How Does a Car Engine Work?

What happens inside an internal combustion engine?

In an internal combustion engine, the kind that runs on gasoline (petrol) or diesel, the fuel is ignited and burns inside the engine itself.^[1] Burning, or combustion, releases energy from a mixture of fuel and air.^[1] At its heart is a fixed cylinder with a moving piston inside: the expanding gases from combustion push the piston, and the piston turns a shaft called the crankshaft.^[1][2]

The burning is not continuous: it happens very quickly at regular intervals, and in between, the engine's parts repeat a sequence of movements called a cycle.^[2] Early car engines used the Otto cycle, developed by N. A. Otto of Germany in 1876.^[2]

An explainer from the U.S. Department of Energy, dated November 22, 2013, describes two kinds of internal combustion engine then in production: the spark-ignition gasoline engine and the compression-ignition diesel engine.^[1] Most of them are four-stroke engines.^[1]

What are the four strokes of a car engine?

A stroke is one movement of the piston along its cylinder, and a four-stroke engine needs four of them to complete one cycle.^[1][2] Valves let gases in and out: an intake valve admits the fuel-air mixture, and an exhaust valve releases the burned gases.^[3][8]

Cams, metal disks turning on a shaft, make sure the valves open and close at the right moments in the cycle.^[4] NASA uses the Wright brothers' 1903 aircraft engine as a teaching example: there, springs hold the valves shut, and a cam-driven rocker arm pushes the exhaust valve open only during the exhaust stroke.^[3][4]

Because burning takes a finite amount of time, the spark usually does not fire exactly when the piston reaches the top of its travel.^[4] NASA calls this adjustment ignition advance and notes that it is used even on modern car engines.^[4]

How does the piston's motion turn the wheels?

Each piston is connected by a rod to the crankshaft; in NASA's example engine, the crankshaft is a single long piece of metal cut into a "snake" shape.^[5] This power train converts the straight-line motion of the pistons into the rotation of the crankshaft.^[5] From the crankshaft, a system of gears in the powertrain carries the motion to the car's wheels.^[1]

The four strokes alternate down, up, down, up, so the piston travels down and back up twice in every cycle.^[6] NASA explains that a four-stroke engine therefore needs a valve camshaft that turns once for every two turns of the crankshaft.^[4]

Only one stroke in four delivers power.^[6] In NASA's example, a large, heavy flywheel on the crankshaft stores energy from the individual firings so that the engine runs more smoothly.^[3][5]

How do gasoline and diesel engines ignite their fuel?

Gasoline and diesel engines differ in how they supply fuel and how they ignite it.^[1]

In a gasoline engine, fuel and air are mixed, the piston compresses the mixture, and then a spark ignites it.^[1] The spark comes from a spark plug at the end of the compression stroke.^[6] In a conventional gasoline engine, the fuel is injected into the intake port and mixes with air on its way into the cylinder; in a direct-injection engine, it is injected straight into the cylinder.^[8]

A diesel engine takes in only air and compresses it.^[1] Fuel is then sprayed into the hot, compressed air at a measured rate, which makes it ignite.^[1] According to fueleconomy.gov, diesel engines have inherently lower losses and are generally one-third more efficient than comparable gasoline engines.^[7]

Where does the fuel's energy go?

An engine turns only part of the energy released by combustion into work.^[1] Fueleconomy.gov, run for the U.S. Department of Energy and the Environmental Protection Agency, says that only about 12% to 30% of the energy in the fuel put into a conventional vehicle is used to move it down the road, depending on the drive cycle.^[7] The remainder is either lost through inefficiencies in the engine and driveline or used to run accessories.^[7]

In gasoline vehicles, the engine itself is where most of the fuel's energy is lost, chiefly as heat.^[7] Lesser losses come from friction inside the engine, from the work of pumping air in and out, and from combustion that is not perfectly efficient.^[7]

When the exhaust valve opens after the power stroke, heat left in the gas passes to the surroundings.^[2] Combustion and piston friction also heat the engine itself; in NASA's example, hoses carry cooling water to a radiator.^[3]

More energy is lost after the engine, in the transmission and the rest of the driveline, and braking turns the vehicle's motion into heat at the brakes.^[7]

How do multiple cylinders work together?

NASA's example engine has four pistons in four cylinders turning one crankshaft, and NASA says this in-line arrangement is identical to that used in modern four-cylinder car engines.^[3] The pistons fire one at a time in a specified order, so the cylinders take turns delivering their power strokes.^[5]

Car engines are also built with six or eight cylinders.^[8] Some use cylinder deactivation, which shuts off the valves to some cylinders when they are not needed, so no fuel and air are pumped into them.^[8] An eight- or six-cylinder engine can then run for a while as a four- or three-cylinder engine, which saves fuel.^[8]