Dezhi Zhou

An Enhanced Primary Reference Fuel Mechanism Considering Conventional Fuel Chemistry in Engine Simulation

A compact and accurate primary reference fuel (PRF) mechanism which consists of 46 species and 144 reactions was developed and validated to consider the fuel chemistry in combustion simulation based on a homogeneous charged compression ignition (HCCI) mechanism. Some significant reactions were updated to ensure its capabilities for predicting combustion characteristics of PRFs. To better predict the laminar flame speed, the relevant C2–C3 carbon reactions were coupled in. This enhanced PRF mechanism was validated by available experimental data references including ignition delay times, laminar flame speed, premixed flame species concentrations in jet stirred reactor (JSR), rapid compression machine (RCM), and shock tube. The predicted data was calculated by chemkin-ii codes. All the comparisons between experimental and calculated data indicated high accuracy of this mechanism to capture combustion characteristics. Also, this mechanism was integrated into kiva4–chemkin. The engine simulation data (including in-cylinder pressure and apparent heat release rate (HRR)) was compared with experimental data in PRF HCCI, partially premixed compression ignition (PCCI), and diesel/gasoline dual-fuel engine combustion data. The comparison results implied that this mechanism could predict PRF and gasoline/diesel combustion in computational fluid dynamic (CFD) engine simulations. The overall results show this PRF mechanism could predict the conventional fuel combustion characteristics in engine simulation.

An Enhanced PRF Mechanism Considering Conventional Fuel Chemistry in Engine Simulation

A compact and accurate primary reference fuel (PRF) mechanism which consists of 46 species and 144 reactions was developed and validated to consider the fuel chemistry in combustion simulation based on a homogenous charged compression ignition (HCCI) mechanism. Some significant reactions were updated to ensure its capabilities for predicting combustion characteristics of PRF fuels. To better predict laminar flame speed, the relevant C2-C3 carbon reactions was coupled in. This enhanced PRF mechanism was validated by available experimental data references including ignition delay times, laminar flame speed, premixed flame species concentrations in jet stirred reactor (JSR), rapid compression machine and shock tube. The predicted data was calculated by CHEMKIN-II codes. All the comparisons between experimental and calculated data indicated high accuracy of this mechanism to capture combustion characteristics. Also, this mechanism was integrated into KIVA4-CHEMKIN. The engine simulation data (including in-cylinder pressure and apparent heat release rate (HRR)) was compared with experimental data in PRF HCCI, partially premixed compression ignition (PCCI) and diesel/gasoline dual-fuel engine combustion data. The comparison results implied that this mechanism could predict PRF and gasoline/diesel combustion in CFD engine simulations. The overall results show this PRF mechanism could predict the conventional fuel combustion characteristics in engine simulation.Copyright

Soot and NO emissions control in a natural gas/diesel fuelled RCCI engine by φ-T map analysis

Soot and NO emissions are considered as major pollutants to the atmosphere from compression ignition engines. Researchers have been dedicated to the reduction of soot and NO emissions. Thus, an advance combustion regime, i.e. reactivity controlled compression ignition (RCCI), was proposed to mitigate the formation of these emissions. In this study, the dynamic ϕ-T (equivalence ratio vs. temperature) map analysis was applied to visualise the combustion processes associated with the in-cylinder temperature and equivalence ratio in an RCCI engine. Therefore, the soot and NO emissions can be efficiently reduced by controlling the combustion process out of the emissions islands on the ϕ-T map. This analysis method employs KIVA4-CHEMKIN and SENKIN code to construct ϕ-T maps under various conditions. To find out the significant parameters of controlling combustion process and emissions formation, four parameters were taken into consideration in a natural gas (NG) and diesel fuelled RCCI engine: NG percentage, the first start of injection (SOI) timing, split fraction of diesel and exhaust gas recirculation (EGR) rate. Each parameter was varied at three levels. Finally, the ϕ-T maps and final soot and NO emissions were compared among varied conditions for each parameter. It is found that the increased NG percentage can significantly reduce soot because of the absence of C-C bond in NG and the reduced diesel fuel impingement on the surface of the piston or cylinder wall. Increasing EGR can decrease the peak combustion temperature due to the dilution effect and thermal effect, consequently maintaining RCCI at low temperature combustion region. This study also indicates that dynamic ϕ-T map analysis is efficient at manipulating the combustion process to mitigate the soot and NO emissions formation.

Application of Dynamic ϕ–T Map: Analysis on a Natural Gas/Diesel Fueled RCCI Engine

In this study, dynamic ϕ-T map analysis was applied to an RCCI (reactivity controlled compression ignition) engine fueled with NG (natural gas) and diesel. The combustion process of the engine was simulated by coupled KIVA4-CHEMKIN with a DOS (diesel oil surrogate) chemical mechanism. The ϕ-T maps were constructed by the mole fractions of soot and NO obtained from SENKIN and ϕ-T conditions from engine simulations. Five parameters, namely NG fraction, 1st SOI (start of injection) timing, 2nd SOI timing, 2nd injection duration and EGR (exhaust gas recirculation) rate were varied in certain ranges individually, and the ϕ-T maps were compared and analyzed under various conditions. The results revealed how the five parameters would shift the ϕ-T conditions and influence the soot-NO contour. Among the factors, EGR rate could limit the highest temperature due to its dilute effect, hence maintaining RCCI combustion within LTC (low temperature combustion) region. The second significant parameter is the premixed NG fraction. It could set the lowest temperature; moreover, the tendency of soot formation can be mitigated due to the lessened fuel impingement and the absence of C-C bond. Finally, the region of RCCI combustion was added to the commonly known ϕ-T map diagram.Copyright

Impact of Urea Direct Injection on NOx Emission Formation of Diesel Engines Fueled by Biodiesel

There are many NOx removal technologies: exhaust gas recirculation (EGR), selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), miller cycle, emulsion technology and engine performance optimization. In this work, a numerical simulation investigation was conducted to explore the possibility of an alternative approach: direct aqueous urea solution injection on the reduction of NOx emissions of a biodiesel fueled diesel engine. Simulation was performed using the 3D CFD simulation software KIVA4 coupled with CHEMKIN II code for pure biodiesel combustion under realistic engine operating conditions of 2400 rpm and 100% load. To improve the overall prediction accuracy, the Kelvin-Helmholtz and Rayleigh-Taylor (KH-RT) spray break up model was implemented in the KIVA code to replace the original Taylor Analogy Breakup (TAB) model for the primary and secondary fuel breakup processes modeling. The KIVA4 code was further modified to accommodate multiple injections, different fuel types and different injection orientations. A skeletal reaction mechanism for biodiesel + urea was developed which consists of 95 species and 498 elementary reactions. The chemical behaviors of the NOx formation and Urea/NOx interaction processes were modeled by a modified extended Zeldovich mechanism and Urea/NOx interaction sub-mechanism. Developed mechanism was first validated against the experimental results conducted on a light duty 2KD FTV Toyota car engine fueled by pure biodiesel in terms of in-cylinder pressure, heat release rate. To ensure an efficient NOx reduction process, various aqueous urea injection strategies in terms of post injection timing and injection rate were carefully examined. The simulation results revealed that among all the four post injection timings (10 °ATDC, 15 °ATDC, 20 °ATDC and 25 °ATDC) that were evaluated, 15 °ATDC post injection timing consistently demonstrated a lower NO emission level. In addition, both the urea/water ratio and aqueous urea injection rate demonstrated important roles which affected the thermal decomposition of urea into ammonia and the subsequent NOx removal process, and it was suggested that 50% urea mass fraction and 40% injection rate presented the lowest NOx emission levels.Copyright

Application of Dynamic Φ-T Map: Analysis on a Natural Gas/Diesel Fueled RCCI Engine

In this study, dynamic ϕ-T map analysis was applied to an RCCI (reactivity controlled compression ignition) engine fueled with NG (natural gas) and diesel. The combustion process of the engine was simulated by coupled KIVA4-CHEMKIN with a DOS (diesel oil surrogate) chemical mechanism. The ϕ-T maps were constructed by the mole fractions of soot and NO obtained from SENKIN and ϕ-T conditions from engine simulations. Five parameters, namely NG fraction, 1st SOI (start of injection) timing, 2nd SOI timing, 2nd injection duration and EGR (exhaust gas recirculation) rate were varied in certain ranges individually, and the ϕ-T maps were compared and analyzed under various conditions. The results revealed how the five parameters would shift the ϕ-T conditions and influence the soot-NO contour. Among the factors, EGR rate could limit the highest temperature due to its dilute effect, hence maintaining RCCI combustion within LTC (low temperature combustion) region. The second significant parameter is the premixed NG fraction. It could set the lowest temperature; moreover, the tendency of soot formation can be mitigated due to the lessened fuel impingement and the absence of C-C bond. Finally, the region of RCCI combustion was added to the commonly known ϕ-T map diagram.Copyright