The homogeneous charge compression ignition (HCCI) and combustion chamber geometry play significant roles in improving the potential of compression ignition engines and their environmental sustainability. However, a major hindrance to the progress of HCCI technology has been the notable decline in brake thermal efficiency and emit high levels of hydrocarbon (HC) and carbon monoxide (CO) which is mainly due to the low temperatures during combustion and low post oxidation of these components. Combustion chambers like hemispherical combustion chamber (TCC) and toroidal combustion chamber (TCC) are known to provide better combustion characteristics due to their increased surface‐to‐volume ratio. Moreover, injection timing and spray position have shown a significant effect on the controlling onset of combustion and emission in HCCI engine. Therefore, the primary objective of this numerical study is to enhance the engine characteristics in HCCI mode using an HCC and TCC piston bowl geometry coupled with multistage injection and compare the results with flat piston bowl geometry. The results show that the toroidal shape piston geometry provides 3%–5% higher brake thermal efficiency compared to the conventional piston geometry. Additionally, the energy balance analysis revealed that the combination of modified piston geometry and multi stage injection improved the combustion process by reducing the energy losses due to incomplete combustion and heat transfer to the engine walls. These findings suggest that the proposed injection strategy combined with modified piston geometry has the potential to enhance the performance of HCCI engines and reduce their environmental impact.
Homogeneous charge compression ignition (HCCI) engines are a promising technology for reducing NO<sub>x</sub> and smoke emissions compared to conventional compression ignition engines. However, uncontrolled combustion and misfires at low loads due to the unavailability of fuel injectors are major challenges in HCCI engine development. The introduction of a direct fuel injection system, particularly multistage direct injection, has shown improved combustion stability and reduced emissions. Modified piston geometry can further enhance air-fuel mixing and combustion stability, resulting in increased power output and reduced energy loss due to incomplete combustion. The aim of this research is to investigate the impact of different piston bowl geometries and spray angles with multistage injection on improving the thermal performance of HCCI engines. The Taguchi method is used to optimize the engine design and ensure that performance is not compromised while meeting emission standards. Combinations A<sub>3</sub> (spray angle: 70 deg), B<sub>2</sub> (RPM: 12 × 10<sup>2</sup>), C<sub>3</sub> [piston geometry: toroidal combustion chamber (TCC)], A<sub>3</sub> (spray angle: 70 deg), B<sub>2</sub> (RPM: 12 × 10<sup>2</sup>), C<sub>3</sub> (piston geometry: TCC), and A<sub>3</sub> (spray angle: 70 deg), B<sub>1</sub> (RPM: 9 × 10<sup>2</sup>), and C<sub>2</sub> [piston geometry: hemispherical combustion chamber (HCC)] produce the least amount of hydrocarbon (HC), CO, and NO<sub>x</sub> emissions, respectively. The results further showed that the TCC geometry is best suited for geometry when operated at higher RPMs. Overall, the performance of HCC and TCC is found to be 12% better than a flat piston.
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