[NSRCA-discussion] Fuel shut-off gizmo

[Troy A. Newman]

Bob, 

it actually happens because the pump doesn't close like the pressure demand regulator in a normal pressurized YS engine. Instead the pump is a positive displacement pump and as such if the pressure is applied to the inlet of the pump the pump has nothing to stop the flow. Yes the pump is actuated but the pushrod, however the seal on the piston of the pump is not a seal. Instead the pump uses some little check valves working against each other to control the flow. These check little valves are referenced to internal pressures in the engine one from the bottom crank case, and one from the top valve cover/airbox. The pressure differentials inside the engine when it is running is what keeps the flow of fuel under control at the given times. When the engine is not running these little check valves don't have the pressure from the engine to keep them in the right spot and by design the fuel will flow around them.

As a result the pressure induced by expansion of the tank while sitting in the sun will cause fuel to siphon to the engine. The engine will completely lock up with fuel...every open space that will allow fuel to fill into the hole will. The engine will be completely full of fuel this goes beyond a flooded condition.

The only way to stop it is to keep the supply line from allowing fuel to flow to the engine. There are many cases and often times that you don't need to plug off or pinch off the line. But if the model is sitting in the sun the tank will warm up and the fuel will siphon to the engine.

Its not an big issue use a fuel dot to plug off the tank until its time to start or use one of those K+S little clips they are quick easy and clean. 

Fancy things like the tettra valve also work well. 

To be 100% factual the exact same thing will happen on a 140RX...except that the way the pump works on the OS 140RX it provides more restriction to the flow than the pump on the YS DZ engine. Due to the less restrictive flow nature of the pump it allows the engine to flood where as the pressure in the tank of the 140RX setup would have to build to a much higher value to over come the restriction in the pump.


By the way one of the causes of this situation is many of us are running a check valve on the vent line of the tank. This prevents the overflow of fuel from the tanks vent line while carrying the model and during flight. If the vent line of the tank was left open to ATM then this would be much less of an issue. There are solutions like looping the vent line behind the tank and other solutions as well. The check valve installed to allow air in and nothing out is the cleanest and easiest, and lightest solution but you do have to have a way to close the fuel off from the engine. 

I used to use a fuel dot....I would plug the line when not running. Once it was time to run I would remove the dot throw it in the TX case and then connect the line to a fuel filter that would take fuel to the engine and pump. Now I use the little K+S clips they weigh nothing and work excellently. they stay attached to the model and fly with it. Literally they weigh about 1g additional to the model. looping the fuel tubing or installing some aluminum tubing that loops behind the tank weighs more and complicates the system further.

Troy Newman
  ----- Original Message ----- 
  From: Bob Richards 
  To: NSRCA Mailing List 
  Sent: Wednesday, December 12, 2007 9:09 PM
  Subject: Re: [NSRCA-discussion] Fuel shut-off gizmo


  I don't know much about the DZs (never owned one) but from what I understand, one of the pushrods is used to operate a pump. Could the flooding happen when the crankshaft is turned to one particular position? 

  Bob R.

  Mark Atwood <atwoodm at paragon-inc.com> wrote:
    Only the DZ line requires the clamp.  "requires" is probably too strong a word as I have run mine successfully for a season without one, but even with being very careful I still had it flood numerous times.  The clamp or a shut off is a very easy, Highly effective, highly recommended fix.

    -M




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