Technology and Engineering · High school

Inside the four strokes

Turn the crankshaft and follow the piston and valves through intake, compression, power and exhaust.

In the learning section below the figure, predict, try, complete the tasks and check yourself with short questions.

Fig. 1Inside the four strokes

Drag the crankshaft, press Play or take the engine apart

372° · Power (combustion)

Parts and inner mechanism

Apart0%

With the engine apart, hover over a part, tap it or pick it from the list.

Pick a part to see what it does and what it is made of in a real engine.

Playback speed

Crank angle (0–720°)

Power (combustion)

The plug has fired: the pressure of the burning gas pushes the piston down. This is the only one of the four strokes that produces work.

Crank angle: 372°
Pressure: 62.1 bar
Cylinder volume: 50 cm³
Valves: Intake: closed
Exhaust: closed

Pressure–volume (P–V) diagram

In this engine the swept volume is 402 cm³ and the combustion chamber 44.7 cm³. Peak pressure is 73 bar and peak temperature 2,200 K; the ideal efficiency is 60%. The ideal work per cycle is 324 J, and the ideal (lossless) power at the speed you entered is 8.1 kW.

A real engine at 3,000 rpm completes these four strokes in 40 milliseconds; the figure is slowed down.

Enter your own engine

Decimal commas or points are fine. The drawing stays the same; the readings, the gauge and the diagram are calculated for the engine you enter.

Engine speed

Compression ratio

Bore

Stroke

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

In this engine, with a compression ratio of 10, the gas volume shrinks 10 times as the piston rises. How many times does the pressure go up?

Purple: fresh air–fuel mixtureGrey: burned gasYellow to red: burning and cooling gasThe numbers come from one model. The connecting rod is taken as 1.75 times the stroke. Pressure follows a simple ideal Otto cycle: intake and exhaust at 1 bar, compression and expansion with no heat exchange (γ = 1.4); combustion happens at TDC, instantly and at constant volume, and raises the gas temperature by 1,446 K. The drawing always shows the default engine; the readings, the gauge and the diagram are calculated for the engine you enter. In a real engine combustion takes a few milliseconds, the valves open before the dead centres, and heat loss and friction take their share; the efficiency is about half of the ideal value.

How does a four-stroke engine work? Intake, compression, power and exhaustRead the article