Quote:
Originally Posted by ecomodded
The premise is that the car will make less hp and use less gas.
My deduction could easily be flawed but here it is in a nut shell. Cold air has more explosive potential and hp than warm air when in a compressed cylinder because the air is denser.
The sensors on the car keep the air fuel mixture correct, the colder denser air would need more fuel to stay at the correct level. With warm air the car would lean out the mixture to correct the air fuel mix. As a bonus the warmer air fuel mix ignites a substantial amount easier.
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I think it's a bit more complicated than this, this is what I am seeing.
A diesel doesn't need to maintain any kind of air fuel mixture, it just injects however much fuel it needs and spits out however much power. Press less on the pedal, less fuel is injected.
The issue is that since colder air has a greater amount of oxygen available, the fraction of fuel injected for the same power level is lower. I'll be taking a sort of metatheoretical view just thinking about temperatures. Less fuel fraction means when the fuel burns, you have a smaller change in temperature and pressure, which is not good. However, a lower temperature to start with increases the maximum possible theoretical efficiency, so it's really not clear what will happen.
Coming back to reality, we see there's another thing about cold air that makes it more complicated. The heat ratio of gases is not truly a constant, and tends to decrease with temperature. In an ideal cycle with ideal gases higher heat ratio is better, but I imagine in the real world the advantage is dulled a little.
This is all not considering combustion quality, which of course can be affected. If you're having trouble starting a diesel engine, that probably means combustion stability is an issue and you'll be seeing a greater effect from that than the ~10-20% absolute temperature changes that happen between seasons.