What is a gasoline engine?

A gasoline engine is an internal combustion engine that runs on a volatile liquid fuel such as gasoline or a gasoline mixture like ethanol, ignited by an electric spark. These engines are versatile, suitable for various applications, most notably in vehicles, aircraft, and power tools. Gasoline engines come in two-stroke and four-stroke designs. In the four-stroke cycle, the piston completes four movements: intake, compression, power, and exhaust. The two-stroke cycle combines the compression and power strokes of the four-stroke cycle, needing only one crankshaft revolution to finish the cycle.

How does a gasoline engine work?

A gasoline engine is an internal combustion engine that transforms chemical energy from a volatile liquid fuel (typically gasoline or a gasoline mixture like ethanol) into mechanical energy, using an electric spark to initiate combustion. Most gasoline engines operate on a four-stroke cycle : 1. Intake: The piston descends, drawing a mixture of gasoline vapor and air into the cylinder. 2. Compression: The piston ascends, compressing the fuel-air mixture. 3. Power: An electric spark ignites the compressed mixture, forcing the piston down and generating power. 4. Exhaust: The piston ascends again, pushing the spent combustion products out through the exhaust valve, and then the cycle repeats.

What fuel does a gasoline engine use?

Gasoline engines use a volatile liquid fuel, typically gasoline or a gasoline mixture like ethanol, to generate power. An electric spark initiates the burning of the fuel. Gasoline itself is a complex mixture of hydrocarbons. It may also contain additives like detergents, anti-icing agents, and antioxidants.

What is the purpose of spark plugs in a gasoline engine?

In a gasoline engine , the purpose of a spark plug is to ignite the air-fuel mixture within the cylinder. It is a component of the ignition system that endures severe conditions, such as high temperatures and pressures within the combustion chamber. The spark plug has two electrodes separated by an air gap. Current from a high-tension ignition system discharges across this gap, creating a spark. The spark plug consists of a steel shell, a ceramic insulating element, and electrodes made of various alloys. The spark gap between the center and ground electrodes must be properly adjusted for the engine to function correctly.

How is the energy from gasoline turned into movement in a car?

In a gasoline engine, energy from gasoline turns into movement through a four-stroke cycle. Intake stroke The piston descends, and a mix of gasoline vapor and air is drawn into the cylinder. Compression stroke The piston ascends and compresses the mixture. Power stroke An electric spark ignites the compressed mixture, and the expanding gas pressure forces the piston down. Exhaust stroke The piston ascends again, pushing the combustion products out through the exhaust valve, and the cycle repeats. The force from the power stroke pushes the piston, which turns a crankshaft. This rotary motion is then transmitted to the wheels, propelling the car.

What are some environmental concerns with using gasoline engines?

Gasoline engines pose several environmental concerns, including air and water pollution. Gasoline engines emit noxious gases, such as unburned hydrocarbons, carbon monoxide, and nitrogen oxides. Nitrogen oxides can cause inflammation and irritation of breathing passages and react in the atmosphere to form nitric acid, which contributes to acid rain. Furthermore, gasoline fumes evaporating from fuel tanks contribute to air pollution. Oil spills, sometimes related to gasoline, can also leak petroleum into bodies of water, causing major environmental problems. The unintentional release of gasoline solvents and lubricants by industries and individuals also greatly aggravates environmental problems.

gasoline engine

Images

Diagram showing four types of gasoline engines: an opposed-piston engine with pistons facing each other, a Wankel rotary engine with a triangular rotor, an in-line engine with pistons arranged vertically, and a V-type (V-8) engine with pistons set in a V shape. Arrows indicate the direction of motion in each engine.

Diagram of a gasoline engine showing a cross-section of a piston and cylinder. Key parts such as the spark plug, valves, camshaft, piston, connecting rod, and crankshaft are labeled.

internal-combustion engine: four-stroke cycle

blower-scavenged two-stroke-cycle engine

Diagram showing three stages in the operation of a Wankel rotary engine: intake of fuel-air mixture, ignition by spark plug, and exhaust release, with labeled parts and rotary motion indicated.

Diagram of a gasoline engine cooling system showing coolant flow from the radiator, through the engine cylinders, and back, with labeled components including the fan, thermostat, pump, and overflow pipe.

Close-up view of a cutaway automobile engine showing metal pistons moving within their cylinders, with visible valves and springs above.

At a Glance

gasoline engine summary

gasoline engine

machine

Also known as: petrol engine

Written by

Orville C. CromerAll

Fact-checked by

Britannica Editors

Last updated

Oct. 28, 2026 •History

Top Questions

What is a gasoline engine?
A gasoline engine is an internal combustion engine that runs on a volatile liquid fuel such as gasoline or a gasoline mixture like ethanol, ignited by an electric spark. These engines are versatile, suitable for various applications, most notably in vehicles, aircraft, and power tools.

Gasoline engines come in two-stroke and four-stroke designs. In the four-stroke cycle, the piston completes four movements: intake, compression, power, and exhaust. The two-stroke cycle combines the compression and power strokes of the four-stroke cycle, needing only one crankshaft revolution to finish the cycle.
2 Britannica Sources
1. four-stroke cycle
2. gasoline engine
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does a gasoline engine work?
A gasoline engine is an internal combustion engine that transforms chemical energy from a volatile liquid fuel (typically gasoline or a gasoline mixture like ethanol) into mechanical energy, using an electric spark to initiate combustion.

Most gasoline engines operate on a four-stroke cycle:
1. Intake: The piston descends, drawing a mixture of gasoline vapor and air into the cylinder.
2. Compression: The piston ascends, compressing the fuel-air mixture.
3. Power: An electric spark ignites the compressed mixture, forcing the piston down and generating power.
4. Exhaust: The piston ascends again, pushing the spent combustion products out through the exhaust valve, and then the cycle repeats.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What fuel does a gasoline engine use?
Gasoline engines use a volatile liquid fuel, typically gasoline or a gasoline mixture like ethanol, to generate power. An electric spark initiates the burning of the fuel.

Gasoline itself is a complex mixture of hydrocarbons. It may also contain additives like detergents, anti-icing agents, and antioxidants.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What is the purpose of spark plugs in a gasoline engine?
In a gasoline engine, the purpose of a spark plug is to ignite the air-fuel mixture within the cylinder. It is a component of the ignition system that endures severe conditions, such as high temperatures and pressures within the combustion chamber.

The spark plug has two electrodes separated by an air gap. Current from a high-tension ignition system discharges across this gap, creating a spark. The spark plug consists of a steel shell, a ceramic insulating element, and electrodes made of various alloys. The spark gap between the center and ground electrodes must be properly adjusted for the engine to function correctly.
2 Britannica Sources
1. gasoline engine: Spark plugs
2. spark plug
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How is the energy from gasoline turned into movement in a car?
In a gasoline engine, energy from gasoline turns into movement through a four-stroke cycle.
1. Intake stroke The piston descends, and a mix of gasoline vapor and air is drawn into the cylinder.
2. Compression stroke The piston ascends and compresses the mixture.
3. Power stroke An electric spark ignites the compressed mixture, and the expanding gas pressure forces the piston down.
4. Exhaust stroke The piston ascends again, pushing the combustion products out through the exhaust valve, and the cycle repeats.
The force from the power stroke pushes the piston, which turns a crankshaft. This rotary motion is then transmitted to the wheels, propelling the car.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are some environmental concerns with using gasoline engines?
Gasoline engines pose several environmental concerns, including air and water pollution. Gasoline engines emit noxious gases, such as unburned hydrocarbons, carbon monoxide, and nitrogen oxides. Nitrogen oxides can cause inflammation and irritation of breathing passages and react in the atmosphere to form nitric acid, which contributes to acid rain. Furthermore, gasoline fumes evaporating from fuel tanks contribute to air pollution.

Oil spills, sometimes related to gasoline, can also leak petroleum into bodies of water, causing major environmental problems. The unintentional release of gasoline solvents and lubricants by industries and individuals also greatly aggravates environmental problems.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

gasoline engine, any of a class of internal-combustion engines that generate power by burning a volatile liquid fuel (gasoline or a gasoline mixture such as ethanol) with ignition initiated by an electric spark. Gasoline engines can be built to meet the requirements of practically any conceivable power-plant application, the most important being passenger automobiles, small trucks and buses, general aviation aircraft, outboard and small inboard marine units, moderate-sized stationary pumping, lighting plants, machine tools, and power tools. Four-stroke gasoline engines power the vast majority of automobiles, light trucks, medium-to-large motorcycles, and lawn mowers. Two-stroke gasoline engines are less common, but they are used for small outboard marine engines and in many handheld landscaping tools such as chain saws, hedge trimmers, and leaf blowers.

Engine types

Gasoline engines can be grouped into a number of types depending on several criteria, including their application, method of fuel management, ignition, piston-and-cylinder or rotor arrangement, strokes per cycle, cooling system, and valve type and location. In this section they are described within the context of two basic engine types: piston-and-cylinder engines and rotary engines. In a piston-and-cylinder engine the pressure produced by combustion of gasoline creates a force on the head of a piston that moves the length of the cylinder in a reciprocating, or back-and-forth, motion. This force drives the piston away from the head of the cylinder and performs work. The rotary engine, also called the Wankel engine, does not have conventional cylinders fitted with reciprocating pistons. Instead, the gas pressure acts on the surfaces of a rotor, causing the rotor to turn and thus perform work.

Piston-and-cylinder engines

Most gasoline engines are of the reciprocating piston-and-cylinder type. The essential components of the piston-and-cylinder engine are shown in the figure. Almost all engines of this type follow either the four-stroke cycle or the two-stroke cycle.

Four-stroke cycle

Of the different techniques for recovering the power from the combustion process, the most important so far has been the four-stroke cycle, a conception first developed in the late 19th century. The four-stroke cycle is illustrated in the figure. With the inlet valve open, the piston first descends on the intake stroke. An ignitable mixture of gasoline vapour and air is drawn into the cylinder by the partial vacuum thus created. The mixture is compressed as the piston ascends on the compression stroke with both valves closed. As the end of the stroke is approached, the charge is ignited by an electric spark. The power stroke follows, with both valves still closed and the gas pressure, due to the expansion of the burned gas, pressing on the piston head or crown. During the exhaust stroke the ascending piston forces the spent products of combustion through the open exhaust valve. The cycle then repeats itself. Each cycle thus requires four strokes of the piston—intake, compression, power, and exhaust—and two revolutions of the crankshaft.

A disadvantage of the four-stroke cycle is that only half as many power strokes are completed as in the two-stroke cycle (see below) and only half as much power can be expected from an engine of a given size at a given operating speed. The four-stroke cycle, however, provides more positive clearing out of exhaust gases (scavenging) and reloading of the cylinders, reducing the loss of fresh charge to the exhaust.

gasoline engine

Engine types

Piston-and-cylinder engines

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Four-stroke cycle