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Discussion and questions related to the course EFI Tuning Fundamentals
I understand how forced induction can result in a VE greater than 100% but can this be achieved in a NA engine?
Are you asking is 102 possible or are you thinking 160% VE
Pressure waves and air velocity in the intake and exhaust system can result in greater than 100% cylinder filling, therefore the VE is > 100%. I have seen well selected intakes, and custom exhaust systems result in 125% volumetric efficiency at peak torque engine speed.
I was just flipping through a book on engine airflow, and came across another reason for VE > 100% -- that is the clearance volume (ie, combustion chamber volume). On an engine with 11:1 compression ratio, the clearance volume is 10% of the swept volume, so if you could fill that entire area with air, it would 110% VE. So that is a reasonable goal of a good intake & exhaust system -- to achieve the complete swept + clearance volume.
Clearance volume is also full of burnt gases which are hotter than intake air, so they take more space.
Clearance volume can be complietely swept in the case the engine has a large overlap and scavenging occurs. Thus fresh air is chasing burnt gases.
In NA application, as mentioned you have air acoustics and dynamic pressure from vehicle beeing at high speed. In that last case the static pressure is upgraded by the dynamic pressure 0.5 x density x speed². So the actual pressure condition (total pressure (= static + dynamic) is higher than the standard condition which are taken into account for the calculation.
In what boosted engines are concernd, VE as defined in the course all have more than 100% VE because you are comparing actual air mass within the cylinder with air mass which would be trapped in standard conditions (lower density).
An other way of expressing VE is comparing actual mass of air trapped into the cylinder against air conditions closely to the intake valve (manifold pressure and manifold tube leeding to the valve temperature). In that way you will have VE which tells you what the cylinder head efficiency is. In that way, even supercharged engines will have VE inferior or equal to 100.
The only way with that last calculation to have more than 100% VE is to have large overlap and thus scavenging (puting the fresh air directly at the exhaust in order for instance to encrease the flow through the turbine and decrease turbo latency at low engine speed). As MAF is measuring the air flow which is crossing it, you cannot know what air is used for combustion and what amount is going out of the cyclinder through the exhaust valves during overlap. We can reach up to 30% of scavened air.
Sorry if my english is difficult to understand.
Sorry, my initial message was cut because of "inferior" sign in "VE inferieur to 100%"