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Volumetric efficiency is a technical term used for comparing performance or some other measurable parameter per unit of physical volume. This figure of merit concept appears in several otherwise unrelated contexts, including design of internal combustion engines, hydraulic pumps, and miniaturized components used in electronic circuits.
Internal combustion engines 
Volumetric efficiency in the internal combustion engine design refers to the efficiency with which the engine can move the charge into and out of the cylinders. More specifically, volumetric efficiency is a ratio (or percentage) of the quantity of air that is trapped by the cylinder during induction over the swept volume of the cylinder under static conditions. VE can be improved in a number of ways, most effectively this can be achieved by compressing the induction charge or by aggressive cam phasing in Normally Aspirated engines as seen in racing applications. In either case VE can exceed 100%.
There are several ways to improve volumetric efficiency, but a system wide approach must be used to fully realize potential.
Many high performance cars use carefully arranged air intakes and tuned exhaust systems that use pressure waves to push air into and out of the cylinders, making use of the resonance of the system. Two-stroke engines are very sensitive to this concept and can use expansion chambers that return the escaping air-fuel mixture back to the cylinder. A more modern technique for 4 stroke engines, variable valve timing, attempts to address changes in volumetric efficiency with changes in speed of the engine: at higher speeds the engine needs the valves open for a greater percentage of the cycle time to move the charge in and out of the engine.
Volumetric efficiencies above 100% can be reached by using forced induction such as supercharging or turbocharging. With proper tuning, volumetric efficiencies above 100% can also be reached by naturally aspirated engines. The limit for naturally aspirated engines is about 137%; these engines are typically of a DOHC layout with four valves per cylinder.
More "radical" solutions include the sleeve valve design, in which the valves are replaced outright with a rotating sleeve around the piston, or alternately a rotating sleeve under the cylinder head. In this system the ports can be as large as necessary, up to that of the entire cylinder wall. However there is a practical upper limit due to the strength of the sleeve, at larger sizes the pressure inside the cylinder can "pop" the sleeve if the port is too large.
Volumetric Efficiency is frequently abbreviated as "VE" when discussing engine efficiency.
Hydraulic pumps 
Volumetric efficiency in a hydraulic pump refers to the percentage of actual fluid flow out of the pump compared to the flow out of the pump without leakage. In other words, if the flow out of a 100cc pump is 92cc (per revolution), then the volumetric efficiency is 92%. The volumetric efficiency will change with the pressure and speed a pump is operated at, therefore when comparing volumetric efficiencies, the pressure and speed information must be available. When a single number is given for volumetric efficiency, it will typically be at the rated pressure and speed.
In electronics, volumetric efficiency measures the performance of some electronic function per unit volume, usually in as small a space as possible. This is desirable since advanced designs need to cram increasing functionality into smaller packages, for example, maximizing the energy stored in a battery powering a cellphone. Besides energy storage in batteries, the concept of volumetric efficiency appears in design and application of capacitors, where the "CV product" is a figure of merit calculated by multiplying the capacitance (C) by the maximum voltage rating (V), divided by the volume. The concept of volumetric efficiency can be applied to any measurable electronic characteristic, including resistance, capacitance, inductance, voltage, current, energy storage, etc.
See also 
- SohoPros. "ENDYN". Theoldone.com. Retrieved 2010-11-07.