Flame structure characteristics and performance comparison of a NG SI engine under stoichiometric and lean combustion using CFD

The development and improvement of efficient compressed natural gas (CNG) engines align with efforts to reduce greenhouse gas and pollutant emissions. The objective of this study is to evaluate the flame structure and compare the performance characteristics of an engine powered by compressed natural gas (CNG) under stoichiometric and lean combustion in wide open throttle. CFD simulation alongside experimental tests are performed. The experimental data were obtained using a Hyundai 2.5-liter HR engine, originally a Diesel engine, adapted for spark ignition operation. Lean and stoichiometric conditions were evaluated at compression ratio 14:1, operating at 1800 rpm in MBT spark timing. The results showed that increasing lambda (λ) had a significant effect on apparent heat release rate, laminar flame speed, flame thickness and flame surface area. While the flame speed decreased in a leaner operating condition, the flame thickness and surface area increased due to reduced reaction rates and extended combustion duration. By reducing the flame speed but increasing its surface area, the amount of unburned fuel does not experience a significant increase, while the heat losses to the cylinder walls are reduced. When comparing the total energy between the two conditions and the indicated thermal efficiency, the lean operation achieved an absolute increase of almost 1% in efficiency, from 37.41% in stoichiometric condition to 38.34% in lean condition. This highlights the need to explore lean operation to increase the efficiency of internal combustion engines using natural gas.