Many people have driven for years without knowing exactly what makes a gasoline-powered car move.
Today, let's clear this up once and for all.
I. Four actions, repeating in a cycle
A gasoline car's power comes from its engine, and the engine's core consists of four strokes: intake, compression, power, and exhaust.
You can visualize these four strokes as a person performing a complete sequence of movements: inhaling, bracing, exerting force, and exhaling.
First action: Intake. The piston moves downward; the intake valve opens while the exhaust valve closes. This creates negative pressure inside the cylinder, drawing in the "air-fuel mixture" (a blend of air and gasoline). It works just like pulling back the plunger on a syringe to draw something in.
Second action: Compression. Both the intake and exhaust valves are tightly closed, and the piston moves upward. The mixture is compressed tighter and tighter; its volume shrinks while its temperature and pressure rise. The compression ratio for standard family cars is usually between 10:1 and 12:1. A higher compression ratio yields more explosive power during combustion. However, it cannot be *too* high; if it is, the gasoline might ignite spontaneously before the spark plug fires-a phenomenon known as "knocking" or pre-ignition. That is why the octane rating of the gasoline you use depends on the engine's compression ratio.
Third action: Power. This is the only stroke that generates power. Just as the piston nears the top, the spark plug fires. The mixture burns instantly, creating high-temperature, high-pressure gas that violently pushes the piston downward. The piston's linear motion is transmitted via the connecting rod to the crankshaft, converting it into rotational motion-which is what makes the wheels turn.
Fourth action: Exhaust. The exhaust valve opens, and the piston moves upward, pushing out the spent exhaust gases. Then, the intake valve opens again, and the next cycle begins.
It takes two full rotations of the crankshaft to complete this four-stroke cycle. When a car is driving, these four actions repeat dozens of times per second.
Working principle of a four-stroke internal combustion engine
II. Two major mechanisms: one generates power, the other handles gas exchange
An engine's structure can be broken down into two major mechanisms and five major systems.
Let's look at the two major mechanisms first. The first is the **crank-connecting rod mechanism**. Its task can be summarized in one sentence: converting the linear up-and-down motion of the piston into the rotational motion of the crankshaft. As the piston moves up and down within the cylinder, it pushes the crankshaft to rotate via the connecting rod; once the crankshaft turns, power is transmitted outward. This mechanism comprises components such as the piston, connecting rod, crankshaft, and flywheel. The flywheel serves an additional purpose: storing energy to ensure smoother engine operation.
The second is the **valvetrain** (or valve train). Its function is to control the opening and timing of the intake and exhaust valves. Opening too early or too late is problematic; the valve timing must be perfectly synchronized with the piston's movement. The core component of the valvetrain is the **camshaft**. A camshaft resembles a rolling pin studded with several "peach-shaped" lobes; as it rotates, these lobes push against the valves, causing them to open.
There are two types of camshaft configurations:
One is **Single Overhead Camshaft (SOHC)**, where a single camshaft controls both the intake and exhaust valves. This design is simple, easy to maintain, and cost-effective, offering good torque at low engine speeds. However, at high speeds, the intake and exhaust processes may struggle to keep up.
The other is **Double Overhead Camshaft (DOHC)**, where separate camshafts control the intake and exhaust valves. This technology is more advanced, resulting in smoother power delivery and superior high-speed performance. The vast majority of modern family cars utilize DOHC systems.
Automotive Engine Basics: A Technical Look at DOHC – Yiche
III. Five Major Systems, Each with Its Own Role
Two mechanisms alone are insufficient; for an engine to function correctly, it relies on five major systems working in concert.
The **fuel supply system** is responsible for mixing gasoline and air in the correct proportions and delivering the mixture into the cylinders. The precise amount of fuel injected and air mixed is calculated based on the depth of the accelerator pedal input.
The **ignition system** is responsible for igniting the air-fuel mixture at the precise moment. Its core components are the spark plug and the ignition coil. The ignition coil transforms the vehicle's 12-volt supply into high-voltage electricity (exceeding 20,000 volts), which the spark plug uses to generate a spark that ignites the mixture inside the cylinder. The lubrication system is responsible for supplying engine oil to all moving parts within the engine. Many engine components-such as the interfaces between the crankshaft and bearings, or between pistons and cylinder walls-operate at high speeds with extremely tight clearances; without oil lubrication, they would wear out and fail within minutes. The oil pump draws oil from the oil sump and delivers it under pressure to the various points requiring lubrication.
The cooling system is responsible for lowering the engine's temperature. Engines generate immense heat during operation-temperatures inside the cylinder can reach 2,200 to 2,800 K during the power stroke. Without cooling, metal components would deform or melt. The cooling system uses a water pump to circulate coolant between the engine and the radiator, carrying away and dissipating the heat.
The starting system is responsible for "waking up" the stationary engine. When you turn the key or press the start button, the starter motor drives the flywheel to rotate the crankshaft and set the pistons in motion, enabling the completion of the initial intake, compression, power, and exhaust cycle. Once this occurs, the engine can continue running on its own.
What are the five major systems of an engine? - Youjia
IV. Three cylinder layouts, each with its own characteristics
The arrangement of an engine's cylinders is a matter of specific design considerations.
A tribute to the N52: The world's best naturally aspirated inline-six engine! _ Chejiahao _ Discovering Car Life _ Autohome
The most common type is the inline engine, where all cylinders are arranged in a straight line. It features a simple structure, low cost, and ease of maintenance. Most family cars with engines under 2.5 liters use an inline-four configuration. BMW's inline-six engine is a notable exception, representing a highly refined design.
Inline, V-type, or horizontally opposed: which engine cylinder layout is best? _ Yiche
The second type is the V-type engine, where cylinders are divided into two banks arranged in a V-shape. This layout is generally used for engines with more than six cylinders. Its advantages include a shorter and lower profile, making it easier to package within the engine bay; additionally, the opposing cylinder banks help cancel out some vibrations. The downsides are a complex structure and more difficult maintenance.
What are the types of cylinder arrangements? _ Engine _ Detailed Specifications _ Car Encyclopedia _ Autohome
The third type is the W-type engine, which can be visualized as two smaller V-configurations joined to form a larger V-shape. The engine is shorter and wider, yet its structure is extremely complex; it is typically found in high-end performance vehicles. Volkswagen's W12 and Bugatti's W16 are classic examples.
Horizontally Opposed Engine (Boxer Engine) – Pacific Auto Encyclopedia
There is also the horizontally opposed engine-or "boxer" engine-where the cylinders lie flat, forming a 180-degree angle. Its low center of gravity and symmetrical layout benefit handling. However, the technology is challenging to implement, and it is currently used primarily by Porsche and Subaru.
V. Over a Century of Technology Dedicated to One Goal
More than a century has passed since the birth of the first gasoline engine in 1886.
Throughout this time, engineers have strived to increase power output, reduce fuel consumption, and lower emissions. Compression ratios have risen from under 5 in the early days to over 10-or even higher-today. Valvetrains have evolved from side-mounted camshafts to overhead camshafts, progressing from SOHC to DOHC. Fuel delivery systems have advanced from carburetors to electronic fuel injection, and finally to direct injection. Ignition systems have evolved from contact-point types to electronic systems, and now to precise computer control.
Yet, the fundamental logic has never changed: converting the chemical energy of gasoline into thermal energy, and then transforming that thermal energy into mechanical energy. Four strokes, two major mechanisms, and five key systems-this framework has remained constant for over a century.
Every spark from the spark plug triggers a micro-explosion. Every downward stroke of the piston represents an energy conversion. Every rotation of the crankshaft delivers power.
It is this exquisitely precise mechanical coordination that allows gasoline-powered vehicles to run.
