Reid Vapor Pressure and Temperature Fluctuations Drive Motorcycle Evaporative Emissions: Overlooked Factors Beyond Regulatory Standards

As exhaust emission standards continue to tighten, evaporative emissions have attracted increasing attention, while information on motorcycles, particularly older models, remains limited. In this study, seven China II and China III motorcycles with engine displacement between 50 and 150 mL were tested using a sealed housing for evaporative determination (SHED), including standardized hot soak and 1 h diurnal breathing loss (DBL) measurements; an additional non-replicated 12 h laboratory-reproduced temperature-profile test was conducted as an exploratory comparison. Across the displacement-matched China II and China III motorcycles, DBL decreased from 6.17–11.12 g to 0.33–0.57 g, corresponding to pairwise reductions of approximately 91–97%, while hot soak loss (HSL) decreased from approximately 1.2–1.6 g to 0.07–0.29 g, corresponding to reductions of approximately 82–95%. These differences coincided with multiple evaporative-emission-control upgrades introduced with China III, including activated-carbon canisters, improved fuel-system sealing, and closed-crankcase ventilation; their individual contributions could not be isolated. In a vehicle-specific comparison between two 150 China III motorcycles, the electronic fuel injection (EFI) equipped motorcycle exhibited 66.9% lower DBL than the carburetor-equipped motorcycle, although this single comparison cannot establish a general EFI effect. The exploratory 12 h test also yielded higher cumulative DBL than the standardized 1 h procedure; however, because test duration, temperature amplitude, and temperature-change rate differed simultaneously, the individual contribution of these factors could not be separated.