This doctoral thesis presents a comprehensive study comprising four published papers in reputable scientific journals, adhering to the Scandinavian model. The research primarily focuses on assessing emissions and fuel consumption under real-world conditions for vehicles primarily powered by internal combustion engines. To improve the accuracy of emission and consumption predictions, an innovative hybrid electric vehicle model was developed based on modal emissions analysis. This model incorporates energy flow diagrams that are based on their purpose. The developed model integrates the longitudinal dynamics of the vehicle and energy flows with fuel consumption and individual emissions. The research demonstrates the successful application of a global rule-based energy management strategy to the developed model across various standard driving cycles and under real-world conditions as defined by Real Driving Emissions (RDE) regulations. The model was validated against control models using the New European Driving Cycle (NEDC), Worldwide Harmonized Light Vehicles Test Cycle (WLTC), and real-world driving conditions. Comparative results show that the proposed model provides more accurate predictions of emissions and consumption, with significantly lower relative error in tested driving cycles compared to existing modal models based on Vehicle Specific Power (VSP) analysis. Additionally, the thesis includes experimental research on exhaust gas emissions from vehicles under real driving conditions. This research was conducted on both fully operational vehicles and those with malfunctioning emission control systems. The findings contribute to a deeper understanding of the impact of real-world driving conditions and long-term vehicle operation on emissions and the performance of emission control systems.