Analysis of an HCCI engine combustion using toluene reference fuel for different equivalence ratios – Comparison of experimental results with CFD and SRM simulations

Experimental engine design studies require very expensive sets thus in recent years computer simulations efforts are becoming more useful considering lower cost and less analysis time.

Combustion is one of the most difficult optimization process in engine design and it is one of the most important phenomena for reducing fuel consumption and exhaust emissions.

Zero and multi-dimensional simulation methods are developed by many researchers and software developers for model engine combustion to increase the engine efficiency and decrease the negative effect of the combustion engines on environment.

The biggest question on using simulation codes is the accuracy of the results and the limitation and reliability.

Therefore these codes are still needed to be investigated in terms of solution consistency.

Many studies show that CFD is one of the most reliable tools for simulating engine combustion.

Kong numerically studied natural gas and dimethyl ether mixture in a homogeneous charge compression ignition (HCCI) engine to investigate effects of the fuel mixture at different operating conditions.

With this study, he showed the CFD model consistency by using detailed chemical kinetic mechanism.

Zheng and Yao simulated the HCCI combustion with detailed and reduced mechanisms at CFD tool and they compared the results with experimental study.

After validating results, they continue their study by using CFD to investigate effects of direct injection ratios and timings at HCCI combustion.

Poorghasemi et al. studied CFD simulation of a natural gas fueled HCCI combustion with using detailed and reduced mechanisms for different operating conditions.

They showed consistency of CFD simulation strategies and investigated detailed and reduced mechanisms effects at simulation results.

Stochastic Reactor Model (SRM) is based on Probability Density Function (PDF) approach developed for an alternative method to simulate engine combustion.

The SRM method can provide low cost simulation regarding CPU time by solving the combustion with chemical kinetic mechanisms which are very important to obtain detailed information about combustion phenomena.

Development of SRM method at engine combustion started in 2000 with the study of Kraft et al. by developing PDF method to simulate HCCI engine.

Various developments for more simulations of the engine combustion by using SRM simulation strategy, and studies to verify SRM technique,,, can be accessed in literature.

Furthermore, different concepts of engine combustion applications were simulated with SRM to show the simulation capability range.

Because of the solution cost of detailed kinetic models at CFD and also SRM simulations, researchers have been developed proper semi-detail or reduced kinetic models.

A semi-detailed mechanism for TRF was developed and tested experimentally at a shock tube and an HCCI engine by Andrae et al..

They obtained good agreement for ignition delay times at shock tube tests, however some disagreement occurred at combustion timing for HCCI engine.

A reduced TRF mechanism was developed by Machrafi and obtained good agreement with experimental studies for cylinder pressure, emission data and ignition delay time.

Qing-Feng et al. were developed a reduced TRF mechanism and validated with experiment for ignition delay time that obtained good approximations.

In this study, an HCCI engine combustion simulations were performed by using a reduced and a semi-detailed chemical mechanisms of TRF.

The CFD and SRM combustion simulation tools used to model HCCI combustion at four different lambda (λ).

One of the major objectives of this work is to evaluate the performance of two different TRF mechanisms and simulation tools by comparing with experimental result.

ANSYS-Fluent 18.1 software for CFD simulations and SRM Engine Suite software for SRM simulations were used to model the HCCI combustion.

The reduced chemical mechanism developed by Machrafi containing 49 species and 62 reactions and, also the semi-detail mechanism developed by Andrea et al. containing 137 species and 633 reactions were used for TRF combustion.

Computational results compared with experimental results in terms of pressure, heat release rate, CO, CO2 and O2.

Also, simulation results of toluene, n-heptane, OH and H2O2 were comparatively given depending on crank angle degree (CAD).

Conclusions

The main objective of this study is to understand the different simulation strategies with different chemical mechanisms behaviors at different air-fuel mixtures of TRF combustion at an HCCI engine. Thus, performance of CFD and SRM methods with a reduced and a semi-detailed mechanisms on TRF combustion were investigated. Four different lambda (λ) were chosen to be able to compare the simulation results with the experimental results for TRF-79 fuel. As can be understood from the study SRM and CFD simulation strategies with reduced and semi-detailed mechanism have advantages and disadvantages over each other. The outcomes can be summarized as:

• It is observed that both simulation methods with reduced mechanism have good agreement for lambda (λ) 3.5 and 4 with experimental pressure and HRR results. Furthermore CFD simulations with reduced mechanism also have good agreement at lambda (λ) 4.75 and 5 for experimental pressure and HRR results.

• Simulations with semi-detailed mechanism were acceptable consistency with experimental pressure and HRR results for all lambda (λ) values. The most important advantage to use semi-detailed mechanism was to prediction of early cold combustion reaction which is important for HCCI combustion.

• Because of the capability of semi-detailed mechanism’s prediction of cold combustion reaction phase, exhaust gas products result have slightly closer to the experimental CO, CO2 and O2 values than reduce mechanism results.

• SRM has easy to use and fast on preparing engine simulation with breathing model which can give reliable emission results than close cycle models.

• The computational cost between CFD and SRM methods is different for identical case that SRM can solve much faster than CFD. Here it can be conclude that because of the huge difference in computational cost SRM would be chosen in HCCI engine design for parametric simulations instead CFD. But it should be noted that recent developments at CFD solvers and new mesh strategies as like adaptive mesh refinement can significantly decrease the solution time.

Reference: G. Coskun, Y. Delil, U. Demir, Analysis of an HCCI engine combustion using toluene reference fuel for different equivalence ratios – Comparison of experimental results with CFD and SRM simulations, Fuel. (2019) 217–227. doi:10.1016/j.fuel.2019.03.046.

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