Dwell is the length of time the valve is open.... also know as duration.... In a "knock open" valve, it is a function of lift.... Flow RATE is a function of lift, but only until the valve is open about 1/4 of the throat diameter.... At that point the "curtain area" (the circumference of the valve times the distance it is open) equals the throat area, so in theory, any additional lift does not increase the flow RATE.... The flow RATE is also a function of the pressure.... and whatever restricts the flow first is likely the point at which the flow rate peaks....
The total flow through the valve, however, is a function of the flow rate through the whole gun, and the dwell.... For a given pressure, throat, and port diameter, additional lift past the 1/4 diameter point doesn't increase the flow RATE, but allows the flow to continue over a longer time.... because it takes the valve longer to make its round trip.... Here is a graph of what the valve lift and flow MIGHT look like, IF the valve opening as a function of time is a SINE WAVE....
If the valve is not flow limited by the throat diameter (ie lift < diameter/4), then if you double the dwell, you get 4 times the area under the curve, so the total flow VOLUME should be 4 times as great.... The three curves shown have relative areas of 1 : 2 : 4..... However, let's assume that once the valve lift equals the height of the smallest curve, the flow RATE no longer increases.... Then we get this situation for the flow RATE....
The total flow is still the area under the curve.... but instead of the largest curve having 4 times the area, it is more like 2.5 times.... and the middle curve is maybe about 1.6 instead of 2 times.... Thinking about how dwell and lift are interconnected, and how varying one changes the other, will help you understand what is happening in a PCP with a "knock-open" style valve.... They self-regulate BECAUSE at higher pressures, the (same) hammer strike meets a greater resistance to not only knocking the valve off the seat, but a higher closing force as well.... That limits the lift, limiting the amount of air released by the valve, and results in a smaller VOLUME of high pressure air being released, compared to a larger volume of lower pressure air later in the shot string....
Bob
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