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My car's FE goes up significantly in winters. I get around 15%-17% more milage. Its not for the fact that the A/C usage is less as i still use A/C all the time because of pollution.

Is it normal? Has anyone noticed this huge jump in milage.

Hi,

One possibility is because of the cooler temperatures the ECU leans out the fuel mixture and another being the AC compressor cuts off faster as the temperatures are arrived at faster and stay there for longer periods hence reducing load on compressor.

Viper

But the jump is quite high. Even if the A/C is off the difference cannot be more than 10% specially in cars with high torque .

Does it mean if there is good cooling mechanism then car can give better FE.

Correct me if I'm wrong, but what I understand is:

Cold weather = denser air = leaner mixture (less fuel for the same amount of air).

Shouldnt the ECU compensate for the lean mixture during cold weather? Why would it cause the mix to become lean? Or is that what you meant?

Rodeo,
My Guess is when cold air expands in the compression chamber it'll result in more volume of air as compared to the warm air expanding at the same temperature. There by increasing the power of engine at low fuel consumption.
Below is an artical from auto.howstuffworks.com
"Cool the incoming air - Compressing air raises its temperature. You would like to have the coolest air possible in the cylinder because the hotter the air is the less it will expand when combustion takes place. Therefore many turbo charged and super charged cars have an intercooler. An intercooler is a special radiator through which the compressed air passes to cool it off before it enters the cylinder."
Another article from thepartsbin.com
"Cold air is denser, offering a higher oxygen content than the usual flow of air into the combustion system, and that denser, higher oxygen level offered by the Acura Integra cold air intake helps to produce a more efficient fuel burn."
Thanks,
Mega

Well, the ECU does not lean out the mixture. Its the other way round.

Cold weather = more air for same volume ( more dense) = more fuel required.

The reason why you see a jump in efficiency is -

1) To maitain your regular cruising speeds, you will be using lesser throttle ( you may or may not notice this ) , as the engine is developing more power than with hotter air... When you calculate the ' richer mixture - lesser throttle requirement ' condition , the average fuel requirement is lowered in many cases.

2) AC thermo switching off for longer periods , as viper said.

3) You get more fuel for the same volume .

karthik.

A large part of its physics.
The IC engine is based on the Carnot cycle wherein efficiency is directly proportional to (T2-T1/T2)
T2-T1 is the temperature difference between ambient temp(external temp) and engine temperature. Since in winters ambient temperature is 15 degrees on an avg in delhi(288K) as compared to 310K in summers.
So your maximum possible efficiency will increase in lower temparatures.

Other reason is Fuel.
You get more fuel in winters as fuel is denser. So in the same volume you pack more energy. In summers also its advisable to get filled up early mornings as Fuel volume is given at 15 degrees.

Does it not make sense to have intercooler for a non turbo charged car if we can get a FE boost???

Next question is what can i do to maintain this FE all thru out the year?

Intercooler is used to cool hot exaust gases. In case of non turbo car something called a "cold air intake" is used to increase efficiency.

Whats required to increase the cold air intake??? K&N??

Thanks.

K&N and other performance filters will increase "air flow". They do not modulate the temparature.
Cold air intake - Wikipedia, the free encyclopedia
Most of these intakes work on reducing effect of engine temperature on the air entering the engine.