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Turbine air start -- too cold?

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Old January 14th 05, 12:40 AM
Juan Jimenez
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Default Turbine air start -- too cold?

The engine on my homebuilt aircraft is a small French turbine, a Microturbo
022 Couguar (one version behind the TRS-18-046). It starts using good ol'
air impingement, which the manual specs out to 145 psi, 1.21 lbs volume.
Now, I always had problems motoring the engine to a high enough RPM where I
thought it would be safe to open the fuel and spark if I pressurized my tank
to 155, so I bought myself a pressure multiplier (I was worried about hot
starts). It's a simple SMC unit that mechanically multiplies air pressure by
4x, up to 280-300 psi or so. I have a converted 100# propane tank to hold
the air, which I pressurize to 280 and regulate to 145 psi. It works GREAT
though it takes a while to do its job.

However, the engine now refuses to start, even though the spark is great and
fuel atomization is also good. I'm concerned that when I dump the air into
the engine, the drop in pressure is cooling the air so much that it
interferes with the start process and the fuel mist doesn't light up like it
did in the past. The temperature drop is significant -- if I remove the air
start hose and open the valve wide open, ice will form on the outlet. Where
I live, the Caribbean, the air is also quite humid, and I'm sure that is
contributing to this even though I have a water trap in the air line.

The original docs on the engine say that the ground servicing rig had air
tanks, two pressure regulators and associated valves. How would one avoid
this problem, and is it in fact something that could interfere with the
start, or should I be looking elsewhere? Fuel pressure is fine, spark is a
whopping blue honker that looks every bit as deadly as I'm sure it is,
atomization is fine, etc.



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