Four-stroke engines are internal combustion engines used in automobiles, like trucks, cars, and some modern motorbikes (most motorbikes work with a two-stroke engine). Four-stroke is referred to as the combustion cycle.
A four-stroke, also called a four-cycle engine, is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder in either direction.
In this reading, we’ll explore what a four-stroke engine is, its diagram, stroke cycle, parts, and working.
Let’s begin!
Related: What is a Two-Stroke Engine? It Parts, Diagram, & How it Works
What Is A Four-Stroke Engine?
A four-stroke cycle engine is an internal combustion engine that uses four different piston strokes, such as intake, compression, power, and exhaust, to complete one operating cycle. A stroke refers to the full travel of the piston along the cylinder in either direction.
The piston makes two complete passes in the cylinder to complete one operating cycle. An operating cycle requires two revolutions (720°) of the crankshaft.
This means that the crankshaft must go around twice, and each piston moves up and down two times, to produce one pulse of power. In other words, the piston goes up-down, up-down, for each firing of the spark plug for petrol engines.
Four-stroke engines are common internal combustion engines mostly used for land transportation such as automobiles, trucks, diesel trains, and motorcycles, and it doesn’t matter if they burn petrol or diesel.
The four-stroke cycle is often called the Otto cycle, after the German engineer Nikolaus August Otto, who designed an engine on that principle in 1876.
Parts Of A Four-Stroke Engine
Several essential parts of a four-stroke engine combine to transform fuel into mechanical energy. An internal combustion engine running on four strokes typically consists of the following parts:
Crankshaft
The component responsible for converting reciprocating motion into rotating movement is the crankshaft.
Connecting Rod
It functions as a lever arm, transferring motion from a piston to a crankshaft.
Piston
An engine’s piston uses a connecting rod to connect the expanding forces of gas to the crankshaft’s mechanical movement.
Spark Plug
It is an apparatus that provides the combustion chamber with electric current, which ignites the fuel and air combination and causes abrupt gas expansion.
Flywheel
The flywheel is a mechanical device that rotates and is used to store energy.
Inlet and Outlet Valves
It enables the ingestion of fresh air with fuel and the expulsion of the exhausted air-fuel mixture from the cylinder.
Related: Difference between SI (Spark ignition) and CI (Compression ignition) engines
The Four-Stroke Cycle
The majority of internal combustion engines, including those in tractors, motorcycles, and cars, use the four-stroke cycle. When the engine is running, it goes through four different phases, or strokes, each of which takes place in a single cylinder. The following are the four strokes:
1. Intake/Inlet Stroke
This phase is the first stroke cycle, also known as the induction or suction stage. The stroke of the piston begins at the Top Dead Center (T.D.C.) and ends at the Bottom Dead Center (B.D.C.).
During this stroke, the intake valve is opened so that the piston can introduce the air-fuel mixture into the cylinder through a partial vacuum (negative pressure). This step occurs during the downward movement of the piston.
2. Compression Stroke
At this stage, the intake valve is closed, blocking the air-fuel mixture from escaping. The compression stroke begins at bottom dead center, or just at the end of the suction stroke, and ends at top dead center.
In this stroke, the piston compresses the air-fuel mixture in preparation for ignition during the power stroke. Both the intake and exhaust valves are closed during this stage.
At the end of the stroke, the availability of a spark plug allows the ignition of the air-fuel mixture, providing the energy required for combustion.
3. Combustion Stroke
This stage is also known as the power or ignition stroke. It is the start of the second revolution of the four-stroke cycle. Furthermore, the crankshaft is said to have completed a full 360-degree revolution at this point.
For gasoline engines, a spark plug ignites the compressed air-fuel mixture. High compression generates heat, which is what powers diesel engines.
Additionally, the engine generates mechanical work to turn the crankshaft by forcefully returning the piston to the bottom dead center.
4. Exhaust Stroke
The discharge phase is the final stage of the combustion cycle, also known as outlet. It occurs when the piston moves back downward and the exhaust valve opens. As it rises again and the valve opens, the piston forces the exhaust gas out.
In other words, the piston once again returns from bottom dead center to top dead center while the outlet valve is open. This action expels the spent air-fuel mixture through the exhaust port.
Related: Difference Between Lean Burn and Rich Burn Engine
Diagram Of Four-Stroke Engine
How A Four-Stroke Enigne Works
The thermal efficiency of an engine is the ability to convert the fuel (chemical energy) to mechanical energy. This energy will vary due to the design and model of the vehicle.
Typically, four-stroke petrol engines are opportune to convert 20% of fuel (chemical energy) to mechanical energy.
The wheels use 15% of it to move, and friction and mechanical elements waste 5% of it. However, a higher compression ratio can improve the engine’s thermodynamic efficiency. The engine chamber’s maximum and minimum volumes determine the ratio.
An engine with a higher ratio will allow the fuel-air mixture to be immense, which will produce higher pressure, make the housing hotter, and increase thermal efficiency.
Related: What Is A Petrol Engine, Its Application, Parts and How it Works?
FAQs
What is the history of the four-stroke engine?
Alphonse Beau de Rochas, a French engineer, invented the four-stroke cycle in 1862. Nikolaus August Otto, a German engineer, created an engine based on this principle in 1876, which is why the four-stroke cycle is frequently referred to as the Otto cycle.
What does a four-stroke stand for?
In gasoline/petrol engines, the four strokes are intake (1), compression (2), power (3), and exhaust (4).
What is the difference between a 4-stroke and a 2-stroke engine?
The difference between a 4-stroke and a 2-stroke engine is that a four-stroke engine completes one power stroke by going through four phases, or two full revolutions, compared to a 2-stroke engine’s single revolution. And also, for a 2-stroke engine to produce one power stroke, it passes through two stages, or one full revolution.
Is a car a four-stroke engine?
The modern standard for passenger cars and trucks is 4-stroke engines. In recent times, they have also been utilized in outboard engines, motorcycles, and lawnmowers. Furthermore, small machines like yard tools often have two-stroke engines. Motorcycles and outboard engines also use them.