Dimethyl Ether (DME) is a new age fuel developed mainly from coal and natural gas to use in compression ignition (CI) engines relatively easily with minimum modifications. One of the advantages of DME combustion in CI engines is the low emission levels of NOx and particulate matter (PM) in comparison with diesel combustion. Therefore, utilization of DME as an alternative fuel in CI engines can potentially meet stricter emission regulations with less effort.
The thesis starts with a review of the body of experimental and numerical research on NOx and PM emissions from DME combustion, with the objective being to identify the most promising methods for emission control in DME fuelled engines. With DME being already available in several countries the current research interest is to optimize the engine performance in a cost-effective way with least modifications to existing technologies while minimizing combustion emissions to meet even the strictest emission regulations.
Gaseous emissions from DME combustion are a well-researched topic while PM emissions, especially UFPM were neglected so far. However, PM emissions will become a major concern under the future emission norms such as Euro VI. A major part of the introduction and literature review discusses some of the novel methods of emission control in CI engines, which are fuel injection strategies, exhaust gas recirculation, and combustion after-treatment.
A major objective of the thesis was to design and build an engine testing facility capable of testing DME and other liquid fuels for combustion performance and emissions. An engine testing facility is thus designed and setup to meet the test requirements and a normal diesel engine system is modified to run on compressed DME fuel. A series of control systems, monitoring devices, analysers and engine management system were setup and programmed to set the test conditions and for measuring various parameters.
Experiments were designed to investigate the influence of injection strategies and exhaust gas recirculation on performance and emissions. For injection strategies, both injection pressure and injection timing were varied for the engine to study its influence on NOx and PM emissions. As expected, injection strategies were found to have some effect on both gaseous and PM emissions. Advancing injection timing for DME showed increase in both NOx emissions and PM emissions. For increase in injection pressure NOx emissions was found to have increased, but the PM emissions were decreased. For studying effect of EGR, the percentage of EGR was varied between 0% to 30% for various engine load conditions to study how emissions varies with EGR rate at various loads. This will help to determine optimal EGR rates to control emissions for different load conditions. With increase in EGR rate NOx emissions found to have decreased drastically for DME. PM emissions from DME was not affected due to change in EGR rate. Observations from DME combustion was compared against test results for ULSD and B20 blends of CSO (cotton seed oil), WCO (waste cooking oil) and BUT (butanol).
DME due to its poor lubrication properties, need to use additives to control premature wear and tear of parts that get in contact with the fuel. Main parts that are susceptible to damage are the fuel pump and the injectors. Two different additives, A1 and A2, were tested with DME to study any effects it has on emissions. As suspected, both gaseous as well as PM emissions seem to be influenced by the type of additive used, which point towards need to identify additives that does not contribute towards emissions.
In the last section of thesis conclusions are drawn towards the effectiveness of using injection strategies and EGR in controlling emissions. Proposals for future research to identify DME additives that does not contribute to emissions was made. This will require in-depth studies to understand chemistry of additives used and nature of their combustion need to be investigated. The thesis concludes by critically evaluating the most likely technologies that can help DME to meet future emission regulations and suggestions are made on future directions for DME research and development.