Performance and Emission Characteristics of Spark Ignition Engine with Sequential and Direct CNG Injection

Introduction: This research aims to compare the performance, efficiency, and emission characteristics of two Compressed Natural Gas (CNG) fuel injection systems, CNG Sequential Injection (CNG-SI) and CNG Direct Injection (CNG-DI), on a modified 4-stroke, 4-cylinder petrol engine. The modifications were designed with the intent of enhancing fuel efficiency and reducing emissions, with potential for patentable innovation. Methods: A 1298cc petrol engine was modified to accommodate direct CNG injection by drilling holes in the cylinder head. Both CNG-SI and CNG-DI systems, currently under patent consideration, were tested on a shared engine test bench equipped with a rheostat-type loading dynamometer, injector emulator, pressure reducer, CNG injector rail, Timing Advance Processor, various sensors, and a Gas Engine Control Module (ECM). Performance testing was conducted across a range of 1000–3100 rpm at 300 rpm intervals. Results: The CNG-DI system demonstrated superior performance compared to the CNG-SI system. It achieved a 3–10% reduction in fuel consumption, a 1–3% improvement in brake thermal efficiency, and a 4–8% increase in volumetric efficiency, operating with a leaner air-fuel ratio. In terms of emissions, CNG-DI showed lower HC emissions at low speeds, though they increased at higher speeds. Both systems reduced CO emissions with increasing engine speed, but CNG-DI provided greater reductions due to enhanced combustion efficiency. While CNG-SI exhibited lower NOₓ emissions at lower speeds, the CNG-DI system maintained more stable NOₓ levels, indicating potential for better NOₓ control at higher RPMs. Discussion: The observed improvements in fuel economy and emission control with the CNG-DI system suggest its promise as a next-generation fuel delivery method. Compared to the conventional CNG-SI system, CNG-DI offers consistent advantages across a range of engine speeds. The leaner combustion achieved in CNG-DI contributes to improved efficiency and reduced CO emissions. However, increased HC emissions at higher speeds and the need for precise fuel-air management are considerations for further development. The results align with existing research on direct injection benefits while presenting unique advantages specific to CNG-fueled engines. Conclusion: The study concludes that the CNG-DI system offers notable advantages over the CNG-SI system in terms of fuel efficiency, combustion performance, and emission reduction. These findings support the potential of CNG-DI technology as a patentable innovation with practical applications in enhancing engine efficiency.