Bu çalışmada, özellikle ağır vasıta dizel motorunda azot oksit (NOx) ve is (C(s)) emisyonlarının oluşumu ve azaltılması, Sayısal Akışkanlar Dinamiği (CFD) modellemesi yardımıyla incelenmektedir. KIVA-3VR2 ve CHEMKIN-II paketi modelleme için kullanılmıştır. Chalmers University of Technology’de geliştirilen n-heptan ve toluen karışımından oluşan dizel yakıt modeli (Disesel Oil Surrogate, DOS), türbülans/yanma etkileşimin içini kısmi karışımlı reaktör modeli (Partially Stirred Reactor, PaSR), detaylı reaksiyon mekanizması ve geliştirilmiş demet modeli KIVA-3VR2'ye uyarlanmış ve modellemeler gerçekleştirilmiştir. Dizel yakıt modeli ve reaksiyon mekanizması sabit hacimli bomba deneylerinde, değişik basınç ve hava fazlalık katsayısı (HFK, l), değerleri için, sıcaklığa bağlı tutuşma gecikmesi (TG) esas alınarak doğrulanmıştır. Sonraki aşamada Volvo D12C ağır vasıta dizel motorunda 18.5 ve 14.0 sıkıştırma oranları, farklı yük, püskürtme zamanı değerleri ve egzoz gazı geri dönüşü (EGR) oranları için modelleme çalışmaları yapılarak deney verileri ile karşılaştırılmıştır. Elde edilen sonuçlara göre, silindir içi basınç ve sıcaklık değerleri, ısı açığa çıkış hızı ve yanma verimi deney sonuçlarıyla uyumludur. NOx ve is emisyonları eğilim olarak deney sonuçlarıyla uyumludur ancak nicel olarak geliştirmeye ihtiyaç vardır. Emisyonlardaki farklılığın nedeni olarak, modelleme için kullanılan detaylı reaksiyon mekanizmasındaki is yanması, NOx ve CO oluşum reaksiyonları arasındaki çok hassas ve birbirini etkileyen dengenin henüz tam olarak sağlanamamış olması gösterilebilir. Bu konuda geliştirmeye ihtiyaç duyulmaktadır. Anahtar Kelimeler: Dizel motoru, modelleme, NOx-is emisyonları. The main emission problem for conventional diesel combustion is NOx-soot tradeoff (diesel dilemma) which could not be completely eliminated with the in-cylinder combustion techniques till now and still after-treatment process is necessary to meet the present emission legislations. Also with the development of the new engines which have different combustion regimes such as Homogeneous Charge Compression Ignition (HCCI), Modulated Kinetics (MK), Low Temperature Combustion (LTC), Premixed Charge Compression Ignition (PCCI), new emissions such as HC and CO became significant for compression ignition engines. This study mainly investigates formation and reduction of NOx and soot emissions in diesel engine combustion, especially in Heavy Duty Diesel (HDD) engines with the help of CFD engine modeling. KIVA-3VR2 and CHEMKIN-II package were used for the modeling purposes. CHALMERS' Diesel Oil Surrogate (DOS) model based on a blend of aliphatic (n-heptane, 70%) and aromatic (toluene, 30%) components, turbulence/chemistry interaction approach with Partially Stirred Reactor (PaSR) model, detailed chemical mechanism and modified spray model were implemented into the KIVA-3VR2 code for the modeling tasks. DOS and detailed chemical mechanism were validated comparing the ignition delay (ID) times with the present available shock tube data for different temperatures at different pressure and excess air ratios, l. Validation of the DOS and its constituents shows that developed reaction mechanism represents well enough ID and Negative Temperature Dependence behavior. Also it is calculated that for different l values (1 / Equivalence Ratios, ER) ignition delay time reduces with the increasing of ER (e.g., for rich mixtures) which is consistent with the experiments. Then modeling results for Volvo D12C engine at MK combustion (Compression Ratio, CR, 18.0) and LTC (CR = 14.0) regimes were compared with the experimental data. Present reaction mechanism is modified in order to improve its NOx-soot emissions behavior which has a good emission calculation tendency, but still quantitatively weak. Different fuel injection times, loads and both EGR-free and EGR cases were studied to extend the modeling capability. The MK combustion regimes in the Volvo D12C DI diesel engine realized under selected retarded fuel injections (e.g. Start of Injection, SOI, varied from -5 till to 10 Crank Angle Degree (CAD) After Top Dead Centre (ATDC) causing the ID to be longer than the injection duration. For injection timings, -5, 0, 5 and 10 ATDC, predicted trends clearly indicate that the combustion mode shifts from conventional diesel-like to HCCI-like mode following from early to late injection cases. This behavior is not clear in the pressure vs. crank angle curves because these retarded injections corresponds to the expansion part of the cycle, and expansion avoids sudden pressure rise sourced from HCCI-like combustion. However it is clearly visible from the Rate of Heat Release (RoHR) curves that, in retarded injection conditions, RoHR maxima increase as expected because of the increased premixed combustion phase, which increases ID times in such a way that ignition occurs after the completion of fuel injection process similar to the HCCI combustion mode. For all cases, calculation results for in-cylinder pressure, temperature, RoHR and combustion efficiency are in a good agreement with experimental results. NOx and soot emissions are reasonably well also. Although tendency of the calculated emissions is good, a quantitative improvement for emission predictions, especially for soot emissions, is required. In diesel combustion, most of the combustion process takes place over soot-formation and soot-oxidation path and soot emission is the difference between these formed and oxidized soot amount, which corresponds to only a small fraction of formed soot ( =1%). Hence modeling of soot emissions is very hard, e.g., even 1% calculation error for soot oxidation will drastically affect the resulted soot emissions. NOx formation is also strictly coupled with the soot-oxidation process because the remained oxygen radical which is necessary for NO formation depends on the soot-oxidation process. If soot-oxidation part is dominant in the mechanism, then excessive soot oxidation process can result lower soot emissions than the real amount by consuming most of the available oxygen radical which will be used later for the NO formation reactions. In conclusion excessive soot oxidation gives less soot emissions and indirectly prevents proper amount of NO formation. For this reason more accurate modeling of NOx-soot emissions in the presented detailed chemistry approach requires a proper balance between soot-oxidation and NO, CO formation reactions. Keywords: Diesel combustion, NOx-soot emissions, CFD modeling.
