All internal combustion engines are powered by the explosive burning of the air/fuel mixture inside the engine's piston cylinders. In a four-stroke motor, four separate and essential steps must take place at the correct time and in the correct sequence in every cylinder of the engine. This process is known as the four-stroke process. The name "four-stroke" is derived from the number of piston strokes required to complete the engine's combustion cycle. Each of the diagrams on this page is of a four-stroke engine cylinder.

In a two-stroke engine, the combustion cycle is completed in just two piston strokes, as steps 1 and 4 and steps 2 and 3 below are combined into single steps. In a two-stroke engine, only two piston strokes are required to complete the engine's combustion cycle.

The four steps of the four-stroke combustion cycle include:

1. Admitting the proper air/fuel mixture into the cylinder
2. Compressing or squeezing the air/fuel mixture so it will combust with greater force, delivering more power
3. Igniting and burning the air/fuel mixture
4. Removing the burned gases from the cylinder so that the process can repeat

A "stroke" is the movement of the piston from its highest position in the cylinder (top-dead-center) to its lowest (bottom-dead-center), or from the lowest to the highest position. The four strokes of a four-stroke engine are called intake, compression, power, and exhaust. Each cylinder of the engine completes the combustion cycle in balance with the other cylinders in the engine to create smooth rotation of the crankshaft.

A four-stroke engine uses valves that open and close. By contrast, a two-stroke engine does not have valves. Instead, the air/fuel mixture is drawn into and the exhaust gasses are expelled out of the cylinder via ports that sit opposite each other in the cylinder wall. Both the intake and exhaust ports in a two-stroke engine are open at the same time as the piston moves past them. The intake of the air/fuel mixture and the removal of exhaust gases occurs simultaneously. This results in a significant portion of the air/fuel mixture being expelled directly into the environment unburned as it mixes with the exhaust gases as they are expelled before combustion takes place.

Additionally, in order to provide lubrication to the piston as it moves against the cylinder wall in a two-stroke, oil is added directly to the fuel. A two-stroke's dependence on this oil/gasoline combination, called "fuel mix," is another reason why two-strokes release a high level of emissions. The release of this combination of unburned and partially burned fuel and oil is the cause of the two-stroke's classic blue smoke characteristic.

Four-strokes are inherently much cleaner than two-strokes for three reasons. First, they burn clean gasoline, not gas mixed with oil. Second, the fuel is burned much more completely and therefore with much greater efficiency than in a two-stroke. Lastly, four-stroke motors draw in the air/fuel mixture and expel exhaust gases on separate strokes of the piston, which means that large amounts of unburned fuel are not released from a four-stoke as in a two-stroke (a fact that also contributes substantially to a four-stroke's much greater inherent fuel efficiency).

A four-stroke engine requires much less fuel to achieve an equivalent amount of output. In fact, the four-stroke outboard motors now in use by the Grand Canyon's professional river outfitters burn forty percent less fuel and release ninety percent fewer hydrocarbon emissions into the environment than the two-stroke motors they replaced.

Intake Stroke

The intake stroke is the first of the four strokes in a four-stroke motor's combustion cycle. As the piston moves away from the top of the cylinder, the intake valve opens. The downward movement of the piston creates a vacuum (negative pressure) in the cylinder. The greater pressure outside the cylinder (ambient or atmospheric pressure) pushes the air/fuel mixture into the cylinder. Just as the piston reaches the bottom of the cylinder, the intake valve closes.

Compression Stroke

The compression stroke comes next and begins as the piston starts to move upward in the cylinder. The intake valve closes, trapping the air/fuel mixture in the cylinder. Upward movement of the piston compresses the mixture into a very small area. This compression or squeezing of the mixture is very important for developing maximum power from the engine. The higher the compression, the greater the pressure exerted on the piston when the mixture is burned. Compression also "preheats" the mixture, which helps it burn more efficiently.

Power Stroke

The third stroke in the four-stoke cycle is the power stroke, which begins as the compressed air/fuel mixture is ignited in the combustion chamber. A spark plug, located in the cylinder head, creates an electrical spark that ignites the mixture, causing it to burn explosively. The burning mixture expands very rapidly, creating very high pressure against the top of the piston. This pressure drives the piston downward, which turns the crankshaft, which drives the motor. The up and down motion of the pistons in all four-stroke motors is converted to rotary motion by the crankshaft.

Exhaust Stroke

The final stoke of the four-stroke cycle is the exhaust stroke. As the piston approaches the end of the power stroke, the exhaust valve opens. Pressure in the cylinder causes the exhaust gases to rush past the valve and into the exhaust system. The piston moves upward, pushing most of the remaining exhaust gases from the cylinder. As the piston nears the top of this stroke, the exhaust valve begins to close as the intake valve begins to open.

The exhaust stroke completes the combustion process. The reopening of the intake valve signals the beginning of a new cycle, which occurs in each cylinder in the engine. The combustion cycle is repeated over and over at a very high rate of speed as long as the engine is running.