Recent developments in the Understanding of the Potential of In-Cylinder NOx Reduction though Extreme Miller Valve Timing
In-cylinder NOx reduction is becoming increasingly important for stationary and marine DI diesel engine applications, where progressively more stringent emission legislation has significantly reduced allowed NOx limits. Extreme Miller Valve timing, coupled with two-stage turbocharging, has shown significant NOx reduction potential, with increased engine efficiency and similar power density as conventional engine setups. Under the Miller cycle, the inlet valve is closed before Bottom Dead Centre, allowing the charge to expand before compression. This leads to a reduced charge air temperature at Top Dead Centre and a reduction in reactant temperatures, resulting in lower adiabatic flame temperature and correspondingly lower NOx formation during combustion. Nonetheless, experimental investigations have shown limitations in the amount of NOx reduction that is possible solely though the reduction of reactant temperature. At extreme Miller degrees, reductions in reactant temperature have been observed to result in increases in NOx emissions, limiting the applicability of the use of Miller valve timing for NOx reduction. The improved understanding of the source of these limitations could lead to improvements in the potential of in-cylinder NOx reduction through Miller valve timing. The current paper aims to provide an understanding of the effects of cyclic variation of in-cylinder soot mass on the overall trend of NOx emissions. At extreme Miller conditions, high cycle-to-cycle variations of in-cylinder pressure and soot concentration, measured using an in-cylinder optical light probe and the method of three-color pyrometry, were observed. Cycles which present with reduced soot concentration also showed increased soot temperature, pointing to the assumption that the reduced soot presence results in reduced flame radiation heat transfer, leading to increased flame temperature. Under conventional diesel engine conditions, the flame radiation heat transfer through the presence of soot particles in the flame leads to flame temperatures well below the adiabatic flame temperature. Thus, the reduction of soot presence results in flame temperatures closer to the adiabatic, leading to increases in NOx production rate. This reduced flame radiation heat transfer at these conditions is understood to contribute significantly to the observed NOx trends with extreme Miller valve timing.
Panagiotis Kyrtatos Klaus Hoyer Peter Obrecht Konstantinos Boulouchos
ETH Zurich,Switzerland Paul scherrer institut,Switzerland
国际会议
上海
英文
1-10
2013-05-13（万方平台首次上网日期，不代表论文的发表时间）