By properly adjusting these fuel-metering devices, the pilot is responsible for the engine’s operating temperature and therefore its reliability and durability. This is true no matter what type of engine is used--a two-stroke or a four-stroke. There are several other types of model engines, such as gas ignition or true diesel, but the two- and four-stroke alcohol-fueled types comprise the majority of the engines that new pilots use.

A quick look at the exploded engine views and photos reveals the differences between the two major engine types. The first (Diagrams 1 and 2), and simplest, is called a “two-stroke” engine. These are almost as operationally simple as engines get. Their design may be complicated, but provide them with the proper fuel/air mixture and a “lit” glow plug (the catalytic enhancer), and they will start and run every time. But how do they work?

To best illustrate how it runs, we start with the engine not running and totally without fuel. The engine’s piston is at Bottom Dead Center (BDC). This means that it is as far down in its movement (called a stroke) as it can get. The engine has not started and there is no fuel anywhere inside it. In fact, there is no fuel anywhere except in the fuel tank. (It is important to have fuel in the tank before trying to run the engine.)

Starting an engine from this position is difficult until fuel flows from the tank, through the fuel lines, and into the carburetor. Therefore, we need to draw the fuel from the tank and into the carburetor. We will use the “suction” effect that permits the engine to run in performing this task.

Where does the suction come from? While at BDC, the Rotary Disk Induction valve--the slot cut into the hollow crankshaft just under the carburetor--is fully closed. The induction valve (for short) connects the carburetor to the engine’s lower crankcase when it is open, allowing fuel and air to flow into this lower area.

As the engine is hand-rotated counterclockwise, the piston begins to move upward and closes all the transfer (intake) ports. These ports are cut into the cylinder wall opposite the exhaust port and connect the lower crankcase section to the sections above the piston and transfer fresh fuel and air into these upper sections.

At this point, the induction valve begins to open. As the piston continues to move upward, the lower crankcase volume begins to increase. As this volume increases with continued upward movement of the piston, a low-pressure area is created in the crankcase. This happens because the now-sealed crankcase volume is bigger than it was, but it still contains only the original amount of air. The air expands to fill the increased volume and therefore has a lower pressure.

But remember the induction valve that was just opening as the transfer ports were closing? It opens more as the piston travels upward. The valve is fully open at this point, and that means that the crankcase section is no longer sealed. If the carburetor throttle barrel is open, air rushes through the carburetor, through the rotary valve (crankshaft), and into the crankcase. Remember this process; it will be repeated shortly once fuel is added to the mix.

Now there is lots of air rushing into and through the engine as we mechanically hand-rotate the propeller. What happens if we put an obstruction such as a thumb over the carburetor’s air inlet?

Click on photo to view large image with caption

Low pressure returns to the lower crankcase since it is again sealed, even when the rotary valve is open. But the piston is still moving and re-creating the low-pressure condition with each revolution. You can actually feel the suction with your thumb. This suction effect draws fuel and air into the carburetor. It is the engine’s only fuel-draw mechanism except for gravity. Since this suction is never as strong as a fuel pump, fuel-tank placement is critical.

The low-pressure condition seeks relief from wherever it can, and since the only possible pressure relief is the small brass fuel inlet inside the carburetor itself—the fuel jet—fuel is drawn from the fuel tank and into the fuel jet.

Remove the obstruction. As the induction valve opens, the crankcase’s lower pressure draws fuel through the fuel jet and air from the atmosphere into the induction valve. As the air is pulled through the carburetor, it speeds up to go through the narrow intake passage. The added velocity means that the intake air gains kinetic energy, and to maintain balance, the potential energy (temperature and pressure) drops.