Engine loading questions...

Cribbs74 wrote:

roddie wrote:I realize that all engines have a max. "design" rpm... and engine "mods." can improve performance. I have heard that; on the Cox engines, it's the ball/socket joint at the piston/conn. rod, that will fail; under too heavy loading... followed by a broken "crank-pin", in extreme cases... I.E. "diesel" conversions.

Stay with me here... because I know this forum consists of mainly "flyers"... and some of my questions could apply to aircraft applications.

I've wanted to try an .049 "reed-valve" engine in a "marine" application (water prop) and I have the "Dumas" kit; complete with flywheel, 1" dia. prop., U-joint, shaft/stuffing tube... that was designed for an .049... but may be more "well suited" to a Tee Dee (rotary-valve) engine.

Question #1. Can a stock reed-valve engine be used "with success", using the Dumas "drive" kit in a model of similar specs. for which the kit was designed?

Question #2. If I didn't have the Dumas running-gear kit, and wanted to try making my own; does the "flywheel" have the most influence on performance... or does the entire running-gear's "rotational-mass" including the Dumas 1" prop.; need to be duplicated? Perhaps a larger dia./steeper-pitch prop. with a lighter weight flywheel would work... Anyone ever try this?

Question #3. Which condition is worse for the engine; "under-loading"/over-revving or "over-loading"/lugging? Will either; result in merely a weakened ball/socket joint... or will more severe damage result to the engine? The "former" can be remedied by re-setting the ball/socket joint with the Cox tool.

Question#4. Does frequent re-setting of the ball/socket joint come with consequences... other than the chore of engine dis/re-assembly... or should this only be done once or twice; before replacing the piston/rod assy. and/or cylinder?

Question #5. Does running a "Killer-Crank" in an otherwise "stock" (glow ignition) .049 engine, cause the ball/socket joint to loosen sooner... or affect the top-end rpm?

I naturally don't expect any "one" member to address all these questions... and I realize that there are other variables involved. I'd just appreciate some feedback.

Thanks, Roger

Roger,

I can only answer questions #3-5

Over revs will kill an engine quicker than anything depending on the parts installed. I say that as if you use a KB crank or similar you can spin the engine a lot faster. Usually the crank is the first to go either up front or the pin. I have never seen a problem with losing the ball socket. Sure it may need reset from time to time, but basically the same piston is used in the reed valves and TD's

A KB crank will rob some power, but not a lot (more mass)

You can reset a ball socket many times as long as you do it properly. Usually once they are set a couple times they hold and need no further reset.

Ron

pkrankow wrote:I answered them all, but 1&2 are not my area.

1:  I don't see why not.  I'm not a model boat guy though.  (Full size is a different story.)

2: The flywheel help prevent over-reving when under no load (like in the cars) and carries the engine through the exhaust part of the cycle when there is load.  There is a range of working amounts of rotational inertia that will work.  Basing on a proven system should make sure that you start in that range.  

Changing the system can reduce or improve performance, depending on the desired definition and measure of performance.  Change your measure or your definition and the same system will evaluate differently.

Again, not a model boat or model car guy.

3: Over-reving will destroy an engine in pretty short order.  Over loading generally will stall the engine or slow the engine, which will reduce maximum stresses by slower operation.

4: Work hardening will eventually come into play.  (These engines never get hot enough to re-crystallize the metal.)  Ron stated after a couple resets they tend to not need them anymore.  This might be a result of said work hardening.

5: A heavier crank will slow the engine due to more rotational inertia.  I doubt it would affect the ball joint any.  

The heavier crank may allow for a lighter flywheel though, but that would depend on your definition of performance.

Phil