Simulations of gasoline engine combustion and emissions using a chemical-kinetics-based turbulent premixed combustion modeling approach

Abstract

This work presents a turbulent premixed combustion modeling approach which is based on chemical kinetics. In this approach, the smallest length scales are of the order of 0.1–1.0 mm for typical engine simulations with a Reynolds-averaged Navier–Stokes turbulence model and, after adaptive mesh refinement technology is used to consider the magnitude of the subgrid field, the Reynolds-averaged Navier–Stokes turbulent flow field can be well resolved. For solution of the flame front, an artificially thickened laminar flame concept is introduced to balance the computational accuracy and the computational cost. Around the artificially thickened laminar flame front, a special grid resolution strategy is designed, i.e. using much finer resolution in the normal direction of the flame front and typical adaptive mesh refinement resolution in the other two perpendicular directions. Then, chemical kinetics can be applied to the chemistry process which occurs in the flame front. To use this chemical-kinetics-based turbulent premixed combustion modeling approach better, a good chemical kinetics mechanism is very important. For this reason a practical primary-reference-fuel chemical kinetics mechanism is improved and validated in present work. The newly improved mechanism resolves several issues in the existing mechanisms, including unrealistically fast autoignition reactions and limited laminar flame speed validation. After reoptimization of those laminar-flame-speed-related reactions, the new mechanism can correctly compute the laminar flame speeds for a wide range of Ford spark ignition engines and for various operating conditions. Using this combustion modeling approach together with the new mechanism, simulations of the combustion and the emissions of several spark ignition engines for typical operating conditions were carried out. The simulated in-cylinder pressures, the simulated burn rates, and the simulated emissions including the brake specific carbon monoxide emissions, the nitrogen oxide emissions, and the unburned hydrocarbon emissions are compared with the experimental data, and very good agreement is found without tuning any model constants.

The challenges of using detailed chemistry model for simulating direct injection spark ignition engine combustion during cold-start

“…It is worth mentioning that all the issues explained so far have less influence on the SAGE model performance under warm operating conditions, where it has been successfully validated in a RANS framework. [7][8][9][10][11] The requirement for a mesh resolution of the order of laminar flame thickness and issues related to capturing diffusivity coefficients, as observed during cold-start conditions, are less significant under warm operating conditions. Under these conditions, the turbulent flame thickness is 1-4 mm, 33 which could be well resolved using a practical grid size of 500 mm.…”

“…In spite of this, the model has been successfully used for capturing premixed gasoline engine combustion as evident from the multiple publications from academia and industry. [7][8][9][10][11][12][13][14] However, most of the validations have been provided for hot operating conditions, where the flame travel is assisted by a significant amount of in-cylinder turbulence. For example, Yang 10 performed extensive validations of the SAGE model in two different spark-ignited engines.…”

An improved TRF mechanism for a new turbulent premixed combustion model with application to engine combustion CFD

“…The base TRF mechanism introduced in Section 2.1 together with the chemical-kinetics-based turbulent premixed combustion modeling approach introduced and justified by Yang 10,11 were used for the combustion CFD simulation of two GTDI engines running at two different operating conditions. In the simulation, all common setup parameters are kept the same as those used in the engine transient cold start operating conditions as presented in Yang’s work 11 except for some specific engine warm-up parameters.…”

Development of a 5-component gasoline surrogate model using recent advancements in the detailed H2/O2/CO/C1-C3 mechanism for decoupling methodology

“…Yang et al [19] improved the mechanisms from Liu et al's work using laminar flame speeds for practical engine operating conditions, and successfully applied the improved mechanism to the simulation of gasoline engine combustion and emissions with very good agreements to experimental data. Using the same methodology, Liu et al [20] expanded their PRF mechanism to include toluene as a component of gasoline surrogate model and achieved good results.…”

“…In the mechanisms of Liu et al [17,18,20], Yang et al [19] and Chang et al [21], the 'core' part CO−C1 sub-mechanism is from Klippenstein et al [22] and Li et al [23] for the ignition of methanol at high 6 pressure by using the ab initio transition state theory, and the transition part C2-C3 sub-mechanism including six species (C2H3, C2H4, C3H4, C3H5, C3H6, and C3H7) is from the model of Patel et al [24] because of its small size. Recently, Kéromnès et al [25], Metcalfe et al [26], and Burke et al [27] have updated H2/O2/CO/C1 detailed sub-mechanism to characterize the kinetic and thermochemical properties of a large number of CO−C1 based hydrocarbons and oxygenated fuels over a wide range of experimental conditions.…”