Four-Stroke Diesel Cycle
The four-stroke diesel cycle is the sequence of intake, compression, power, and exhaust that a diesel engine's piston repeats to convert fuel into rotating torque. Unlike a petrol engine, it ignites fuel using heat from compression alone, with no spark plug.
01Definition
The four-stroke diesel cycle is the sequence of intake, compression, power, and exhaust that a diesel engine's piston repeats to convert fuel into rotating torque. Unlike a petrol engine, it ignites fuel using heat from compression alone, with no spark plug.
02Physical Principle
On the intake stroke the piston draws in only air. On compression, that air is squeezed hard enough — often 14 to 22 times smaller — that its temperature alone exceeds diesel fuel's ignition point. Fuel is injected right at that moment and ignites immediately on contact with the superheated air, driving the piston down on the power stroke; the exhaust stroke then clears the cylinder for the next cycle. Every crankshaft revolution moves the piston through two strokes, so a four-stroke engine fires each cylinder once every two revolutions.
03Mathematical Foundation
04Interactive Diagram — Four-Stroke Cycle Animation
Press play to step through one full four-stroke cycle — intake, compression, power, exhaust — two crankshaft revolutions per cycle.
Piston motion is simplified for clarity, not exact crank geometry.
05Interesting Facts
- Because diesel ignition needs no spark, diesel engines can run at far higher compression than petrol engines — one reason they extract more work from the same fuel energy.
- Turbocharging (see Chapter link below) exists largely to fix diesel's one weakness at altitude and low RPM: not enough air packed into the cylinder to burn the fuel completely.
- The largest mining haul trucks run two of these engines' industrial cousins side by side, purely because no single diesel yet built is compact enough to deliver the combined output alone.