2 Stroke Vs 4 Stroke: What Are the Key Differences?

When we rev our motorcycles and go on a scenic drive, combustion of air-fuel mixture repeatedly happens below us to power the vehicle. This process is so well-synchronized that we don’t even understand multiple steps are taking place inside the engine.

Today, we will uncover the engine head of modern single cylinder motorcycles and take a closer look at the working principle of two types of internal combustion engines. Whether you have a motorcycle or plan to buy one, let’s see the key differences between 4 stroke vs 2 stroke engine.

What Is a 2 Stroke Engine? How It Works?

We will only be talking about internal combustion engines in this discussion to keep things simple. Before entering the engine, you must be familiar with a few basic terms.

The first thing you should know is the Stroke. What does it mean? In simple words, stroke means the traveling of the piston inside the cylinder. It can be more precisely defined by the movement of the piston from the Top Dead Center to the Bottom Dead Center.

What Is a 2 Stroke Engine

Top Dead Center means the topmost position of the piston when it is closest to the valves. And Bottom Dead Center is the bottom-most position of the piston when it is furthest from the valves.

When the piston travels from the top dead center to the bottom dead center, it is considered to complete a stroke. Inside the engine, the combustion of the air-fuel mixture happens in four steps. These are:

  • Intake
  • Compression
  • Combustion
  • Exhaust

In the first step, the piston moves to the bottom dead center so that the air-fuel mixture can enter the cylinder through the intake valves. The piston then moves to the top dead center compressing the mixture.

This mixture is then ignited by the spark plug to create a combustion. The power produced in this step pushes the piston downward. In the final step, the piston goes up again and pushes the burnt mixture out through the exhaust valve.

In 2 stroke engines, the first two strokes are combined in just one stroke. The next two strokes are also integrated into one. In such an engine, the air-fuel mixture enters the cylinder while the piston is traveling up, compressing the mixture at the same time.

After the compression, the fuel mixture is ignited, and the piston is pushed downward. Power is generated in this stroke while the exhaust gas also leaves the cylinder in the same stroke.

What Is a 4 Stroke Engine? How It Works?

In 4 stroke engines, all four events happen during a cycle of four strokes. The piston moves back and forth four times in one revolution of the engine. Power is generated in the combustion stroke only. Let’s get into more detail on how a four stroke engine works.

What Is a 4 Stroke Engine

Intake Stroke

Intake valves are opened at the beginning of this stroke, but exhaust valves remain closed. The piston starts moving downward to make room for the air-fuel mixture to get into the cylinder.

Compression Stroke

After reaching the bottom dead center, the piston starts moving upward and compresses the air-fuel mixture in its way. Both intake and exhaust valves remain closed in this stroke.

Once the fuel mixture is fully compressed and the piston is in its topmost position, the spark plug creates a spark igniting the fuel mixture.

Combustion Stroke

This stage is also called the power stroke because the engine produces power only in this stroke. When the fuel mixture is ignited, it bangs the piston downward. The piston moves back and forth with this power for three other cycles.

This power is delivered to the transmission system via the crank shaft which sits at the top of the cylinder head. Intake and exhaust valves remain close during this stroke too.

Exhaust Stroke

This is the final stroke when the piston starts moving upward, and the exhaust valves open up to release the burnt gases. Once all the burnt gases are released, onc cycle of the engine is completed, and the cylinder is ready to intake another load of air-fuel mixture for the next cycle.

Differences Between 2 Stroke and 4 Stroke Motorcycles

Motorcycles with two stroke engines and four stroke engines are completely different from each other. We will evaluate their differences in terms of multiple factors.

Engine Size & Weight

Two stroke engines are smaller and lighter than four stroke engines. 4 stroke engines require a lot of space over the cylinder heads. This is why 4 stroke engines are much larger.

2 Stroke Engine

Power Stroke

In 2 stroke engines, every revolution of the crankshaft includes a power stroke as one of the two total strokes is a power stroke. But in 4 stroke engines, power is only generated in one of four total strokes.

The crankshaft makes two revolutions within this time. To be simple, the crankshaft makes one revolution per one power stroke. On the other hand, the crankshaft in four stroke engines makes two revolutions per one power stroke.

Flywheel

Flywheels in two stroke engines are lighter. As the engine produces power in every revolution, the flywheel can rotate pretty easily. This is why two stroke engines have more lightweight flywheels.

But in four stroke engines, the power is generated in only one stroke of the four total strokes. The crankshaft needs to make revolutions until the piston goes through the combustion stroke again.

This requires a bit of time and the crankshaft still needs to make revolutions. Heavier flywheels gain greater kinetic energy to keep the crankshaft moving until the next blow of power comes. For this reason, four stroke engines have heavier flywheels.

Torque

The torque generated in two stroke engines are more uniform than four stroke engines. As the power is smoothly delivered in each returning strokes, the torque remains uniform all the way up to a higher RPM range.

4 Stroke Engine

But four stroke engines don’t deliver uniform torque as the power delivery isn’t smooth in these engines. Though they have more torque at the lower RPM range, the torque tends to be uneven and more inadequate at the higher RPM range.

Power Delivery

As you might already assume, two stroke engines deliver more power than four stroke engines, 1.5 times more, to be specific. This is because two stroke engines have a power stroke for every two strokes or one revolution of the crankshaft.

On the other hand, four stroke engines have a power stroke for every four strokes of the piston or two revolutions of the crankshaft. This latency causes four stroke engines to generate less power than their two stroke counterparts.

Valve Mechanism

You will see intake and exhaust valves in four stroke engines that aren’t present in two stroke engines. Intake and exhaust valves control the flow of air-fuel mixture and exhaust gases.

Camshafts operate these valves. But there are no such valves in two stroke engines. Instead, they have intake and exhaust ports controlled by the piston’s movement inside the cylinder.

Combustion of Fuel

As respective valves remain fully closed during the combustion inside a four stroke engine, the air-fuel mixture is burnt completely. The intake valve doesn’t open for the next cycle until all the burnt gases are pushed out through the exhaust valve.

But as there is no valve in two stroke engines, the fuel mixture doesn’t get burnt completely. While going out, these half-burnt gases are mixed with fresh fuel, which decreases the quality of the combustion.

Thermal Efficiency

This factor is correlated with the previous factor. Due to the incomplete combustion of the fuel mixture, the thermal efficiency of two stroke engines is much lower than four stroke engines.

As the fuel mixture burns completely in four stroke engines, they generate power at high efficiency. The fuel cost is decreased in four stroke engines due to higher efficiency.

Durability

Two stroke engines have fewer moving parts. As a result, the components of these engines wear a lot less than that of four stroke engines. But four stroke engines are made of many different moving parts that catch wear and tear during the operation.

4 Stroke

Cost

As there are fewer parts in two stroke engines and they are easy to build, they cost a lot less than four stroke engines. Those engines are very difficult to manufacture as they require bigger and heavier parts.

Systems like valve mechanisms also add to the cost of such engines.

Environment-friendly

We have already mentioned that the fuel mixture doesn’t get burnt completely in two stroke engines. As a result, these engines produce a lot of smoke while running. They also generate a louder noise than four stroke engines.

But as the fuel mixture gets completely burnt in four stroke engines, they produce much cleaner power than two stroke engines. So, four stroke engines are more environmentally friendly than two stroke engines.

This is the main reason two stroke engines aren’t that common nowadays. Certain emission norms have been created to eliminate two stroke engines and replace them with four stroke engines.

Pros and Cons of a 2 Stroke Engine

Both the engines have their advantages and disadvantages. Let’s have a quick overview of the pros and cons of a two stroke engine.

2 Stroke Engine

Pros

  • As two stroke engines are lightweight and easy to build, they cost a lot less.
  • Two stroke engines deliver more power and torque through a wide RPM range.
  • There are fewer moving parts in two stroke engines, so they wear less and are very durable.

Cons

  • Though you have more power in two stroke engines, they aren’t very clean. Exhaust gases of two stroke engines contain harmful compounds to the environment.
  • As power is generated every two strokes, these engines consume more fuel.

Pros and Cons of a 4 Stroke Engine

Four stroke engines are a more modern and economical solution, yet they have some disadvantages. Check them here.

4 Stroke Engine

Pros

  • Four stroke engines produce more power and torque in the low RPM range.
  • The power produced in four stroke engines is very clean, and harmful components in the exhaust gas are very low.
  • Four stroke engines consume less fuel as they generate power every four strokes.

Cons

  • Four stroke engines are usually more complex and heavier. So, they cost more than two stroke engines.
  • The power delivery isn’t uniform through the higher RPM range.

Is a 4 Stroke Better Than 2?

These two engines perform differently. They are used in different scenarios, so it is difficult to pick one over another. But in terms of performance, fuel economy, and environmental impacts, four stroke engines are way better than two stroke engines.

Two stroke engines have a greater power to weight ratio, so they are commonly used in dirt bikes or racing bikes. But four stroke engines are used in road bikes more often due to their smooth power delivery and low noise.

Is a 2 Stroke Engine Bad for the Environment?

Yes, two stroke engines are really bad for the environment. They are considered environmental hazards. The main reason behind this is the incomplete combustion in two stroke engines.

This incomplete combustion of the engine releases toxic gases into the environment. The most common pollutants from a two stroke engine include Carbon Monoxide, Nitrous Oxide, etc. 

According to a joint study of U.S. EPA and World Bank, two stroke engines are a major contributor of Particulate Matter (PM), Volatile Organic Compounds (VOC), and Carbon Monoxide (CO) in the Asia region.

These compounds are highly toxic and terribly pollute the air. Two stroke engines also create a lot of noise that increases noise pollution to a great extent.

Why Is a 2 Stroke More Powerful?

In simple words, two stroke engines deliver power twice as fast as four stroke engines. The combustion cycle inside two stroke engines are performed in two quick strokes. This ensures a more uniform power flow from the engine.

There are no intake or exhaust valves in two stroke engines, so they waste no time in opening and closing the valves. The power stroke happens in each revolution of the crank shaft rather than that of every two revolutions in four stroke engines.

How Long Does a 2 Stroke or 4 Stroke Engine Last?

Though two stroke engines seem to be more durable, they actually last less than four stroke engines. Drawing a specific conclusion is difficult because engines are made of different qualities.

Usually, two stroke engines need to be replaced every 8000-10000 miles. Four stroke engines run for much longer. Some engines run even more than 50000 miles without any issues.

Considering the performance of the engines, four stroke engine will be a winner in this section. But with proper maintenance and care, two stroke engines can also last for a long time.

Final Verdict

Though both two stroke and four stroke engines work in the same principle, the main difference is in their power cycle. Other major differences include power delivery, fuel consumption, cleanliness, and efficiency.

Now that you know 2 stroke vs 4 stroke: what are the key differences, you can better understand which engine will be more suitable for your motorcycle. Understanding the impact of these engines on the environment is also crucial.

To conclude, two stroke engines are suitable when you need more consistent power delivery. But four stroke engines will produce smoother and cleaner power while giving a better efficiency. Choose the right engine depending on your purposes.

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