Reaction-space analysis of homogeneous charge compression ignition combustion with varying levels of fuel stratification under positive and negative valve overlap conditions

Abstract

Full-cycle computational fluid dynamics simulations with gasoline chemical kinetics were performed to determine the impact of breathing and fuel injection strategies on thermal and compositional stratification, combustion and emissions during homogeneous charge compression ignition combustion. The simulations examined positive valve overlap and negative valve overlap strategies, along with fueling by port fuel injection and direct injection. The resulting charge mass distributions were analyzed prior to ignition using ignition delay as a reactivity metric. The reactivity stratification arising from differences in the distributions of fuel-oxygen equivalence ratio (f FO), oxygen molar fraction (x O2) and temperature (T) was determined for three parametric studies. In the first study, the reactivity stratification and burn duration for positive valve overlap valve events with port fuel injection and early direct injection were nearly identical and were dominated by wall-driven thermal stratification. nitrogen oxide (NO) and carbon monoxide (CO) emissions were negligible for both injection strategies. In the second study, which examined negative valve overlap valve events with direct injection and port fuel injection, reactivity stratification increased for direct injection as the f FO and T distributions associated with direct fuel injection into the hot residual gas were positively correlated; however, the latent heat absorbed from the hot residual gas by the evaporating direct injection fuel jet reduced the overall thermal and reactivity stratification. These stratification effects were offsetting, resulting in similar reactivity stratification and burn durations for the two injection strategies. The higher local burned gas temperatures with direct injection resulted in an order of magnitude increase in NO, while incomplete combustion of locally over-lean regions led to a sevenfold increase in CO emissions compared to port fuel injection. The final study evaluated positive valve overlap and negative valve overlap valve events with direct injection. Relative to positive valve overlap, the negative valve overlap condition had a wider reactivity stratification, a longer burn duration and higher NO and CO emissions associated with reduced fuel-air mixing.

Computational Fluid Dynamics Simulation of Gasoline Compression Ignition

“…This analysis is similar to the reactivity stratification analysis per formed by Kodavasal et al for HCCI ([54,56,57]). We observe that just based on computing ignition delays without any consider ation of the thermodynamic-time history of the charge from SOI up to that point, the -18 deg aTDC SOI case shows a shorter minimum ignition delays within the charge compared to the -30 deg aTDC SOI case, which would lead us to believe that this case would ignite before the optimum -30 deg aTDC SOI case.…”

The effects of boost pressure on stratification and burn duration of gasoline homogeneous charge compression ignition combustion

“…Using this approach, the computational expense of the chemical kinetics is reduced by an order of magnitude relative to detailed chemistry in every cell. 24 The kinetics calculations in this work use a reduced 312-species reaction mechanism based on the detailed gasoline reaction mechanism from Mehl et al 25 along with the matched four-component gasoline surrogate fuel 26 shown in Table 2. This surrogate and reduced gasoline mechanism captures gasoline fuel chemistry attributes such as the low, negative temperature coefficient (NTC) and high temperature ignition predicted by the detailed parent mechanism.…”

A thermodynamic model for homogeneous charge compression ignition combustion with recompression valve events and direct injection: Part I — Adiabatic core ignition model

“…Using this approach, the computational expense of the chemical kinetics is reduced by an order of magnitude relative to detailed chemistry in every cell. 25 The kinetics calculations use a reduced 312-species mechanism based on the detailed gasoline mechanism from Mehl et al 32 along with the matched fourcomponent gasoline surrogate 33 shown in Table 2. This surrogate and reduced gasoline mechanism captures gasoline fuel chemistry attributes such as the low, negative temperature coefficient (NTC) and hightemperature ignition predicted by the detailed parent mechanism.…”

“…Even though the temperature of the hottest charge is correctly predicted up to the time of ignition for both cases, the use of the mean composition for HCCI ignition modelling with NVO valve events may be problematic because of stratification. To assess the validity of using the mean composition along with the adiabatic core temperature for ignition modelling, the cumulative distribution of reactivity 25 is visualized with the ignition delay calculated in every CFD cell using Goldsborough's correlation 42 at the onset of ignition (u IGN = 212.5°CA). The differences in the iso-octane and gasoline ignition delays are essentially negligible for temperatures greater than 1000K [43,26].…”