Deterioration in Emissions from Light Vehicles in Use Converted to CNG (Bi-Fuel)

Abstract

Natural gas is a cleaner fuel for combustion and with less carbon intensity than traditional ones (gasoline and diesel); however, it requires an adequate calibration of the electronic management of the internal combustion engine, as well as improvements in the catalytic selectivity of exhaust gas post-treatment systems, to preserve its comparative environmental advantages. The conversion of Otto cycle engines with three-way catalytic converters (TWC) to the alternative use of compressed natural gas (CNG, Bi-fuel) is one of the most used options at international level for light-duty vehicles. However, its implementation with conversion kits applied to engines in use, originally designed and optimized for gasoline, requires a specific evaluation in emissions to demonstrate that the engine is capable of complying with the standardized emission limits (EURO 1 to 6) for both alternative fuels and of maintaining this behavior during the guaranteed useful life of the vehicle. In the present work, a EURO 3 vehicle with an Otto cycle engine in use with gasoline that is converted to CNG Bi-fuel is evaluated. The objective was to verify compliance with the emission limits for the use of both fuels (gasoline and CNG), initially using a new TWC that is aged for more than 100,000 km of driving. The analysis of pollutants with an effect on health and climatic conditions allows evaluating some typical characteristics of the environmental behavior of this technology, in the contribution of both local and global (GHG) pollutants, also determining its relative deterioration in emissions for both fuels. All emissions evaluations have been carried out in the laboratory in agreement with international certification standards (70/220/CEE and later), but in the case of vehicle deterioration due to driving over 100,000 km, it was developed under real urban and extra-urban traffic conditions. The results show the increase in NOx and CH4 emissions with the use of CNG and the deterioration of the TWC activity above 60,000 km of use. According to what was evaluated, the implementation of the CNG by conversion of in-use vehicles can deteriorate the emissions of local health-effect pollutants and global GHG.