Variations in the in cylinder flow field which result from differences in the intake flow are known to have important effects on the performance and emissions behavior of diesel engines. The intake flow and combustion in a heavy duty DI diesel engine with a dual valve port have been simulated using the computational fluid dynamics code KIVA-3. Variation of the in-cylinder flow field has been achieved by varying the intake valve timing. Variations in the in-cylinder flow, including a range of length scales, degrees of inhomogeneity in a number of scalar and vector quantities, and the persistence of various flow structures, are compared, and their significance to combustion and emissions parameters are assessed. The interaction of fuel spray parameters, particularly spray-wall interaction with structures present in the flow field are evaluated.
INCREASINGLY STRICT REQUIREMENTS for NOx and soot emissions continue to drive the effort to improve computational models related to diesel combustion. In recent years, experimental studies and computational modeling work have led to improvements in the emissions performance of diesel engines as well as advancements of the models themselves. One particularly interesting and difficult problem in optimizing diesel engines is the simultaneous reduction of NO and soot emissions. Recently, a number of important advancements have been in areas such as the understanding and modeling of high pressure fuel spray behavior, autoignition, fuel droplet combustion, NOx production, and soot production and oxidation. Researchers have been successful using combined experimental and computational approaches to accurately measure and reproduce trends in emissions behavior [1]. Further, modeling efforts have been instrumental in suggesting strategies for simultaneously reducing soot and NO emissions [2].
It has been known for many years that intake flow has important effects on the combustion behavior and emissions performance in diesel engines. For example, in small direct-injected (DI) diesel engines, swirl can increase the rate of fuel-air mixing, reducing the combustion duration at retarded injection timings [3]. Swirl interaction with compression induced squish flow increases turbulence levels in the combustion bowl, promoting mixing [4]. On the other hand, it has been shown that there exists a level of swirl beyond which the fuel-air mixing in the piston can actually be diminished due to adverse swirl-squish interaction [5]. In addition, creation of excess swirl can also diminish engine efficiency due to increased pumping losses. It is now possible to compute intake flow in detail, including the effects of moving intake valves. Examples of such simulations include the modeling of intake flow through a dual valve port in a heavy-duty industrial diesel engine [6] and of flow in a 4 valve per cylinder direct injected gasoline engine [7]. Previous work has also focused on the effects on fuel spray of changes in the in-cylinder flow field generated by changes in the piston bowl geometry [8]. A further step in understanding intake flows through modeling is to study their effect on combustion. The availability of higher performance computers and CFD codes with advanced grid structure has made detailed modeling of intake flow and its effect on fuel-air mixing a reality. In addition, modeling of the effect of intake flow on global combustion parameters, such as heat release rate, average pressure, and average turbulent kinetic energy has been performed in both diesel [9] and spark ignited engines [10].
Pollutant formation and oxidation processes are very dependent on local conditions such as temperature and species densities. For this reason, detailed studies of the effect of intake generated in-cylinder flow on emissions performance need to emphasize local flow details in addition to cylinder averaged quantities.
In this work, we attempt to relate the emissions performance of a heavy duty dual port DI diesel engine to changes in the intake flow. Table 1 gives operating parameters for the Caterpillar 3406 single cylinder engine. Differences in the intake flow are accomplished by varying the intake valve lift profiles. Because the formation of pollutants occurs at length scales smaller
than the dimensions of the combustion chamber, we compare flow field details at several length scales. Since details of the intake flow field are often dissipated prior to fuel injection, the persistence of structures in the flow field during the compression stroke is also assessed. In the next section, details of the models used as well as the methods used to analyze the flow field details are given.
than the dimensions of the combustion chamber, we compare flow field details at several length scales. Since details of the intake flow field are often dissipated prior to fuel injection, the persistence of structures in the flow field during the compression stroke is also assessed. In the next section, details of the models used as well as the methods used to analyze the flow field details are given.