ventilated but only using cabin air, which is the reason I don't switch them on until the cabin has chilled or you are just blowing hot air through them! It doesn't take long but is noticeable. 

I suppose my point was that the Japanese naturally expect a high level of climate performance, as they have such  hot, humid summer/autumn.

The RC has dedicated A/C went which kind of blows towards rear passenger seats, but could as well be picked-up by front seats... in that respect it technically makes them "cooled", but not to the same degree as LS where vent is dedicated to the seat. Previous gen IS/GS didn't even have vent under the seat, so the seats were definitely just ventilating. Althought my point remains the same - what other make offers even ventilated seats? 

I don't know about Japanese expectations, just know that americans expect very high performance as deciding factor (or at least one of major ones) in car purchase. As well Japanese are weird - hate leather smell etc. so who knows what works for them, I don't even want start trying to understand them 😄 

[DavidCM]

Mazda offer ventilated seats on their top spec models eg Mazda 6,Cx5 and the newest sub.. CX 60?

[Linas.P]

Very interesting, never seen Mazda with ventilated seats, but on other hand I have not looked at Mazdas for long time as FWD is showstopper for me.

[Herbie]

I never switch off the AC and/or Climate Control, so like someone else further up, I have no comparison. Surely though, it should all work out in the wash?

During hot weather the petrol engine fires up for the AC; during cold weather the engine fires up for cabin heat.