Turbulence–chemistry interactions in a heavy-duty compression–ignition engine

Gasoline Compression Ignition (GCI) is a promising engine operating mode that can reduce maximum pressure rise rate (MPRR) without knock tendency and better control the combustion phasing compared to the Homogeneous Charge Compression Ignition (HCCI) by using a late direct-injection (DI). In this study, a 107species reduced mechanism and a 207-species skeletal mechanism were developed using the Computer Assisted Reduction Mechanism (CARM) and validated under engine conditions for a newly developed 5-component surrogate for a Haltermann 437 certification gasoline (AKI=93). Then, 3D computational fluid dynamics (CFD) simulations with an optimized grid size determined by a grid convergence study were performed with the 107species reduced mechanism and the 5-component certification gasoline surrogate. Two experimental boosted GCI cases with similar, moderate MPRR and heat release parameters, but different second DI timings (-52˚ aTDC and -5˚ aTDC), were validated and analyzed. For the -52˚ aTDC DI case, the combustion can be interpreted as a partially sequential auto-ignition due to the competition between the charge cooling effect and the equivalence ratio (ϕ)-sensitive effect of the stratified mixture, which is responsible for mitigating the MPRR. For the -5˚ aTDC DI case, the combustion can be decoupled into a partially sequential auto-ignition and a subsequent nonpremixed combustion by the DI fuel near top dead center in the compression stroke. The MPRR is relaxed through the slow, mixing-limited combustion between the injected fuel and the premixed mixture.

Section: Grid Convergence and Validationsupporting

confidence: 55%

Development of a reduced chemical mechanism targeted for a 5-component gasoline surrogate: A case study on the heat release nature in a GCI engine

Chen

Wolk

Mehl

et al. 2017

“…Soot modeling has been a challenging task and has drawn a lot of attention due to the importance of emission predictions at engine conditions. Some effort has been focused on developing different levels of details of soot models [52][53][54]. In this section, an empirical Hiroyasu soot model with C 2 H 2 as a soot precursor is used to assess its validity in the context of high-fidelity LES simulations.…”

Section: Soot Predictionsmentioning

confidence: 99%

Large eddy simulation of a reacting spray flame with multiple realizations under compression ignition engine conditions

Pei

Som

Pomraning

et al. 2015

174

An n-dodecane spray flame (Spray A from Engine Combustion Network) was simulated using a δ function combustion model along with a dynamic structure large eddy simulation (LES) model to evaluate its performance at engine-relevant conditions and to understand the transient behavior of this turbulent flame. The liquid spray was treated with a traditional Lagrangian method and the gas-phase reaction was modeled using a δ function combustion model. A 103-species skeletal mechanism was used for the n-dodecane chemical kinetic model. Significantly different flame structures and ignition processes are observed for the LES compared to those of Reynolds-averaged Navier-Stokes (RANS) predictions. The LES data suggests that the first ignition initiates in a lean mixture and propagates to a rich mixture, and the main ignition happens in the rich mixture, preferably less than 0.

“…A detailed investigation of in-cylinder radiative heat transfer is performed for a Volvo 13L production six-cylinder heavy-duty diesel truck engine. This is the same engine that was the subject of an earlier modeling study [15] that focused on the influence of unresolved turbulent fluctuations in composition and temperature on combustion chemistry and pollutant emissions (turbulencechemistry interactions -TCI). Key geometric parameters are a bore, stroke and compression ratio of 13.1 cm, 15.8 cm and 15.8:1, respectively.…”

Section: Engine Configuration and Operating Conditionsmentioning

confidence: 99%

“…The differences between results obtained using WSR versus tPDF models with respect to combustion and emissions (TCI effects) have been discussed in [15] . Here the focus is on radiative heat transfer.…”

Section: Model Combinations Consideredmentioning

confidence: 99%

A detailed modeling study of radiative heat transfer in a heavy-duty diesel engine

Paul

Fernandez

Haworth

et al. 2019

In recent years, the importance of radiative heat transfer in combustion has been increasingly recognized. Detailed models have become available that accurately represent the complex spectral radiative properties of reacting gas mixtures and soot particles, and new methods have been developed to solve the radiative transfer equation (RTE). At the same time, the trends toward higher operating pressures and higher levels of exhaust-gas recirculation in compression-ignition engines, together with the demand for higher quantitative accuracy from in-cylinder CFD models, has led to renewed interest in radiative transfer in engines. Here an in-depth investigation of radiative heat transfer is performed for a heavy-duty diesel truck engine over a range of operating conditions. Results from 10 different combinations of turbulent combustion models, spectral radiation property models, and RTE solvers are compared to provide insight into the global influences of radiation on energy redistribution in the combustion chamber, heat losses, and engine-out pollutant emissions (NO and soot). Also, the relative importance of the individual contributions of molecular gas versus soot radiation, the spectral model, the RTE solver, and unresolved turbulent fluctuations in composition and temperature (turbulence-radiation interactions-TRI) are investigated. Local instantaneous temperatures change by as much as 100 K with consideration of radiation, but the global influences of radiation on heat losses and engine-out emissions are relatively small (in the 5-10% range). Molecular gas radiation dominates over soot radiation, consideration of spectral properties is essential for accurate predictions of reabsorption, a simple RTE solver (a first-order spherical harmonics-P 1-method) is sufficient for the conditions investigated, and TRI effects are small (less than 10%). While the global influences of radiation are relatively small, it is nevertheless desirable to explicitly account for radiation in in-cylinder CFD. To that end, a simplified CFD radiation model has been proposed, based on the findings reported here.