Mechanical · engine cycle

2 & 4-Stroke Engine Simulator

Piston, valves or ports, and spark firing together in phase — one animated cylinder, one synced timing diagram.
crank-slider kinematics
4-stroke · 2400 rpm

Engine

2400
3.5
20°

Presets

Cylinder cross-section

Intake θ = 0°
sim speed
Real-world timing at this RPM
Field notes

Reading the cycle

Using this tool

Every stroke is the same slider-crank, timed differently

The piston's motion here isn't a canned animation — it's computed live from the actual slider-crank equation that governs every reciprocating engine, using the rod-to-crank ratio you set. What changes between 2-stroke and 4-stroke isn't the mechanism, only when the valves or ports open relative to that motion: a 4-stroke uses a camshaft turning at half crank speed to open valves once per two revolutions, while a 2-stroke lets the piston itself uncover ports cut into the cylinder wall, timed purely by height.

Worked example

At 2,400 rpm, a 4-stroke engine completes one full 720° cycle in 50 ms — but the power stroke that actually drives the crank only lasts a quarter of that, around 12.5 ms, which is why multi-cylinder engines exist: to overlap those short power pulses into smoother output.

Why does a 4-stroke engine need two crankshaft revolutions per cycle?

Four distinct strokes — intake, compression, power, exhaust — each need one piston pass (one half-revolution) to complete, and four halves make two full revolutions, which is also why a 4-stroke camshaft (which opens the valves once per cycle, not once per revolution) always turns at exactly half crankshaft speed.

How does a 2-stroke engine fit intake, compression, power and exhaust into one revolution?

It overlaps them: the exhaust and transfer ports open together near the bottom of the power stroke so fresh mixture entering low in the cylinder helps push spent gas out (scavenging) at the same time, while compression happens on the way back up and the next charge is simultaneously drawn into the crankcase beneath the piston.

Why does the spark fire before the piston reaches top dead centre?

Combustion takes a small but real amount of time to spread through the cylinder, not an instant. Firing a few degrees early (spark advance) times peak cylinder pressure to land just after TDC, when the crank angle is most effective at converting that pressure into rotating torque, instead of wasting it while the piston is still travelling upward against it.

Is the port timing shown here exact for every 2-stroke engine?

No — this models a typical piston-ported, crankcase-scavenged design (the classic chainsaw or small outboard layout) where port height alone, not a camshaft, decides timing. Reed-valve and rotary-valve 2-strokes intake differently, and exact port heights vary by engine, but the sequence and phase relationships shown are representative.