Microscopy of damage mechanisms in diesel engine pre-combustion chambers in a cyclic oxidising –carburising environment

Abstract

The performance of pre-combustion chamber tips manufactured from Nimonic Alloys N80A and N105 is examined. The pre-combustion chamber provides an additional volume to the main chamber enabling primary ignition and transfer of burning fuel/air charge to the main combustion chamber of a compression ignition two cycle engine. The engine is used for light aircraft propulsion, is of innovative design and high power density, up to 65 bhp/L. The pre-combustion chamber is exposed to rapidly cycling high temperature oxidising and reducing atmospheres in addition to thermal shocks and erosion by pyrolytic carbon particles. Early field operation of the engine revealed unacceptable scatter in the service life of the pre-combustion chambers. Optical and scanning electron microscopy (SEM) with energy dispersive analysis of X-rays (EDX) were used to identify failure mechanisms of the N80A pre-combustion chamber tips. Cracking around fuel spray distribution orifices was observed associated with the cyclic oxidising–carburising environment. This led to loss of the pre-combustion chamber tip and could lead to potentially catastrophic engine failure. A material change to N105 considerably improved service life and minimised catastrophic failures; however, indications of impending failure were observed after 50 h engine laboratory trials. To improve service life further chemical vapour deposition (CVD) was used to increase surface aluminium content with the aim of providing a more mechanically robust, protective alumina scale on the pre-combustion chamber surface. Pre-combustion chambers were then tested in the laboratory using isothermal tests to establish mass changes and rank treatments accordingly. The performance of uncoated and coated systems was evaluated using field emission gun (FEG) SEM and environmental SEM studies. Further testing took place on engine test beds with pre-combustion chambers being inspected at set intervals. Detailed analysis of engine pre-combustion chambers run under oxidising/carburising conditions was also completed to assess pre-combustion chamber life using optical, SEM and focused ion beam (FIB) sectioning of the cyclic oxidised, then carburised, interface to elucidate the mechanism of corrosion damage propagation in service for both coated and uncoated parts.