Structure and Working Principle of a Car Engine

Cars are tools we use every day, and today we’ll take a look at the structure and working principle of a car engine!

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Why do we say that engine power comes from an explosion?

The power of a car comes from the explosive force produced when gasoline or diesel fuel burns.

However, if you pour gasoline into an open bowl and ignite it, it only burns—it doesn’t explode. Why?
Because the bowl is not sealed, it’s open to the air.

If gasoline and air are placed in a sealed container and then ignited, an explosion will occur.
This is the basic principle on which car engines are designed.

When gasoline and air are mixed in the most suitable ratio for combustion and then compressed strongly, the temperature rises. At that moment, if the mixture is ignited, a much greater explosive force is generated.

This force is then transmitted through a series of mechanical components to the car’s wheels, propelling the vehicle forward.

What Are Internal Combustion Engines and External Combustion Engines?

We often refer to car engines as internal combustion engines (ICEs), but are there external combustion engines as well? Yes, they do exist. Examples include the steam engines used in early trains, steam turbines in power plants, and engines on ships—all of these are external combustion engines.

In an external combustion engine, the fuel burns outside the engine cylinder to generate power. For example, early steam engines burned fuel such as wood, coal, gas, or diesel to heat water in a boiler. This produced high-pressure steam, which entered the cylinders and pushed pistons to do work, generating power.

An internal combustion engine, on the other hand, burns fuel inside the cylinder. Today, the gasoline and diesel engines used in cars are all internal combustion engines.

How to calculate engine displacement and compression ratio:

Cylinder displacement refers to the volume of air-fuel mixture that the piston sweeps from the bottom dead center (BDC) to the top dead center (TDC). It depends on the bore (cylinder diameter) and stroke (piston travel).

Engine displacement is the total volume of all cylinders, usually expressed in cc (cubic centimeters), mL (milliliters), or L (liters). Since cylinders are cylindrical, their volume rarely comes out to an exact liter, which is why you see figures like 1998 cc or 2397 cc, often rounded approximately as 2.0 L or 2.4 L.

The larger the engine displacement, the more air-fuel mixture it can intake per cycle, generating more power during combustion. This is similar to a person with a larger appetite: the more they eat, the more energy they have.

Engine compression ratio is the ratio of the cylinder volume before compression to the cylinder volume after compression (i.e., the combustion chamber volume).

Principle of Gasoline Engine Combustion

The main component of gasoline is hydrocarbons, which are molecules composed only of carbon and hydrogen atoms. During combustion, hydrocarbons chemically react with oxygen from the intake air. Specifically, one carbon atom combines with two oxygen atoms to form one molecule of carbon dioxide (CO₂), and two hydrogen atoms combine with one oxygen atom to form one molecule of water (H₂O).

If the amount of intake air is insufficient, there will be less oxygen available to combine with carbon atoms, preventing complete formation of carbon dioxide and resulting in the production of some carbon monoxide (CO).

During the explosive combustion process, the extremely high temperatures can also cause nitrogen in the air to oxidize, forming nitric oxide (NO) and nitrogen dioxide (NO₂).

Therefore, the main components of automobile exhaust are carbon monoxide (CO), carbon dioxide (CO₂), nitric oxide (NO), and nitrogen dioxide (NO₂).

Principle of Gasoline Engine Combustion

The main component of gasoline is hydrocarbons, which are molecules composed only of carbon and hydrogen atoms. During combustion, hydrocarbons chemically react with oxygen from the intake air. Specifically, one carbon atom combines with two oxygen atoms to form a molecule of carbon dioxide (CO₂), and two hydrogen atoms combine with one oxygen atom to form a molecule of water (H₂O).

If the intake air is insufficient, there will be less oxygen available to react with carbon atoms, preventing the complete formation of carbon dioxide and resulting in the production of some carbon monoxide (CO).

During the explosive combustion process, the extremely high temperatures can also cause nitrogen in the air to oxidize, forming nitric oxide (NO) and nitrogen dioxide (NO₂).

Therefore, the main components of automobile exhaust are carbon monoxide (CO), carbon dioxide (CO₂), nitric oxide (NO), and nitrogen dioxide (NO₂).

Gasoline Engine Operating Cycle

The piston in a cylinder must complete four strokes—intake, compression, power, and exhaust—to finish one operating cycle. During this process, the piston moves up and down twice in the cylinder, while the crankshaft rotates two full turns.

Looking at the tachometer on a car, you can see how fast this process occurs. If the tachometer points to 6, it indicates the engine is running at 6,000 revolutions per minute (RPM), which is 100 revolutions per second. In this case, each piston completes 50 operating cycles per second (since the crankshaft rotates twice per cycle), meaning each cylinder experiences 50 combustion events per second. For a 4-cylinder engine, this amounts to 200 combustion events per second. The “popping” sound of a running engine is actually the sound of these rapid fuel combustions.

Similarly, if the tachometer points to 3, the engine is running at 3,000 RPM, or 50 revolutions per second. Each piston then completes 25 operating cycles per second, meaning each cylinder experiences 25 combustion events per second. For a 4-cylinder engine, this results in 100 combustion events per second.

Why a Gasoline Engine Can Produce Continuous Power

The piston moves up and down inside the cylinder. The lowest point the piston reaches is called the bottom dead center (BDC), and the highest point is the top dead center (TDC). The distance between the top and bottom dead centers is called the stroke. When the piston is at TDC, the space above the piston is called the combustion chamber.

Intake Stroke: When the piston moves from TDC to BDC, the intake valve opens and the exhaust valve closes. This creates a partial vacuum in the cylinder, allowing a fresh mixture of air and gasoline to be drawn into the cylinder.

Compression Stroke: Both the intake and exhaust valves are closed, and the piston moves upward from BDC to TDC, compressing the air-fuel mixture. The more mixture drawn into the cylinder, the closer the piston approaches TDC, increasing the compression pressure. Compressing the mixture makes it more uniform and raises its temperature, which promotes better combustion and greater power output.

Power Stroke: After both intake and exhaust valves remain closed, the spark plug emits a high-voltage spark at the right moment to ignite the compressed mixture. The combustion produces high pressure that pushes the piston from TDC to BDC. The spark plug receives high-voltage energy from the ignition coil, which is amplified and distributed to each cylinder by the electronic control unit (ECU) in sequence, igniting the compressed air-fuel mixture.

Exhaust Stroke: The piston moves upward from BDC to TDC, with the intake valve closed and the exhaust valve open. The burned gases are pushed out of the cylinder through the exhaust valve and exhaust manifold into the atmosphere. After passing through the muffler, the exhaust gases are silenced to reduce noise.

This continuous sequence of four strokes in each cylinder allows the engine to produce power continuously.

Rotary Engine

Most automobiles use reciprocating engines, where the pistons move back and forth in a straight line. This applies to both gasoline and diesel engines. In contrast, a rotary engine has pistons that rotate within the cylinder.

The rotary engine commonly referred to today is the triangular rotor engine designed by German engineer Felix Wankel in the 1950s. Therefore, the rotary engine is also called a Wankel engine.

The main components of a rotary engine are structurally simple, its size is compact, it produces high power, operates smoothly at high speeds, and has good performance. This once drew considerable attention from the automotive industry, leading to various experimental developments. However, after decades of testing, it has been shown that this type of engine cannot yet match traditional reciprocating piston engines. Issues include significant rotor tip wear and relatively high fuel consumption.

In a rotary engine, the piston is a flat triangular shape, and the cylinder is a flattened housing. The piston is eccentrically placed within the cavity. As the piston moves in a planetary motion, the volume of the working chamber changes periodically with the rotation of the piston, completing the intake, compression, power, and exhaust strokes. Each full rotation of the rotor completes one four-stroke cycle. The four-stroke cycle in a rotary engine is the same in principle as that of a reciprocating piston engine, with the difference being the shape of the piston and its motion path.