Indicator Diagrams & Engine Performance

How a pencil, a drum and a spring turn cylinder pressure into power, balance and fault-finding.

9 min read

Intermediate

Marine Propulsion & Diesel Engines

Key Principles at a Glance 5 points

Ten parameters move together with load — MIP, compression, max pressure, pump index, exhaust and scavenge readings plus turbo speed tell one story.
Indicated power is diagram area turned to pressure (Pm = a/l × k) times swept volume times firing rate — PmLAN with the right n.
Cards are taken by ritual: soot blown, paper stretched, atmospheric line first, short hot exposure, fuel-cut compression line, clean-and-oil after.
Four cards, four lenses: power for work and MIP, compression for sealing health, draw for combustion timing, light-spring for gas exchange.
Balance by pump index and cams, spares ready for instant valve swaps — but main-engine timing stays untouched at sea.

1. The Load Picture & Power Definitions

Engine load is read against a fixed baseline — clean hull, normal trim, matched propeller. Hold those three steady and ten parameters move together with load.

Baseline rule

Draught, trim and hull condition sit outside the engine but decide what "normal" looks like. Power pushed to the propeller becomes thrust against hull resistance — so compare readings only at like-for-like hull and propeller conditions.

Group	Parameters that track load
Cylinder	Mean indicated pressure · Compression pressure · Maximum combustion pressure
Fuel setting	Fuel-pump index (load-indicator position)
Gas path	Exhaust gas pressure · Exhaust gas temperature · Scavenge air pressure · Scavenge air temperature
Turbocharger	Turbocharger speed · Uptake pressure after the turbocharger

10Load-linked parameters

2Conditions logged: loaded + ballast

+1Set after every major overhaul

Logged every voyage in both conditions, plus a fresh set after each major overhaul, this record tracks completeness of combustion and cylinder health over time.

Indicated power (IP)

Power born inside the cylinder, read off the indicator diagram. Fuel burns, gas pushes the piston — this is the gross work before any losses.

Brake power (BP)

Power available at the flywheel for real work. Friction and attached losses take their cut, so BP is always below IP. Measured on a hydraulic dynamometer or torsionmeter.

Mechanical efficiency

How much cylinder power survives friction. BP / IP × 100% — the tighter the engine, the closer to 100.

Thermal efficiency

How much fuel heat becomes work. Output (IP or BP basis) over fuel mass flow × calorific value — quoted as indicated or brake thermal efficiency.

SFOC

Specific fuel oil consumption — fuel burned per unit power. The price tag that links every parameter above to the bunker bill.

2. PmLAN — Area Into Power

Planimeter (or mid-ordinate) the card area a, divide by length l, multiply by spring scale k — that mean height is Pm in bar. Power per unit is Pm × piston area × stroke × firing rate, with n = rev/s on two-strokes and rev/s ÷ 2 on four-strokes (single-acting). Bar·m·m²/s resolves through 10⁵ N/m² into watts; kN/m² lands directly in kilowatts.

Indicator diagram area converted to mean pressure rectangle of equal area — **Figure 1:** Area becomes height, height becomes pressure — Pm is the rectangle hiding inside the loop.

Power card with peak pressure marked above the work loop — **Figure 2:** Peak pressure crowns the loop — height for firing violence, area for work done.

3. Taking Cards Without Lying to Yourself

Spring matched to expected peak.

Paper drum-tight.

Cocks blown through firing strokes to clear soot.

Atmospheric line first, drive hooked, cock open — power card drawn, cock shut, instrument spared prolonged heat.

Drum hand-turned clear, fuel cut, compression line drawn for sealing truth.

Every unit in turn, then strip, clean, oil and stow the instrument — the engine's stethoscope deserves clinical care.

Lubricate against seizure, prove free piston sliding, keep cocks carbon-clear, touch the stylus lightly, cool between units — each precaution guards one error source.

4. Four Cards, Four Lenses

Card	Taken how	Reveals
Power	Drum in phase, normal firing	Work area, MIP, Pmax, Pcom — the master card
Compression	Fuel cut, same phasing	Sealing health — coincident lines prove sync; sagging height blames liner, rings, air or exhaust valve
Draw (90° out)	Fuel on, drum quarter-turn displaced	Combustion timing and injector faults in stretched detail
Light spring	Soft spring, in phase	Exhaust/scavenge pressures magnified — gas-exchange faults exposed

Card marking maximum and compression pressures — **Figure 3:** Pmax vs Pcom — firing vs sealing.

Light-spring card magnifying exhaust opening and scavenge events — **Figure 4:** Soft spring, big picture of small pressures.

At-sea discipline: exhaust temperature per unit flags imbalance daily.

Leaky fuel valves, wrong valve events and choked exhaust passages read straight off the cards.
Bench-tested spare injectors and exhaust valves stand ready for momentary-stop swaps — defective valves never keep running.
Main-engine cams and timing wait for port.