Diesel Engine Basics — Terms, Strokes & Timing

Stroke, swept volume, compression ratio — then how four strokes collapse into two.

7 min read

Beginner

Marine Propulsion & Diesel Engines

Key Principles at a Glance 5 points

Stroke is crank-length in metal: TDC-to-BDC travel fixed by crank throw; swept volume is piston area times that stroke.
Compression ratio is total volume over clearance volume — the single number governing diesel ignition.
Four strokes separate every event: suction, compression, power, exhaust across two crank revolutions with valves in sequence.
Two strokes merge events into one revolution — ports and a blower replace two valve strokes, and the camshaft runs at crank speed.
A timing diagram is the cycle drawn as crank angles: read every opening, closing, injection and blow-down off TDC and BDC.

1. The Geometry — Stroke, Volumes, Ratio

Every diesel dimension starts at the crank. Three volumes build on one another:

Stroke

TDC-to-BDC travel, set by crank length.

Swept volume

Piston area times stroke.

Clearance volume

What remains above the piston at TDC.

Compression ratio — total over clearance — decides whether compression heat alone can fire the fuel.

Piston stroke as TDC to BDC travel set by crank length — **Figure 1:** Stroke — crank throw written in piston travel.

Swept volume as piston area times stroke within total cylinder volume — **Figure 2:** Swept volume — area times stroke, the engine's breathing capacity.

Compression ratio formula as swept plus clearance volume over clearance volume — **Figure 3:** The one formula to own — (swept + clearance) / clearance.

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).

Supercharging principle with turbocharger forcing extra air into the cylinder — **Figure 4:** Forced breathing — exhaust energy drives the extra air that doubles two-stroke output.

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.

Two-stroke timing diagram with compression, injection, power, blow-down and scavenging angles — **Figure 5:** The cycle as angles — compression, injection, power, blow-down, scavenging in order.