Four-stroke internal combustion engine – valves, ignition and the work diagram
Design & TechnologyMechanical EngineeringAges 16–17
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Mechanical Engineering
A cutaway view of one cylinder of a four-stroke engine: the piston, connecting rod, rotating crankshaft, and the intake and exhaust valves opening and closing in the right phase. Click Run to watch the four strokes — intake, compression, power (combustion and expansion), exhaust — play out over two crankshaft revolutions (720°), with the spark plug firing (petrol engine) or the injector spraying fuel (diesel engine) near the end of the compression stroke. The p–V work diagram is drawn in real time; change the rpm and load to see power and torque change.
Lesson: How the four-stroke internal combustion engine works: the intake–compression–power–exhaust cycle, valve timing, ignition and fuel injection
What it shows
A four-stroke engine completes one working cycle in two crankshaft revolutions (720°): intake, compression, power and exhaust. Only the power stroke delivers work; the other three are driven by the crankshaft and flywheel. A petrol engine compresses a fuel–air mixture about ten times and ignites it with a spark plug, while a diesel engine compresses air about eighteen times until it is hot enough for injected fuel to ignite by itself. The p–V diagram plots cylinder pressure against volume, and the area of the loop represents the work done per cycle.
How to use
Raise the Animation slow-down factor and press Run, or use Step 30° to turn the crank in steps while watching the piston, both valves and the crank angle. Tick Show p–V work diagram and follow the pressure through each stroke. Press Switch to Diesel to compare ignition and compression ratio with the petrol engine. Finally change the Crankshaft rotation speed and Load and note how power and torque respond.
Parameters you can change
- Engine type Petrol engine (spark ignition, ε = 10), Diesel engine (compression ignition, ε = 18)
- Crankshaft rotation speed 600–6000 rpm
- Load (throttle opening / fuel amount) 20–100 %
- Show p–V work diagram
- Animation slow-down factor (to observe each stroke) 20–400 ×
Questions to explore
- During which strokes are both valves closed, and how does the cylinder pressure change in each?
- Why does a diesel engine need no spark plug, and how does its higher compression ratio help?
- How many crankshaft revolutions does one full cycle take, and how many strokes in it produce work?
