Mechanical Engineering - Theses
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ItemMinimising cold start fuel consumption and emissions from a gasoline fuelled engine( 2011)The catalytic converter, which is in the exhaust of most vehicles on road today, has led to a dramatic improvement in the quality of the air in industrialised cities despite a large growth in traffic. As a result, almost all of the gaseous pollutant emissions from cars are now released in the first minutes of driving while the catalyst warms to its operating temperature. Current emissions control approaches therefore mostly aim at reducing this warm up time, and are developed during extensive testing by experienced engineers. This involves significant time and cost, and may not guarantee an optimal outcome. In this thesis, a novel approach to this warm-up problem is proposed. Dynamic optimisation procedures are applied to a mathematically compact model of an engine and its exhaust system to identify new engine control strategies and trends for prescribed driving conditions. The order of the model, which can be calibrated from steady state engine maps and a single transient engine test, is high enough to capture most of the major phenomenon involved, but is of low enough order to permit dynamic optimisation studies. Both the model itself and some of the optimised strategies found are validated using a transient engine dynamometer and emissions facility. Several constrained optimisation problems are considered, in which different tailpipe emissions regulations are the constraints under which the fuel consumption is minimised. The solutions of these optimisation problems indicate that optimised spark advance is always characterised by retarded timing during cold start, followed by a transition to near maximum brake torque (MBT) timing. These results suggest that a bang-bang type of approach might apply. To test this, an ignition policy, defined initially by retarded spark timing limited by drivability considerations and MBT timing afterwards, is prescribed. It is demonstrated that provided the switching time is optimised, overall fuel consumption and tailpipe emissions approach the results of the dynamic optimisation. Further, higher degree of freedom optimisation, particularly incorporating air-fuel ratio and cam timing, suggest that additional gains may be achievable.