Volumetric efficiency calculation

I'm confused as to why the theoretical volumetric efficiency calculation assumes that at BDC (since the stroke was used in calculating cylinder volume) the cylinder is only filled with air. Shouldn't the air-fuel mixture be taken into account when determining theoretically how much air moves through the engine?

Hey Connor, as far as the VE calculation goes we're really thinking of the engine as a simple air pump and 100% VE is simply the engine displacement. Yes the injected fuel could be considered to displace some volume of air however it's actually not as straight forward as it might seem since the cooling effect of the fuel as it goes through a phase change form liquid to vapour actually has the effect of increasing air density which in turn can help improve the VE - this is more notable on DI engines since the fuel is injected directly into the cylinder and hence the fuel absorbs heat directly from the air rather than absorbing heat from the intake port wall/valves etc. In essence though from our perspective it's not considered to be relevant for the purposes of our calculations.

Hi Andre,

Thanks for your response. That makes more sense to me now. Excited to continue working my way through the course.