Study and Development of Techniques to Improve Engine Stability and Reduce Emissions from Natural Gas Engines

Abstract: The objective of this thesis was to find ways to improve combustion and reduce emissions from supercharged natural gas engines. In-cylinder flow measurements were made with laser doppler velocimetry, LDV, heat release was calculated from cylinder pressures and emissions were measured at various locations. The original combustion chamber was a low turbulence-generating geometry, resulting in an overall slow combustion. Four new combustion chambers were designed, to generate high turbulence and hence a fast combustion. The piston-geometry with the highest turbulence was more tolerant for highly diluted mixtures in terms of engine stability, which is favorable for lean burn operation or high amounts of EGR at stoichiometric operation. The flow-measuring tests were made on a single-cylinder engine. Base-engine performance measurements were conducted on a multi-cylinder version of the engine. The results show that pulse-width fuel-injection close to the cylinders (at the throttle) resulted in variations in air/fuel ratio between the cylinders. Cycle-to-cycle variations were high in cylinders with leaner mixtures, and NOX emissions were high from cylinders with richer mixtures. Late ignition timing, high boost pressure and lean mixture led to the need for a small spark gap in order to avoid misfires with the original ignition system. A larger gap results in higher spark energy, but the ignition system must be powerful enough not to cause misfires. The original ignition settings were retarded to suppress NOX formation. The HC and CO emissions were also lower than at maximum brake torque ignition (MBT) due to higher temperatures during expansion and exhaust, leading to more post-oxidation. Load and efficiency were reduced with the retarded ignition timing. A new engine control system was installed with high power ignition modules, enabling a larger spark gap. Idle quality was improved and maximum load was increased with this new system. Cylinder-individual control of fuel injection and ion-current measurements in all six cylinders made it possible to adapt port fuel-injection and cylinder balancing. Both cycle-to-cycle and cylinder-to-cylinder variations were reduced with cylinder balancing at lean operation. The ion-current integral and variations in the integral were used to perform the cylinder balancing. Lean burn operation was compared to stoichiometric operation diluted with EGR. The raw emissions of NOX and HC were higher at lean burn operation than for the EGR case. NOX emissions after a three-way catalyst were up to 700 times higher at lean operation (29.5 g/kWh vs. 0.042 g/kWh), and HC emissions approximately 20 times higher (2.7 g/kWh vs. 0.13 g/kWh). The early flame period (ignition to 5% burned) was much longer for the EGR case, since EGR has a stronger influence on laminar flame speed than excess air. The main combustion duration (10% to 90% burned) was similar for both cases. The ion-current signal was very weak for the lean burn cases, but a strong signal was found for the EGR case.