Homogeneous Charges with Direct Injection Strategy to Achieve High Efficiency and Clean Combustion in Diesel Engines

Reactivity-Controlled Compression Ignition (RCCI) has emerged as the most promising strategy to achieve high efficiency and clean combustion in diesel engines without any compromise on the achievable load range. Nevertheless, the complexity of the system hardware due to dual fueling and higher unburned fuel emissions are the major challenges to be addressed in RCCI. Although various approaches are proposed in the literature to reduce higher unburned emissions in RCCI, single-fuel strategies without any reactivity stratification that result in higher thermal efficiency and lower unburned emissions are not available. In the present work, a single-fuel novel Homogeneous Charge with Direct Injection (HCDI) strategy is proposed to address the limitations of RCCI in terms of higher unburned emissions. In HCDI, the premixed diesel vapor-air mixture inducted during the engine suction stroke is compressed and subjected to autoignition along with an early direct injection of diesel fuel during the compression stroke. The direct-injected (DI) fuel is used to control the autoignition of the premixed fuel-air mixture in HCDI combustion. To give better insight into HCDI combustion in comparison to other modes, the present research work compares the performance and emissions of HCDI with RCCI and the base conventional combustion in the best-optimized condition. A production light-duty single-cylinder diesel engine used for agricultural and power generation applications is modified to implement HCDI and RCCI strategies through suitable changes in the intake manifold and fuel injection system. The operating parameters in RCCI and HCDI combustion modes are optimized to achieve maximum brake thermal efficiency using a suitable controller. Unlike Homogeneous-Charge Compression Ignition (HCCI), the DI diesel fuel is found to result in improved control over combustion phasing through the absorption of latent heat of vaporization and charge stratification effects. The energy analyses in RCCI and HCDI under optimal operating conditions are also compared among themselves and with that of conventional diesel combustion. The results obtained show that the engine can be operated over the entire operating load range in RCCI and HCDI with higher brake thermal efficiency and near-zero oxides of nitrogen (NOₓ) and smoke emissions. The brake thermal efficiency is higher in HCDI compared to RCCI with a maximum increase of ~14%. The unburned hydrocarbon (HC) and carbon monoxide (CO) emissio