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Powertrains 103: Advanced Guide To Two Stroke RC Engines

Article by Kenny McCormick

Two-stroke engines are very commonplace in RC. Fact is, cars use them 100% (to date no ready-to-run has ever come out with a 4-stroke, and OS has discontinued the two short-run car 4-stroke they did make), and aircraft still use them by majority. They're simple, dependable workhorses... but why are they called 2-strokes? What makes them so ubiquitous in RC?

Simplicity, in a nutshell. The average glow two stroke has... seven moving parts. That's it. Seven. Piston, con-rod, wrist pin, crankshaft, drive washer/flywheel, and front/rear bearings. This simplicity means they can fit in a very small space, last a good long time, and make boatloads of power. Now, think back to Powertrains 102. Engines need to do four things to run. In a two stroke these things are all done more or less at once.

A two stroke engine requires just two strokes to produce power, hence the term. The first, upward, stroke in the 2-stroke cycle combines intake and compression into one smooth motion. Intake occurs because, as the piston travels upward, it closes off the intake ports and begins to draw a vacuum on the crankcase. At this time the crankshaft port, or reed for gassers and most 1/2A glow engines, opens. This vacuum allows air and fuel to be drawn into the crankcase. At the same time, the piston is compressing the charge trapped in the cylinder. As the piston nears the end of the stroke the crank port/reed closes, and combustion happens. Now the engine gets very busy.

Combustion gases force the piston back down on the second stroke. In the crankcase, the fuel/air mix drawn in on the first stroke is now being lightly compressed. When the piston uncovers the exhaust port, the exhaust is vented out the back, readying the engine for the fresh charge. When the piston nears BDC (bottom dead center), it uncovers the transfer ports. The compressed fuel/air mix in the crankcase then shoots through these ports, filling the chamber up and pushing the remaining exhaust out. Momentum carries the crankshaft around, and the piston begins a new cycle, first closing the intake ports and then heading up on compression.

Seems simple enough. But why do we use this method? Why do some rc airplane guys not use them? Well, the answer lies in fuel economy and output. Because a two-stroke fires every time, and because some unburned fuel flies right out the exhaust port before it closes, two strokes are very thirsty little things. They also require a total loss oil system, none of the oil going into the engine stays there very long. As a result they tend to be quite messy as well as quite thirsty. They're also far faar touchier regarding their fuel mixture, as if you run it lean you starve it of oil. Some engines just have no tolerance whatsoever for this and will explode after one lean run. Most sport engines are fairly tolerant but it still won't do them any favors.

What two strokes have going for them is RPM, simplicity, and power. Two strokes, by nature of their simplicity, are born to rev. They absolutely love it. The higher they rev the smoother they run. They also make tons of power. Horsepower is actually a derived unit, a function of torque, RPM and a constant. Two strokes, by nature of revving so damn high, make far more top-end HP than a four stroke would, all other things being equal. Lastly, they tend to be cheaper by a significant factor. A quality two-stroke that will last you for years costs half as much as a quality four stroke of the same power range, and it doesn't take much expertise to make one that runs well.

In our final installment of powertrains, we take a look at 4 cycle rc engines.

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