Quote:
Originally Posted by Daox
What factors effect benefit from cold air and what factors penalize for it?
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The cooler air benefits in 2 ways:
First cooler air means more dense air and therefore, you'll run a higher AFR (at least on most diesels--some more modern ones have some sophistcaed electronic controls). The higher AFR increases the combustion efficiency. Fundamentally, the thermodynamic benefit comes from the fact that mixtures with higher AFR have a higher specific heat ratio (gamma). Secondly, the lower air temperature itself increases the specific heat ratio and thus the combustion efficiency. You can see the dependence of specific heat ratio on temperature and AFR here (pg. 2392):
http://muhserv.atauni.edu.tr/makine/..._files/A/c.pdf
and the also the dependence of theoretical diesel efficiency on the specific heat ratio here:
The Diesel Engine
Now, for the downside:
In a diesel the ignition takes place due to compression, not due to spark as it does in a gasser. Therefore the ignition doesn't take place until the pressures and temperatures reach the point where the fuel evaporates and the mixture self-ignites. The time between when the fuel in injected and when ignites is the "ignition delay". Cooler air temps can prolong the ignition delay to the point where the ignition doesn't take place until very late in the cycle or, in extreme case, the ignition never happens and you get a misfire. Later combustion typically means lower efficiency because the heat release happens in a larger volume. Now, how much the ignition delay is affected depends on how cold it is as well as the type of fuel system. Newer common rail systems have extremely high injection pressures (30,000 psi+) so the ignition delays are very short and less affected by temperature. Also, many of the new systems will electronically advance the injection timing for cold air temps to compensate. The older mechanical fuel injection systems were more affected because the injection pressures were relatively low and the ignition delays were quite long.