Volumetric efficiency is one of the key ratios that help determine the degree of performance of an automotive engine or a hydraulic pump. In short, it indicates the amount of air and fuel that is in the cylinder at a specific moment of its operation. Volumetric efficiency is commonly expressed in percentage form. Usually the higher the percentage, the more powerful and faster an engine is.
More specifically, volumetric efficiency deals with two things: the theoretical maximum amount of air pumped into an engine and the actual amount of air being pumped. The theoretical maximum is determined by calculating the engine’s overall full volume, also called displacement — in short, how much air can fit into an engine’s cylinders. A cylinder has its maximum volume when the piston inside is at its lowermost position, also known as “bottom dead center.”
In simple terms, if you can pump 0.13 gallons (0.5 liters) of air-fuel mixture into a cylinder with a volume of 0.13 gallons (0.5 liters) at bottom dead center, you’ve achieved 100% volume efficiency. By contrast, if only 0.10 gallons (0.4 liters) enter the cylinder, you’ve achieved 80% volume efficiency.
In the real world, the actual amount of air pumped into the engine seldom equals its theoretical maximum. This is due to various factors, including friction loss, leaks, and the engine’s design. When air has to go through many turns and valves in order to reach the cylinders, some air resistance is bound to occur, leaving some of that air behind. On a naturally-aspirated engine — meaning any engine without pumping enhancements such as turbochargers — this is usually the case. Automotive enthusiasts often tweak with various parts of the engine to bring the actual amount of air closer to its theoretical maximum; in other words, increase its volumetric efficiency.
Devices such as turbochargers and superchargers actually increase an engine’s volumetric efficiency more than 100% because they substantially increase air density inside the cylinders. Under static conditions, such as when the engine is turned off, the air molecules inside the cylinder would be able to fill all of its maximum displacement, but they would be relatively far apart. Pumping air aggressively into the cylinder, by contrast, brings those molecules closer together, resulting in a higher air density.
Higher air densities result in a higher energy output because the more air you can fit into the cylinder in its intake stage — that’s when the cylinder goes down to drop dead center — the more fuel you can mix with it. This ultra-potent air-fuel mixture is then compressed by the rising piston until it ignites, providing more power to the engine’s output and resulting in faster, stronger engine performance.