Mechanical Engineering - Theses
Permanent URI for this collection
Search Results
1 - 3 of 3
-
ItemAlternative ignition systems for CNG in diesel applications( 2003)Ignition and combustion enhancement of lean homogeneous mixtures offers the potential to simultaneously lower pollutant emissions and improve the thermal efficiency of internal combustion engines. A single cylinder, high compression ratio (16.5:1), open chamber diesel engine has been converted to operate on homogenously charged compressed natural gas (CNG) with the aim of minimising pollutant emissions such as oxides of nitrogen, particulate matter and carbon dioxide. Three ignition systems were tested to examine how effectively they could ignite lean mixtures of CNG with the ultimate aim of achieving simultaneously high thermal efficiency and low oxides of nitrogen emissions. The ignition systems examined were spark ignition (SI), diesel pilot ignition (DPI) and hydrogen assisted jet ignition (HAJI). Irrespective of ignition system used, the efficiency of the engine operating on CNG was significantly reduced at part load compared to diesel. This was predominantly due to a greater amount of unburnt hydrocarbons, higher cycle-by-cycle variability, slow and partial burns and increased heat transfer to the walls. DPI and HAJI systems were able to extend the lean limit to lambda 2.7 and 3.3 respectively, however this did not result in efficiency gains over SI systems. HAJI proved to be superior to DPI with higher peak efficiency, lower carbon dioxide, carbon monoxide and particulates, and significantly lower oxides of nitrogen in the absence of a locally rich ignition source. (For complete abstract open document)
-
ItemHybrid chemical recuperation of a spark ignited natural gas internal combustion engine( 2012)This work proposes means of improving the efficiency of natural gas fuelled power plants by using chemical recuperation as a bottoming cycle. The integration of a renewable energy source to boost the recuperation process is also examined. The work is in three parts. A thermodynamic study of both the conventional (i.e. fossil input only) and hybrid (i.e. both fossil and renewable energy inputs) power cycles is first undertaken. The financial performance of the hybrid cycle is then examined by calculating its levelized cost of electricity and comparing it to that of standard fossil and non-fossil power plants. Finally, the performance of the engine when running on four different natural gas reformates is examined and analysed. The presented work shows that the proposed hybrid cycles should have baseload capability, with comparable cost and greenhouse emissions to natural gas fuelled combined cycle (NGCC) power plants, but at a significantly smaller scale. This is thought to be a significant finding, since NGCCs are commonly considered to be the most cost-effective, low emission baseload form of fossil fuelled power generation currently available.
-
ItemPerformance of a spark ignition, lean burn, natural gas internal combustion engine( 2012)Relative to gasoline and diesel, use of natural gas as a transport fuel can produce significantly lower emissions of particulate matter and greenhouse gases. Future emission standards for commercial transportation, combined with projections in transport demand and gas and oil production, are resulting in increased interest in natural gas use in road vehicles. Lean-burn, natural-gas fuelled spark ignition engines have particular potential in terms of both regulated emissions and increased thermal efficiency. This work therefore first presents a comparison of an engine’s performance fuelled with gasoline and natural gas. Particular emphasis is placed on the natural gas engine’s lean burn behaviour. Analysis of the in-cylinder pressure traces for the natural gas engine is then undertaken. This analysis is used to explain how the observed variations in emissions and efficiency of the natural gas engine vary with air/fuel ratio via the turbulent flame propagation. In particular, it is argued that the commonly observed optimal efficiency at a given lean condition is due to a trade-off between reducing heat losses and increasing flame quenching with increasing air/fuel ratio.