Experimental and numerical study of combustion and emissions performance in a hydrogen-enriched Wankel engine at stoichiometric and lean operations

Based on a retrofitted Wankel engine, fundamental experiments and numerical simulations were conducted to investigate the role of the hydrogen addition under stoichiometric and lean combustion conditions. The testbench recording of in-cylinder pressure, heat release, cycle-to-cycle fluctuation, and exhaust emissions were used to characterize fuel combustion. The verified CFD model clarified the inherent mechanisms behind experimental data. Results showed that the introduction of hydrogen addition caused combustion temperature to increase, the timing taken for active radical formations was significantly advanced, and the peak concentrations increased. This contributed to the increased pressure and the shortened combustion phasing in experiments. There was a proportional correspondence of the early flame growth and cyclic variation. Hydrogen addition has a minimal effect on the turbulent intensity after spark timing. However, adopting a highly diluted mixture caused the enhanced turbulent intensity, which was responsible for improving the stability of the engine. Due to the synergistic effects of the work delivery, cyclic fluctuation, and exhaust temperature, a larger hydrogen addition with stoichiometric operation showed lower higher brake thermal efficiency. Nitrogen oxide emissions increased with hydrogen enrichment and reduced with a higher degree of mixture dilution owing to a close positive correlation between the mass elevated-temperature and NOx formation. Benefits from sufficient oxygen concentration and elevated combustion temperature, the chain transfer process can be accelerated. This explained why CO emissions decreased at a larger hydrogen-enriched and leaner condition in the experimental investigation. Increasing the hydrogen content provided improvement in the desired HC emissions for all operating points.

Automated summary

Based on experimental and numerical simulations, the addition of hydrogen in a retrofitted Wankel engine was found to increase combustion temperature and pressure, optimizing combustion processes and improving engine stability. However, it also led to increased nitrogen oxide emissions, which could be mitigated by diluting the mixture.