Early Pilot Injection Research Articles

Improving low load combustion, stability, and emissions in pilot-ignited natural gas engines

Stringent environmental policies and the ever-increasing demand for energy have triggered interest in novel combustion technologies that use alternative fuels as energy sources. Of these, pilot-ignited natural gas engines that employ small diesel pilots (∼1-5 per cent on an energy basis) to compression ignite a premixed natural gas-air mixture have received considerable attention. This paper discusses the effect of intake charge temperature and pilot injected quantity on the onset of ignition (ΔIGN) and combustion (ΔCOM) in a pilot-ignited natural gas engine with specific focus on early diesel pilot injection [begining of injection (BOI) at about 60° before top dead centre (BTDC)] for low-load operation. Both ΔIGN and ΔCOM had a strong influence on performance and emissions at 60° BTDC. At advanced BOI for both half and quarter-load operation, the best performance and hydrocarbon (HC) emissions could be obtained by optimally advancing ΔIGN relative to TDC and minimizing the cyclic variability in the ΔIGN. Furthermore, a clear dependence of ΔCOM on ΔIGN was observed with the optimally advanced and the least-variable ΔIGN producing the least ΔCOM variations. Engine performance, stability, and emissions were more sensitive to intake charge temperatures in comparison with pilot injected quantities. The best improvement in performance and emissions was obtained with increasing intake temperature at half load, where fuel conversion efficiency (FCE) increased from approximately 31 per cent to 38 per cent, coefficient of variation of indicated mean effective pressure (COVIMEP) decreased from about 11 per cent to 4 per cent, and HC emissions decreased from 72 to 23 g/kW h, while oxides of nitrogen (NOx) emissions increased from 16 to 142 mg/kW h. Performance and emissions trends at quarter load were similar to those observed at half load.

THREE-DIMENSIONAL MODELING OF SELF-IGNITION IN HCCI AND CONVENTIONAL DIESEL ENGINES

This paper addresses the problem of self-ignition modeling of diesel-related fuels in the framework of engine combustion three-dimensional computational fluid dynamics simulations. A model based on the evolution of a self-ignition progress variable is proposed and implemented in the KMB code. The evolution of the progress variable is determined cell by cell and is a function of a local self-ignition delay that depends on the cell fresh mixture thermodynamic conditions. These are temperature, pressure, equivalence ratio, and residual gas percentage. The local delays are calculated via interpolation inside an extensive ignition delay database created a priori. Each delay in the database is determined through complex chemistry calculations using SENKIN along with a kinetic mechanism from the literature developed for self-ignition modeling of n-heptane. The mechanism has been validated over a broad range of thermodynamic conditions. The ignition database covers the cold flame regime allowing application of the model to homogeneous charge compression ignition (HCCI) combustion. The model is validated through a series of tests including constant-volume self-ignition and homogeneous adiabatic compressed charge self-ignition. Results of the model are finally compared to experimental data of HCCI combustion as well as conventional diesel engine combustion with early pilot injection where cold flame self-ignition is a first-order phenomenon. The comparisons show the feasibility and accuracy of the model in computing industrial configurations.