Page 4 Canandaigua C.H.I.E.F.S. RC Flying Club, Canandaigua, N.Y. return to Hawk's Nest

The Hawks' Nest

February 2004

Vol 2, Issue 2

Engine Basics (con't)

The two stroke model aircraft engine carburetor is designed to mix the fuel with air in the required proportion. The open cylinder at the top of the carburetor is known as the venturi (air inlet). The engine crankshaft is hollowed out internally to form a tube. The mixed fuel and air enters the engine through this tube.

An inlet is machined at the front of the tube. The inlet is designed so that its position lines up with the bottom of the carburetor and within the cylinder so that the inlet is open when the piston is moving up towards the TDC (top dead center) of the cylinder. The inlet is closed off by the piston when the crankshaft has rotated a specified distance (arch in degrees) towards TDC (top dead center).

On one side of the engine cylinder is a cut out slot positioned so that the burnt fuel/air mixture can leave the cylinder when the piston reaches the bottom of its travel. On the other side of the cylinder is a machined transfer port to allow the fuel/air mixture to move from the engine crankcase below the piston, up into the top of the cylinder above the piston.

This transfer is opened by holes drilled through the side or “skirt” of the piston and is closed off when the piston rises to (TDC) top dead center of the cylinder.
Two stroke engine operation creates a partial vacuum in the crankcase as the piston moves upward in the cylinder. A fresh addition of the fuel/air mixture rushes in from the carburetor to fill the void left by the rising piston.

The fuel/air mixture above the piston is compressed due to the upward travel of the piston. The mixture is ignited by the heated glow-plug creating an expanded volume of gases that forces the piston downward.

The fresh fuel/air mixture in the crankcase below the piston is compressed and is forced through the transfer port to the top of the cylinder when the piston reaches (BDC) bottom dead center of the cylinder. This transfer of the fresh fuel/air mixture helps to push any of the remaining ignited fuel gases out the exhaust slot.

All that is needed to keep this cycle going is a supply of fuel and air, along with the right size flywheel connected to the crankshaft, to push the piston upward. The prop acts as the flywheel in a two stroke model engine.


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