It occurred to me today, having finally got the aircon fixed, that aircon introduces another dimensions into MPG calculations.
It is generally reckoned that a car with windows closed has less drag than one with them open, but adding aircon adds a *constant* power drain, irrespective of car speed.
Which means for best MPG you want to travel *faster* with aircon, than without it. In order to shorten journey times and hence energy loss per mile from the aircon.
I'm afraid that there's rather a serious flaw in that argument. Whereas the aircon itself may use less energy by that method, the engine is likely to use *more*. The faster you go, the higher the tractive effort required to overcome the drag. Remember that aerodynamic drag is proportional to the square of the speed. Since work done (= energy consumed) = force x distance, the energy consumed per mile will increase with speed, as the drag increases.
This, in turn, leads to more fuel consumed per mile - i.e. lower MPG. This may be partially offset if, by going faster, you can run the engine at a more efficient point on its fuel map - but the overall effect is still going to be less MPG the faster you go.
The best way of saving fuel is to travel for long distances at a constant speed without a lot of starting and stopping or accelerating or braking. My car achieves far better MPG at a steady 70mph on a motorway that it does around town at a far lower average speed. *But* just to emphasise the point made above, it does far better still at a constant
60pmh - all with the aircon on all the time.On 16/09/2023 23:09, Roger Mills wrote: . *But* just to
On many UK motorways travelling at a constant 60 usually means a HGV in front of you, A HGV up your arse and a HGV beside you for the miles it takes it to overtake at 60.05 mph. :)
The general rule of thumb used to be - below 40mph, open windows are more efficient than aircon and above 40mph it is the other way around.
*More* efficient than what? Cooling the car's interior? If that's what you mean, surely it's the other way round. The faster the air flow, the cooler it will appear to be. You lose heat by evaporative cooling, so the faster air flow will remove sweat more quickly.
One question though - is it better to have the aircon recirculating the cooled internal air, or better to have fresh air being cooled all the time? I guess it depends on the outside temperature and how quickly the car's interior suffers from any "greenhouse" effect and perceived "stuffiness" with closed windows. I always have fresh air coming in (although in the past when stuck behind a smoking, stinking diesel I would close the outside vents).
In message snipped-for-privacy@mid.individual.net>, Roger Mills snipped-for-privacy@gmail.com writes snip
Momentary idle thought.. presumably lines of traffic travelling at similar speeds will create local air flows not linked to wind speed/direction.
On a UK motorway HGVs are mechanically limited to *56mph* (90km/h).
The fact that it looks like 60mph to you is because 99.999% o9f all speedometers read about 7% too high.
The process of one overtaking the other I call the "Commercial Two-Step"
(quick quick slow slow)
That sounds about right for a 20 year old car...
I wonder if it holds today.
Anyone else remember air brakes aka 'quarter lights'
If you really want to save fuel, you'd slipstream an HGV ... needs balls of steel though.
I think I would only recirculate when there was a need for massive temperature change, quickly.
The build up of water vapour rapidly leads to high humidity which to an extent reduces perceived cooling in hot weather and induces misting up in cold weather...One of the less appreciated uses of aircon is to dry extremely damp air by refrigerating it, and then heating it again.
Oh for sure. My first vehicle was a Bedford CA MkII van. Barely capable of 70mph. Slipstreaming the big HGVs allowed me to tale my foot half off the accelerator. I will never forget my surprise when I first fitted a full set of radial tyres, too.
Another 10% on mpg.
The aircon is not a constant load, at least on modern cars. The compressor has a clutch or other mechanism that varies the amount of load it puts on the engine belt, so it only takes the power it needs to maintain the cabin temperature. On hybrids and EVs it's an electric compressor that achieves the same thing.
Agreed it's not related to speed, merely to cooling load.
A car a/c compressor consumes about 2 kW. An engine is ballpark 80-150kW. So it doesn't take much increased speed (air resistance) to completely negate any load from the compressor. Likewise from this it's not hard to imagine that opening the windows could easily create more than 2kW of drag.
Theo
And brakes at least better than the HGVs...and lightning fast reflexes. Basically you can drive a reaction time away from the HGV.
Any loss of concentration could be a disaster though.
I used to do it back in the day, but these days I leave the longest possible gap to traffic ahead.
I do that, but people keep pulling into it. Sigh.
Nit picking. That's like saying that the average power input to a heating system is not constant as your boiler switches on and off.
What I meant by constant was persistent and (on average) time invariant.
But not constantly eh?
An engine is ballpark 80-150kW.
Mine is up to 200kW
But not all the time. At cruise, most cars are running at at most about
20kW, and many a lot less.2kW is a 10% change in mpg
Again, no numbers, just wet finger waving.
Her are a few numbers someone else prepared earlier:
"I might be being dim, but for an approximation, can you take a car's BHP and maximum speed (assuming no limiter) and work back from there? Power required = velocity ^ 3.
Obviously your results are only good for a car of that weight and aero.
For example: my car can supposedly do 137mph with 165bhp (I should be so lucky on either count), so were that idea valid and my maths correct, it'd need a mere 22bhp to do 70mph. "
That sorta gybes with my recollection of many old cars that were around
40bhp that could do around 60mph with shit aerodynamics and cross ply tyres.In fact a shade better. Ford popular 100E was 36bhp from a side valve engine and had a top speed of 69.9mph
3-4KW on top of that for aircon would have made a 10% difference to mpg.And if it was cruising at say half throttle at its comfortable 55mph sort of speed at around 18bhp, adding another 3kW would reduce its mpg by ~16%.
Likewise from this it's not hard to
I presume many have found a way around the limiters then? Many travel well in excess of that speed.
Indeed they do, and I am sure they thank you for it
One of the most dangerous things that happens is a nose to tail train of cars all too nervous or underpowered to overtake the car at the front of the queue, and you are neither, but you can only do it in two hops, but the guy with the gap deliberately closes it when he sees you coming.
That's one of the reasons I like to run with an excess of horsepower.
50-100mph in a couple of seconds gets you past a LOT of nervous Sunday drivers without the need to cut back in.And it also means that you can be a quarter of a mile back with far better visibilitu and still be past in a flash, if it's you at the queue head.
Annoying, innit. It's a bit like the sections of motorway marked with arrows and "keep two arrows apart" signs - fat chance. Mind you, that's not such a daft idea - if everyone suddenly backed off 2 arrows distance, the end car in the queue might well have to engage reverse to cope.
That's one of the things a semi autonomous car could do ...
In a stationary car is the only safe way - you would get quickly anoxic and become a danger to yourself and everyone else if you kept it on recirculate whilst driving along for some time.
It is only the recent fuel price increases that have persuaded me to ever turn mine off, and then not very often. A nice cool, ventilated car is about the biggest contribution to safety on the roads that one can make. As you remark, demisting on a damp winter's morning takes a fraction of the time with the aircon left on. I often wish some of the morons who drive off with one tiny hole in the condensation on their windscreens knew that. However, I suspect that comes under the old rule that education cannot cure stupidity.
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