Engine gains speed when fuel is low?

[frank mccune]

    Hi All:

     While running an engine yesterday, I again noticed that the engine gained a lot of rpms when the tank was almost empty.  This also happens if I disconnect the fuel line from the tank while running an engine on a test stand.   The fuel line from the tank to the engine is about 6 inches.  I have noticed this since I have been playing with model airplane engines.

     What causes this? Does this mean that the engine has reached its optimum fuel/air ratio?  Is the engine running too lean?  If I could get the engine to run this fast all of the time, it would be like getting the performance of a larger engine out of the smaller engine.  I can not duplicate this increase in speed via the needle valve.

      Any thought about this observation?

                                                                                                                                  Tia,

                                                                                                                                   Frank

[Brett Buck]

    Hi All:

     While running an engine yesterday, I again noticed that the engine gained a lot of rpms when the tank was almost empty.  This also happens if I disconnect the fuel line from the tank while running an engine on a test stand.   The fuel line from the tank to the engine is about 6 inches.  I have noticed this since I have been playing with model airplane engines.

     What causes this? Does this mean that the engine has reached its optimum fuel/air ratio?  Is the engine running too lean?  If I could get the engine to run this fast all of the time, it would be like getting the performance of a larger engine out of the smaller engine.  I can not duplicate this increase in speed via the needle valve.

      Any thought about this observation?

                                                                                                                                  Tia,

                                                                                                                                   Frank

   I don't think I understand, are you referring to it leaning out right before it quits? That's because it starts sucking in air as the fuel pickup uncovers. You are almost always running richer than ideal for maximum power, so as it runs out, it gets leaner, speeds up for while, then slows down, then quits.

    In many cases, the added acceleration from eating a few bubbles moves more fuel to the back, which richens it back up by covering the pickup again, this causes the fuel to slosh forward, etc, so it frequently cycles back and forth for a lap or more, then finally quits.

   Brett

[frank mccune]

      Hi Brett:


      Thanks for the reply.

      As I said, I noticed this on a test stand where one can see the last of the fuel being drawn into the engine through the clear fuel line.  It is then the engine goes much faster and quits very abruptly.  If we could tune our engines to operate at this increased speed, it would provide a much more efficient engine run.

      I will play with an engine on a test stand to confirm my findings as soon as I am able.

                                                                                                                      Stay well,

                                                                                                                       Frank

[Brett Buck]

      Hi Brett:


      Thanks for the reply.

      As I said, I noticed this on a test stand where one can see the last of the fuel being drawn into the engine through the clear fuel line.  It is then the engine goes much faster and quits very abruptly.  If we could tune our engines to operate at this increased speed, it would provide a much more efficient engine run.

   ??  You can of course tune the engine to peaked out at any time. In stunt it usually isn't set that way because you want the load of maneuvering to help with speed stability, but you certainly can.

    Brett

[Tim Wescott]

If we could tune our engines to operate at this increased speed, it would provide a much more efficient engine run.

If we were racing or speed flying or flying endurance then we'd probably want that (or at least we'd want the higher speed).

But what we want is an engine that doesn't sag in the maneuvers (which you get if it's too lean), doesn't fly too fast or too slow, and -- if we're lucky -- actually gives us more power when the plane is slowing down a bit.  All of that comes when we use the right engine, sized to be bigger than it needs to be for controlled level flight, and is running way richer than would be "right" for an RC plane.

Brett and others have written on this extensively.  If you can't find their statements on this (try searching on "engine run" or "stunt run") then ask, and maybe some kind soul will point you to some threads.  I think there's a sticky at the top of this very forum that goes into how and why you set up for a stunt run.

[Phil Krankowski]

I recommend you either buy or borrow a copy of "2-stroke glow engines for RC aircraft" by C.David Gierke
http://www.amazon.com/gp/product/0911295305/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1

Check the local library, this is an expensive book that you might read through 2-3 times and not look at much after this.  The low priced used copies on Amazon are generally stained inside with greasy fingerprints, I have one of these copies, and felt it was worth the purchase even though it does not get looked at regularly.

A whole lot of questions regarding engine operation will be answered, even questions you never even thought of! (Well, that was my experience at least)  

Another benefit is reading what people (Brett Buck, et al) have to say here will make more sense.  Control line engine operation is quite different from RC engine operation.

Phil

[frank mccune]

     Hi All:

     Thanks for the replies.

      I was not speaking only of Stunt.  I was wondering why the engine speeds up so much when the fuel line is removed from the tank. This does not happen when the fuel line is pinched off to stop the engine.  The fuel is just sucked along the fuel line until all of it is used.  During this time, the engine speeds up a large amount.  Is there less "drag" on the fuel when only the amount of fuel in the line has to be drawn into the engine.

     Heck with this question.  I only know that it happens and perhaps nobody has studied this question enough to come to an understanding of what is really happening.  Perhaps for my dissertation! Lol

                                                                                                                           Stay well,

                                                                                                                           Frank

 

[Brett Buck]

     Hi All:

     Thanks for the replies.

      I was not speaking only of Stunt.  I was wondering why the engine speeds up so much when the fuel line is removed from the tank. This does not happen when the fuel line is pinched off to stop the engine. 

   Because it slowly goes lean instead of cutting quickly.

    Brett

[Jim Thomerson]

If you can't get it to lean out like that with the needlevalve, perhaps you have a restrictive venturi, where the amount of air entering the venturi is the limit to speeding the engine up.  When you get bubbles in the fuel line more air is going into the engine along with less fuel.  Speed and racing engines generally have large venturis.

[Phil Krankowski]

The engine is charged with fuel, and may flood because there is no restriction on the fuel.  Then the engine can draw air through the fuel line, and the venturi, so it can burn off the fuel charge going more lean than expected.

If speed is the name of the game, pressure feed systems such as bladders, and large bore venturies make for screaming engines.  Start on prime then let the fuel run!  As venturi bore size increases ease of starting decreases, until it reaches a point of impossible after passing through really annoying. 

A couple of Cox TD engines a .049 and a .051 arrived with over-bored intakes, much larger than recommended.  Swapping to stock suits my desires and needs much better than having nearly impossible to operate engines.  (I like one and two flip starts)

Phil