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How Does a Car Engine Work?

Image courtesy of Peter Bonnett on Flickr, licensed under CC BY-SA 2.0

Learning about the basics of an internal combustion engine can help you make informed decisions about automobile purchases and repairs. This article will walk you through the basics of these complex machines; by the end, you should understand how a regular gasoline engine transforms the energy released during combustion into motion.

The Combustion System

At its most basic level, combustion is the process of igniting an energy-dense fuel in a controlled environment. Today’s automotive engines most often have a four-stage process of harvesting energy from fuel, which is why they are known as four-stroke engines. The four stages of the process are known as intake, compression, combustion and exhaust.

The Four-Stroke Process Explained:

"4StrokeEngine Ortho 3D Small" by Zephyris - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.
“4StrokeEngine Ortho 3D Small” by Zephyris – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

In the intake stroke (1), air and fuel are injected into one of the engine’s cylinders as a movable piston moves down. During the compression stroke (2), the piston moves towards the top of the cylinder, compressing the fuel mixture. Next, a spark plug fires, igniting the fuel during the combustion stroke (3). Once the piston reaches the bottom of its movement, the exhaust valve opens and the piston moves up again, forcing the waste gases out of the cylinder during the exhaust stroke (4).

The basic combustion cycle may seem simple, but making all the components work properly is a substantial engineering challenge. Let’s talk about these components, and see what their role in the process is.


Image courtesy of Nathanael Burton on Flickr, licensed under CC BY-SA 2.0
Image courtesy of Nathanael Burton on Flickr, licensed under CC BY-SA 2.0

Cylinders are the heart of your engine; they are the site of combustion and the source of power in your engine. Each cylinder is a sturdy chamber, the base of which is a movable piston. As combustion occurs, it causes the pistons to move up and down, thereby supplying power to the vehicle.


Some engines have more cylinders than others; each cylinder provides additional combustion opportunities, allowing engines with more cylinders to deliver more power to the vehicle. Engines contain either four, six or eight cylinders, arranged in one of three ways: inline, V or flat.

  • In an inline engine, the cylinders are arranged in a row above the crankshaft.
  • In engines with a V configuration, the cylinders are arranged in two offset banks, at an angle to the crankshaft, resembling a V.
  • Flat engines also split the cylinders, but arrange the cylinders in a flat configuration on either side of the crankshaft.
  • Most 4-cylinder engines use an inline configuration; six- and eight-cylinder engines typically use a V configuration, which is the origin of the terms V-6 and V-8.

Spark Plugs

Image courtesy of Aidan on Flickr, licensed under CC BY 2.0
The spark plug ignites the fuel. Image courtesy of Aidan on Flickr, licensed under CC BY 2.0

Each cylinder houses a spark plug at the top of the chamber. During the combustion stroke, the spark plug generates a spark that ignites the fuel. This spark must be precisely timed for maximum efficiency; it must occur just before the piston reaches the top of the cylinder, otherwise the engine might suffer damage.

Valves and the Valve Train

The valve train regulates the timing of engine valves. Photo by  Wapcaplet at the English language Wikipedia / CC BY-SA 3.0
The valve train regulates the timing of engine valves. Photo by Wapcaplet at the English language Wikipedia / CC BY-SA 3.0

Each cylinder has an intake valve and an exhaust valve. The intake valve allows air and fuel to enter the cylinder, and the exhaust valve opens to vent the waste gases from each combustion stroke. These valves are precisely timed to only open during the correct stroke of the combustion cycle. They remain sealed during compression and combustion.

The valve train, also known as the camshaft, regulates the timing of the engine valves. The camshaft runs above each valve; a small lobe, also called a cam, is attached to the camshaft above each valve. As the camshaft rotates, each cam’s motion opens and closes a corresponding valve at the appropriate time.

Modern engines position the camshaft directly above the valves; this is referred to as an overhead camshaft. The camshaft is linked to the crankshaft via a series of gears; the gears are engineered to rotate the camshaft at exactly half the rate of the crankshaft.


  • Since regular engines have two valves per cylinder, most vehicles have a single overhead cam (SOHC).
  • High-performance engines, however, have dual overhead cams (DOHC); each cylinder has four valves, which requires the addition of a second camshaft.

Pistons and Piston Rings

Pistons are cylindrical metal pieces attached to the crankshaft of the engine; as the pistons move up and down, they cause the crankshaft to rotate. Pistons make use of the energy released during combustion.

Piston rings provide a sliding seal for the cylinder; they keep the contents of each cylinder contained, and they prevent engine oil from leaking into the cylinder. If the rings don’t seal properly, oil may occasionally leak into the cylinder, causing the engine to burn oil.

Connecting Rod

Each piston is connected to the crankshaft via a connecting rod. This high-strength part can rotate on both ends, allowing both the piston and the crankshaft to move without interference.


The crankshaft translates the up and down motion of the pistons into circular movement. The crankshaft is connected to the car’s transmission system; the transmission ultimately turns the drive wheels of the car.


The sump surrounds the crankshaft and serves as a collection point for engine oil.

Fuel Delivery

Each cylinder needs a steady diet of air and fuel to maintain the engine. There are three main methods of fuel delivery used in internal combustion engines:

  • Carburetor – Carburetors are one of the oldest forms of fuel delivery, but nowadays they’re only found in older cars. Carburetors mix air and fuel in a large chamber; this mixture is then pushed into the fuel delivery system before the valves draw the mixture into the cylinders. Carburetors are relatively simple and don’t require precision timing, but they can’t deliver the exact air and fuel mixtures demanded by modern engines, and they’re also not as efficient as modern fuel injection systems.
  • Port Fuel Injection – In port fuel injection, fuel and air are mixed in a small port above the engine valve; when the valve opens, the mixture is drawn into the cylinder. Port injection is more efficient than using a carburetor, but it does require some amount of precision to deliver the correct amount of fuel at the correct time.
  • Direct Fuel Injection – In direct fuel injection, fuel is injected directly into the cylinder; the engine valves regulate the flow of air into the cylinder. Direct fuel injection delivers more power and better fuel efficiency than port fuel injection. However, the timing must be exceptionally precise, as well as the amount of fuel delivered. The parts needed for direct fuel injection must withstand high temperatures and deliver the fuel at high pressure, so direct fuel injection systems are more expensive than other systems.

As technology has evolved, direct fuel injection has become increasingly common.

Electrical Systems

Image courtesy of PublicDomainPictures on Pixabay.com, hosted under CC0.
Image courtesy of PublicDomainPictures on Pixabay.com, hosted under CC0.

Modern engines include several electrical systems, including the ignition system, the battery, the alternator and the starter. Each of these systems plays a key role in the operation of the engine.

Ignition System

The ignition system controls the precise operation of the spark plugs. It consists of a distributor, the spark plugs and several wires. The distributor has a single wire leading into the unit and a number of wires leading away from it; the number of wires matches the number of cylinders. A charge flows into the distributor through the central wire. Inside the distributor, a rotor spins. As it spins, it periodically makes contact with a distributor cap; when it does, it completes a circuit with one of the spark plugs, sending a charge to the spark plug, which ignites the contents of a cylinder. If you ever need to locate your distributor, you can identify it by the wires connected to it.

Battery and Alternator

Image courtesy of Mike Mozart on Flickr, licensed under CC BY 2.0
The battery provides electricity before the engine starts. Image courtesy of Mike Mozart on Flickr, licensed under CC BY 2.0

The battery and the alternator provide the electricity to the rest of the engine. Before the car has started, the battery provides electricity. Once the vehicle is in motion, the car uses some of its energy to spin the alternator; the alternator generates electricity to power some of the engine subsystems and charge the battery.

The Starting System

Once the vehicle has started, it uses the energy released combustion to maintain the motion. However, before the engine has started, it must rely on the starter motor. The starter motor spins the crankshaft when you turn the ignition key, which in turn begins operating all of the systems that rely on the crankshaft. In addition to the starter motor, all vehicles have a starter solenoid, which switches a large electric current to the starter motor after receiving a small control current from the ignition switch.

Engine Lubrication

Image courtesy of Sean MacEntee on Flickr, licensed under CC BY 2.0
Image courtesy of Sean MacEntee on Flickr, licensed under CC BY 2.0

Engines have many moving parts, and each of these parts must be able to move freely. Many of the parts aren’t naturally slippery, so oil is used to lubricate the engine, especially the pistons and the bearings around the various shafts. Engine oil collects in the sump; once in the sump, a pump draws the oil through a filter and then sprays the oil wherever it is needed. The oil drips back down into the sump, and the process starts again.

Air Intake Systems

Image courtesy of fortfan on Flickr, licensed under CC BY-ND 2.0

Combustion requires air, so all engines include various systems to deliver air to the cylinders. Most engines are naturally aspirated, which means that they simply draw air from around the vehicle and allow it to flow into the cylinder. High-performance engines may include turbochargers or superchargers, which pressurize the air and allow engines to pack more air and fuel into each cylinder. This allows each cylinder to deliver more power.

Cooling the Engine

Exploding fuel within your engine can create quite a bit of heat. Left unchecked, this heat can easily damage your engine. Typically, engines are outfitted with an elaborate network of water pipes; water flows next to the heat-generating components of your engine and absorbs the heat. The water then flows to the radiator, which exposes the water to the cooler outside air. This cools the water, and the water is then recirculated through the engine to continue cooling it.

The Exhaust System

Image courtesy of ClearFrost on Flickr, licensed under CC BY-SA 2.0
Image courtesy of ClearFrost on Flickr, licensed under CC BY-SA 2.0

The exhaust system controls the waste gases from the engine and consists of three main parts: the exhaust pipe, the muffler and the emissions control system. The exhaust pipe is simply a channel through which the exhaust gases can exit the vehicle. The muffler suppresses the sound of the engine; without it, you would hear the sounds of thousands of tiny explosions every time you drive your car.

The emissions control system consists of several parts. The main part is a catalytic converter, which helps to reduce the pollution created by combustion. The system is also home to a number of sensors to monitor the composition of the gases leaving the engine. Based on feedback from these sensors, the engine adjusts the amount of fuel or air injected into the engine.

Differences between Gasoline and Diesel Engines

Image courtesy of Clay Junell on Flickr, licensed under CC BY-SA 2.0
Image courtesy of Clay Junell on Flickr, licensed under CC BY-SA 2.0

This article has mostly focused on gasoline engines, but diesel engines work similarly. The key difference between the two is the fuel used and the method of ignition. Diesel engines don’t have spark plugs; instead, they ignite the fuel by compression. As the fuel is compressed, its temperature increases, eventually leading to spontaneous combustion.

In Conclusion

The history of the internal combustion engine is full of innovation. The drive for better performance and increased efficiency has compelled automakers to constantly evolve their designs, leading each engine subsystem to consistently improve over time.

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