Optimizing the Design of the Air Flow Orifice or Restrictor for Race Car Applications (2007-01-3553)

Several race car competitions seek to limit engine power through a rule that requires all of the engine combustion air passes through a hole of prescribed diameter. As the approach and departure wall shapes to this hole, usually termed orifice or restrictor are not prescribed, there is opportunity for innovation in these shapes to obtain maximum flow and therefore power.This paper reports measurements made for a range of restrictor types including venturis with conical inlets and outlets of various angles and the application of slotted throats of the ‘Dall tube’ type.Although normal venturis have been optimized as subsonic flow measuring devices with minimum pressure losses, at the limit the flow in the throat is sonic and the down stream shocks associated with flow transition from sub-sonic to sonic are best handled with sudden angular changes and the boundary layer minimized by the corner slots between the convergent and divergent cones.The measurements made here were on a quasi-steady flow rig using a Roots type blower to simulate repeatable flows and for a restricted range of operating temperatures over which small corrections to datum conditions could easily be applied.The conclusions made are that best results are obtained for the conical inlet nozzle conical diffuser devices with slots at the throat, quite consistent with Dall’s original claims as found in older editions of the BS 1042 flow standard. This design maximizes the mass flow integral over a range of volumetric flow rates from Mach number M=0.6 to a wide range of M=1 conditions simulating increasing downstream MAP with increasing engine speed. This suggests ability to outperform more conventional devices by a few per cent, which could be important in delivering more flexible and powerful engines with more conventional nozzles.