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Thousands of small fires a minute move your car
A complete, step-by-step look inside the internal combustion engine, made for people who have never opened a hood
Burning fuel in a sealed space pushes metal
Combustion releases energy
Burning is a chemical reaction between fuel and the oxygen in air. It releases energy as heat, which makes the gases expand fast.
It happens inside
In an internal combustion engine the fuel burns within the engine itself, in a closed chamber, rather than in a separate boiler as in a steam engine.
Pressure pushes a piston
The engine is a fixed cylinder with a piston that can slide. Expanding hot gas shoves the piston down the cylinder.
Pushing becomes turning
The moving piston rotates a crankshaft. Gears in the drivetrain then pass that rotation on to the wheels.
Fact source: U.S. Department of Energy, Internal Combustion Engine Basics
Six parts do the essential work
Cylinder: the sealed tube where fuel burns Piston: the sliding plug that gets pushed Valves: doors that let air in and exhaust out Spark plug: makes the spark that lights the mix Connecting rod and crankshaft: turn pushing into spinning
Fact sources: U.S. Department of Energy; Pilot Institute, Four-Stroke Engines
Every cylinder repeats four strokes, endlessly
1. Intake
Piston moves down, drawing in air and fuel
2. Compression
Valves shut, piston rises and squeezes the mix
3. Power
Spark ignites mix, pressure drives piston down
4. Exhaust
Exhaust valve opens, piston pushes gases out
Fact sources: ScienceDirect, Four-Stroke Cycle overview; Energy Education, University of Calgary
Only one stroke in four makes power
Fact sources: ScienceDirect, Intake Stroke overview; Summit Racing Help Center; EBSCO, Nikolaus August Otto
The crankshaft turns pushes into rotation
Offset crank throws work like pedals on a bicycle Connecting rods link each piston to its throw Heavy flywheel stores momentum between power strokes Camshaft, belt-driven at half speed, opens each valve on time
Fact sources: U.S. Department of Energy; Summit Racing Help Center
Five support systems keep the cylinders firing
Fuel and air
Injectors meter fuel against incoming air. About 14.7 kg of air is needed to burn 1 kg of gasoline completely.
Ignition
The ignition system fires each spark plug near the top of the compression stroke, timed to the piston's position.
Cooling
Coolant flows around the cylinders and through the radiator, carrying away heat that would otherwise damage the metal.
Lubrication
An oil pump coats moving parts with a thin film of oil, so metal slides on oil rather than grinding on metal.
Exhaust treatment
A catalytic converter turns carbon monoxide, hydrocarbons and nitrogen oxides into less harmful gases. It works best at the 14.7:1 mixture.
Engine computer
The control unit reads oxygen sensors and adjusts fuel in real time to hold the mixture near the ideal.
Fact sources: ScienceDirect, Air-to-Fuel Ratio; USPTO patents 5953905 and 9140161; fueleconomy.gov
Diesels light fuel with heat, not sparks
Gasoline (spark ignition)
Fuel and air mixed before or during intake Spark plug ignites the compressed mixture Compression limited to prevent premature ignition Common in cars and light vehicles
Diesel (compression ignition)
Only air drawn in, then squeezed hard Compression heats air to around 550 °C Fuel sprayed in near top; ignites by itself No spark plug; higher thermal efficiency
Fact sources: U.S. DOE, Diesel Engine Vehicles; USPTO patent 8561581
Most of the fuel's energy becomes heat
12–30%
Share of fuel energy that actually moves a gasoline car, depending on driving
71–75%
Energy lost inside the engine in city driving, mostly as heat
5–7%
Parasitic losses in city driving: water, fuel and oil pumps, and other engine loads
6%
Lost to idling in stop-and-go city driving
Fact source: U.S. DOE and EPA, fueleconomy.gov, Where the Energy Goes: Gasoline Vehicles
The four-stroke idea is 150 years old
1861
Beau de Rochas describes the four-stroke principle in a pamphlet
1876
Nikolaus Otto builds his four-stroke engine at Deutz
1876–1886
More than 30,000 Otto engines sold
1886
Otto's patent revoked; many makers build Otto-cycle engines
Late 1880s
Automobiles using Otto's engine appear on European roads
Fact source: EBSCO Research Starters, Otto's Practical Internal Combustion Engine and Nikolaus August Otto
Where these facts come from
U.S. Department of Energy, Internal Combustion Engine Basics; Diesel Engine Vehicles U.S. DOE and EPA, fueleconomy.gov, Where the Energy Goes: Gasoline Vehicles EBSCO Research Starters, Otto's Practical Internal Combustion Engine ScienceDirect (Elsevier), Four-Stroke Cycle; Air-to-Fuel Ratio overviews USPTO, patents 8561581 and 9140161 (diesel combustion; air-fuel ratio)
Supplementary: Energy Education, University of Calgary; Summit Racing Help Center
Open a hood and find these parts yourself
Start with the U.S. DOE's Internal Combustion Engine Basics at energy.gov, then trace intake, ignition, cooling and exhaust on a real engine