In these studies, three important works examined to get ultra-low emission for a single cylinder dieselengine. The first study was performed for single fuel and compression ratio (CR), intake and exhaust valve timings,mass flow rate were optimized for a range of engine speed. Then for the same engine injection parameters such asstart of injection (SOI), injector cone angle, and split injection structures were examined to get optimum parametersfor the diesel engine. In CR studies, different combustion chambers were tested according to injector cone anglesand fuel-wall interaction. In the second study, in addition to the above studies, dual fuel compressed biogas (CBG)and diesel combustion were analyzed under different engine loads both experimentally and computationally.Optimized single fuel diesel cases were compared with CBG + Diesel dual fuel cases which employed portinjection for CBG fuel. In dual fuel engine applications, CBG fuel and air mixture is induced from intake port andthis air-fuel mixture is ignited by pilot diesel fuel near top dead center (TDC). In dual fuel engine mode, exhaustemissions reduced considerably especially in NOx and particulate matter (PM) because of methane (CH4) rate andoptimized engine parameters. The third study is focused on aftertreatment systems to minimize residual exhaustemissions. The emissions of the diesel engines consist of various harmful exhaust gases such as carbon monoxide(CO), particulate matter (PM), hydrocarbon (HC), and nitrogen oxides (NOx). Several technologies have beendeveloped to reduce diesel emissions especially NOx reduction systems in last decades. The most promising NOxemission reduction technologies are exhaust gas recirculation (EGR) system to reduce peak cylinder temperaturethat reduces NOx form caused by combustion and active selective catalyst reduction (SCR) system using reducingagent such as urea-water-solution for exhaust aftertreatment system. In this study, computational fluid dynamic(CFD) methodology was developed with conjugate heat transfer, spray, deposit and chemical reaction modelingthen emission prediction tool was developed based on the CFD results with deposit prediction mechanism. CFDand deposit results were correlated with image processing tool in flow test bench.