Hi.
Can anyone please advise the workability of bleeding a small amount of air from the Combustor, to provide a pressure feed for fuel?
Dave
Bleeding Pressure
Moderator: Mike Everman
-
Eric
- Posts: 1859
- Joined: Sat Oct 04, 2003 1:17 am
- Antipspambot question: 0
- Location: United States
- Contact:
Re: Bleeding Pressure
Best way to do it is to get a RC 2 stroke engine one way valve that connects from the muffler to the gas tank, should be able to get one for 2-3 bucks.
The valve will allow the engine to pressurize the fuel tank up to its maximum internal pressure.
Using a small 1/8" pressure tap tube should provide plenty of flow, and run it through 8-12" of metal tube before connecting it to the plastic valve / fuel tank.
You will likely need a small pressure regulator, once again made for the RC 2 stroke engines, and a needle valve to control the fuel flow rate.
Eric
The valve will allow the engine to pressurize the fuel tank up to its maximum internal pressure.
Using a small 1/8" pressure tap tube should provide plenty of flow, and run it through 8-12" of metal tube before connecting it to the plastic valve / fuel tank.
You will likely need a small pressure regulator, once again made for the RC 2 stroke engines, and a needle valve to control the fuel flow rate.
Eric
Talking like a pirate does not qualify as experience, this should be common sense, as pirates have little real life experience in anything other than smelling bad, and contracting venereal diseases
Re: Bleeding Pressure
Thats most helpful Eric.
I was concerned about affecting the airflow, inside the combustor. But from what you say its not unduly critical.
I havent elicited much response from my 3 legged entity (see my other thread). Do you have any views on that please?
Is it too radical, or has it been tried?
Dave
I was concerned about affecting the airflow, inside the combustor. But from what you say its not unduly critical.
I havent elicited much response from my 3 legged entity (see my other thread). Do you have any views on that please?
Is it too radical, or has it been tried?
Dave
-
Bruce Simpson
- Posts: 39
- Joined: Tue Mar 09, 2010 9:04 pm
- Antipspambot question: 125
Re: Bleeding Pressure
You can bleed combustion pressure to generate fuel pressure, my valved engines do this. By choosing the right size of pressure hole and jet-size, the engines will run reliably over a wide range of fuel-head (+/- 300mm with ease), something an aspirated valved-engine won't normally do.
Be aware however that, in the case of larger engines, you can end up with a bomb (a tank containing fuel under pressure) right beside your engine. Any split in the tank or fixture failure can produce a spray of highly inflammable fuel that would almost certainly ignite if it came into contact with the engine or if its vapor was near an exhaust flame.
Note also that you will need a regulator otherwise it's quite likely the engine will become somewhat erratic/unreliable. That's because, if the engine suffers a cycle or two of poor combustion, the fuel pressure will drop which means less fuel will be injected and thus the chamber pressure will fall further. Without a regulator you end up with a positive feedback loop.
You can regulate the fuel pressure as its delivered to the engine or the gas-pressure within the tank. Both options have their pro's and cons.
There are other ways to pump the fuel using combustion pressure, albeit they are more complex.
Be aware however that, in the case of larger engines, you can end up with a bomb (a tank containing fuel under pressure) right beside your engine. Any split in the tank or fixture failure can produce a spray of highly inflammable fuel that would almost certainly ignite if it came into contact with the engine or if its vapor was near an exhaust flame.
Note also that you will need a regulator otherwise it's quite likely the engine will become somewhat erratic/unreliable. That's because, if the engine suffers a cycle or two of poor combustion, the fuel pressure will drop which means less fuel will be injected and thus the chamber pressure will fall further. Without a regulator you end up with a positive feedback loop.
You can regulate the fuel pressure as its delivered to the engine or the gas-pressure within the tank. Both options have their pro's and cons.
There are other ways to pump the fuel using combustion pressure, albeit they are more complex.
Re: Bleeding Pressure
Cheers Bruce.
Its interesting to note the V1 used 2 compressed air tanks, for controls and fuel delivery, in-flight.
I wonder if the former was not a requirement the method Im interested in using would have been adopted.
Thanks for reminding me that pressurised fuel tanks are a bit of a liability, Id envisaged using a nylon RC type, + accompanying valves etc, in exactly the way you have confirmed.
Dave
Dave
Its interesting to note the V1 used 2 compressed air tanks, for controls and fuel delivery, in-flight.
I wonder if the former was not a requirement the method Im interested in using would have been adopted.
Thanks for reminding me that pressurised fuel tanks are a bit of a liability, Id envisaged using a nylon RC type, + accompanying valves etc, in exactly the way you have confirmed.
Dave
Dave
-
Bruce Simpson
- Posts: 39
- Joined: Tue Mar 09, 2010 9:04 pm
- Antipspambot question: 125
Re: Bleeding Pressure
Note that you don't actually need a non-return valve in the pressure line -- in fact things work much better without one.
The advantage of not using such a valve is that if/when the engine stops, the tank pressure can rapidly bleed out the pressure hole rather than continuing to force fuel into the (now not running) engine with a resultant fireball.
It's quite surprising how much pressure you get in a tank with the right size/location of pressure tap. On my valved engines I place the pressure tap on the valve-plate so that the hot exhaust gases are effectively cooled as they pass through the plate. This means regular silicone tubing can be used for the pressure line without fear of it melting or burning. Mounting a pressure tap on the combustion chamber wall is far more problematic as it will deliver *hot* gases down the line.
The advantage of not using such a valve is that if/when the engine stops, the tank pressure can rapidly bleed out the pressure hole rather than continuing to force fuel into the (now not running) engine with a resultant fireball.
It's quite surprising how much pressure you get in a tank with the right size/location of pressure tap. On my valved engines I place the pressure tap on the valve-plate so that the hot exhaust gases are effectively cooled as they pass through the plate. This means regular silicone tubing can be used for the pressure line without fear of it melting or burning. Mounting a pressure tap on the combustion chamber wall is far more problematic as it will deliver *hot* gases down the line.
Re: Bleeding Pressure
Many thanks Bruce.
The temperature of the inner walls of combustors rises locally where surface imperfections are present but locating the bleed off on the valveplate, as you kindly suggest would still be cooler than at the constricted end of the combustion ch' although on valveless designs only the rear of the tailpipe and part of the intake pipe remain 'fairly' cool but the pressures here would be relatively low.
Driving a thick wall tube down the centreline of the jet may be another means to tap pressure, or if water was injected to generate a head of steam within a fuel pump servo.
Dave
The temperature of the inner walls of combustors rises locally where surface imperfections are present but locating the bleed off on the valveplate, as you kindly suggest would still be cooler than at the constricted end of the combustion ch' although on valveless designs only the rear of the tailpipe and part of the intake pipe remain 'fairly' cool but the pressures here would be relatively low.
Driving a thick wall tube down the centreline of the jet may be another means to tap pressure, or if water was injected to generate a head of steam within a fuel pump servo.
Dave