A Cylinder Pressure Measurement System for Main Diesel Engines

Abstract

Cylinder pressure measurement of diesel engines can be a step toward enabling condition-based maintenance, moving beyond traditional maintenance strategies based solely on engine running hours. This thesis investigates the application of a continuous cylinder pressure measurement system for condition monitoring of marine diesel engines. It addresses the question how such a system can be used effectively for the analysis of engine performance, combustion, and fault detection and diagnosis of a naval vessel’s main engines. To answer this question, a comprehensive methodology is proposed and tested on an ocean-going patrol vessel of the Royal Netherlands Navy. The main diesel engines of this vessel have been retrofitted with a permanent, multi-cylinder pressure measurement system, enabling simultaneous, high-frequency, crank-angle-resolved pressure acquisition. The setup includes piezoelectric sensors, a crank-angle encoder, and a custom synchronisation mechanism. In order to obtain useful information from the raw cylinder pressure data, these data have to be preprocessed first. Preprocessing steps applied in this thesis include:
1. Synchronising pressure data with crank angle using a method that determines the TDC shift
based on a temperature-entropy diagram of a non-firing engine.
2. Applying the correct pressure offset, preferably using the air intake manifold pressure. Alternatively, if this pressure is too unsteady, a linear least-squares regression method is used.
3. Averaging the cylinder pressures over 25 cycles to reduce random noise.
4. Applying cubic-spline smoothing to reduce non-random noise. A tunable smoothing parameter dictates the trade-off between curve smoothness and fidelity to the original data.
The preprocessed data are subsequently used to derive several performance and combustion-related indicators, using a single-zone heat-release model. Performance indicators include indicated mean effective pressure, compression pressure, and peak pressure. Combustion characteristics are assessed through net heat release, thermodynamic efficiency, and crank angles corresponding to key energy release thresholds. Cylinder-to-cylinder comparison of these parameters enables the identification of anomalies that may indicate underlying faults. Rejection criteria should be defined based on manufacturer guidelines, though these may need to be adapted. Notably, since OEM specifications only cover peak pressure and exhaust gas temperature, these criteria can be supplemented with operational experience. A structured diagnostic approach involves distinguishing whether the root cause lies in compression (e.g., valve leakage, excessive blowby) or combustion (e.g., injector malfunction).
In this study, particular attention was given to a suspected malfunctioning fuel-injection pump, identified through deviating exhaust gas temperature. However, analysis of pressure-derived parameters did not support this diagnosis; instead, another cylinder was found to exhibit signs of underperformance. This study further argues that IMEP is a more reliable parameter than peak pressure, while the angle of peak pressure is of limited diagnostic value. Exhaust gas temperature is also found to be less effective for localising faults.
In conclusion, the results of this study suggest that cylinder pressure measurement holds substantial promise for condition monitoring applications. However, validation of rejection criteria requires actual fault cases, which highlights the need for a database of known faults, including their symptoms. The study also recommends routine system checks, automation of data processing, and clear assignment of system ownership. It also recommends to implement a method for determining the correct TDC shift for the engine under load.