Gasoline compression ignition (GCI) is a low temperature combustion (LTC) concept that lias been gaining increasing interest over the recent years owing to its potential to achieve diesel-like thermal efficiencies with significantly reduced engine-out nitrogen oxides (NOx) and soot emissions compared to diesel engines. In this work, closed-cycle computational fluid dynamics (CFD) simulations are peifarmed o f this combustion mode using a sector mesh in an effort to understand effects o f model settings on simulation results. One goal o f this work is to provide recommendations fo r grid resolution, combus tion model, chemical kinetic mechanism, and turbulence model to accurately capture experimental combustion characteristics. Grid resolutions ranging from 0.7mm to 0.1 mm minimum cell sizes were evaluated in conjunction with both Reynolds averaged Navier-Stokes (RANS) and large eddy simulation (LES) based turbulence models. Solu tion o f chemical kinetics using the multizone approach is evaluated against the detailed approach o f solving chemistry in every cell. The relatively small primary reference fuel (PRF) mechanism (48 species) used in this study is also evaluated against a larger 312-species gasoline mechanism. Based on these studies, the following model settings are chosen keeping in mind both accuracy and computation costs-0.175 mm minimum cell size grid, RANS turbulence model, 48-species PRF mechanism, and multizone chem istry solution with bin limits o f 5 K in temperature and 0.05 in equivalence ratio. With these settings, the performance o f the CFD model is evaluated against experimental results corresponding to a low h a d start o f injection (SOI) timing sweep. The model is then exercised to investigate the effect o f SOI on combustion phasing with constant intake valve closing (IVC) conditions and fueling over a range o f SOI timings to isolate the impact o f SO! on charge preparation and ignition. Simulation results indicate that there is an optimum SOI timing, in this case -3 0 deg aTDC (after top dead center), which results in the most stable combustion. Advancing injection with respect to this point leads to significant fuel mass burning in the colder squish region, leading to retarded phasing and ultimately misfire fo r SOI timings earlier than -4 2 deg aTDC. On the other hand, retarding injection beyond this optimum timing results in reduced residence time avail able fo r gasoline ignition kinetics, and also leads to retarded phasing, with misfire at SOI timings later than -IS d e g a T D C .

No abstract

In this work, we have studied cycle-to-cycle variation in a spark-ignited engine using large-eddy simulation in conjunction with the G-equation combustion model. A single cylinder of a four-cylinder port-fueled spark-ignited engine was simulated. A total of 49 consecutive full cycles were computed. The operating condition studied in this work is stoichiometric and stable and represents a load of 16 bar brake mean effective pressure and an engine speed of 2500 r/min. The computational fluid dynamics simulation shows good agreement in terms of in-cylinder pressure prediction with respect to the experiments and is also able to capture the range of cycle-to-cycle variation observed in experiments. Furthermore, neither the simulation nor the experiments show any distinguishable pattern in the sequence of high and low cycles. We numerically decoupled the effects of variations in equivalence ratio fields and velocity fields to isolate the effects of differences in the velocity field and differences in the equivalence ratio field on flame development and propagation. Based on this study, we inferred that for this engine, under the operating conditions studied, the differences in burn rates can be attributed to the differences in the velocity flow-field in the region around the spark gap during ignition. We then performed an analysis to identify the correlation between peak cylinder pressure and flame topologies over all the simulated cycles. We found that high cycles (higher peak cylinder pressure values) are strongly correlated to flatter flame volume shapes (flattened in the piston-to-head direction) and volumes that are more symmetric about the ignition axis. In addition, these kinds of flame volumes were found to correlate well with lower values of prior-to-ignition velocity going from the intake to the exhaust side (mean flow caused by tumble) at the spark and also higher values of prior-to-ignition velocity in the piston-to-head direction.

The effects of thermal and compositional stratification on the ignition and duration of homogeneous charge compression ignition combustion In this work, the effects of thermal and compositional stratification on the ignition and burn duration of homogeneous charge compression ignition (HCCI) combustion are studied with full-cycle 3D computational fluid dynamics (CFD) simulations with gasoline chemical kinetics performed during the closed portion of the cycle, from intake valve closing (IVC). The stratification was varied through the use of negative valve overlap (NVO) and positive valve overlap (PVO) breathing strategies. To remove charge energy and phasing effects from the simulation results, the fuel mass and ignition timing were held constant, while mean composition effects were isolated from those of local stratification by maintaining the same mean oxygen concentration, fuel-oxygen equivalence ratio and charge heat capacity. Fuel was premixed with the intake to avoid potential stratification effects arising from direct injection. With NVO, the incomplete mixing of the fresh charge with the large mass fraction of product gases retained within the cylinder from the previous combustion cycle leads to a 23% increase in the preignition thermal stratification, and an order of magnitude increase in the levels of stratifications in fuel to oxygen equivalence ratio and oxygen mole fraction relative to the PVO strategy, which employs a premixed mixture of fresh and product gases. Under the conditions studied, the use of NVO resulted in a 30% increase in the 10-90% burn duration (CA10-90) compared to the PVO condition. Two additional analyses were performed to decouple the effects of thermal and compositional stratification. The first examined the reaction space (based on the ignition delay distribution within the charge prior to ignition) for both breathing strategies to quantify the inherent reactivity stratification. The second examined the more stratified NVO case with a quasidimensional multi-zone model. These analyses revealed that under the conditions studied, HCCI reactivity and combustion duration are governed primarily by thermal stratification and are largely insensitive to compositional stratification at the time of ignition.

We have developed an accelerated multi-zone model for engine cycle simulation (AMECS) of homogeneous charge compression ignition (HCCI) combustion. This model incorporates chemical kinetics and is intended for use in system-level simulation software. A novel methodology to capture thermal stratification in the multi-zone model is proposed. The methodology calculates thermal stratification inside the cylinder based on a single computational fluid dynamics (CFD) calculation for motored conditions. CFD results are used for tuning zone heat loss multipliers that characterize wall heat loss from each individual engine zone based on the assumption that these heat loss multipliers can then be used at operating conditions different from those used in the single CFD run because the functional form of thermal stratification is more dependent on engine geometry than on operating conditions. The model is benchmarked against detailed CFD calculations and fully coupled HCCI CFD chemical kinetics calculations. The results indicate that the heat loss multiplier approach accurately predicts thermal stratification during the compression stroke and (therefore) HCCI combustion. The AMECS model with the thermal stratification methodology and reduced gasoline chemical kinetics shows good agreement with boosted gasoline HCCI experiments over a range of operating conditions, in terms of in-cylinder pressure and heat release rate predictions. The computational advantage of this method derives from the need for only a single motoring CFD run for a given engine, which makes the method very well suited for rapid HCCI calculations in system-level codes such as GT-Power, where it is often desirable to evaluate consecutive engine cycles.