The real-world emission factors (EFs) and emission performance of bi-fuel (CNG/GAS) vehicles have been seriously investigated by the emission scientists and urban planners. In the present study, we propose an almost new practical methodology and perform on-road experiments on 60 bi-fuel vehicles to discuss the impacts of vehicle speed, fuel type, usage, and accumulated mileage on the CO, HC, and NOx EFs. 80% and 20% of the tested vehicles were private cars (PCs) and taxis (TXs), respectively. TXs and PCs are compared together based on their emission performances, emission characteristics, vehicular specific power (VSP) distributions, and type of fuel. The results show that in the entire cases except for GAS-PCs, CO EFs increased with vehicle speed. Although CNG vehicles show less HC EFs, they produce higher NOx mileage-based EFs compared to gasoline ones. NOx mileage-based EFs decrease with vehicle accumulated mileage, whereas the opposite trend was observed for the CO and HC EFs. Burning CNG rather than gasoline decreases 70% and 42% of CO, and 70% and 85% of HC mileage-based EFs in PCs and TXs, respectively. The influence of burning CNG on NOx emission of TXs seems to be insignificant, but NOx was more emitted by a factor of 1.7 in CNG PCs. Accumulated mileage has a destructive effect on combustion quality, especially in CNG vehicles. The factors of usage and accumulated mileage could considerably develop the VSP distribution and fuel consumption rate of GAS vehicles, while they have no detectable impact on that of CNG vehicles.
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