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post #15 of (permalink) Old 23rd December 2006
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Originally Posted by davesss View Post
Talking about lower mpg in winter.

There is one advantage with cold weather apart from the negligable increase in mpg perhaps due to more electricity use.

The car engine will actually produce more power as lower temprature air means denser air, the denser air going into the engine will increase mpg if you keep to your usual speeds. this is a scientific fact. works like a mini turbo !

So mpg should be the same, winter or summer.
No I dissagree. Your thinking sounds sensible but is flawed. A warm air/fuel mix, because the surface tension of the vapourised fuel molecules is decreased - vapourises and ignites more completely.
I ran a Fiat diesel for a few years and used that as a test bed for various fuel tests. ( ran it on veg oil etc) I read up by chance some info on air density/ignition points which got me thinking. I took off the forward facing air scoop and added a section that ran the intake behind the engine into a warmer air flow. Over a 2 week test period the fuel consumption went down. I refitted the original system to check this out and it went up again!
I'm sure this is another reason why fuel consumption increases in winter along with the other air temperature density/ electrical factors. Other factors that increase fuel consumption are as you say "more dense air" Any debatable perceived advantage of cold air intake to the engine are negated to a larger extent by the cars mass using more power to move it through cold "more dense" winter air.
If you doubt what I say, try running your car in winter and do the following; as soon as your fuel light comes on in daylight put 2 ltrs of fuel in it and run it on none or as little electrical components as possible. See how far it will go before the light comes back on. OK you will be cold for a half hour or so but hang in there! Store that info till the summer then do the same test. I bet you will be surprised how much warm air benifits your engine and your cars mpg.
Not a very scientific test I agree but it works.

Here is a thought related more to petrol engines. Why when a petrol engine is cold does it need the choke? Surely a cold air/fuel mix is more efficient? Not so - a cold engine needs a good rich fuel mix to start it and run it till it gets to it's warm efficient working temperature.

Diesels are not much different. They need warmed up too. The cold air thing is a bit of a red herring, The air must be at the engines comfortable operating temperature. Whether it is warmed beforehand or by the ignition process. It has to be warm.

Last edited by rodmaker; 23rd December 2006 at 12:56 AM. Reason: missed out a words
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