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Here is a simple exercise to illustrate the PRV pumping loss reduction concept:
1. With your thumb and forefinger, partially pinch your nose and inhale deeply. Your diaphragm represents the piston and your lungs represent the cylinder. The sensation on your diaphragm is the pumping loss of a piston pulling on a partial pressure manifold. Your nasal passage represents the intake valve pressure drop
2. Repeat without pinching you nose at first, then a slight pinch at the end of the inhale cycle. This exercise represents PRV draw-down from a Venturi starting from full atmospheric pressure.
3. Note that your lungs pulled the same number of air molecules. It was much easier without the vacuum induced by your fingers. At the bottom of the piston cycle, the same number of molecules are pulled into the cylinder, given the same engine load, whether PRV or conventional throttle. This is because at the end of intake cycle, intake pressure – the pressure just immediately upstream of the intake valve -- must be the same for the same amount of air (and fuel). So, for the last millimeter of piston pull, pumping loss is identical...but PRV facilitates flow at the beginning where full atmospheric intake valve pressure is available. As intake valve pressure draw-down continues, the Venturi effect is applied but diminished until the last molecule of air is ingested. Note that the plenum pressure into the Venturi is always fully atmospheric and pressure reduction occurs only ahead of the intake valve. The overall effect is reduced cumulative pumping loss, and consequently, improved engine efficiency.
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