Impact Assessment of Induced System Faults on the Performance of Natural Gas Engines

Internal combustion engines remain pivotal in transportation and industrial sectors, with natural gas engines (NGEs) emerging as a low-carbon alternative. However, on-board diagnostics (OBD) system failures in aftertreatment and exhaust gas recirculation (EGR) systems can compromise emission control, particularly amid stringent regulations such as Euro VII. This study investigates the effects of induced faults, namely thermal aging of three-way catalytic converters (TWCs) and high-flow EGR malfunctions, on a turbocharged NGE. Experiments were conducted under world harmonized transient cycle (WHTC) conditions, measuring power, fuel consumption, and emission (CO, HC, NOx, and NH₃) characteristics using a full-flow dilution sampling system and Horiba MEXA-ONE-DC analyzers. The results indicate that the thermal aging treatment of the TWC exerts a primary and significant influence on pollutant emissions from NGEs, whereas its impact on power performance and fuel economy is negligible. Thermal aging of the TWC leads to a reduction in the HC and NOx conversion efficiency. Specifically, the thermal aging process of the TWC causes a 6.4% escalation in HC emissions and a 3.91-fold increase in NOx emissions during the WHTC. CO emissions decrease after catalyst thermal aging treatment. Compared to a thermally aged TWC, NGEs equipped with a fresh TWC detect a significant amount of NH₃ signals after more than 1600 s in the WHTC, and the NH₃ emissions produced in the hot WHTC are relatively lower. For EGR faults, induced high flow rates decrease intake air mass and cause pronounced oscillations in the intake charge between 1600 and 1700 s. The torque response of NGEs demonstrates a consistent trend with variations in intake air flow rate. HC and CO emissions increase significantly; however, the weighted NOx emissions drop from 289.6 mg/kW·h in the normal engine to 250.1 mg/kW·h. The exhaust gas of NGEs with EGR orifices contains a higher O₂ concentration, thereby resulting in lower NH₃ levels in the reduction products.