Which term describes the overall breathing efficiency of an internal combustion engine?

Volumetric efficiency refers to the efficiency with which an internal combustion engine fills its cylinders with air and fuel mixture during the intake stroke. It is a measure of how effectively the engine can draw in the maximum amount of air, relative to the total cylinder volume.

High volumetric efficiency indicates that the engine can fill the cylinders more completely, allowing for better combustion and increased power output. Factors affecting volumetric efficiency include the design of the intake system, the timing of the intake valves, and the pressure and temperature of the incoming air. Essentially, a high volumetric efficiency leads to improved engine performance, making it a fundamental concept in evaluating an engine's breathing capabilities.

The other terms refer to different aspects of engine performance. Combustion efficiency relates to how effectively the fuel is burned, scavenging describes the process of clearing exhaust gases from the cylinders, and thermal efficiency pertains to how well the engine converts fuel energy into mechanical work. Each of these is important, but none encompasses the concept of an engine's total air intake efficiency as directly as volumetric efficiency does.