Modern engines have undergone many developments. Still, they operate either as 2-stroke or 4-stroke engines. 2-stroke vs 4-stroke – what is the difference between them?
The primary difference between a 2-stroke and a 4-stroke engine is that to complete one power stroke, the former must go through two stages, completing one revolution. Meanwhile, to complete one power stroke, a 4-stroke engine needs to go through 4 stages which complete two revolutions.
In a 2-stroke engine, there is combustion in every crankshaft revolution. This system can generate more power compared to a 4-stroke engine. The delivery of this power is also more instantaneous.
This is one of the reasons why the preferred engine for motorcycles is a 2-stroke engine. But there are also certain advantages of 4-stroke engines, making them the preferred engines for cars and trucks.
Read on to learn more about the differences between a 2-stroke and a 4-stroke engine and other relevant details about them that you should know.
2-Stroke Vs 4-Stroke
Manner of Delivering Power Strokes
The primary difference between the 2-stroke and the 4-stroke engine is how they deliver their power strokes. A 2-stroke engine has to go through two stages to complete one power stroke, enabling it to make one revolution.
A 4-stroke Engine Have to Go Through Two Stages to Deliver One Power Stroke
A 4-stroke engine needs to go through two stages to deliver one power stroke, enabling it to make two revolutions. Combustion occurs whenever the crankshaft completes one revolution in an internal combustion engine. Another way of saying this is that they differ in how their crankshaft and piston move.
2-stroke Engines Generate More Power
So, 2-stroke engines generate more power than 4-stroke engines. The power generated in a 2-stroke engine is also more instantaneous. That is why many people prefer 2-stroke engines as the motive power of motorcycles, up to this day.
Lighter and Noisier 2-stroke Engines
In 4-stroke vs 2-stroke, 2-stroke engines are essentially lighter and noisier than 4-stroke engines. 4-stroke engines are also nearly heavier because of their bigger and heavier flywheels.
Regarding fuel efficiency, 4-stroke engines are more fuel-efficient since they only consume fuel at the fourth stroke, while 2-stroke engines consume fuel every second stroke.
2-stroke vs 4-stroke Engines Application
2-stroke engines are the ones that are typically used in smaller applications such as the following:
- Brush cutters,
- Dirt bikes,
- Motors for boats,
- Power lawn tools, and so forth.
Meanwhile, 4-stroke engines are usually installed in the following:
- Dirt bikes,
- Lawnmowers, and
- Engines that run our modern cars, trucks, and other automotive vehicles.
What Is the Meaning of Stroke?
Know the Basics of an Internal Combustion Engine
To understand the meaning of ‘stroke,’ you should know the basics of an internal combustion engine. ICE’s operate under the principle of the combustion cycle. A combustion cycle has pistons moving up and down inside the cylinders.
TDC and BDC
There are terms called TDC or Top Dead Center in an engine’s combustion cycle and BDC or Bottom Dead Center. They refer to the position of the piston inside the cylinder. TDC is the piston’s position closest to the valves, while BDC is furthest from the valves.
One stroke is the piston’s movement from TDC to BDC or vice versa. A combustion cycle or combustion revolution is the complete process of sucking air and gas into the piston, igniting it, and then ejecting the exhaust.
How Does an Engine Deliver Power
The complete process that the engine takes to deliver power is enumerated below:
The piston goes down the cylinder admitting the gas and air mixture inside the combustion chamber or the cylinder.
The piston goes back up the cylinder in which the intake valve is closed, thereby compressing the air and gas mixture inside it.
A spark from the spark plug ignites the air and gas mixture.
The piston moves back up the cylinder, and the exhaust valve opens to eject the unburned gasses.
More on 2-Stroke Engines
The steps in the combustion cycle of a 2-stroke engine are less than a 4-stroke engine. This engine combines the compression and ignition stages when the piston goes up and the power and exhaust as the piston goes down.
One advantage of this fewer number of steps is you will only need fewer parts. This makes for easier and shorter maintenance. However, this engine type generates smaller torque than the 4-stroke engine.
Two-stage Process of 2-stroke Engines
The two-stage process of 2-stroke engines is as follows:
The air and fuel mixture enters the cylinder, and the piston goes up. Compression and ignition take place.
Upon ignition of the fuel and air mixture, the piston goes down. Then, the ejection of the exhaust from the cylinder takes place.
Fuel in a 2-stroke Engine
The fuel used in a 2-stroke engine has a small quantity of motor oil mixed into it. The motor oil takes care of the lubrication of the cylinder, pistons, and piston rings inside the combustion chamber (cylinder).
No Intake or Exhaust Valves
Unlike 4-stroke engines, there are no intake or exhaust valves in 2-stroke engines. What 2-stroke engines have are scavenging ports or small holes in the walls of the cylinder. These holes admit the air and expel the exhaust.
Generates More Power
Since there is combustion for every crankshaft revolution in a 2-stroke engine, it generates more power than a 4-stroke engine. The resulting power is also instantaneously delivered. That is why 2-stroke engines are great for motorcycles and other smaller applications.
More Mechanical Efficiency
So, a 2-stroke engine has more mechanical efficiency than a 4-stroke engine. That’s because a 2-stroke engine can produce the same power in only one revolution, which a 4-stroke engine can only produce in two revolutions.
Again, 2-stroke vs 4-stroke engine – what is the difference? Compared to a 4-stroke engine requiring four strokes, a 2-stroke engine completes all five cycle functions in two piston strokes.
More on 4-Stroke Engines
As their basic term implies, 4-stroke engines operate in four stages. They are as follows:
The step where the intake valve opens, admitting the fuel and air mixture inside the combustion chamber.
This is when the piston goes up and the air and involves fuel mixture compression.
3. Power (or Ignition)
A spark from the spark plug ignites the compressed air and fuel mixture producing the power needed to turn the engine.
This is the step where the exhaust valve opens, allowing the ejection of the exhaust or unburned gasses from the cylinder.
Run on Pure Gasoline
Unlike 2-stroke engines, 4-stroke engines run on pure gasoline without mixed motor oil. Motor oil is delivered to the engine and its cylinders through a different channeling system that will lubricate the moving metal parts of the engine.
Pistons Move Up and Down Two Times
For each combustion cycle, the pistons move up and down two times. That is the reason why it is called 4-stroke. This configuration makes it necessary for the engine to have intake and exhaust valves. These valves need to operate with high precision so that the combustion cycle will operate efficiently.
Heavier Than 2-stroke Engines
With more parts and more steps in the combustion cycle, 4-stroke engines are more complex, are heavier, and have some advantages over 2-stroke engines. The power they can provide is stable. They are also more fuel-efficient, and their emissions are more environmentally friendly.
More Volumetric Efficiency
There are many advantages of 4-stroke engines over 2-stroke engines. 4-stroke engines have more volumetric efficiency because they have more time to mix air and fuel before they are compressed and ignited.
More Thermal Efficiency
They also have more thermal efficiency than 2-stroke engines. Thermal efficiency is the relation between work output and heat input. In equation form, thermal efficiency = work output/heat input.
In theory, the work output of 2-stroke engines is double that of 4-stroke engines for one complete revolution of the crankshaft. The same goes for its fuel input. In reality, however, the work output of 2 strokes is less than four-strokes because they have no individual valves.
That is why part of the fuel mixes with the exhaust and leaves the combustion chamber before being fully burned. So, the thermal efficiency of 2-stroke engines is less than 4-stroke engines.
Difference in Combustion Cycles
The rate by which the combustion cycle is completed is also a significant difference between 2-stroke and 4-stroke engines. Since a 2-stroke engine only takes an intake and an exhaust step, its combustion cycle is faster.
Generally, 4-strokes have more pistons than two strokes, so every combustion cycle of these engines is lower. Let’s look at this area more closely:
2-Stroke Combustion Cycle
Performed by One Piston
The combustion cycle in a 2-stroke engine is performed by just one piston in one piston stroke. This stroke consists of a compression stroke immediately followed by the ignition or power stroke.
The exhaust is ejected when the piston is in its return stroke or returning to its original position before the power stroke. Then a fresh air and fuel mixture is admitted inside the cylinder.
Spark Plug Fires Once Every Revolution
The spark plug fires once every revolution, which generates the power once every two strokes of the piston. Motor oil is added to lubricate the parts.
4-Stroke Combustion Cycle
Each Piston Completes Two Strokes for Each Revolution
For 4-stroke engines, each piston completes two strokes for each revolution of the crankshaft. One stroke is for the compression of the air and fuel mixture, and the second stroke is for the ejecting of the exhaust.
A return stroke follows each stroke. The spark provided by the spark plugs comes only once every other crankshaft revolution. Every four strokes, power is generated by the piston.
No Premixing of Fuel and Motor Oil
There is no premixing of fuel and motor oil with 4-stroke engines because they have a separate lubrication system for the moving metal parts of the engine.
Table Showing the Differences Between 2-Strokes and 4-Strokes
The table below will give you an overview of the many differences between 2-stroke and 4-stroke engines:
|2-Stroke Engine||4-Stroke engine|
|Cheaper and easier to manufacture.||More expensive and more difficult to manufacture because of additional parts like valves and lubrication system.|
|One revolution of the crankshaft for every power stroke||Two revolutions of the crankshaft for every power stroke|
|Bigger ratio in terms of power to weight||Smaller ratio in terms of power to weight|
|Engines are lighter and noisier.||Engines are heavier because they have flywheels which are heavy and less noisy.|
|Uses ports or holes for inlet and outlet of the fuel||Uses valves for inlet and outlet of the fuel|
|More wear and tear because of poor lubrication.||Less wear and tear because it has a separate lubricating system.|
|Lower thermal efficiency||Higher thermal efficiency|
|It emits more smoke making it less fuel-efficient.||It emits less smoke making it more fuel-efficient.|
|Produces higher torque||Generates lesser torque|
|More lubricating oil is required because some oil burns together with the fuel.||Requires less lubricating oil|
Which Is Better?
You need to look at the pros and cons of these two engine types to see which one of them is better. Let’s take up their pros and cons one by one:
1. Fuel Efficiency
4-stroke engines are better since they consume fuel once every four strokes.
Four strokes are friendlier to the environment because they generate less exhaust smoke than two strokes.
3. Engine Design
Two strokes have simpler designs than 4-strokes. That makes them easier to repair and maintain.
Two strokes are lighter than four because they are smaller and have fewer parts.
Two strokes generate higher torque at higher RPM, while four strokes produce higher torque at lower RPM.
Two strokes are less durable than four-strokes because they are designed to run higher RPMs. Running at elevated RPMs, the parts of two strokes will wear out faster.
7. Pre-Mixing of Oil
Four strokes are more convenient to operate because they don’t require premixing of motor oil. Two strokes need to premix motor oil with the fuel before running the engine.
8. Sound Produced
Two-stroke engines are considerably louder when running compared to four strokes. They generate high-pitched buzzing sounds while running.
Conclusion: 2-Stroke Vs 4-Stroke
The fundamental difference between a 2-stroke and a 4-stroke engine is their power strokes. After two stages, a 2-stroke engine completes one power stroke to complete one revolution. Meanwhile, to complete one power stroke, a 4-stroke engine has to go through 4 stages or two complete revolutions.
Each of these two engine types has its advantages and disadvantages. Weighing their pros and cons will help you determine which of them will better serve your needs.