Difference between four-stroke and two-stroke engine

The knowledge of the strokes (four-stroke and two-stroke ) of an engine is an essential arsenal for vehicle owners, especially when buying a vehicle. Furthermore, to understand that knowledge, an understanding of the engine cycle is cardinal.An engine to a machine is like a heart to a human being. Its principal purpose is to circulate energy. The engine cycle is the cycle of events that the engine must accomplish in transforming chemical energy into mechanical energy.

Difference between two-stroke and four-stroke[/caption]The cycle is the process of transforming whatever fuel it uses into the power to move the vehicle. For an engine to operate, it repeats the following sequence of events, over and over, again to produce sustained power. Therefore, one complete series of these events in an engine is a cycle.

Intake: this is when a combustible mixture pulls into the cylinder.

Compression:  this occurs when the combustible mixture compresses into a smaller space.

Power: this is when the compressed combustible mixture expands and produces power because it ignites.Exhaust: This is the process of expelling burnt gases from the cylinder. For a better understanding of its mechanism please read our article on a Car exhaust system.

Engines commonly have either a four-stroke cycle or a two-stroke cycle though you will find, now, that most engines operate on the four-stroke cycle.

The Stroke

The stroke of an engine means the completion of the piston movement in the cylinder from the TDC (Top Dead Centre) to BDC (Bottom Dead Centre) of the cylinder.

The stroke is, in simpler terms, the up or down movement of the piston in the cylinder between the top and the bottom positions. Each "stroke" represents the motion of the piston inside the cylinder. One revolution of the crankshaft equals two strokes.

In a four-stroke engine, each cylinder "fires" on every second revolution of the crankshaft.Stroke of the engine means some revolutions of engine crankshaft per cycle of the car engine. Stroke also means the distance traveled by the engine piston.

Types of Power Cycles

We can describe power cycles by the number of strokes necessary to complete a Cycle. Therefore, the most common designs for engines are two-stroke and four-stroke. Other less common designs include five-stroke engines, six-stroke engines, and two and four-stroke engines.

The Two-Stroke Engine

What is a two-stroke engine?A two-stroke engine has a different design from the four-stroke engine. The two-stroke engines complete a power cycle every two strokes.Two-stroke engines cannot power cars or light trucks. They are more commonly used in typically large marine engines, outdoor power tools, like Lawnmowers and chainsaws and, motorcycles, and so on. In two-stroke engines, the two strokes are the upward stroke and the downward stroke.

How does a two-stroke engine work?

Here the cycle is completed by only two strokes the up and down movement of the piston.

  • Upward Stroke

During this stroke, the piston moves upward from the bottom dead center to the top dead center. This movement happens by compressing the charge air petrol mixture in the combustion chamber of the cylinder.Due to the upward movement of the piston, a partial vacuum comes up in the crankcase.

And a new charge is drawn into the crankcase through the uncovered inlet port.The exhaust port and transfer port are covered when the piston is at the top dead center position. The compressed charge is ignited in the combustion chamber by a spark given by the spark plug.

  • Downward Stroke

As soon as the charge ignites, the hot gases compress the piston that moves downward, rotating the crankshaft, thus doing helpful work. During this stroke, the piston covers the inlet port, and the crankcase compresses the new charge.

Further downward movement of the piston uncovers first the exhaust port and then the transfer port, and hence the exhaust starts through the exhaust port. As soon as the transfer port is open, the charge forces its way into the cylinder. This charge strikes the deflector on the piston crown, rises to the top of the cylinder, and pushes out the exhaust gases.

The piston is now at the bottom dead center position. The cylinder is filled with a fresh charge, although it is somewhat with the exhaust gases. The cycle of strokes repeats, with the piston making two strokes for each revolution of the crankshaft.

Four-stroke engine

What is a four-stroke engine?

The four-stroke engine is the most popular and most environmentally friendly type of engine currently being used. During engine operation, pistons in this type go through four events to achieve each power cycle. The definition of an event is an up or down piston motion or stroke.

The Four-stroke engines complete a power cycle every four strokes, which means a power cycle will be complete after every two crankshaft revolutions. Most automotive engines are a four-stroke design.

How does a four-stroke engine work?

The four-stroke engine has a full-combustion cycle that consists of two revolutions of the crankshaft and four strokes:

  • Intake stroke

The intake stroke is always the first of the sequence. In the intake stroke, the piston moves down, creating a vacuum above it. The camshaft opens the intake valve drawing the air from the intake manifold.The intake valve starts opening at the end of the exhaust stroke of the previous cycle. As the piston moves down, the air fills the cylinder. Soon after the piston reaches the bottom position, the intake valve closes. The exhaust valve is closed during the intake stroke.

  • Compression Stroke

This stroke is the second stroke of the sequence. During the compression stroke, the intake and exhaust valves are closed. As the piston moves up, it compresses the air trapped in the cylinder. The direct fuel injector injects gasoline under very high pressure into the cylinder during the compression stroke, when the piston is closer to the top.

Just before the piston reaches the top position, the spark between the spark plug electrodes ignites the air/fuel mixture. The topmost point of the piston is called Top Dead Center or TDC. The combustion happens in the combustion chamber. The combustion chamber is the space between the top of the piston and the cylinder head.

  • Power stroke

This stroke is the third stroke of the sequence. In the power stroke, the pressure of hot gases created during the combustion pushes the piston down with great force.The power stroke provides the energy to turn the wheels in a car. After the power stroke, the crankshaft keeps rotating due to the inertia of the heavy components attached to the crankshaft.

In cars with a manual transmission, it's the flywheel. In vehicles with an automatic transmission, it's the torque converter.During the power stroke, both the intake and exhaust valves are still closed. As the piston approaches the bottom position in the power stroke, the exhaust valve starts opening, allowing hot exhaust gases to rush out. In some literature, the power stroke is the 'expansion stroke' or 'combustion stroke.'

  • Exhaust stroke

This stroke is the fourth stroke of the sequence. During the exhaust stroke, the exhaust valve is open, and the intake valve closes. The piston moves up, pushing the remaining exhaust gases out of the cylinder and into the exhaust manifold.The exhaust stroke is the last stroke of the cycle. As the piston approaches the top position (TDC), the intake valve starts opening for the intake stroke of the next combustion cycle. The exhaust valve closes right after the piston reaches the TDC.

Difference between four-stroke and two-stroke engine

Comparison between two-stroke and four-stroke engine[/caption]Two-stroke engines differ from four strokes in three key areas:

  • They Use Ports, Not Valves

Two strokes flow air, fuel, and exhaust through the engine using ports. The use of machined ports with the engine casing itself allows two strokes to avoid using valves.

  • They Burn Oil In The Combustible Mix Of Air And Fuel

Mixing oil and gas can be either a manual process where the operator physically combines both components in a gas can or an automatic process using oil injection systems. Either way, 2-stroke engines burn oil to provide lubrication to moving parts.

  • They Power Twice as Often

In a two-stroke engine, suction and compression take place at the same time. In a four-stroke engine, all steps happen individually, thereby causing two revolutions of the crankshaft per cycle.Two strokes also take advantage of the airspace below the piston. Each piston stroke is pressurized and acts upon two chambers simultaneously.

There is also no camshaft required to open or close the valve, and fewer parts mean that two-strokes are lighter and more compact than four-stroke engines.

Common questions about four-stroke vs. two-stroke

You might be asking, if two strokes are lighter, smaller, and more powerful than four strokes, why are they not more common? The two-strokes have a few distinct downsides that include it being:

  • Less fuel-efficient
  • Noisier and more prone to vibration
  • A considerably higher emitter of pollution

The two-stroke oil is also expensive to purchase, and mixing to the correct ratio can be challenging.

Conclusion and final words from the expert

To better understand the process of an engine cycle and how the two-stroke engine and the four-stroke engine works, and advice to you would be to find the animated representations (GIF pictures) of those two power cycles.It would be hard not to find one when you search for the keywords.