doodleengine.

HOW IT WORKS

Four strokes,
one spark.

A petrol engine repeats one simple loop, over and over.

Draw in fuel and air, squeeze it, ignite it, push the spent gas out. Nikolaus Otto built the first four-stroke engine in 1876, and the idea of compressing before igniting is still in every combustion engine today.

Step through it Read the short answer
The open bonnet of a car in a quiet home garage, showing a clean engine bay lit by soft daylight from the doorway
AI-generated image, not a photograph of a real person or place.
01 Intake, compression02 Power, exhaust03 Then do it againThen the loop repeats

THE SHORT ANSWER

Squeeze it,
then light it.

Fuel holds energy as chemistry.
The engine turns a small explosion into a turning shaft.

THE CYCLE

Four movements of a piston

NASA describes the cycle as intake, compression, power and exhaust. In the power stroke, hot burning gas pushes the piston and turns the crankshaft.

Step through it

THE SPARK

Petrol sparks, diesel squeezes

A petrol engine uses a spark plug to light the fuel and air. A diesel uses compression ignition instead, with no spark.

What else is in there

EFFICIENCY

Most of the fuel is lost

The US Department of Energy says petrol cars turn about 12 to 30 percent of the fuel's energy into power at the wheels. Electric cars do far better.

See the numbers

STEP THROUGH THE CYCLE

What is the piston
doing now?

Pick a stroke to see what happens inside one cylinder. The bar shows how high the pressure is in the cylinder at that point.

Based on NASA Glenn Research Center's explanation of the four-stroke internal combustion engine.

Cycle check

Choose a stroke
1. IntakeBreathing in

The piston moves down and the intake valve opens. Fuel and air enter the cylinder at near-atmospheric pressure.

2. CompressionSqueezing

Both valves close and the piston moves back up. The volume shrinks and the pressure rises.

3. PowerThe push

A spark ignites the mixture. Hot, high-pressure gas drives the piston down, and the rod turns the crankshaft.

4. ExhaustBreathing out

With pressure back near atmospheric, the exhaust valve opens and the rising piston pushes the spent gas out.

Source: NASA Glenn Research Center, Four Stroke Internal Combustion Engine.

TWO NUMBERS WORTH KNOWING

Where the fuel
actually goes.

How much of the energy you pay for
ends up turning the wheels.

PETROL CARS · US DOE

Fuel energy that reaches the wheels

12–30<small>%</small>

FuelEconomy.gov says conventional petrol vehicles convert only about 12 to 30 percent of the energy in petrol to power at the wheels.

ELECTRIC CARS · US DOE

Grid energy that reaches the wheels

77<small>%+</small>

The same source says electric vehicles convert over 77 percent of the electrical energy from the grid to power at the wheels.

The US Alternative Fuels Data Center puts the average at about 20 percent for petrol cars, with the rest lost largely to engine inefficiencies, parasitic loads such as water pumps and alternators, and drivetrain friction.

An electric car parked on a driveway with a charging cable plugged into a wall-mounted home charger
AI-generated image, not a photograph of a real person or place.

FROM TANK TO TYRE

What happens
when you press go.

The four strokes are the heart of it, but a petrol car needs a team of parts around the engine to get power to the road.

  1. Fuel is deliveredA pump moves petrol from the tank along the fuel line to the fuel injection system, which feeds it in to mix with air.
  2. The computer sets the timingAn electronic control module manages the fuel mixture, ignition timing and the emissions system.
  3. The cylinders fireSpark plugs ignite the mixture and the pistons turn the crankshaft, which turns twice for every turn of the cams.
  4. Power goes out, gas goes outThe transmission carries power to the wheels while the exhaust, with its three-way catalyst, takes gases to the tailpipe.

THE ELECTRIC ALTERNATIVE

No strokes,
no tailpipe.

A battery electric car has no combustion engine at all. A traction battery feeds an electric motor that drives the wheels, and a power electronics controller sets the motor's speed and torque.

With no fuel burned, there is no tailpipe exhaust and no fuel tank, pump or line. The US Department of Energy says these cars turn over 77 percent of the electricity from the grid into power at the wheels.

A sectioned display engine in a museum, cut open to show pistons, valves and the crankshaft inside the cylinder block
AI-generated image, not a photograph of a real person or place.

QUESTIONS PEOPLE ASK

Frequently asked

Who invented the four-stroke engine?

The Science Museum Group credits German engineer Nikolaus August Otto with building the first four-stroke engine in 1876. A working replica built in Manchester in 1982 is held at the Science and Industry Museum.

Why is it called four-stroke?

NASA explains that the piston makes four movements, or strokes, before the firing sequence repeats: intake, compression, power and exhaust.

What is the difference between petrol and diesel engines?

Petrol engines are spark-ignited: a spark plug lights the fuel and air. Diesel engines use compression ignition instead.

Why does compression matter?

The Science Museum Group notes that compressing fuel and air before igniting it was the key to Otto's engine, and that the principle is still used in all present-day internal combustion engines.

Are electric cars more efficient?

At the wheels, yes. FuelEconomy.gov says electric cars convert over 77 percent of grid energy to power at the wheels, against about 12 to 30 percent of petrol's energy for conventional cars.

SOURCES

Where the facts come from

Each figure on this page comes from the sources below. Check them for the full detail and context.

  1. NASA Glenn Research Center: Four Stroke Internal Combustion Engine
  2. Science Museum Group: Replica Otto four-stroke engine
  3. Alternative Fuels Data Center: How do gasoline cars work?
  4. FuelEconomy.gov: All-electric vehicles
  5. Alternative Fuels Data Center: How do all-electric cars work?
  6. Alternative Fuels Data Center: Energy losses in light-duty gasoline vehicles