Diesel Engine Basics — Terms, Strokes & Timing

1. The Geometry — Stroke, Volumes, Ratio

Every diesel dimension starts at the crank. Three volumes build on one another, and the compression ratio follows from them:

• Clearance volume — what remains above the piston at TDC.
• Swept volume — piston area times stroke.
• Stroke — TDC-to-BDC travel, set by crank length.

Clearance volume plus swept volume gives the total volume, and the compression ratio — total over clearance — decides whether compression heat alone can fire the fuel.

2. Four Strokes — One Event at a Time

Two crank revolutions, four dedicated strokes, valves in strict sequence: the piston draws charge down (suction), squeezes it up (compression), takes the firing push down (power), and sweeps the burnt gas out up (exhaust) — drawing in, compressing, firing and expelling, each with its own stroke and its own valve timing.

3. Two Strokes — Everything Doubled Up

One revolution must do all four jobs, so events overlap: fuel fires every revolution, ports uncovered by the piston replace valve strokes for gas exchange, and a blower forces fresh air in while driving exhaust out — without it the cylinder would breathe its own burnt gas. Because every revolution fires, the camshaft runs at full crank speed (not half, as on four-strokes).

4. Reading the Timing Wheel

The timing diagram wraps the whole cycle onto crank degrees around TDC and BDC: compression climbing toward the top, fuel injection straddling TDC, power expanding down, exhaust blow-down, then the scavenge band sweeping the cylinder clean near the bottom before compression restarts. Learn to point at any event and name its crank angle — every timing question in the orals starts exactly there.