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Comparing and Contrasting Four-Stroke and Two-Stroke Engines

Posted by Scott Goldfarb on

Both four-stroke engines and two-stroke engines utilize the combustion of fuel to harness the energy necessary to run their respective engines. If they are accomplishing the same goal through the same process, how different can they be? Let's take a deeper look into how four-stroke and two-stroke engines function.

Four-Stroke Engines

Inside a four-stroke engine, there is a crankshaft. Piston rods connect the pistons to the crankshaft, causing the pistons to move up and down with each turn of the crankshaft.

Above the crankshaft and pistons, there is a diesel camshaft. The camshaft and the crankshaft are connected by a timing belt. While the crankshaft turns and moves the pistons up and down, the camshaft rotates and allows the inlet and exhaust valves to open and close.

In a four-stroke engine, the piston completes four separate strokes during two separate revolutions of the crankshaft, which makes one single thermodynamic cycle. Here are the four steps or strokes that the engine performs.

  1. Intake. The inlet valve opens (while the exhaust valve remains closed) and allows the fuel and air mixture to be drawn in from the injection pump delivery valve above the piston as the piston is pulled down.
  2. Compression. The inlet valve closes. Now with both valves closed, the piston travels upward, compressing the fuel and air mixture. Just before the piston reaches the top of the compression stroke, the spark plug emits a spark.
  3. Combustion. The spark from the spark plug ignites the compressed fuel and air mixture. This combusts the mixture and sends the piston shooting back down. The power from this combustion is what powers the engine. During this cycle, both the intake and the exhaust valves remain closed.
  4. Exhaust. The exhaust valve opens, and the intake valve remains closed. The piston travels upward and pushes the exhaust out the exhaust valve. At the top of this stroke, the exhaust valve closes and the inlet valve opens, beginning the cycle again.

This entire process is completed in four piston strokes or two complete crankshaft revolutions.

Four-stroke engines are used in many different applications, from a lawnmower to a vehicle to a ship.

Two-Stroke Engines

Instead of taking four steps to complete a combustion cycle like in the four-stroke engine, the entire combustion cycle of a two-stroke engine is completed in just one piston stroke. The piston in a two-stroke engine has many jobs. It acts as a piston, compressor, intake valve, and exhaust valve.

When the piston is at the top of its rotation, it compresses fuel and air. A spark plug emits a spark that ignites and combusts the mixture, sending the piston downward with a lot of force. As the piston is lowered, it uncovers a port that allows the exhaust to escape to the exhaust pipe.

The piston continues to be pushed down, and a fraction of a second later, it uncovers a second port. This is the intake port. It sends in a mixture of fuel and air into the cylinder. Once the cylinder is filled with the fuel and air mixture, the piston moves upward again, compressing the air and fuel. Once compressed, it is ignited with a spark from the spark plug, and the process repeats.

This whole process is done in two piston strokes or just one complete crankshaft revolution.

Two-stroke engines are common choices for model airplanes, weed whackers, chainsaws, hedge trimmers, and leaf blowers.

The Differences Between Two- and Four-Stroke Engines

Because they are simpler machines than the four-stroke engine, two-stroke engines are easier to maintain. Two-stroke engines also take up less space. Since the piston in a two-stroke engine does three jobs instead of just one, two-stroke engines are a lot lighter than four-stroke engines. A two-stroke engine can also operate in any orientation; it can be upside down or right side up and still function correctly.

The four-stroke engine is a more complex machine, but it also has a longer lifespan than a two-stroke engine because the wear and tear on the parts is more evenly distributed. A four-stroke engine can also deliver higher levels of torque at a lower RPM than a two-stroke engine.

Of the two options, four-stroke engines are more fuel-efficient and create less exhaust than two-stroke engines.

Both engines are the right choice for different applications. Neither is better than the other; they are just used for different purposes.

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