EV Design: Wind resistance, etc

[ChrisBarron]

I was of the opinion that a lower air temperature imporved internal combustion efficiency.

Chris it depends what you mean by efficiency ASFAIK the opposite is true, lower temperature increases charge density and you get higher power at cost of increased fuel consumption. That's what some people want, more power!

I've always been led to believe that the colder air means more oxygen, which means fewer pumping losses and that means increased efficiency for any given power output.

Quoting myself now, but the penny just dropped for me.

I took my motorbike out this morning, at -2C, and as is usual on a cold morning the bike is much more powerful at any given throttle position, then when the air is warm.
It is a 4 cylinder 4-stroke 550C engine with 4 manual carbs which have no temperature compensation.

Because the throttle position is the same no extra fuel is supplied but more power is created. Efficiency is output divided by input, and so the bike is more efficient in colder air, and I see better mpg in cold weather.

The carbs are set to run well for a wide range of operating conditions, and as a best compromise, because the fuel mixture setting is fixed all across the throttle opening range and can't be adjusted on the run.

However, a fuel injected car changes it's mixture all the time and usually works in a closed loop system which monitors unburnt hydrocarbons and oxygen in the exhaust, so if that sort of system were fitted to my bike then presumably in the summer it would lean out the mixture as the air temperature rose.

I agree with you now Peter (!), in the case of a fuel injected car, like the Prius is, the traditional temperature effects don't come into it anywhere near as much because the engine management takes car of that already.

Chris

[JonSpence]

.. therefore a smaller throttle opening can be used to create the same power with colder air than for a wider opening and a hot air intake.

I've always been led to believe that the colder air means more oxygen, which means fewer pumping losses and that means increased efficinecy for any given power output.

Just thought that someone should point out that closing the throttle (restricting the intake) increases pumping losses rather than reduces them. This is if course one reason why larger petrol engines are less efficient for a given load (because of increased pumping losses).

[Jeremy]

The peaks on my plot show summers, the troughs, winters. The gradual decline is almost certainly a combination of me driving a bit less economically, plus variations in average temperature and humidity.

The issue of engine efficiency variation with cold air is a red herring. Cold air is more dense, so a given volume will contain more molecules of oxygen. This will allow engine peak power to increase (more oxygen molecules can be packed into the cylinder for a given volume) but this will also cause the carburettor (or fuel injection system) to supply more fuel. Both carburettors and fuel injection systems meter on mass flow, not volume, as the engine mixture ratio is held more or less constant. If you use the extra power in cold weather, then you will use extra fuel!

The reason that fuel economy is much worse in winter is due mainly to the increase in aerodynamic drag with a decrease in air temperature. Drag is linearly proportional to air density, so a 2% increase in density will cause a 2% increase in drag. However, power required to overcome drag is proportional to the square of the drag, so power (and hence fuel used) is affected non-linearly by small changes in aerodynamic drag. This sensitivity to small changes in drag also helps to explain variations due to surface wind speed and direction, particularly as drag is proportional to the square of air speed.

There are other temperature effects too. Cold tyres tend to be slower at returning deflection energy from the road surface, so will have a slightly higher rolling resistance until they warm up. The engine will take longer to get to operating temperature (the UK Prius doesn't have the Thermos system) and the car will expend more energy in running things like heaters, heated screens and lights.

Jeremy

[qdos]

I think you'll also find that cold weather means the engine runs rich for a much more considerable length of time before it reaches a good working temperature and the fact that it's effectively on choke means the fuel consumption plummets. Also in winter you run with the heater on to de mist or even de ice the screen. The lights are on nearly all the time plus the windscreen wipers and all this puts a load on the alternator which takes power out of the engine and also causes your fuel consumption to plummet. hmmm or do I mean sky rocket. Anyways I'm sure you get the picture.

[ChrisBarron]

.. therefore a smaller throttle opening can be used to create the same power with colder air than for a wider opening and a hot air intake.

I've always been led to believe that the colder air means more oxygen, which means fewer pumping losses and that means increased efficinecy for any given power output.

Just thought that someone should point out that closing the throttle (restricting the intake) increases pumping losses rather than reduces them. This is if course one reason why larger petrol engines are less efficient for a given load (because of increased pumping losses).

Of course that's true, and that's why there were two paragraphs describing two seperate things. Perhaps I didn't make clear enough the comparative nature of my comments.

Chris

[ChrisBarron]

I think you'll also find that cold weather means the engine runs rich for a much more considerable length of time before it reaches a good working temperature and the fact that it's effectively on choke means the fuel consumption plummets. Also in winter you run with the heater on to de mist or even de ice the screen. The lights are on nearly all the time plus the windscreen wipers and all this puts a load on the alternator which takes power out of the engine and also causes your fuel consumption to plummet. hmmm or do I mean sky rocket. Anyways I'm sure you get the picture.

I think this is called a multivariable system !

I'm glad the question came up though, because looking into it some more has made me learn that moist air is less dense than dry air, which is completely counter-intuitive itself !

[Jeremy]

I'm glad the question came up though, because looking into it some more has made me learn that moist air is less dense than dry air, which is completely counter-intuitive itself !

It seems counter intuitive unless you're an aviator or parachutist, particularly the latter (parachutes fall faster in moist air). The key to this is to look at the most common form of very moist air we have, clouds. The fact that they float indicates that they are less dense than the dryer air they are floating in.

Generally speaking, overall engine efficiency for any given power output will be poorer in cold weather, for all the multitude of reasons given here. Cold air does make for more available power though, something many will have noticed when their ICE powered vehicle feels a bit more lively on a cold, dry, day.

Jeremy

[ChrisBarron]

I'm glad the question came up though, because looking into it some more has made me learn that moist air is less dense than dry air, which is completely counter-intuitive itself !

It seems counter intuitive unless you're an aviator or parachutist, particularly the latter (parachutes fall faster in moist air). The key to this is to look at the most common form of very moist air we have, clouds. The fact that they float indicates that they are less dense than the dryer air they are floating in.


Jeremy

I always thought, and common sense leads you to think, that the reason that the clouds are up in the sky is because they usually form either at the top of rising air columns or at level a cool/warm interface and are only made visible because the dew point has been reached and the moisture is collecting as large droplets. The relative humidity (and therefore moisture content) of the invisible air immediately below a cloud can be identical to the that of the visible air within the cloud and the only difference is temperature.

Lol, it all comes back to temperature again !

[MalcolmB]

Fascinating stuff – the closer you look the more complicated things get.

Just to add to the confusion: When does moist air become denser than dry air and why doesn't fog rise?

[ChrisBarron]

Fascinating stuff – the closer you look the more complicated things get.

Just to add to the confusion: When does moist air become denser than dry air and why doesn't fog rise?

Yet more confusion, what type of fog do you mean exactly ?
http://www.crh.noaa.gov/jkl/?n=fog_types