How 2 stroke and 4 stroke Outboards Work.
Learn about how outboards generate power, and why different engines use different systems, and what it all means.
Have you ever wondered how an outboard engine really works?
Modern outboards, similar to engines found in other products such as cars or motorcycles, use internal fuel combustion to move pistons, which in turn rotate a drive shaft. All engines of this type require three elements to work together for combustion and propulsion—
The engine has systems in place to determine the amount of each and when they need to be applied. In the case of an outboard, the combustion creates rotational force on the crank shaft that in turn is used to spin a propeller.
4-stroke engines need over 100 more moving parts than a 2-stroke engine.
2 Different Technologies
There are two primary technologies used in outboard engines to create power using combustion. Each has similarities and differences to achieve the same goal – turning the propeller to create propulsion.
One type of outboard is called a 4-stroke and the other is called 2-stroke. The reason they are named this way comes from the way the engine is set up to time the necessary functions to effect combustion.
A “stroke” is when a single piston moves from one end of the cylinder to the other. One type of engine used in outboards requires four strokes for every combustion, so it is called a 4-stroke engine. Another requires just two strokes for each combustion, so it is called a 2-stroke engine.
A 4-stroke engine needs one stroke to complete each of the engine’s basic necessities.
How a 4-Stroke Engine Works
A 4-stroke engine goes through four steps to effect combustion, and each involves the piston moving the length of the cylinder, or making a “stroke.”
1. First the piston moves down in the cylinder, creating a vacuum. As this happens, a valve, located at the top of the cylinder opens, letting in a mixture of air and fuel. This is called the intake stroke. The valve is held shut by a spring mechanism and is opened by a cam (a raised bump on a camshaft) that pushes the valve and compresses the spring. Once the cam passes the valve, the spring closes the valve again.
As the piston moves down (indicated by the pink arrow), the intake valve opens (shown by yellow arrow), admitting fuel-air mixture to the cylinder.
2. The piston then moves back up to compress the mixture of air and fuel in the combustion chamber. This is called the compression stroke. When the piston reaches the top of the cylinder, the air-fuel mixture is compressed.
As the piston moves up, it compresses the fuel-air mixture in the combustion chamber at the top of the cylinder.
3. With the piston at the top of the cylinder, a spark plug ignites the mixture, creating an explosion that drives the piston down. This is when the piston makes its third transit of the cylinder. This is the combustion stroke, or “power” stroke.
As the spark plug ignites the fuel-air mixture, it forces the piston down in the cylinder.
4. The fourth stroke is when the piston comes up again, the exhaust valve opens, and the spent gas is pushed out into the exhaust manifold. It is called the exhaust stroke.
As the piston moves back up the cylinder (shown with pink arrow), it pushes the exhaust gases out of the now-open exhaust valve (indicated by yellow arrow).
How a Conventional Carbureted 2-Stroke Engine Works
A 2-stroke engine goes through two steps to effect combustion, and each involves the piston moving the length of the cylinder, or making a “stroke.”
1. As the piston begins to move up, it compresses the air/fuel mixture in the cylinder and closes off both the intake and exhaust valves. On a 2-stroke engine, the valves are holes in the cylinder wall, rather than near the top of the cylinder in the combustion chamber, as they are on a 4-stroke. So, the first stroke in a conventional carbureted 2-stroke engine completes both the intake and the compression function.
This is a drawing of a conventional carbureted 2-stroke engine. As the piston moves up, the air/fuel mix in the crankcase is forced into the cylinder through the intake valve (shown by the yellow arrow). The yellow arrow shows the exhaust port where exhaust gases recently exited the chamber. Both of these valves will soon be blocked by the piston moving up in the cylinder.
When the spark ignites the fuel-air mixture, the piston moves down, compressing the air in the crankcase.
2. When the piston is at the top of the cylinder, the spark plug ignites the fuel-air mixture, and an explosion occurs, pushing the piston down at the beginning of its second stroke. As it passes down the cylinder, the piston uncovers the exhaust valve and the spent gasses exit the chamber. In this way, this engine accomplishes both the power stroke and the exhaust function in one stroke.
At the same time, the bottom of the piston compresses the air in the crankcase, pushing the air/fuel mix -- through the recently opened intake valve -- into the cylinder. And, the process is repeated.
After combustion, the piston moves down (shown by pink arrow), and pressurizes the crankcase. This forces the crankcase air into the cylinder through the intake valve (indicated by the yellow arrows) and also opens the exhaust valve so the gases can escape (shown by orange arrow) into the exhaust manifold.
Do the Math
All of this means that the natural mechanics of a 2-stroke combustion engine generates twice as many power strokes for every revolution of the crankshaft. As we’ve seen, a 4-stroke outboard has its piston make two additional transits of the cylinder, one to push exhaust gases out, and another to draw in the air/fuel mixture.
On a 2-stroke, the intake and exhaust stages are managed by the piston’s downward pressure on the crankcase air, which pushes into the piston as the cylinder-sidewall valve opens.
2000 Power Strokes vs. 1000
To look at it another way: At 2000 rpm, each cylinder in a 2-stroke engine is firing – and creating energy – 2,000 times. It doesn’t matter how many cylinders there are. Whether there’s one piston turning the crankshaft on a 2-stroke outboard, or four, or six, or eight, they all do the complete up-down cycle each time the crankshaft turns.
In a 4-stroke engine operating at the same 2000 rpm, each cylinder is firing – and creating energy – 1,000 times, exactly half as often. The piston of the 4-stroke must make two more transits of the cylinder with no appreciable addition of power.
There are many other differences between 2-stroke and 4- stroke engines, but a core difference is the number of strokes necessary for combustion.