Effect of mixing CO2 with natural gas-hydrogen blends on combustion in heavy-duty spark ignition engine

Because further reduction in carbon dioxide (CO2) will require upgrading the existing engine technology or developing a new type of engine, the addition of hydrogen to natural gas is considered as an alternative that could meet the required emission standards. However, the carbon monoxide/CO2 generated in the reforming process of natural gas can affect the engine performance and emissions characteristics. The content of CO2 in the reformed gas can be varied by varying the ingredients or the conditions of the process. Therefore, it is essential to control the air-fuel mixture condition and combustion phasing. In this research, the performance and emission characteristics of an 11 L spark ignition engine using natural gas-hydrogen blends with various CO2 contents were examined, and an optimization strategy for controlling the excess air ratio and the spark advance timing was assessed. The thermal efficiency decreased with the increased content of CO2 under a given excess air ratio condition. The increased heat capacity of the mixture reduced the combustion temperature, so that the generation of nitrogen oxides was suppressed. However, total hydrocarbon (THC) emissions and the methane portion of the THC emissions were increased by the introduction of CO2. The decrease in the exhaust gas temperature also resulted in low conversion efficiency of the oxidation catalyst. (C) 2012 Elsevier Ltd. All rights reserved.