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The pressure profile of a PRV induction system is entirely different for two reasons. First, the PRV intake pressure is always ambient (or turbocharger discharge) pressure. Second, when the cylinder valve closes, air from the plenum can flow around the pintle, equalizing the Venturi discharge pressure with the plenum. There is plenty of time for this to happen because inlet valves spend at least twice as long closed as open. Third, in comparison with a butterfly throttle which is as obstructive as one could imagine, the PRV is as streamlined as possible.
The throttle control fluid mechanics of a conventional throttle plate and PRV induction system are entirely different, contributing to the improved fuel economy performance derived from PRV. Throttle control with PRV is accomplished by the limitation of sonic velocity at the throat. The velocity at the throat of the Venturi is determined by the speed of the piston and the annular or area between the pintle and Venturi. Initially, the piston moves slowly and air easily flows through the streamlined Venturi. However, as the piston accelerates away, at some point the air flow through the Venturi will approach sonic velocity. At sonic velocity, the pressure in the intake duct can no longer influence the flow through the Venturi. The Venturi is said to be choked -- and in this way, the streamlined PRV controls the power of the engine.
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