### Luftflödet förbi en trottel

The relationship between the pressure loss and air mass flow past the
throttle is interesting to understand. Today the mass flow is calculated
by using a formula out of its range. It is necessary to multiply a
correction factor to get a good result. This factor is not a
constant.
Simulations and measurements are used to explain the correction
factor. The air mass flow is calculated for throttle plate angles up to
40 deg . The flow is supposed to be compressible and stationary.
The conclusions are that the flow is turbulent and compressible in
normal engine conditions. The main pressure loss depends on expansion
of the cross-section. The throttle plate generates a jet. The minimum
cross-section of the jet is called the vena contracta. The smallest
area which all flow goes through is the area of the sum of the two vena
contracta. The cross-section expansion is calculated as the ratio
between the area of the vena contracta and the cross-section area of
the pipe. The contraction of viscous flow is calculated with an
approximated expression based on the simulations. The expression
depends on the angle between the flow direction just before the throttle
and the flow direction in the vena contracta. The connection between the
pressure loss and air mass flow past the throttle depends on the area of
the vena contracta and the velocity in the vena contracta. The
velocity depends on the static pressure in the vena contracta divided
by the total pressure in the vena contracta. The total pressure is
approximated with the total pressure just before the throttle. The
static pressure is calculated with known pressures and the loss
coefficient.

*Mattias Krysander*

**2000**

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Last updated: 2020-04-16