Effects of variable valve timing on the exhaust thermal management of a turbocharged & intercooled diesel engine

Emissions from diesel engines have become recently a significant problem due to their positive effect on global warming and negative effects on human health. Recently, NOx and PM emission regulations for diesel engines have become quite strict and the emission limits have decreased to very low numbers. Engine producers generally use exhaust thermal management systems to reduce the high rates of emissions discharged from diesel engines. Some typical aftertreatment systems are SCR, DPF and DOC. SCR is used to decrease NOx emissions, DPF is utilized to reduce PM emissions and DOC is mostly applied in a diesel engine system to diminish unburned hydrocarbons (HCs) and carbon monoxide. Although these aftertreatment systems are indeed effective to meet stringent emission regulations on diesel engines, they have a major drawback. These systems generally require high exhaust gas temperatures in order to have high emission conversion efficiencies. In other words, they are temperature-dependent and mostly need higher than 250oC catalyst temperatures to perform efficiently. However, exhaust gas temperatures are generally lower than 250oC for diesel engines operating at lower engine speed and lower engine loading conditions on the performance map. That not only leads to inefficient aftertreatment systems but also insufficient emission reduction for those engine performance points. One of the current methods to achieve greater than 250oC exhaust gas temperatures for these lower engine speed and lower engine loading areas is to use VVT. Changing the opening and closing timings of intake and exhaust valves, VVT can be applied to a diesel engine at any speed and at any load. Therefore, the aim of this study is to try to increase the exhaust gas temperatures of a diesel engine above 250oC for different engine speeds and different engine loadings via utilizing VVT. Change of inlet and exhaust maximum valve lifts is also examined in order to rise exhaust temperatures more in the system. The intention is to attain those high exhaust gas temperatures without causing any fuel consumption penalty in comparison to nominal valve timings. Firstly, a six-cylinder turbocharged and intercooled diesel engine system is modelled to analyze exhaust gas temperatures and also exhaust gas flow rates by using Lotus Engine Simulation (LES) program. The simulation has different elements (cylinders, pipes, ports, valves, turbocharger, intercooler etc.) and these components are explained in a detailed manner with mathematical formulations. Secondly, validation of the simulation is achieved for earlier and later intake valve closing (IVC) timings on a particular engine speed and engine loading point (1200 rpm and 2.50 bar bmep). The results of the simulation for turbine out temperature and for volumetric efficiency are generally compatible with the experimental results. It is shown on the validation that advancing IVC timing 65 degrees CA or retarding it 100 degrees CA from nominal timing is adequate in the system to reach 250oC turbine exit temperature (TET). Up to 55o C TET rise is obtained by sweeping IVC timings forward or backward. Also, when effect of IVC on other diesel engine performance parameters is examined, it is seen that advanced and retarded IVC timings results in fuel-saving. However, these timings also cause high exhaust flow rate reduction compared to nominal valve timings in the system. Early IVC (EIVC) leads to more exhaust flow reduction, but it is also found to be more fuel-efficient than later IVC. Thirdly, other VVT options are searched to combine with EIVC so as to attain higher than 250oC exhaust gas temperatures without causing a substantial reduction on exhaust flow rate. EEVC, LIVO and EEVO are all examined. EEVO is found to be the best option to combine with EIVC. Therefore, combined EIVC&EEVO is applied to the system. IVC is advanced 40 degrees CA from stock IVC timing and EVO is also advanced 70 degrees CA from nominal EVO timing. Although it is successful to satisfy 250oC TET in the system, it also causes a little bit more than 1 % bsfc penalty in comparison to nominal valve timings. Later, inlet and exhaust maximum valve lifts are analyzed to reduce the fuel penalty on the system. It is seen that when lower inlet and higher exhaust maximum lifts compared to nominal maximum lifts is utilized, not only does it enable the system to acquire greater than 250oC TET, but also it decreases bsfc penalty down to zero percent in comparison to nominal valve timings. Then, the method is implemented for different engine speeds (1000 rpm to 2000 rpm) and several engine loadings (1.0 bar to 5.0 bar bmep). When it is compared with TET and exhaust flow figures at nominal valve timings calculated on the previous part of the study, it is seen that 250oC target TET line can be decreased to lower engine loadings. For instance, for 1200 rpm engine speed, TET is obtained at only 2.50 bar bmep rather than satisfying the same temperature at 4.50 bar bmep with nominal valve timings and nominal maximum lifts. More importantly, this is achieved without requiring any extra fuel consumption compared to stock valve timings. Also, reduction of exhaust gas flow rates is not as high as the case where only IVC is used to meet the target TET for more efficient exhaust thermal management. Finally, general evaluation of the study is done by discussing the results achieved in the analysis and the study is concluded on the final part with recommendations for future work. This study is expected to provide insight into the utilization of VVT technique on diesel engines to raise TET particularly on lower engine speed and lower engine loading points of the engine performance map. In this way, more efficient aftertreatment systems can be performed to fulfill the stringent emission criteria.