IT LOOKS DIFFERENT from the .40 cu. in. engine that came with your RTF trainer. There is a "bump" on top, and the glow plug points from the head at an angle. The carburetor appears to be upside down, and the throttle arm is on the wrong side. The sound is also different from your engine; it's lower in pitch with a "crack" to it. The owner calls it a "four-stroke" and says he wouldn't fly with anything else.

Despite its different appearance and sound, the model four-stroke engine is identical to its two-stroke cousin except for the manner in which the fuel/air mixture enters the combustion chamber and the way in which the burnt gases escape the chamber after combustion.

The four-stroke is fuel and air cooled, is fuel lubricated, runs on alcohol-based fuel, uses glow catalytic ignition, usually has carburetor induction, and relies on fixed, mechanical timing for operation—just like a two-stroke engine.

O.S. 120 valve and rocker arm assembly is visible with valve cover removed. Thin tubes in front house pushrods that operate valves.

Operationally, there is no difference in user technique or equipment between a two- and four-stroke, with the possible exception of fuel and glow plugs. This commonality makes it easy for the newer model pilot to enjoy both types of power plants without learning new techniques or buying additional field equipment.

So then, why the different name and appearance?

The induction/exhaust characteristics that differentiate a four-stroke from a two-stroke do have some effect after all. Although they do not change the way the engine is used, they do change almost everything else. The label "four-stroke" is derived from these differences.

Unlike an engine that produces power on every up and every down piston stroke—two strokes—the manner in which the gases enter and leave the combustion chamber in a four-stroke requires that it produce power only on every other up and down piston stroke, which is four strokes.

How Those Parts Work Together: To understand why this happens, let's look closer at four-stroke operation. As we do, keep in mind that the engines being discussed are normally aspirated sport engines intended for sport, high-drag models.

Camshaft determining engine's timing is located in round housing just under pushrods. "Upside-down" carburetor is connected to intake manifold that leads to intake valve.

Similar to the engine in your automobile, except for rotary-powered cars, the model four-stroke uses intake and exhaust valves driven by a camshaft. Most four-strokes also use pushrods from the camshaft to move the valves, but a few use belt-driven overhead camshafts.

The induction/exhaust cycle is similar to that in your automobile's engine. In theory, the cycle begins with the piston at the top of its stroke, called Top Dead Center (TDC). The intake valve opens as the piston begins its first downward stroke (stroke 1). This creates a low-pressure area in the combustion chamber above the piston.

A fuel/air mixture from the carburetor is pushed into the intake manifold through the open intake valve and into the combustion chamber by the greater atmospheric pressure trying to fill the internal low-pressure area. After the fuel/air mix is in place, the intake valve closes and the piston starts its upward stroke (stroke 2).

Again, in theory, the piston compresses the fuel/air mix until it reaches TDC. The intense pressure, plus the catalytic effect from the hot glow-plug element, ignites the mixture. This controlled burning, called combustion, forces the piston onto a downward stroke (stroke 3), producing power and turning the propeller that is connected to the rotating crankshaft.

Once the piston reaches Bottom Dead Center (BDC) again, the exhaust valve opens and rotational momentum of all the moving parts causes the piston onto another upward stroke (stroke 4). As it moves upward, the piston pushes the burned gases out the exhaust port. The exhaust valve closes and the cycle repeats.

Four piston strokes produce one power stroke. The three other piston strokes are required to get the cycle to repeat. As I have discussed in previous articles in this series, a model two-stroke produces one power stroke with just one additional stroke required for operation. In theory, the two-stroke should produce twice the power of an equivalent-size four-stroke. In practice, it is not that simple.

Two-stroke engines have their own inherent inefficiencies that rob power. In addition, what extra power two-strokes have is often unusable by the modeler because it occurs at high engine speeds (rpm) that are difficult to reach in sport models running on sport fuels.