[Diesel_Dave]

You say that cold air has "more oxygen" and warm air has "less oxygen". That's sort of true. Cold air does have more oxygen per unit volume--but only because there's more air per unit volume. The temperature doesn't affect the percentage of oxygen in air--therefore the amount of oxygen per unit mass doesn't change.

Now, you might be tempeted to think that because the stoke of the engine is a fixed volume, that volume is what matters. However, closed loop control keeps the air-fuel ratio basically constant (near stoich)--and AFR is mass-based, not volume based--the throttle will adjust to keep the same mass of air regardless of temperature (by changing the intake pressure). So warmer temps mean higher intake pressures--to keep the density the same. Higher intake pressure means lower pumping losses and also lower throttle losses, therefore, better fuel efficiency.

Also, warmer air increases the flame speed, so the combusion happens more quickly, giving it more room to expand. That also increases fuel efficiency.


Diesels are a different story because they're typically not at constant AFR and also don't have flame propogation. I've gone into the details on how a CAI benefits a diesel in other threads, but since you asked about a gasser, I'll stop now.