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Abstract



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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Senast uppdaterad: 2019-12-12