Volumetric efficiency
Volumetric efficiency in internal combustion engine engineering is defined as the ratio of the equivalent volume of the fresh air drawn into the cylinder during the intake stroke to the volume of the cylinder itself. The term is also used in other engineering contexts, such as hydraulic pumps and electronic components.
Internal combustion engines
Volumetric Efficiency in an internal combustion engine design refers to the efficiency with which the engine can move the charge of fresh air into and out of the cylinders. It also denotes the ratio of equivalent air volume drawn into the cylinder to the cylinder's swept volume. This equivalent volume is commonly inserted into a mass estimation equation based upon Boyle's Gas Law. When VE is multiplied by the cylinder volume, an accurate estimate of cylinder air mass can be made for use in determining the required fuel delivery and spark timing for the engine.The flow restrictions in the intake and exhaust systems create a reduction in the inlet flow which reduces the total mass delivery to the cylinder. Under some conditions, ram tuning may either increase or decrease the pumping efficiency of the engine. This happens when a favorable alignment of the pressure wave in the inlet plumbing improves the flow through the valve. Increasing the pressure differential across the inlet valve typically increases VE throughout the naturally aspirated range. Adding intake manifold boost pressure from a supercharger or turbocharger can increase the VE, but the final calculation for cylinder airmass takes most of this benefit into account with the pressure term in n=PV/RT which is taken from the intake manifold pressure.
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 four-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 130%; these engines are typically of a DOHC layout with four valves per cylinder. This process is called inertial supercharging and uses the resonance of the intake manifold and the mass of the air to achieve pressures greater than atmospheric at the intake valve. With proper tuning, VE's of up to 130% have been reported in various experimental studies.
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.