Volumetric efficiency

Volumetric efficiency

Volumetric efficiency in internal combustion engine design refers to the efficiency with which the engine can move the "charge" into and out of the cylinders. More correctly, volumetric efficiency is a ratio (or percentage) of what volume of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. Therefore, those engines that can create higher induction manifold pressures - above ambient - will have efficiencies greater than 100%. Volumetric efficiencies can be improved in a number of ways, but most notably the size of the valve openings compared to the volume of the cylinder and streamlining the ports. Engines with higher volumetric efficiency will generally be able to run at higher speeds (commonly measured in RPM) and produce more overall power due to less parasitic power loss moving air in and out of the engine. There are several standard ways to improve volumetric efficiency. A common approach for manufacturers is to use larger valves or multiple valves. Larger valves increase flow but weigh more. Multi-valve engines combine two or more smaller valves with areas greater than a single, large valve while having less weight. Carefully streamlining the ports increases flow capability. This is referred to as Porting and is done with the aid of an air flow bench for testing.

Today, automobile engines typically have four valves per cylinder. Many high performance cars in the 1970s used carefully arranged air intakes and "tuned" exhaust systems to "push" air into and out of the cylinders, making use of the resonance of the system. Two-stroke engines take this concept even further with expansion chambers that return the escaping air-fuel mixture back to the cylinder. A more modern technique, 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 120%, 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.

Volumetric "Efficiency" should in no way be construed to be a measure of engine efficiency, the thermal efficiency of the engine, although it may have an effect on it. For instance, when a standard gasoline engine is at idle or otherwise at less than full throttle, the effective compression ratio of the engine is reduced, resulting in reduced cylinder temperature and pressure during combustion. The thermodynamic laws which apply to all heat engines dictate that the engine will be therefore operating with less than its optimum thermal efficiency.

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