Engines are most widely used all over the world for numerous applications. A four-stroke engine is a famous type of internal combustion (IC) engine. This engine completes a power cycle in four strokes of the piston. The piston reciprocates inside the engine cylinder. This article explains the 4-stroke engine working, types, components, and applications.
What is a Four-Stroke Engine?
A 4-stroke engine is an IC engine that uses four strokes of the piston to complete a working cycle. It converts the thermal energy of the fuel into useful mechanical work due to the upward and downward movement of the piston. Therefore, it belongs to the category of the reciprocating engine.
A four-stroke engine completes a power cycle after the completion of two revolutions of the crankshaft or 4 strokes of the piston.
The 4-stroke engines are most commonly used in motorcycles, automobiles, and other vehicles, as well as in generators and other machinery. These engines are famous because of their high reliability and efficiency. They are relatively easy to repair and maintain. These are heavy engines than the 2-stroke engines.
The main difference between the 2-stroke and 4-stroke engines is that a 2-stroke engine completes a working cycle in just two strokes while a four-stroke engine completes a working cycle in four strokes of the piston. A 4-stroke engine produces fewer emissions compared to a 2-stroke engine.
How does a 4-Stroke Engine Work?
A four-stroke engine completes a power cycle with four strokes of the piston. It works in the following way:
- Intake Stroke
- Compression Stroke
- Power Stroke
- Exhaust Stroke
1) Intake Stroke
- As the piston reciprocates toward BCD from TDC (downward), a vacuum starts producing at the suction side of the compression chamber (cylinder).
- When the vacuum is generated at the suction side, the exhaust valve closes, and the inlet valve opens.
- When the inlet valve opens, the air-fuel mixture starts entering the compression chamber.
2) Compression Stroke
- As the internal pressure of the compression chamber becomes equal to the outer pressure, the inlet valve closes, and compression stroke starts.
- As the piston moves upward (from BCD to TDC), it compresses the air-fuel mixture inside the compression chamber and increases the temperature and pressure of the air-fuel mixture.
3) Power Stroke
- The power stroke is also known as a combustion stroke.
- When the compression stroke is nearly to be complete, a spark plug burns the compressed air-fuel mixture.
- As the fuel gets ignited, the power is generated so that the piston moves from TDC to BDC by expanding the chemical reaction. Therefore, this stroke is called a power stroke.
- Due to this burning process, the temperature and pressure of the mixture become very high. Due to an increase in pressure, the air-fuel mixture pushes the piston to move downward (toward BCD from TDC) and drives the crankshaft, which further moves the vehicle.
- During this process, both the inlet and exhaust valves remain shut.
4) Exhaust Stroke
- After the completion of the power stroke, the exhaust stroke starts.
- In this stroke, the piston again moves upward (from BDC to TDC).
- During this stroke, the inlet valve is closed, while the exhaust valve is opened. The piston pushes the exhaust gases out of the combustion chamber.
- After completing the exhaust stroke, the piston again moves downward (from TDC to BDC), sucks the air-fuel mixture and the whole cycle repeats. This final stroke forces the spent gasses/exhaust out of the cylinder.
Read Also: Working of 2-stroke Engine
Four-Stroke Engine Cycle
The following PV diagram represents the working cycle of the 4-stroke engine. A four-stroke engine completes a working cycle in the following steps:
Four-stroke cycle- Isobaric Process (0 to 1): In the isobaric process, the piston moves downward and generates a vacuum inside the combustion chamber. During vacuum creation, a pressure difference generates between the atmospheric pressure and the chamber’s inner pressure. Due to this pressure difference, the intake valve opens, and the air-fuel mixture enters the combustion chamber.
- Adiabatic Process (1 to 2): After the completion of the isobaric process, the inlet valve closes, and the piston moves upward and pressurizes the air-fuel mixture. During this process, the piston rises the temperature and pressure of the mixture, but its heat doesn’t change.
- Isochoric Process (2 to 3): A spark plug ignites the air-fuel mixture at the end of the compression stroke (adiabatic process). This process increases the temperature and pressure of the air-fuel mixture and transforms it into a high-temperature and pressure mixture. This ignition process also increases the entropy (heat) of the air-fuel mixture.
- Power Stroke (Process 3 to 4): In this stroke, the heat produced due to the ignition process is used to push the piston to move down, which further moves the crankshaft. The movement of the crankshaft moves the vehicle. Therefore, this process is called a power stroke.
- Exhaust Phase (4 to 1): In this phase, the piston again moves up, and the exhaust valve opens, which discharges the waste heat from the combustion chamber. Due to the removal of useless heat, the kinetic energy of the air-fuel mixture’s molecules decreases. Again, the pressure difference generates between the atmospheric pressure and the chamber’s inner pressure, and the whole cycle repeats.
Read Also: Working of Diesel Engine
Types of Four-Stroke Engines
1) 4-Stroke Gasoline Engine
A 4-stroke gasoline engine is a type of internal combustion engine that runs on gasoline or petrol. It works on the base of the otto cycle. In this engine, the ignition process occurs due to the spark provided by a spark plug. It uses petrol or gasoline as a working fluid. The gasoline engine is less efficient than the diesel engine. It has a low compression ratio.
4-stroke gasoline engines are commonly used in motorcycles, light trucks, and passenger cars due to their relatively smooth operation, high power output, and easy availability of gasoline fuel. They are also popular because they are easier to start in cold weather than diesel engines.
2) 4-Stroke Diesel Engine
The four-stroke diesel engine works on the base of a diesel cycle. In this engine, ignition occurs due to the high compression of the air mixture. It uses diesel as a fuel. It uses a low amount of fuel.
The high torque and efficiency of these engines make them ideal for heavy-duty vehicles like buses, trucks, and industrial machinery. These engines are more fuel-efficient than gasoline engines due to their higher compression ratios. However, diesel engines produce more noise than gasoline engines.
Four-Stroke Engine Power Output Limitations
The output power of the engine depends on the amount of air drawn in. The performance of a piston engine (whether it is a 4-stroke engine or a 2-stroke engine) depends on the speed (RPM), the calorific value of the fuel, loss, air-fuel ratio, volumetric efficiency, oxygen content in the fuel-air mixture and combustion chamber size. Ultimately, the speed of the engine controls through lubrication and material strength.
The connecting rod, piston, and valve of the engine face strong acceleration forces. High engine speeds can lead to engine damage, loss of power, piston ring flutter, or other physical damage. When the piston ring vibrates vertically in the piston groove in which the piston ring is located, the piston ring flutters.
The purpose of the ring flutter is to settle the seal between the cylinder wall and the ring, which leads to a loss of power and pressure in the cylinder.
If the engine turns too fast, the valve spring will not be able to close the valve fast enough. This is often known as a “valve float” and causes the piston to hit the valve and cause a serious engine break.
At high speeds, the piston-cylinder wall interface’s lubrication tends to be damaged. Therefore, the piston speed of an industrial engine is limited to up to 10 m/s.
Read Also: Different types of Engines
Parts of 4-Stroke Engine
The four-stroke engine has the following major components:
- Fuel injector
- Piston
- Inlet valve
- Outlet valve
- Crankshaft
- Spark plug
- Connecting rod
- Engine block
- Flywheel
- EGR valve
- Engine cylinder
1) Piston
The piston is one of the most vital parts of the four-stroke engine. It reciprocates inside the engine cylinder. It is attached to the crankshaft via a connecting rod. It transfers its motion to the crankshaft via a connecting rod. The piston moves downwards and upwards inside the cylinder to complete the power stroke.
When the piston moves up, it sucks the air inside the cylinder, while it compresses the air when it moves downward. Due to this piston movement, the temperature and pressure of the air-fuel mixture inside the cylinder increase.
The engine piston has a complex construction with a steel crown and a ductile cast iron skirt. This skirt uses pressure lubrication to ensure oil delivery to the cylinder liner underneath each working situation. Oil feds toward the cooling duct at the top of the piston via the connecting rods.
Read More: Piston Working and Parts
2) Piston Rings
All piston rings are chrome-plated to resist wear. The piston ring contains a spring-compatible oil control ring and two guiding compression rings. The piston ring groove has excellent wear resistance and is stabilized.
3) Cylinder Linear
This component of the four-stroke engine has a high, rigid collar to lessen deformation. This linear material is a gray cast iron alloy with high strength and brilliant wear resistance. To avoid the bore polishing risk, the linear is equipped with a protective polishing ring.
The space between the cylinder liner and the cylinder block is sealed with a double O-ring. The upper end of the linear equips with an anti-polish ring that prevents the internal bores from being polished and reduces the consumption of lubricating oil.
4) Connecting Rod
This component of the 4-stroke engine connects the engine crankshaft and piston. It is usually made of Alloy steel and forged in one piece.
A connecting rod is machined in a circular cross-section. The lower side of the connecting rod splits in the horizontal direction so that the connecting rod and the piston can be removed from the cylinder liner.
All bolts of the connecting rod are tightened hydraulically. The holes in the connecting rod direct oil to the pistons and gudgeon pin bearing. This component of the engine transfers the motion of the piston to the crankshaft, which further moves to the vehicle’s wheel.
Read More: Connecting Rod Working and Construction
5) Crankshaft
The crankshaft transforms the reciprocating motion of the engine piston into rotary motion and moves the vehicle wheels. It is an essential component for all engines. This part transfers final power in the form of mechanical power.
It is made in the form of one piece. It is connected to the piston via a connecting rod.
Read Also: Working of Crankshaft
6) Engine Block
An engine block is a part of the four-stroke engine that contains the cylinder, piston, and other internal parts of the engine. The engine block is also known as a cylinder block.
The engine block is constructed of ductile iron. The engine blocks also have oil galleries and coolant passages. The cylinder head is utilized to close the upper portion of the cylinder block.
Read Also: Working of Engine Block
7) Camshaft
It is used to open and shut input and exhaust valves. It plays a vital role to ensure a proper supply of fuel to the combustion chamber.
Read Also: Working of Camshaft
8) Spark Plug
The spark plug is only used in 4-stroke petrol engines. It is used to provide the spark to the air-fuel mixture to ignite it.
9) Fuel Injector
It is used to inject the fuel inside the engine cylinders. Some engines use a fuel pump instead of a fuel injector.
10) Flywheel
The flywheel is one of the most important components of the four-stroke engine. It is mounted on a cast iron pole. It stores energy as inertia.
11) Inlet and Outlet Valves
The inlet valve is used to introduce fuel into the engine cylinder. The outlet valve is used to discharge the exhaust gases.
12) EGR Valve
It is a part of the engine which is used to regulate the emission of exhaust gases into the environment. It allows a controlled amount of exhaust gas to flow into the intake manifold.
Read More: EGR Valve Working and Parts
Advantages and Disadvantages of 4-Stroke Engines
The four-stroke engine has the following advantages and disadvantages:
Advantages of Four-Stroke Engine
- Reliability: These types of engines are more reliable and efficient than 2-stroke engines.
- Durability: These engines have high durability than 2-stroke engines.
- Environmental friendly: These engines are environmental-friendly because a 4-stroke engine releases less dangerous fumes than a 2-stroke engine.
- These engines are best for heavy loads and heavy vehicles.
- Fuel efficiency: These engines have high fuel efficiency than 2-stroke engines.
- Noise: These have a quiet operation than two-stroke engines
- More torque: At low speeds, four-stroke engines produce more torque than 2-stroke engines.
- More fuel efficiency: This type of IC engine has higher fuel efficiency than a two-stroke engine.
- No extra oil requirement: This engine does not require any additional lubrication or oil to add fuel. Only the turning components require lubrication immediately.
Disadvantages of Four-Stroke Engine
- Power: This engine has lower power than the two-stroke engine.
- Expensive: A four-stroke engine has many parts. Therefore, it has high cost than a two-stroke engine.
- Weight: These engines have high weight than 2-stroke engines
- Required area: They required a large area for installation.
- Piston strokes: It requires more piston strokes to complete a power cycle.
- Design: These engines have a complex design.
Applications of Four-Stroke Engines
- Automobiles: The 4-stroke engines are most commonly used in cars, trucks, and other motor vehicles. These have high efficiency and reliability.
- Construction equipment: They are commonly used in construction equipment such as loaders, excavators, and bulldozers.
- Marine engines: The 4-stroke diesel engines are used in boats and ships to power their propellers.
- Lawn and garden equipment: Four-stroke engines are used in different leaf blowers, lawn mowers, and other outdoor power devices. These engines are easy to start.
- Generators: These engines are used in stationary and portable generators to transform fuel into electricity.
FAQ Section
What is meant by a 4-stroke engine?
An engine that completes a power stroke in four strokes of the piston is known as a 4-stroke engine.
What are examples of a 4-stroke engine?
Four-stroke engines are most commonly used in heavy applications such as trucks, buses, dirt bikes, vans, tractors, and other heavy vehicles.
Which engine produces less pollution, 2-stroke or 4-stroke?
A 2-stroke engine produces more pollution than a 4-stroke engine. This is because a 2-stroke engine uses ports for the suction and discharge of the fuel.
Which is faster, 2 stroke or 4 stroke?
A 2-stroke engine has lower parts than a 4-stroke engine. In comparison, a two-stroke engine completes a working cycle (in just 2 strokes of the piston) faster than a four-stroke engine. Therefore, a 2-stroke engine is faster than a 4-stroke engine.
Is there a six-stroke engine?
A 6-stroke engine is a most modern version of an IC engine that is based on a 4-stroke engine design, but this engine has two extra electric strokes to decrease emission and improve efficiency. A 6-stroke engine utilizes fresh air (clean air from the atmosphere) for the 5th-stroke breath of the 2nd suction.
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