Larry Cottrill wrote:
Rossco wrote:I have posted before about being able to get my engines going much harder by "pumping them up".
What i mean by this is get it going as hard as it will, self sustaining. While running, and hot, give the intake a sudden blast of air (as high pressure and volume as you can). At the same time crank the gas up a bit, then let it sustain at this new level. I can do it about 5 times with mine, and at a guess, i get about twice the thrust and heat level than at the highest throttle position that it would self sustain at originaly.
My theory on this is that all of my current engines have undersized intakes. By doing this to them it induces a lower intake phase pressure, and therefor suck more air, and will hence tollerate more fuel!
I doubt that you can hang this on "undersizing" of the intake [OK, maybe you can in its final condition, where you can't 'pump it up' further, but the intake must be doing OK up to that point!]. I do agree that there must be a lowering of average
pressure in the suction phase of the cycle. What I mean by this is that it could
be that the deepest pressure isn't lowerered further, but rather that the low part of the pressure graph is made broader in time value under 'pumped' conditions.
What I'm saying is, the pressure curve of an engine running hard may not
be a vertically magnified version of the pressure curve of the very same engine running easy. There!
Whatever the exact situation is, it has to mean that you're getting a better energy conversion per cycle than at the last plateau [more work in the same period of time]. It all boils down to the momentum you actually achieve in the tailpipe 'piston' - more momentum means more thrust, but also means a more powerful draw at the front end [all other things being equal].
What this observation proves is that it is not necessary to alter system geometry to achieve stable operation at various power levels. I have usually assumed that an engine runs up [in the first few cycles] until it reaches a limiting condition where negative feedback catches up and we have a stable condition beyond which we can't go. Obviously, this is not true [in the case of your engine, at least] and that is important. It means that we should not necessarily give up on a design that sustains in a 'weak' thrust condition - it may simply be operating in the lowest stable state that we can find [or, some low stable state we happened upon]!
Remember that if your engine were in motion, the action you're describing would not involve coordination between an artificial air source and the fuel valve, but merely a careful synchronization of air- and fuel-throttling. ['merely' - ha!]
The next time you get this to work, you should see if temporarily restricting the air causes a step downward to a lower-powered stable state, and try the same with fuel restriction, i.e. is the transition air-led or fuel-led, or must it be a coordinated adjustment? Even if the theory isn't quite immediately grasped, a set of practical guidelines for how it works could be invaluable.
At the risk of once again providing proof of myself as an utter fool, I propose The Cottrill Postulate:
For any given self-sustaining pulsejet where the fuel input is an independent variable, there will be a stable operating state at a definite average machine temperature for any given energy input level [rate of fuel consumption], with higher average temperatures corresponding to higher input levels.
There - I've said it. What this means is that your engine will eventually achieve a certain stable rate of heat radiation [average temp] where it will throttle down its intake air if it becomes hotter, and conversely, increase its intake air if it becomes cooler, so that it always tries to return to equilibrium for a particular fuel input level. If, then, you can bump it up to a higher temperature, you will achieve a stable state of equilibrium between energy input [fuel consumption] and energy output [radiated heat and air movement]. You do this by setting up more fuel delivery and excess air until the operating temperature comes up, and then you can relax the fuel back to a level that will be at equilibrium at the higher output point while the engine controls its own air input to achieve the stable state [thermal equilibrium] at that new level.
I feel confident that to seasoned practitioners such as Bill Hinote, M, and some others, my Postulate will seem obvious and practically trivial. But for guys like us, it might be helpful to have in mind while searching for such desirable features as throttleability.
Also keep in mind that this statement does not imply that there is a high degree of linearity or that there aren't limits as to how high or low you can go -- merely that for any one design there is no one "perfect" operating point [constituting the only condition where it will work], but rather a range of values that will satisfy a stable running condition.
Basically, I think that any pulsejet can be run hard or easy -- you just have to discover how to properly set up the exact running condition for the power level you want.
It is harder to apply this to a carbureted engine, since the fuel input loses its 'independent variable' aspect, and tries to track the air input in a roughly proportional manner. In theory, the principle still applies, though - your method of control would be trickier, imposing tight synchronization of air and fuel delivery [since the carburetor action will not be perfectly linear].