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.

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.

In reality, even the actual four-stroke cycle is more complex than I have described. The operations described do not occur in the simple order pictured. Many of the operations overlap; the intake valve begins to open before the piston first reaches TDC. Why?

Since the piston slows its normally rapid motion as it nears the top of each stroke, it creates a slight area of negative pressure just above itself. This happens because the gases being pushed by the piston are moving at the piston's rapid speed, and their inertia carries them away from the piston, and through the exhaust valve, as the piston suddenly slows.

The advanced intake-valve opening uses this sudden negative pressure to begin accelerating the fresh intake gases into the chamber even before the piston begins traveling on its downward, intake stroke.

This "advance timing" also allows the intake valve enough time to open and the fuel/air mix in the carburetor more time to begin to move, or accelerate, through the intake manifold and the open intake valve. The intake gases have inertia and cannot instantly move at top speed. At this point, the exhaust gases from the previous cycle are still quickly exiting the chamber. The extra low pressure their exit creates also helps overcome the intake gases' inertia.

The intake valve remains open even after the piston reaches BDC and starts upward again, to allow the quickly moving intake gases more time to "pack" as much gas into the chamber as possible. Again, this extra movement is caused by the gases' inertia—this time, fast-moving inertia. The intake valve only begins to close after the piston has completed roughly 25% of its upward travel and is fully closed before the piston reaches the 50% point.

The exhaust valve actually opens before the piston reaches BDC after the power stroke. The burning gases still have extra pressure at this point, which helps accelerate the exhaust gases through the opening, but not yet fully open, exhaust valve.

Once the piston starts up on its "exhaust stroke," the spent gases are already on their way out of the chamber and the exhaust valve is fully opened. The exhaust valve only begins to close after TDC to allow extra time for the exhaust gases to escape. As the exhaust gases escape the chamber, they help create the initial low-pressure area that begins to move the fresh intake fuel/air mix.

As I mentioned, the intake valve also starts to open as the piston nears the top of the exhaust stroke. This means that for a brief moment both valves are open at the same time. This is called "valve overlap" and is important for producing maximum power. The amount of overlap and its relationship to the actual combustion event is called the engine's "timing."

Sport engines designed for good power and good fuel economy usually have "mild timing and overlap, meaning that although there is some overlap, it is not excessive and will not waste fuel out of open exhaust ports. High-performance engines use more overlap to produce extra power, but they lose fuel economy as some unburnt fuel escapes through the exhaust port or some exhaust gases may actually enter the intake area. 

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