Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
then the valve curtain area, At, is given from the value of x as:
whence
At = 7tf d ° s +dis )\(L - d ° s dis tan
The simple criterion for maximum valve lift, ignoring the effect of the valve stem diameter,
can be found from Eq. A5.1 by inserting the area ratio, k, as a maximized value of unity,
which produces the information that it is 25% of the inner seat diameter. From the above
theory this is manifestly much too simple a conclusion. The effect of a (conventional) valve
seat angle of 45°, and typical values for valve stem diameters, is to empirically raise that ratio
from 0.25 to more nearly 0.3 in practice. Empiricism is quite inadequate in this context, as the
above theory must be employed by the designer to ensure that the flow areas of the poppet
valve and its port are carefully matched at the design stage. The mathematical reality for the
assessment of maximum valve lift is that the discharge coefficient must be incorporated into
the debate as:
CdaAt = Aseat eff (A5.6)
and the maximum lift is then found from the curtain area term, At, by iteration for valve lift
until an equality for Eq. A5.6 is obtained.
To ensure accuracy regarding the prediction of mass flow rates and of the magnitude of
pressure wave formation, this analysis of valve curtain area must be incorporated into both
the reduction of measured data for the determination of the actual coefficient of discharge,
Cda, measured in steady flow experiments and also into the replay of such data within any
engine simulation incorporating unsteady gas flow. For C