We’ve built a big PJ (we expect over 100 lbf), and the closest thing to running we can get is what sounds like an idling nitro-burning funny-car engine (besides a really great boom) and of course, big billowy plumes of fire.
The dimensions are these:
- Combustion Chamber - 9 in. dia., 13 in. len.;
- Exhaust Tube - 6 in. dia., 36 in. len.;
- Reduction Cone - 16 in. len.
The fuel is propane; and is injected inside the chamber by two jets.
We've tried:
- .006, .008, and .010 in. thick spring-steel material for the valves;
- fuel in both gaseous and liquid forms;
- fuel pressure from 0 to 100 psi;
- with and without continuous (gas powered) leaf-blower air;
- extending the exhaust tube length with a "trombone" sleeve another 12 inches;
- shortening the exhaust tube length with metal cutters by 6 inches;
The best we get is by choosing a pressure, say about 20 psi, turn on the fuel, hit the spark, and give it the air blower. If we give it the air right away, we get something like, BbBbbbBBbBbbBBbbbB… (though it only lasts for a few seconds). Or if we hold off on the air for a couple seconds, and when we give it we get a great BOOM! In any event, it just ends up an over-achiever’s barbeque flame dispenser.
We figure there has to be something fundamentally wrong because there is such a wide margin of “as good as it getsâ€