Miller cycle and exhaust gas recirculation for a naturally aspirated lean-burn gas engine

Resumen

Motivated by tightening emission standards for small natural gas driven cogeneration gas engines, this thesis explores an alternative working process that extends the pre-vailing lean-burn operation with Miller cycle and cooled exhaust gas recirculation (EGR). The combination of these well-established means is expected to improve the trade-off between engine efficiency, NOx emissions and indicated mean effective pres-sure (IMEP) of a naturally aspirated gas engine. Preliminary experiments on the combustion of varying mixture composition under qui-escent flow conditions were conducted using a constant volume combustion chamber. Evaluating the measured pressure trace reveals how relative air-fuel ratio, exhaust gas concentration, pressure and temperature of the unburnt zone affect laminar burning velocity and temperature of the burnt zone. Subsequently, the effect of replacing a part of excess air with exhaust gas on the performance of the baseline engine (Otto valve timing) was studied. After first trials centred on the trade-off between engine efficiency and NOx emissions at constant IMEP, the potential to increase IMEP at con-stant NOx emissions was assessed. A representative comparison between both approaches to realise a Miller working cy-cle, namely early and late intake valve closing, is a core element of this work. The effective compression ratio of the baseline engine, as criteria for comparability, was maintained constant by adjusting the geometrical compression ratio. To this end, a method was applied that accounts for the effects of gas dynamics, such as ram effect and reverse flow. A detailed model of the test bed engine was built using one-dimensional engine simulation software. The model predicts realistic engine behaviour and enabled analysing the influence of intake valve closing on engine performance. Fundamental differences between early and late Miller cycle could be explored and configurations of same expansion/compression ratio for experimental engine testing determined. Using the same piston design for both configurations, ensures identical squish and area to volume ratio, thus favouring comparability. 3D CFD gas exchange calculations were conducted to gain a deeper insight into the influence of intake valve closing on charge motion and homogenisation. The numerical results help explaining different combustion characteristics of both Miller configurations in experimental en-gine testing. Comprehensive series of measurements were carried out in engine operation with Mil-ler valve timing and varying EGR rate. The main focus was put on the potential that the working process offers in improving the trade-off between engine efficiency and NOx emissions at constant IMEP of the baseline engine’s reference operating point.