DIY Network

Gas-Powered Model-Airplane Engines -- Engine Basics From "Radio Control Hobbies" episode RCH-203

Model-engine expert Randy Lisalato of Mecoa provides technical information and sound advice about model-airplane engines.

In this segment, model-airplane expert Randy Lisalato of Mecoa Engines explains some of the principles involved in the operation of gas-powered model-engines. Randy's company manufactures all types of gas-powered engines -- 4-stroke, 2-stroke, racing engines, sport engines, helicopter engines and more.

Important: This demonstration shows provides some of the basic information regarding these particular model engines. There may be variations in operation for each particular model. Read and carefully follow the manufacturer's instructions that come with yours, whether it is this particular model or another. Additional information about assembly as well as running, troubleshooting and fine-tuning the model can be found in the manufacturer's instructions.

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	Figure A
	Figure B
	Figure C
	Figure D
	Figure E
	Figure F
	Figure G
	Figure H

Engine Basics

• Since most amateur RC pilots start with 2-stroke engines, Randy explains some basics about miniature-sized internal-combustion using a cut-away of a 2-stroke engine (figure A). On a 2-stroke engine, the carburetor -- which mixes the fuel with appropriate amounts of air -- is located just above the crankshaft. On the crankshaft, there is a valve that opens and closes. As the piston travels upward in the sleeve, it creates a vacuum in the crankcase to bring in the fuel/air mixture through the crankshaft. As the piston moves back down, the valve closes, and the fuel and air is compressed in the crankcase. As the piston comes down further, it opens up transfer ports in the sleeve so that the fuel/air charge is transferred into the combustion chamber. As the piston comes back up, it brings in another charge of fuel and air while compressing the charge that's in the combustion chamber. When the piston reaches the top of its stroke, the fuel is ignited and the piston is driven back down as the exhaust port opens to expel exhaust gasses, and the series starts over again. In summary, there are always two processes occurring simultaneously in a 2-stroke engine: at every stroke of the piston, combustion is occurring at the top while induction is occurring at the bottom.

• The OS-300 (figure B) is a twin 4-stroke engine. On a 4-stroke engine, the induction of fuel and air occurs at the carburetor, and the mix travels down through an intake manifold directly into the combustion chamber (not through the crankcase as on the 2-stroke engine). Rocker arms located on the side of the engine (figure C) open and close a poppit valve -- just as they do on an automobile engine.

• One important distinction among model airplane engines is ABC engines vs. ringed engines. A ringed engine uses a cylinder piston equipped with a ring (figure D) that occupies the clearance between the piston and cylinder -- just like the piston in an automobile engine.

• An ABC engine doesn't have a ring. Instead it has an aluminum piston and brass liner with a chrome wear-surface inside (figure E) -- hence the "A-B-C" moniker signifying aluminum, brass and chrome.

• Since ABC engines don't have a ring, they are characterized by a very tight fit between piston and cylinder as seen in the diagram (figure F). The sleeve is tapered, and the parts are fitted for when they reach running temperature. While engine is running, the top of the cylinder is hotter than the bottom of the cylinder, and the expansion rates of brass and aluminum differ, so the fit of the elements must be tight at the top. This is why the prop on an ABC engine is relatively hard to turn and feels like it has a "tight spot" at a certain position of the rotation.

• The break-in procedure for ABC engines versus ringed engines is somewhat different because of this basic structural difference. ABC engines should always be run at operating temperature. Running one at a cold temperature can wear out the engine prematurely.

• To successfully tune a model engine, you need to be able to properly adjust the carburetor. There are two needle-valve adjustments that regulate the fuel-mixture intake: a high-speed adjustment and a low-speed adjustment. The two adjustments are located opposite one another. The low-speed valve is adjusted with a screwdriver (figure G).

• As seen in the diagram (figure H), the tapered tip of the needle valves incrementally opens and closes the openings of a spray bar to regulate the flow of fuel in proportion to the air being drawn into the carburetor. Adding more fuel in proportion to a fixed amount of air is known as richening the mix. The low-speed valve is used to specifically meter the fuel mix for when the engine is at idle -- when it needs less fuel than it does at full-throttle.
  

Understanding these basics of engine operation will enhance your ability to tune your model engines. In the segment that follows, Chris and Randy demonstrate the procedures in bench-tuning a model engine.

Mecoa Engines

Crankshaft......hardened steel
Cylinder ...... hard chrome ABC
Carburetor...... all metal
Manufactured ...... USA
Street Price ... all ranges.