EVALUATING ENGINE PERFORMANCE AND EMISSION OUTPUT IN ENRICHED-CO2 NATURAL GAS APPLICATIONS

Abstract

This study investigates the effect of varying air-fuel ratios (AFR) on the performance and emission characteristics of a spark-ignition direct-injection (SI-DI) compressed natural gas (CNG) engine fuelled with enriched-carbon dioxide (CO2) natural gas mixture. The experimental work was conducted on a single-cylinder, four-stroke SI-DI engine operated at a constant speed of 3000 rpm and varying AFRs from stoichiometric (λ = 1) to lean mixtures (λ = 1.1, 1.2, and 1.3). The engine performance parameters analyzed include brake torque and brake specific fuel consumption (BSFC), while emission characteristics measured were carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (uHC), and carbon dioxide (CO2). The results indicate that the brake torque decreases progressively with increasing AFR, with a significant reduction of 50.5% at λ = 1.3 compared to the stoichiometric mixture. The BSFC initially decreases at λ = 1.1 but increases with further lean mixtures due to the reduced energy density of the enriched-CO2 fuel mixture. Emission analysis shows that NOx emissions decrease sharply with increasing AFR, reaching an 84.2% reduction at λ = 1.3. Conversely, unburned hydrocarbons increased due to incomplete combustion in leaner mixtures, peaking at 600.1 ppm/kW. CO emissions decrease by 81.8% at λ = 1.2 before a slight increase at λ = 1.3, while CO2 emissions peak at λ = 1.2 and decline thereafter. These findings demonstrate the trade-offs between engine performance and emission control with varying AFR and highlight the potential of enrihed-CO2 natural gas as an alternative fuel with optimized air-fuel ratios.