What is the primary mechanism by which exhaust backpressure affects volumetric efficiency in an internal combustion engine?

Exhaust backpressure primarily affects volumetric efficiency by increasing the residual gas fraction in the cylinder. Volumetric efficiency is a measure of how effectively the engine cylinder fills with fresh air-fuel mixture during the intake stroke, relative to the cylinder's displacement. Exhaust backpressure is the pressure opposing the expulsion of exhaust gases from the cylinder. High exhaust backpressure impedes the exit of exhaust gases, resulting in a greater amount of burnt gases remaining in the cylinder after the exhaust stroke. This remaining gas is known as residual gas. Because the cylinder volume is fixed, a higher residual gas fraction reduces the space available for the fresh air-fuel mixture during the intake stroke. This means the cylinder fills with less fresh charge, decreasing the volumetric efficiency. The reduction in fresh charge directly reduces the amount of oxygen available for combustion, leading to lower power output. In essence, the engine is working harder to pump out exhaust gases, reducing its ability to efficiently draw in fresh air. An example is a clogged catalytic converter. A partially blocked catalytic converter significantly increases exhaust backpressure, reducing the amount of fresh air-fuel mixture that can enter the cylinder on the intake stroke. This leads to a noticeable decrease in engine power, especially at higher engine speeds where the effect of the increased backpressure becomes more pronounced.