Effect of spark timing on laser ignition and spark ignition modes in a hydrogen enriched compressed natural gas fuelled engine

In this experimental study, a prototype laser ignited engine was developed and fuelled with hydrogen enriched compressed natural gas (HCNG; mixture of H-2 and CNG). Engine performance, emissions and combustion characteristics were compared for laser ignition (LI) and spark ignition (SI) modes. The maximum brake torque (MBT) timing was employed to reduce cycle-to-cycle variations (CCV) and to improve the engine performance, combustion and emission characteristics. Composition of HCNG mixture was changed by dynamically blending H-2 with CNG on a volumetric basis. MBT timing was determined for naturally aspirated engine in conventional SI mode, by varying the spark timing (ST) between 22 degrees CA bTDC to 46 degrees CA bTDC and by varying relative air-fuel ratio (lambda) from rich-to-lean. Optimum torque for HCNG mixtures was observed at 31 degrees CA bTDC. The 10% mass fraction burn (MFB10) duration reduced continuously with advancing ignition timing up to 31 degrees CA bTDC, which then increased with further advanced ST. For any particular ST, MFB10 and MFB90 decreased with increasing H2 enrichment of CNG. At high lambda (lean mixtures), CCV would be higher due to lower engine efficiency and increased emissions. LI mode exhibited lower coefficient of variation in indicated mean effective pressure (COVIMEP) compared to SI mode. The COVIMEP for all HCNG mixtures were within 2% up to lambda = 1.2 and it increased further with increasing. but the variation was within +/- 6%. Lower CCV (< 2% of COVIMEP), reduced emissions and improved brake thermal efficiency (BTE) were observed at 31 degrees CA bTDC MBT timing compared to other spark timings. This experimental study indicated that laser ignition is a suitable technology for deployment in HCNG engines.