Effect of hydrogen enrichment of compressed natural gas on combustible limit and flame kernel evolution in a constant volume combustion chamber using laser ignition

Laser ignition of hydrogen-enriched compressed natural gas (HCNG) enables lean-burn combustion, a key technology for future emissions norms compliance for internal combustion (IC) engines. HCNG overcomes the limitations of low volumetric energy density and lowers the flame speed of CNG fueled spark ignition (SI) engine-powered vehicles. In this study, laser beam profile and beam quality (M2) of a Q-switched Nd: YAG laser at fundamental wavelength of 1064 nm, having 6-9 ns pulse duration, were characterized. During the Constant volume combustion chamber (CVCC) experiments, relative air-fuel ratio (2) of different HCNG blends were varied to identify the lean-burn limits, and observe flame kernel evolution, pressure-time history, and detect the start of combustion (SoC). At 10 bar initial chamber filling pressure, lean flammability limit increased due to hydrogen (H-2) enrichment of CNG. It was found to be lambda = 1.6, 1.8, 1.9, 2.0, and 2.1 for CNG, 10HCNG (10% v/v hydrogen and balance CNG), 20HCNG, 30HCNG, and 40HCNG respectively. Flame kernel displacement (Early stages of flame initiation and its development in mu s time-scale) was the highest for lambda = 1.1 for all HCNG mixtures, irrespective of the test fuel composition. Knocking or two-stage combustion increased with lowering lambda at 10 bar initial chamber filling pressure. SoC advanced and combustion duration (CD) decreased with increasing H-2 enrichment of CNG. Increased H2 enrichment of CNG accelerated the flame kernel evolution and made it a predominantly a 3D combustion phenomenon (volumetric combustion). Lower lambda and higher H-2 enriched CNG mixtures were effective in reducing the CD.

Automated summary

Laser ignition of HCNG enables lean-burn combustion, key for future IC engine emissions compliance. Experimental results in CVCC showed that lean flammability limit increased with H2 enrichment, while combustion duration decreased.