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.

Thoughts?

Juan,
Cooling would take one leg of the combustion triangle out of the
picture. I dare say it would take a large amount of cooling to do so.

I suspect that a more plausable reason would be the fuel air ratio being
outside the LEL.

The manual calles for 145 psi and 1.21 lbs volumn. are you getting the
volumn with the setup you are using ?

all the best.
Sean

Juan Jimenez wrote:
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.

Thoughts?

Yes, the volume is more than enough, as evidenced by N1 reaching close to
15%.

Juan

"Sean Trost" wrote in message
. ..
Juan,
Cooling would take one leg of the combustion triangle out of the picture.
I dare say it would take a large amount of cooling to do so.

I suspect that a more plausable reason would be the fuel air ratio being
outside the LEL.

The manual calles for 145 psi and 1.21 lbs volumn. are you getting the
volumn with the setup you are using ?

all the best.
Sean

Juan
I believe that N1 is related to the velocity of the airstream
introduced. Which is good. But not related to the ignition problem
Now You have said that you have an excellent spark and good fuel. the
only other thing you need for start is the right airflow. or volumn.
Dont know about your engine specifically but most everything else runs
right around 14 to 1 on the ratio wise.

I can get a HIGH volume low pressure spray gun to flow 10 psi but it
still wont spray anything unless i get the volmume behind it to work
properly. Not that this has anything to do with jet engines or
cumbustion but to illustrate that velocity and volumn are not
interchangable.

Sean


Juan Jimenez wrote:
Yes, the volume is more than enough, as evidenced by N1 reaching close to
15%.

Juan

"Sean Trost" wrote in message
. ..

Juan,
Cooling would take one leg of the combustion triangle out of the picture.
I dare say it would take a large amount of cooling to do so.

I suspect that a more plausable reason would be the fuel air ratio being
outside the LEL.

The manual calles for 145 psi and 1.21 lbs volumn. are you getting the
volumn with the setup you are using ?

all the best.
Sean

I understand what you are saying. The way I see it, the proof that there is
sufficient volume is that the engine is turning up to the desired RPM needed
before putting fuel into the mixture.

I think, from testing I did today, that I have a stuck fuel control spill
valve. I hope it's not that, but I think that's what it is.

"Sean Trost" wrote in message
news
Juan
I believe that N1 is related to the velocity of the airstream introduced.
Which is good. But not related to the ignition problem
Now You have said that you have an excellent spark and good fuel. the
only other thing you need for start is the right airflow. or volumn. Dont
know about your engine specifically but most everything else runs right
around 14 to 1 on the ratio wise.

I can get a HIGH volume low pressure spray gun to flow 10 psi but it still
wont spray anything unless i get the volmume behind it to work properly.
Not that this has anything to do with jet engines or cumbustion but to
illustrate that velocity and volumn are not interchangable.

Sean


Juan Jimenez wrote:
Yes, the volume is more than enough, as evidenced by N1 reaching close to
15%.

Juan

"Sean Trost" wrote in message
. ..

Juan,
Cooling would take one leg of the combustion triangle out of the picture.
I dare say it would take a large amount of cooling to do so.

I suspect that a more plausable reason would be the fuel air ratio being
outside the LEL.

The manual calles for 145 psi and 1.21 lbs volumn. are you getting the
volumn with the setup you are using ?

all the best.
Sean

Sean Trost wrote:
Juan
I believe that N1 is related to the velocity of the airstream
introduced. Which is good. But not related to the ignition problem
Now You have said that you have an excellent spark and good fuel. the
only other thing you need for start is the right airflow. or volumn.
Dont know about your engine specifically but most everything else runs
right around 14 to 1 on the ratio wise.

Actually, jets are a different beast from piston engines. Between
about 25:1 and 50:1 is normal, even for lightoff fuel flow.
Stoichiometrically correct combustion is too hot.

I stand corrected. Thanks Jim. Learned somthing new everyday.
Sean

Jim Carriere wrote:
Sean Trost wrote:

Juan
I believe that N1 is related to the velocity of the airstream
introduced. Which is good. But not related to the ignition problem
Now You have said that you have an excellent spark and good fuel. the
only other thing you need for start is the right airflow. or volumn.
Dont know about your engine specifically but most everything else runs
right around 14 to 1 on the ratio wise.


Actually, jets are a different beast from piston engines. Between about
25:1 and 50:1 is normal, even for lightoff fuel flow. Stoichiometrically
correct combustion is too hot.

"Juan Jimenez" wrote in message
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.

With the air multiplier, you have the pressure, but do you still have the
volume (cubic feet per minute)?

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.

APUs are designed to start at altitude where the temperature may be minus
40C.

D.

"Capt.Doug" wrote in message
news
"Juan Jimenez" wrote in message
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.

With the air multiplier, you have the pressure, but do you still have the
volume (cubic feet per minute)?

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.

APUs are designed to start at altitude where the temperature may be minus
40C.

The Microturbo Couguar is not an APU. It's a turbojet engine designed to
power small aircraft or drones.

Juan

Juan Jimenez wrote:

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.

Thoughts?

My thought is that the air impinges on the turbine wheel, not the
compressor. So that cold air has nothing to do with lighting the mixture.