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#1
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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? |
#2
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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? |
#3
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"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. |
#4
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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. |
#5
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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 |
#6
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"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 |
#7
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"Colibri" wrote in message ... 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. But the cold air goes through the engine after it hits the turbine wheel... |
#8
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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 |
#9
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"Richard Riley" wrote in message news Propane tanks are normally rated for 250 LBS. That rating is normally at 120 degrees. 280 PSI air is probably a lot hotter than that. It's been too long since I've done ideal gas law equations, but I seem to remember 250 psi air getting to 300 or 350 F. At a temperature higher than it's rating, a tank is weaker than it's rating. You lost me here. In my experience, 250# air can be any temperature you want it to be. Vaughn |
#10
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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 |
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