Background
The technique predates electronic instrumentation by more than a century. The mechanical engine indicator, attached to a tap on the cylinder head and producing a paper trace as the piston traversed its cycle, was a standard tool of marine engineering from the 1880s onwards. The electronic indicators that have largely replaced it (DEPAS, MARLIN/Premet, Kistler, Lehmann & Michels) record the same physical quantity (cylinder pressure as a function of crank angle or volume) at much higher resolution, with automatic data processing, and with the ability to capture transient behaviour that the mechanical instrument could not.
This article describes the fundamentals of combustion in the marine diesel cycle, the construction and interpretation of P-V and P-θ (pressure-versus-crank-angle) diagrams, the operation of modern electronic indicators, the key performance parameters extracted from the diagrams, the diagnostic significance of common combustion irregularities, and the integration of combustion analysis into performance monitoring and the engine room automation framework.
Combustion Fundamentals
The diesel cycle, in its idealised form, consists of compression of air to a pressure and temperature at which fuel injected near top dead centre (TDC) ignites spontaneously. The fuel injection continues for a fraction of the power stroke, with the released chemical energy producing a near-constant-pressure expansion (in the ideal cycle) that drives the piston downward. Exhaust valves open near bottom dead centre and the burnt gases are expelled.
Real diesel combustion deviates from the idealisation. After fuel injection begins, a measurable ignition delay precedes the actual start of combustion. During this delay (typically 1 to 3 degrees of crank angle on a slow-speed two-stroke), fuel accumulates in the cylinder. When ignition occurs, the accumulated fuel burns rapidly in the premixed combustion phase, producing a sharp pressure rise. Subsequent combustion proceeds in the diffusion phase, controlled by the rate of mixing of remaining fuel with surrounding air, producing the more gradual pressure pattern characteristic of the latter portion of the power stroke.
The total chemical energy released per cycle is determined by the mass of fuel injected times its lower heating value. Of this energy, a fraction is converted to mechanical work (the indicated power), a fraction is lost to the exhaust, and a fraction is rejected to the cooling jackets and lubricating oil. The thermodynamic efficiency of the cycle, expressed as a percentage of the fuel energy converted to indicated power, is typically 50 to 55% for modern slow-speed engines and 45 to 50% for medium-speed.
P-V Diagrams
The P-V diagram (pressure plotted against cylinder volume) is the classical thermodynamic representation of the engine cycle. The closed loop traced out per cycle has area equal to the indicated work per cycle. Multiplying by cycle frequency gives the indicated power. The shape of the loop, and particularly the area near TDC where compression and combustion overlap, conveys most of the diagnostic information.
In the idealised dual cycle (a Sabathé cycle, the standard idealisation for marine diesel), the loop comprises an isentropic compression from BDC to a pressure P1, a constant-volume combustion phase to a higher pressure P2, a constant-pressure combustion phase to a volume V2, and an isentropic expansion to BDC. The actual diagram differs in the rounding of corners, the shape of the combustion phase (which is neither pure constant-volume nor pure constant-pressure), and the presence of pumping work for four-stroke engines.
The compression line on a P-V diagram should follow approximately P × V^n = constant, where n is in the range 1.30 to 1.37 for typical marine engines. Deviation from this exponent indicates leakage past piston rings, valve leakage, or air starting valve leakage.
Indicator Diagrams
The P-θ diagram (pressure plotted against crank angle) is the form most commonly produced by electronic indicators today. It presents the same information as the P-V diagram in a form that more directly relates to engine timing. Combustion timing, ignition delay, and the relative positions of injection, combustion, and exhaust events are all directly readable.
The indicator diagram is drawn either as a single cycle or as an average over many cycles. The averaged diagram smooths cycle-to-cycle variation and gives a representative view of the engine’s behaviour. Cyclic variation itself is a diagnostic, with high variation indicating fuel injection inconsistency, ignition difficulty, or air supply problems.
A typical slow-speed two-stroke engine indicator diagram extends from about 60 degrees before TDC (compression starting after exhaust port closure) through TDC to 60 degrees after TDC (where the rapid pressure rise of combustion has subsided), with the remaining expansion and exhaust stroke shown at lower resolution.
Electronic Indicators
Several manufacturers supply electronic indicator systems for marine engines:
DEPAS (Diesel Engine Performance Analysis System) by Doctor of Engineering Diesel Service is a long-established system designed for slow-speed two-stroke engines. It connects to the indicator cock on each cylinder, samples pressure at high frequency, and computes a comprehensive set of performance parameters. The system is portable and can be used by the engineering staff during voyages.
MARLIN by Lemag (formerly Premet) provides similar functionality with strong analytics in fault diagnosis. Its software base for trend analysis across multiple measurement campaigns is widely used for fleet performance monitoring.
Kistler supplies pressure transducers and complete combustion analysis systems, with marine variants of their cylinder pressure transducers used for both periodic measurement and continuous monitoring.
Lehmann & Michels offer marine-specific engine performance analysers with integrated indicator diagram, FIVA timing analysis on electronically controlled engines, and trending across the fleet.
The modern systems use piezoelectric pressure transducers that fit the indicator valve on each cylinder. Crank angle is measured either by a TDC sensor on the engine flywheel or by an optical encoder on the camshaft. The pressure samples are recorded against crank angle at typically 0.1 to 0.5 degree resolution.
The system typically computes parameters automatically and produces a per-cylinder report. The chief engineer compares results across cylinders, against the previous campaign, and against the engine builder’s reference values.
Key Parameters
Several parameters extracted from the indicator diagram are central to performance analysis:
Pmax is the peak combustion pressure. It is the highest pressure observed in the cycle, occurring shortly after TDC. Pmax is the principal driver of mechanical loading on the crankshaft and main bearings, piston, and connecting rod, and the engine builder specifies a maximum allowable Pmax which must not be exceeded.
Pcomp (compression pressure) is the pressure at TDC if no combustion occurred. It is measured by either inhibiting fuel injection on a single cylinder (compression test) or by extrapolating the compression line to TDC. Low Pcomp indicates leakage past piston rings, valve leakage, or compression ring damage.
Pfired is the peak pressure during firing. Where combustion timing is correct, Pfired is achieved a few degrees after TDC. Excessive Pfired indicates over-fuelling, advanced injection timing, or low-temperature combustion at poor air conditions.
IMEP (indicated mean effective pressure) is the average pressure that, applied uniformly through the power stroke, would produce the same work as the actual cycle. IMEP times piston area times stroke gives the indicated work per cycle. IMEP is therefore directly proportional to indicated power per cylinder. Comparing IMEP across cylinders identifies cylinder-to-cylinder load imbalance.
Pscav is the scavenge air pressure at the moment exhaust ports close in a two-stroke engine. It governs the amount of air available for combustion.
Combustion timing parameters include the angle of ignition (where pressure first deviates from the compression line), the angle of Pmax, and the angle of 50% mass fraction burned. Each timing parameter is referenced to TDC.
Combustion Timing
Combustion timing is the relationship between the fuel injection event, the start of combustion, and the resulting pressure rise. On older engines with mechanical fuel injection and fixed cam-driven timing, the timing is essentially fixed at design and varies only with wear. On modern engines with variable injection timing (VIT) or Common Rail injection, the timing is adjustable from the engine control system.
Optimum timing balances Pmax (which rises with advanced timing) against fuel efficiency (which is best at moderate timing producing combustion close to TDC). Retarded timing reduces Pmax at the cost of higher exhaust temperature and worse fuel economy; advanced timing increases Pmax at the cost of higher mechanical loading.
The injection timing, observed via the fuel pump cam position or, on Common Rail engines, the FIVA valve actuation timing, is referenced against the actual combustion timing observed in the indicator diagram. The difference is the ignition delay.
Combustion Irregularities
Several irregularities are commonly diagnosed from indicator diagrams:
Mis-firing is the failure of one or more cycles to ignite. The pressure trace shows compression but no combustion-induced pressure rise. Mis-firing produces high cyclic variation, low IMEP for the cylinder, and characteristic pulsation in the exhaust temperature. Causes include water in the fuel, fuel pump failure, atomiser blockage, and severely worn valve.
After-burn is combustion continuing far into the expansion stroke, evidenced by elevated pressure at 60 to 90 degrees after TDC and high exhaust temperature. Causes include retarded injection timing, poor atomisation, low scavenge air pressure, or low compression.
Knock or pre-ignition is auto-ignition occurring earlier than the injection event, producing a sharp pressure spike before the normal ignition timing. Knock is rare in marine diesels because the cylinder air temperature at injection is well below the auto-ignition threshold of conventional fuel. It can occur with very heavy residual fuels or with deposits on combustion chamber surfaces.
Combustion roughness is excessive rate of pressure rise, indicated by a steep slope on the P-θ trace immediately after ignition. Causes include long ignition delay (cold starting, low cetane fuel) leading to large premixed combustion phase. Roughness is associated with high-frequency vibration and accelerated bearing fatigue.
Troubleshooting
Combustion analysis is a primary tool in troubleshooting power imbalance, fuel consumption increases, and exhaust temperature anomalies. A typical investigation proceeds:
The chief engineer takes a full set of indicator diagrams across all cylinders at the operating load. The diagrams are compared against the engine builder’s shop test values and against the previous reference campaign in the planned maintenance system.
Cylinder-to-cylinder differences in Pmax and IMEP are flagged. A cylinder with low Pmax and low IMEP is under-fuelled; with low Pmax and normal IMEP is timing-retarded; with high Pmax and high IMEP is over-fuelled.
The compression line is compared. Reduced compression points to mechanical wear: piston ring damage, cylinder liner wear, or valve leakage on four-stroke engines.
The ignition delay is compared. Increased delay indicates degraded fuel quality, low scavenge air pressure, or poor atomisation from worn injectors.
Crew action follows: fuel pump rack adjustment for cylinder balance, injection timing adjustment if available, cleaning or replacement of atomisers, valve grinding for valve leakage, or piston ring overhaul if compression is severely degraded.
Performance Optimisation
Beyond troubleshooting, regular combustion analysis enables performance optimisation. The chief engineer can adjust injection timing within engine builder tolerances to balance fuel consumption against Pmax, while remaining within the safe operating envelope.
For Common Rail engines, the engine control system applies adaptive logic to optimise fuel injection profile in real time, using cylinder pressure transducer feedback. The chief engineer’s role becomes monitoring the system’s behaviour rather than direct adjustment.
Trend analysis across thousands of operating hours, as recorded in fleet performance monitoring systems, reveals slow degradations that single-campaign analysis misses. Hull fouling effects, propeller condition, and sea margin all show up as gradual rises in fuel consumption that combustion analysis can apportion among engine and hull factors.
The integration with the operator’s emissions reporting (EEXI, CII) is increasingly important. Combustion analysis provides the raw data on engine efficiency that feeds the carbon intensity calculation under MARPOL Annex VI.
Related Wiki Articles
- Marine Diesel Engine
- Marine Engine Performance Monitoring
- Marine Engine Crankshaft and Main Bearings
- Marine Engine Cylinder Liners and Pistons
- Marine Engine Camshaft and Valve Train
- Marine Engine Fuel Injection Systems
- Marine Engine Common Rail Technology
- Marine Engine Turbocharging
- Marine Engine Room Automation and Monitoring
- Marine Auxiliary Engines and Generators
- Marine Fuel Oil Systems
- Marine Spare Parts and Maintenance Management
See also
Calculators
Related wiki articles
References
- ISO 3046-1, Reciprocating Internal Combustion Engines - Performance Declaration
- ISO 15550, Internal Combustion Engines - Determination and Method for the Measurement of Engine Power
- ISO 8178, Reciprocating Internal Combustion Engines - Exhaust Emission Measurement
- IACS Unified Requirement M51, Type Testing of Diesel Engines
- IACS Unified Requirement M44, Documents for the Approval of Diesel Engines
- MAN Energy Solutions Service Letter SL2014-587, Performance Evaluation
- MAN Energy Solutions Service Letter SL2017-642, Cylinder Pressure Measurement
- Wartsila Service Bulletin RT-65, Cylinder Pressure Measurement on Two-Stroke Engines
- IMO MARPOL Annex VI, Regulations for the Prevention of Air Pollution from Ships
- IMO Resolution MEPC.328(76), 2021 Revised MARPOL Annex VI
- CIMAC Recommendation No. 27, Specification for Heavy Fuel Oil
- DNV Class Guideline CG-0339, Engine Performance Monitoring