Four-stroke engine

Today internal combustion engines in cars, trucks, motorcycles, aircraft, construction machinery and many others, most commonly use a four-stroke cycle. The four strokes refer to intake, compression, combustion (power) and exhaust strokes that occur during two crankshaft rotations per working cycle of the Gasoline engine and Diesel engine.

A four-stroke engine is characterized by four strokes, or reciprocating movements of a piston in a cylinder:

  1. intake (induction) stroke
  2. compression stroke
  3. power stroke
  4. exhaust stroke

In this example animation, the right blue side is the intake and the left yellow side is the exhaust. The cylinder wall is a thin sleeve surrounded by cooling water.

The cycle begins at top dead center (TDC), when the piston is furthest away from the axis of the crankshaft. On the intake or induction stroke of the piston, the piston descends from the top of the cylinder, reducing the pressure inside the cylinder. A mixture of fuel and air is forced (by atmospheric or greater pressure) into the cylinder through the intake (inlet) port. The intake (inlet) valve (or valves) then close(s), and the compression stroke compresses the fuel–air mixture.

The air–fuel mixture is then ignited near the end of the compression stroke, usually by a spark plug (for a gasoline or Otto cycle engine) or by the heat and pressure of compression (for a Diesel cycle or compression ignition engine). The resulting pressure of burning gases pushes the piston through the power stroke. In the exhaust stroke, the piston pushes the products of combustion from the cylinder through an exhaust valve or valves.

The Otto cycle

The four-stroke engine was first patented by Eugenio Barsanti and Felice Matteucci in 1854, followed by a first prototype in 1860. It was also conceptualized by French engineer, Alphonse Beau de Rochas in 1862.

However, the German engineer Nicolaus Otto, in collaboration with Gottlieb Daimler and Wilhelm Maybach, was the first to develop a functioning four-stroke engine, in 1876, which is why the four-stroke principle today is commonly known as the Otto cycle and four-stroke engines using spark plugs often are called Otto engines. The Otto Cycle consists of adiabatic compression, heat addition at constant volume, adiabatic expansion and rejection of heat at constant volume.

The four strokes of the cycle are intake, compression, power, and exhaust. Each corresponds to one full stroke of the piston, therefore the complete cycle requires two revolutions of the crankshaft to complete.

Intake. During the intake stroke, the piston moves downward, drawing a fresh charge of vaporized fuel/air mixture. The illustrated engine features a 'poppet' intake valve which is drawn open by the vacuum produced by the intake stroke. Some early engines worked this way, however most modern engines incorporate an extra cam/lifter arrangement as seen on the exhaust valve. The exhaust valve is held shut by a spring .

Compression. As the piston rises the poppet valve is forced shut by the increased cylinder pressure. Flywheel momentum drives the piston upward, compressing the fuel/air mixture.

Power. At the top of the compression stroke the spark plug fires, igniting the compressed fuel. As the fuel burns it expands, driving the piston downward.

Exhaust. At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism. The upward stroke of the piston drives the exhausted fuel out of the cylinder.