With a strong legal basis and regulatory authority, cost-effective transient emission sensors that reflect real-driving emissions are key factors for accomplishing environmental requirements. It is difficult for the existing NO_x emission monitoring techniques to achieve a balance between accuracy and timeliness. Fundamentally, in-cylinder combustion is the thermodynamic cause of NO_x emissions and the main excitation source for engine vibration and noise emissions. A novel vibration-based virtual NO_x sensor is developed based on these critical relationships for real-time NO_x monitoring. First, the correlation between vibration and NO_x emission was characterized in-depth. Then, a technique of constructing two-dimensional filters for vibration signals is proposed to extract combustion-related information. A principal component regression (PCR) model for NO_x prediction was established based on the reconstructed in-cylinder pressure. Finally, the virtual NO_x sensor is tested and validated on a single-cylinder diesel engine bench. The virtual NO_x sensor is proven to meet the accuracy requirement of vehicle emission monitoring for both steady-state and transient conditions and has a better frequency response compared to the emission measurement system.

Reactivity-controlled compression ignition (RCCI) is an advanced combustion mode. Its uses two fuels with different physical and chemical properties to form a combustible mixture with active stratification. RCCI can flexibly control the combustion process by changing the concentration and activity of the combustible mixture. It can also reduce the emission of NOx and particulate matter in the engine without significantly reducing the thermal efficiency. Among various fuel combinations, methanol as an oxygen-containing fuel, has a high latent heat of vaporization, which is conducive to reducing combustion temperature and achieving low-temperature combustion. This experimental study explores the potential of Methanol-Diesel Reactivity Controlled Compression Ignition (RCCI) in achieving low emissions and high thermal efficiency and compares this with the conventional dual fuel mode. Low-temperature combustions such as Reactivity Controlled Compression Ignition (RCCI) have been shown to be a promising way to reduce pollutants at the exhaust, i.e. NOx and soot emissions. and increase the thermal efficiency of future engines. The methanol to diesel energy share (MDES) could be enhanced to 56% in the RCCI mode with proper setting of the injection parameters from 45% in the dual fuel mode. A higher quantity in the second diesel pulse that occurred close to TDC led to higher thermal efficiency and good combustion stability. Engines working in a dual-fuel mode need special conditions to ignite an air-fuel mixture without a spark plug in a good moment with high combustion efficiency.