problems starting a lpg/propane pj

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Bruce Simpson
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Re: problems starting a lpg/propane pj

Post by Bruce Simpson » Fri Jul 09, 2010 3:42 am

Not enough gas for a start and, lighting the gas at the tailpipe is not a very good way to get an engine going -- it's much better to have the ignition source in the combustion chamber (a spark-plug, sparkler or what-ever).

I suspect that if you wind up the gas-flow you'll still have problems getting the engine going because you'll need so much air blown in the front that the flamefront won't be able to travel back to the combustion chamber due to the gasflow in the tube exceeding the speed of deflagration (flame-burn-rate).

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Re: problems starting a lpg/propane pj

Post by Rocket Man » Fri Jul 09, 2010 7:31 pm

Turn on the air first then turn on the propane. You need a pressure regulator in the propane line so you can adjust the correct amount of fuel.

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Re: problems starting a lpg/propane pj

Post by metiz » Fri Jul 09, 2010 8:26 pm

Rocket Man wrote: You need a pressure regulator in the propane line so you can adjust the correct amount of fuel.
Handy with very small engines but even then not necessary. I'd start the propane up first and then apply a puff of air so you get ignition. Apply air and increase fuel flow. If the flame shoots to the tailpipe decrease gas and give a small puff of air again until the flame recedes back into the cc. Rince repeat. Eventually the cc will get hot enough for you to turn up the gas and blast it with air; it will start right up (usually)
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Re: problems starting a lpg/propane pj

Post by Rocket Man » Fri Jul 16, 2010 3:45 am

metiz wrote:
Rocket Man wrote: You need a pressure regulator in the propane line so you can adjust the correct amount of fuel.
Handy with very small engines but even then not necessary. I'd start the propane up first and then apply a puff of air so you get ignition. Apply air and increase fuel flow. If the flame shoots to the tailpipe decrease gas and give a small puff of air again until the flame recedes back into the cc. Rince repeat. Eventually the cc will get hot enough for you to turn up the gas and blast it with air; it will start right up (usually)
You have too much fuel.

Rule #1, turn the air on first then turn on the fuel.

What are the demensions of your engine?

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Re: problems starting a lpg/propane pj

Post by metiz » Fri Jul 16, 2010 7:46 am

Rule #1, turn the air on first then turn on the fuel.

That's bs, Rocketman. You'll only waste startup air. turn on the gas *ignition* THEN start blowing.
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Re: problems starting a lpg/propane pj

Post by Rocket Man » Sat Jul 17, 2010 2:26 pm

metiz wrote:Rule #1, turn the air on first then turn on the fuel.

That's bs, Rocketman. You'll only waste startup air. turn on the gas *ignition* THEN start blowing.

Here is what happens if you turn on the fuel first. You get a very large flame out the back of the engine. You have to blow a very large amount of air through the engine to blow out all the excess fuel. Then when the extra fuel is gone the fuel air mixture is correct and it fires up. You waste fuel like this. Air is free you should be more concerned with wasting fuel. What works best is to turn the air on only about 1/2 second before the fuel. I start my engines at about 20% throttle then I turn the fuel up to how ever much thrust I want. I can run an engine for several hours at 100% static thrust. 100% static thrust is not maximum thrust once the engine picks up speed ram air makes the engine run lean so I can throttle up it to about 140% to 150% thrust depending on the speed of the engine and the design of the air intake. The more air that gets rammed into the air intake the more fuel the engine will burn and the higher the thurst is.

If your starting a valveless engine you have to turn the fuel on first.

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Re: problems starting a lpg/propane pj

Post by Mark » Sat Jul 17, 2010 10:43 pm

I recall Tharratt had a single lever that controlled the fuel, starting air, and I guess the spark too. He remarked he could start and stop the duct 15 times in as many seconds, as fast as you could throw the lever. He did not waste fuel or air. ha
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Re: problems starting a lpg/propane pj

Post by Bruce Simpson » Tue Jul 20, 2010 12:22 am

The startup sequence varies depending on the design.

The ECU I built for a liquid-fueled valveless engine starts with spark, adds air, adds fuel, listens for combustion (vibration sensor) and if the engine doesn't sustain, shuts off gas, waits (to allow the air to flush the chamber) then resequences.

Once pulsating combustion is detected, it waits for the combustion chamber and vaporization plate to achieve sufficient temperature then ramps up the liquid fuel injection while ramping down the gas.

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