Volumetric efficiency
Encyclopedia
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 specifically, volumetric efficiency is a ratio (or percentage) of what quantity 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, but with added complexity. 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. Another major aspect of design is to use a crossflow cylinder head
, which has become the standard configuration in modern engines.
Many high performance cars use 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 chamber
s 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 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.
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.
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
design
Design
Design as a noun informally refers to a plan or convention for the construction of an object or a system while “to design” refers to making this plan...
refers to the efficiency with which the engine can move the charge into and out of the cylinders
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
. More specifically, volumetric efficiency is a ratio (or percentage) of what quantity 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
Poppet valve
A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...
or multiple valves
Multi-valve
In automotive engineering a multi-valve or multivalve engine is one where each cylinder has more than two valves. A multi-valve engine has better breathing and can operate at higher revolutions per minute than a two-valve engine, delivering more power.- Multi-valve rationale :A multi-valve design...
. 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, but with added complexity. Carefully streamlining the ports increases flow capability. This is referred to as porting
Cylinder head porting
Cylinder head porting refers to the process of modifying the intake and exhaust ports of an internal combustion engine to improve the quality and quantity of the air flow. Cylinder heads, as manufactured, are usually suboptimal due to design and manufacturing constraints...
and is done with the aid of an air flow bench
Air flow bench
An air flow bench is a device used for testing the internal aerodynamic qualities of an engine component and is related to the more familiar wind tunnel.Used primarily for testing the intake and exhaust ports of cylinder heads of internal combustion engines...
for testing. Another major aspect of design is to use a crossflow cylinder head
Crossflow cylinder head
A crossflow cylinder head is a cylinder head that features the intake and exhaust ports on opposite sides. The gases can be thought to flow across the head...
, which has become the standard configuration in modern engines.
Many high performance cars use carefully arranged air intakes and tuned exhaust systems to push air into and out of the cylinders, making use of the resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
of the system. Two-stroke engines take this concept even further with expansion chamber
Expansion chamber
An expansion chamber is an exhaust system used on a two-stroke cycle engine to enhance its power output by improving its volumetric efficiency. It makes use of the energy left in the burnt exhaust exiting the cylinder to aid the filling of the cylinder for the next cycle...
s that return the escaping air-fuel mixture back to the cylinder. A more modern technique, variable valve timing
Variable valve timing
In internal combustion engines, variable valve timing , also known as Variable valve actuation , is a generalized term used to describe any mechanism or method that can alter the shape or timing of a valve lift event within an internal combustion engine...
, 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
Forced induction
Forced induction is the process of compressing air on the intake of an internal combustion engine . A forced induction engine uses a gas compressor to increase the pressure, temperature and density of the air...
such as supercharging
Supercharger
A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...
or turbocharging
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
. 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
Multi-valve
In automotive engineering a multi-valve or multivalve engine is one where each cylinder has more than two valves. A multi-valve engine has better breathing and can operate at higher revolutions per minute than a two-valve engine, delivering more power.- Multi-valve rationale :A multi-valve design...
.
More "radical" solutions include the sleeve valve
Sleeve valve
The sleeve valve is a type of valve mechanism for piston engines, distinct from the usual poppet valve. Sleeve-valve engines saw use in a number of pre-World War II luxury cars and in USA in the Willys-Knight car and light truck...
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 in a hydraulic pump
Hydraulic pump
Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic.Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement cannot be adjusted, or variable displacement pumps, which have a more...
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.