Four-Stroke Engine

A four-stroke motor is an internal combustion engine that utilizes four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle. A complete operation in a four-stroke motor requires two revolutions (720⁰) of the crankshaft. In this article, let’s study the four-stroke motor.

Parts of a Four Stroke Engine

Piston

In a motor, a piston transfers the expanding forces of gas to the mechanical rotation of the crankshaft through a connecting rod.

Crankshaft

A crankshaft is a part that converts the reciprocating motion to rotational motion.

Connecting Rod

It transfers motion from a piston to a crankshaft, acting as a lever arm

Flywheel

The flywheel is a rotating mechanical device that is used to store energy.

Inlet and Outlet Valves

It allows us to enter fresh air with fuel & to exit the spent air-fuel mixture from the cylinder.

Spark Plug

It is a device that delivers electric current to the combustion chamber, which ignites the air-fuel mixture leading to abrupt gas expansion.

Four-Stroke Engine Cycle

The four strokes of the Motor go by the following names:

Suction/Intake Stroke

Intake stroke occurs when the air-fuel mixture is introduced to the combustion chamber. In this stroke, the piston moves from TDC (Top Dead Center – the farthest position of the piston to the crankshaft) to BDC (Bottom Dead Center – the nearest part of the piston to the crankshaft.) The movement of the piston towards the BDC creates a low-pressure area in the cylinder. The inlet valve remains to open a few degrees of crankshaft rotation after BDC. The intake valve then closes, and the air-fuel mixture is sealed in the cylinder

• Key points
• Inlet Valve – Open
• Outlet Valve ­­ – Closed
• Crankshaft Rotation – 180⁰

Compression Stroke

In compression stroke, the trapped air-fuel mixture is compressed inside the cylinder. During the stroke, the piston moves from BDC to TDC, compressing the air-fuel mixture. The momentum of the flywheel helps the piston move forward. Compressing the air-fuel mixture releases more energy when the charge is ignited. The charge is the volume of compressed air-fuel mixture trapped inside the combustion chamber ready for ignition. The inlet and outlet valves must be closed to ensure that the cylinder is sealed, resulting in compression.

• Key points
• Inlet Valve – Closed
• Outlet Valve ­­ – Closed
• Crankshaft Rotation – 180⁰ (Total 360⁰)

Power/Combustion Stroke

The second rotation of the crankshaft begins when it completes a full rotation during the compression stroke. The power stroke occurs when the compressed air-fuel mixture is ignited with the help of a spark plug. Ignition or Combustion is the rapid, oxidizing chemical reaction in which a fuel chemically combines with oxygen in the atmosphere and releases energy in the form of heat. The hot expanding gases force the piston head away from the cylinder head.

• Key points
• Inlet Valve – Closed
• Outlet Valve ­­ – Closed
• Crankshaft Rotation – 180⁰ (Total 540⁰)

Exhaust Stroke

As the piston reaches BDC during the power stroke, combustion is complete, and the cylinder is filled with exhaust gases. The exhaust valves open during this stroke, and the inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC, and one operating cycle has been completed.

• Key points
• Inlet Valve – Closed
• Outlet Valve ­­ – Open
• Crankshaft Rotation – 180⁰ (Total 720⁰)

Comparison Between a 2-stroke engine and a 4-stroke engine

1. A 4-stroke motor weighs 50% heavier than a 2-stroke Motor.
2. A 4-stroke Motor is more efficient than a 2-stroke motor because fuel is consumed once every 4 strokes.
3. A 2-stroke engine creates more torque at a higher RPM, while a 4-stroke Motor creates a higher torque at a lower RPM.
4. A 4-stroke motor is quieter than a 2-stroke Motor.
5. 2-stroke motors tend to wear out fast because they are designed to run at a higher RPM.
6. 2-stroke motors are easier to fix because of their simple construction. 4-stroke engines have complex designs with more parts, making them more expensive, and repairs cost more.

Two-stroke engines are typically found in smaller applications such as chainsaws, boat motors, and dirt bikes. Four-stroke engines are found in go-karts, lawnmowers, and combustion engines in your car.

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Frequently Asked Questions – FAQs

What is a four-stroke engine?

A four-stroke motor is an internal combustion engine that utilizes four distinct piston strokes to complete one operating cycle.

What are the various parts of a four-stroke engine?

Various parts of the four-stroke engine are:

1. Piston
2. Crankshaft
3. Crankshaft
4. Connecting Rod
5. Inlet and Outlet Valves
6. Spark Plug

Which is heavier? Two-stroke engine or four-stroke engine?

A four-stroke motor weighs 50% heavier than a two-stroke engine, and hence four-stroke engine is heavier.

Which type of engine tends to wear out fast?

The two-stroke motor tends to wear out fast since it is designed to operate at a higher RPM.

What are the three applications of four-stroke engines?

Four-stroke motors are used in the following ways: Combustion engines-carts lawn mowers