On the Use of the Discharge Coefficient Concept in the IC Engine Valve Mass Flow Rate Calculations
1. mr Dobrivoje Ninković, University Belgrade, Serbia
Calculation of the instantaneous mass flow rate through IC engine valves is almost invariably carried out by multiplying the mass flow rate passed by an isentropic nozzle with a quantity referred to as discharge coefficient. The latter accounts for all deviations of the real valve flow from the ideal nozzle flow, and is customarily determined from the data obtained on a steady-flow test rig. While this is a valid approach to calculating the mass flow rate, there are serious problems when one attempts to integrate the discharge coefficient concept into a broader gas dynamics context of IC engine valve flow simulation.
The paper traces back this method to its origins, describes its evolution, and presents the formulae currently used in the simulation work. Differences between the factors defining the discharge coefficient values in incompressible and compressible flows are pointed out. An analysis is presented as to the unsuitability of this concept for inclusion into the lost work framework that dominates modern thinking about losses in thermodynamics. Superiority of the Fanno flow theory over the discharge coefficient concept in predicting compressible flow through a thick orifice is demonstrated by a negligible deviation of a former method's result from the experimental data.
Ključne reči : Discharge coefficient Fanno flow IC engine valve Loss modelling.
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Datum:
11.02.2013.
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