Do not use gas regulators for direct injection engines

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Viv
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Do not use gas regulators for direct injection engines

Post by Viv » Tue Jan 20, 2004 10:07 am

( originally posted in another thread but moved here for easy referance )

You can not use a gas pressure regulator in the supply line to a pulse jet! we have spoken about this before but it was on the old forum, so to reiterate.

The problem is nothing to do with the maximum flow rate of the pressure regulator as is normally assumed, the regulator mechanism maintains a constant pressure drop at varying flow rates using the outlet pressure as the reference pressure.

If the regulator is connected to your engine by a pipe that goes directly in to the combustion chamber when the engine fires a pressure pulse will be transmitted back along the pipe to the pressure regulator, that pulse will alter the reference pressure for the regulator mechanism, the mechanism has a certain amount of inertia so the small pulses add up to a steady high pressure, and that will fool it in to shutting the flow down to lower the pressure.

So your engine fires and shuts down its fuel supply! now I know there are cases were it is the shear flow rate that is the problem but they have different injector locations.

Resonance in the fuel system can be a real problem, changing from soft to hard gas hose changed the way a test engine ran due to the hard hose transmitting the pressure pulses from the combustion chamber better, in the end we had to fit check valves behind the injectors to stop this.

Remember! when the fuel valve on top of the gas bottle is open, you have effectively added it as another component to the resonant system that is your engine!

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Mike Everman
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Post by Mike Everman » Wed Jan 21, 2004 10:38 am

so put a needle valve as close to the combustion chamber as possible, with the tank valve wide open?
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Post by Viv » Wed Jan 21, 2004 11:04 am

Mike Everman wrote:so put a needle valve as close to the combustion chamber as possible, with the tank valve wide open?
Yes but think of a spring and mass modell, you have just switched things around, call it a helmholtz chamber with a varible opening.

Its still resonant but now you can tune it:-)

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Post by Mike Everman » Wed Jan 21, 2004 11:26 am

I think we talked about this before, using a coil of tubing with metering valve at the engine, and check valve at supply end as an accumulator whose length and therefore resonant frequency you can adjust simply by changine the tube length.
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Post by Viv » Wed Jan 21, 2004 12:12 pm

cant remember but to keep it simple to start with I would put the check valve at the engine end so as to remove the resonance in the fuel system as a varible.

When the engine was ok then I would start trying to tune the fuel system, the trouble is we are blessed with too many varibles in pulsating combustion.

I am realy becoming quite keen on pressure jets you know as a jentle side line:-)

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Post by Mike Everman » Wed Jan 21, 2004 12:18 pm

I am realy becoming quite keen on pressure jets you know as a jentle side line:-)
I've noticed!
It's too bad Luc can't submerge a gear-pump in the liquid propane...
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Post by Viv » Wed Jan 21, 2004 12:24 pm

Mike Everman wrote:
I am realy becoming quite keen on pressure jets you know as a jentle side line:-)
I've noticed!
It's too bad Luc can't submerge a gear-pump in the liquid propane...
Yes although I want to confirm that nozzle is not the problem before we start on that problem.

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