Volumetric efficiency

The volumetric efficiency describes in an internal combustion engine with internal engine combustion, the ratio of the fresh charge actually contained in the cylinder after the completion of a charge cycle to the theoretical maximum possible filling.

Gasoline engine

In a conventional gasoline engine fresh gas (petrol -air mixture ) is drawn. The necessary for the filling pressure gradient between the cylinder and the intake manifold is achieved either alone by the intake stroke of the piston or by compressor. If the charge exchange process would take place infinitely slowly, the volume of the intake mixture would correspond exactly to the engine capacity, provided that the intake valves close exactly at bottom dead center (BDC ) and the exhaust valves are closed during the entire procedure. But because a gasoline engine running at speeds of about 600-17000 1/min, remains very little time for the water streams. The result is that the incoming mixture undergoes a first and second flow resistance has to be accelerated and decelerated. Through in the intake and cylinder pulsating pressure waves unfavorable currents and the valve overlap is further reduced the already introduced into the cylinder gas quantity. As a result, upon completion of the charge cycle in the cylinder remaining amount of fresh gas be higher or lower than would correspond to the displacement.

The cylinder charge, however, is largely responsible for the torque and, consequently, also for the performance of the engine. A motor with high volumetric efficiency therefore has a high -capacity specific performance.

The volumetric efficiency for a gasoline engine is defined:

Diesel engine

In a diesel engine only fresh air flows into the combustion chamber. The diesel fuel is injected only after the completion of the intake stroke during the compression stroke and thus has no effect on the volumetric efficiency. The same holds as for a gasoline engine for a diesel engine. In order to develop a lot of torque, plenty of air must be present in the combustion chamber, so much fuel has sufficient oxygen available for combustion.

The volumetric efficiency of a diesel engine, defined by:

Settings

There are many ways to increase the volumetric efficiency:

  • The intake valves close usually considerably after the UT, and flows through the kinetic energy of the mixture even after a further mixture UT ( reload effect). The cylinder charge can thus be larger even with naturally aspirated would correspond to the displacement.
  • With a manifold gas dynamics can be adapted to the speed in the suction, so that the recharging effect can be used in a wider rpm range.
  • More intake valves give more flow cross-section ( three-valve, four-valve, five-valve technology).

The gas equation for air

Leads to two other methods to increase the filling of the combustion chamber:

  • The supercharging of the engine (eg, turbocharger, compressor) increases the pressure p of the fresh gas.
  • The intercooling reduces the temperature T of the fresh gas.

Both measures increase the value of the fraction, and thus the fed to the engine air mass.

Note: By definition, the theoretical air mass () is calculated with the same values ​​for density, pressure and temperature as the mass of air actually supplied (). That is, increase charging and intercooling, although the cylinder filling, but not the volumetric efficiency.

Other terms

Air effort

Volumetric efficiency is the amount of fresh gas relative to the theoretically possible (geometric ) quantity of fresh gas that passes through the combustion chamber during a working cycle. The fresh gas shares so it will be counted, which immediately leave the combustion chamber again without having been involved in the combustion. This can happen during the valve overlap ( simultaneous opening of the intake and exhaust valve), or by flowing back of the mixture in the intake pipe.

Trapping efficiency

The ratio of actually remaining in the combustion chamber fresh gas and air effort is called trapping efficiency, because the proportion of the make "captured" was.

Spülgrad

The Spülgrad however, indicates how the ratio of fresh gas to the charge (fresh gas and residual gas could not flow ) is. The idea that the residual gas (which is what in the combustion at the end remains ) completely escapes through the exhaust after combustion, is an ideal that is not entirely correct. Hence the name of the Spülgrads.

Trapping efficiency and Spülgrad are mainly in two-stroke engines are important parameters because at this motor type of outlet is almost always open, even if the inlet is open. It is therefore difficult to avoid that fresh gas again escapes through the outlet. Development goal is always to keep these scavenging loss small.

Engine displacement
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