Air Compressor Report

stay at full lift for the full time it is open; and c) the valve does not close exactly at the dead center. All of these factors affect both the capacity and the power of the compressor. A simple method of calculating the power loss due to the pressure drop across the valve was given in the section on theory. However, this assumed that the valve was at full lift for the entire time gas was flowing through it. For a more accurate estimate of the power loss, the weighted average valve lift should be used. This can be calculated from the valve lift diagram, Fig. 2.4.

Fig 2.3: Typical Valve Dynamics Diagram Obviously, the average lift is less than the full lift and so the average valve flow area is less than the full lift flow area. Fortuitously, that method also contains an error that makes it overestimate the power loss and in many cases it gives a good estimate of the true loss. One assumption of that method is that the gas is incompressible. That is, it is assumed that at valve opening, the pressure loss increases instantaneously to the value calculated from the piston velocity. In fact, due to the compressible nature of the gas and as shown in Fig. 12, the pressure drop rises gradually from zero at the instant the valve opens. 19