Study of ignition and combustion processes of methane and ammonia using both conventional and advanced ignition methods

Abstract

The ignition system plays a major role in igniting the air-fuel mixture in internal combustion engines (IC) to produce maximum power and least emission possible. Excessive ignition retard can lead to higher Hydrocarbon (HC) emission and too much advance can lead to higher Oxides of Nitrogen (NOx) emission. The conventional ignition system is energy intensive, prone to electrode erosion, limited to a single ignition point and less effective for lean combustion of fuels with lower ignitability. Therefore, innovative ignition systems must be explored and developed to support ultra-lean burn combustion of future low carbon and zero carbon fuels. Certain sectors of transport like the shipping and power generation, ammonia has a significant potential for decarbonization. Ammonia being the fuel with lower burning velocity and higher ignition energy than most fossil fuels, successfully igniting ammonia for complete combustion is necessary in the adoption of ammonia as a zero carbon fuel in IC engines. Hence, in the present work the combustion characteristics of ammonia were studied in addition to methane. The first phase of the current work involves setting up a new experimental test rig including a pressurized constant volume chamber, multiple-fuel intake system, ignition system, control system and data acquisition system. The chamber of 3.5 litre capacity was designed by the author and manufactured by an external company. The second phase focused on the design of a new Barrier Discharge Ignition plug with different ceramic materials and dimensions. High speed flame imaging with Schlieren imaging system was used to capture the development of flame from the time of ignition. Methane and ammonia air mixtures were tested in the constant volume chamber to compare the energy requirement and the flame development. The ignition systems evaluated include Transistorized Coil Ignition (TCI) with J-hook spark plug, Nanosecond Repetitively Pulsed Discharge (NRPD) with J-hook spark plug and Barrier Discharge Ignition (BDI) plug. The methane-air mixtures were tested at 1 bar (gauge) initial pressure and a temperature of 20OC using all the ignition systems. The minimum ignition energy required to ignite the methane air mixtures is 45 mJ, 30 mJ and 6 mJ for TCI, NRPD with J-Hook and NRPD with BDI respectively at all lambda values. The spectroscopic studies of methane air mixtures showed more oxygen and hydrogen atoms near the spark plug that resulted in successful ignition and subsequent flame propagation. Lesser atoms near the spark plug tip has resulted in partial burning and incomplete combustion confirmed by the corresponding pressure traces. Unlike methane-air mixtures, the ammonia air mixtures could not ignite at ambient temperature and required pre-heating of the chamber in steps of 50OC from ambient upto 200OC. The Barrier Discharge Ignition was able to achieve reliable ignition of the stoichiometric ammonia air mixtures at and above 50 OC thanks to its distributed streamers. Both the Transistorized Coil Ignition and Nanosecond Repetitively Pulsed Discharge with J-Hook spark plug was capable of igniting the ammonia from 150 OC. The minimum amount of ignition energy required to ignite the ammonia air mixtures is 118 mJ, 400 mJ, and 64 mJ for TCI, NRPD with J-hook spark plug and NRPD with BDI plug respectively. The Barrier Discharge Ignition system was able to ignite the methane air mixtures reliably up to 1.8 lambda and the ammonia air mixtures up to 1.4 lambda. The breakdown voltage for the BDI and the J-hook spark plug is 22 kV and 14 kV respectively influenced by the spark plug geometry as well as the air-fuel mixture resistance in the chamber. Among all the ignition systems tested with ammonia and methane air mixtures, BDI outperformed in terms of faster flame propagation and better heat release rate.