A study of premixed and stratified charge combustion of CH4/H2/air and H2/air mixtures under constant volume conditions
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Mechanical Engineering
The aim of this research was to conduct fundamental charge stratification studies in a constant volume combustion chamber using hydrogen/methane/air and hydrogen/air mixtures. Charge stratification allows the benefits of a rich, or richer, mixture at initiation to be combined with the benefits of a lean overall mixture. Thus benefits can be achieved in terms of combustion efficiency and/or emissions. The investigation was divided into two main sections; the first dealing with the hydrogen enrichment of methane and the second, the reduction of nitric oxide emissions when burning hydrogen only in air. The aim of the first section was to determine the improvements to the combustion characteristics of methane that could be achieved through the homogeneous (pre-mixed) and heterogeneous (stratified charge) addition of hydrogen. The parameters measured were the peak pressure and time to peak pressure while the proportion of hydrogen added was varied. The aim of the second section was an attempt to reduce the concentration of nitric oxide formed during the combustion of hydrogen/air mixtures through the use of charge stratification. The proportion of the hydrogen injected was varied along with the ignition delay in order to alter the level of stratification. The concentration of nitric oxide in the exhaust gases was measured using a mass spectrometer. In addition to the above, optical probes were used to investigate flame propagation and to determine the cause of the 'double hump' pressure trace. For all investigations a cylindrical constant volume combustion chamber of internal volume 2.06 litres was used and the stratified charge was created by injecting pure hydrogen through the core of a specially modified spark plug. Investigations were carried out to improve the level of stratification of the injected hydrogen along with the accuracy of test mixture creation. The results from the first section of this investigation indicate that the addition of hydrogen is a very efficient way of improving combustion characteristics. In particular, significant improvements were observed in the ignition delay and rate of pressure rise leading to higher peak pressures and reduced times to peak pressure. Further benefits were achieved through the use of a stratified charge. Similar improvements to homogeneous charges were observed whilst using up to ten times less hydrogen. The second section indicated that the use of a stratified charge is an effective means for reducing the emissions of nitric oxide. Maximum reduction of nitric oxide (30%) was achieved when igniting the stratified mixture with a minimum ignition delay after the end of injection. An increasing ignition delay leads to a less stratified region and nitric oxide levels tending to homogeneous values. Much work is still required in the area of stratified charge formation and initial results indicate that further improvements could be achieved. In conclusion, this project has indicated that the use of hydrogen enrichment could pave the way for the general acceptance of alternative transport fuels, including hydrogen itself and that the charge stratification concept remains one of the most promising alternatives available for meeting the anticipated environmental and legislative requirements of the future.