Effects of engine misfire on regulated, unregulated emissions from a methanol-fueled vehicle and its ozone forming potential

Abstract

Methanol is a feasible and promising alternative fuel for passenger cars, particularly in China. However, methanol-fueled vehicles struggle with stronger low-temperature operating instability and increased chance of misfire. In prior publications, the impacts of engine misfire on the regulated and unregulated emissions from methanol-fueled vehicles have been merely discussed. In this paper, regulated emissions, unburned methanol, carbonyl and VOC emissions from a China-5 (equivalent to Euro-5b+IUPR) certificated methanol-fueled car were measured over new European driving cycle (NEDC). By using a SAE J2901-compatible generator, two misfire rates (6% and 9%) were employed to mimic ordinary and severe engine malfunction statuses. Emission results were also put into comparison with the current Euro-5 and the upcoming Euro-6c OBD threshold limits, and used to estimate Carter MIR-based ozone forming potential. The results demonstrated that, both CO and THC emissions increased with misfire rate while NOx emission persistently decreased with intensified engine misfire. Even with a misfire rate up to 9%, all the regulated emissions were significantly lower than their thresholds regulated in Euro-5 and Euro-6c, which meant the current OBD requirements were too tolerant for methanol vehicles. It is proposed that, countries regarding methanol as a future option, shall promulgate methanol-specific emission standards in advance. Unburned methanol emission increased by at least 1.6 and 5.7 times when misfire rates of 6% and 9% were used. Both carbonyl and VOC emissions increased corresponding to higher misfire rates. Formaldehyde and toluene were found the key pollutants of carbonyls and VOCs. An increase in anticipated ozone forming potential (OFP) together with a decrease in specific reactivity (SR) was also noticed with the rising of misfire rate. Among all the pollutants, CO, toluene, xylene and formaldehyde were the chief species contributing to secondary ozone formation.