What is Volumetric Efficiency?

Volumetric efficiency is a form of efficiency measurement, most frequently used in reference to combustion engines and hydraulic pumps. While the basic concept of volumetric flow for combustion engines and hydraulic pumps is roughly the same, the details are different enough to warrant further scrutiny. This article will attempt to delineate those differences, show how volumetric efficiency is determined for both types of mechanisms and discuss the processes by which volumetric efficiency can be altered.

Volumetric Efficiency In Combustion Engines

In regard to combustion engines, volumetric efficiency in combustion engines is defined as the ratio of volume of working substance actually admitted, measured at a specified temperature and pressure, to the full piston displacement volume. With engine or gas compressors, Access Science defines volumetric efficiency as the ratio of volume of working substance admitted, measured at specified temperature and pressure, to the full piston displacement volume. With liquid-fuel engines (i.e. diesel engines), volumetric efficiency is the ratio of volume of air drawn into a cylinder to the piston displacement. For a gas-fuel engine, volumetric efficiency is determined on the charge of fuel and air drawn into the cylinder.

Formula for Volumetric Efficiency in Combustion Engines

The formula for determining volumetric efficiency in a typical combustion engine is to multiply 3456 by the engine air flow rate (measured in cubic feet per minute) and then divide the result by the amount achieved by multiplying the engine size, displacement or volume (measured in cubic inches) by the engine speed (measured in revolutions per minute). For the sake of clarity, the formula for volumetric efficiency in a combustion engine looks like this: 3456 x Cubic Feet Per Minute/Cubic Inch Displacement x Revolutions Per Minute.

Volumetric Efficiency in Hydraulic Pumps

In terms of hydraulic pumps, volumetric efficiency refers to the percentage of actual liquid flow out of the pump contrasted to the flow out of the pump minus leakage. As an example, if the flow from a 100cc pump is 45 cc, the volumetric efficiency for that pump at that time would be 45 percent. Volumetric efficiency changes in conjunction with the speed and pressure with which a pump is operated. Because of this, when comparing volumetric efficiencies, the speed and pressure information should be made available. The formula, then, for determining volumetric efficiency in a hydraulic pump is to divide the actual output of the pump by the theoretical output of the pump, at a set speed and pressure. The resulting amount will be calculated as a percentage.

How Volumetric Efficiency Can Be Altered in a Combustion Engine

Since engines with higher volumetric efficiency are better able to handle higher speeds and produce more overall power (as a result of less power loss due to air moving in and out of the engine), there is concerted interest in being able to alter or improve volumetric efficiency in a combustion engine. The most common and effective way to enhance volumetric efficiency in an engine is to increase the size or number of valves in the engine. While larger valves increase flow, they can have the negative effect of causing the engine to weigh more. Using multiple valves can increase the flow of an engine without adversely affecting the weight. By streamlining ports, a technique known as "porting," one can increase flow capability and, by proxy, the volumetric efficiency of the engine.

Supercharging

Supercharging is the process of introducing air for combustion into the cylinder of an internal-combustion engine at a pressure in excess of that which can be obtained by natural aspiration. Superchargers are powered mechanically by belts, gears, shafts, or chains connected to an engine's crankshaft. The two main types of supercharger are positive-displacement and dynamic compressors. Positive-displacement compressors provide a constant level of boost regardless of engine speed, while dynamic compressors provide growing boost with growing engine speed.

Turbocharging

A turbocharger is a turbine-propelled supercharger driven by the exhaust gas pressure of the engine. As with any other type of supercharger, the purpose of a turbocharger is to increase volumetric efficiency through forced induction. It is composed of a radial fan pump, which is propelled by the engine's exhaust gases.
With a turbocharger, exhaust heat energy is transformed into power, propelling the compressor and compressing surrounding air and then sending the compressed air to the air intake manifold of the engine at an increased pressure. This results in an increased mass of air entering the cylinders.