Tech Talk - 2 Stroke Engines
by Pettero Schwantholtzberger, Lower Slobbovia
"Was looking for something to add to our Web Site and one of our good members reminded me that it is about time to tell the story of 2 stroke engines. Nothing remarkable and nothing that the old timers don't already know but we do have a lot of new members just getting into flying RC and from what I've seen at the field, a few could use some help, at least along the lines of basics.
I guess by this time everyone reading this knows the difference between a two stroke and a four stroke engine. Two strokes fire every time the piston goes up and four strokes fire every-other time the piston goes up. Nothing at all mysterious about that.
I'm going to keep this really, really short because there are only three things that really make a difference in the operation of your engine and they are as follows:
"The Sweet Spot." No, get that off your mind. That's not what I'm talking about. The Sweet Spot is a needle valve setting. It applies to two stroke engines and four stroke engines. Very simply stated it is where you run your needle valve in so that the engine reaches maximum RPM. You can find this spot either by the sound of the engine or using a tachometer. Do you want to take off with this setting? Heck no, because once you let her rip, the engine unloads and will pick up even more RPM and then, by golly, you will be too lean. Two strokers will often pick up 400 RPM once airborne and this is going to be a very lean setting for your engine.
On one flight alone, you can cook what may have been a great engine.
Aha, now comes the "Sweet Spot." Before you take off and once you have found the maximum RPM on your engine, richen the needle valve so that the engine loses 200 or so RPM. It's that simple. Once in the air and flying, you will regain that RPM you lost when you richened the needle valve and off you go with a happy engine.
"Castor Oil." Regardless of the advertising you have read, castor oil should be added to your fuel. Some brands already have some in there but many do not. Always check the label on your fuel can to find out. If there is any doubt, add some. About 2 ounces to the gallon is usually enough. Yes, it smokes more and to some, it stinks, but the fact is that castor burns at a considerably higher temperature than does the synthetic oil used in our fuels. Tower sells Castor Oil and you can buy in half pints or you can get some from your local motorcycle shop but make sure it is racing castor and double refined. Otherwise, it builds up too much gum.
Synthetics are fine but with their limited ability to carry off heat from the engine because they break down sooner than castor oil, adding some castor oil is like buying some insurance for that expensive engine.
Yes, I know. Joe Hot Rod runs pure synthetic but his engines burn out at a regular pace.
"ABC." No, this is not practicing the alphabet. Essentially, we have two types of pistons. One with a ring on the piston and the other with pistons that fit the cylinder so tightly (lapped pistons) that you sometimes can't turn the engine over and if you do, you will hear some squealing from inside. Ringed engines need to be run rich, very rich, when you break them in. They need the extra cooling of a rich mixture as well as oil to properly seat the rings to the cylinder walls.
But, ABC engines should never be broken in by running them rich and slow. Why, you ask? Well, gee whiz, when an ABC engine runs, it must heat up the top part of the cylinder above the port to the point that the cylinder wall expands to allow the piston to move freely in that part of the cylinder. By running an ABC engine rich, you can ruin the piston/cylinder seating in a couple of minutes. Tower engines are real strong running engines and you can hear the squeal before you even run them for the first time when you try to turn them over by turning the shaft. ABC simply means Aluminum/brass/chrome and there are several combinations of this such as aluminum/aluminum/chrome etc. So before you run your new engine the first time, check to see what the metallurgy is. And if it is an ABC or related engine, run it at high RPM for a few minutes, at a slightly rich mixture, shut it down, let it cool and run it again until the engine holds RPM at a lean mixture. Some folks actually use a propane torch to start a new ABC engine in order to get the top part of the cylinder to expand as it should. By running the engine for a few minutes at a time, then letting it cool down, you allow the molecules in the metal to properly align themselves, or so they say. Pretty fancy stuff for me.
So here we are:
Rule #1: Before take off, find the "Sweet Spot" which is a couple of hundred RPM less on the rich side than your maximum RPM setting on the needle valve.
Rule #2: Always use no less than 20% oil, preferably 22%, and add a couple of ounces of Castor Oil to protect the engine in the event you mess up and take off too lean or the engine goes lean in flight.
Rule #3: Always figure out whether your engine is a ringed piston or a lapped piston and break them in accordingly. Rich and slow for ringed engines and at a high enough RPM to heat up the top part of the cylinder if it is an ABC engine.
In general, you can expect a good 100 hours running time on any decent 2 or 4 stroke engine but it does require a proper break-in, some castor oil and flying with the proper needle valve setting."
Now go fly something.
Pettero (Commodore) Schwantholtzberger
Macon Aero Modelers, Inc.
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