Effects of auto-mechanical diesel pump utilization and diesel injection timing on the fuel energy share, combustion, performance, and emissions of NG-diesel naturally aspirated engine at constant speed and intermediate to high load

• Naturally aspirated engine contains mechanical diesel injection pump and NG-air mixer. • Combustion, performance and emissions study at various load and diesel injection timing. • Diesel energy share was reducing with advancing diesel injection in dual fuel mode. • Combustion and performance improved with advancing diesel injection in dual fuel mode. • CO, CO 2 and smoke drooped, but NO X raised with advancing diesel injection in dual fuel. The studies on the switching from diesel to natural gas (NG)-diesel fuel mode of naturally aspirated diesel engines fueled with diesel by full auto mechanical diesel injection pump (MDIP) are insufficient. This study investigated the effects of diesel injection timing (DIT) and engine load on fuels (NG and diesel) energy share, combustion, engine performance, and emissions. Tests were conducted at DITs of 6° to 20° BTDC with 2° angle intervals of crank angle, engine loads of 50, 67, and 83 %, and speed of 1500 rpm. NG was supplied with constant pressure to the intake manifold through a gas-air mixer positioned after the air filter. Results show that advancing DIT increases NG energy share (NGES) and decreases diesel energy share (DES) at constant load while increasing load raises DES and lowers NGES at constant DIT. Peak in-cylinder pressure and heat release rate (HRR) rise with load and advancing DIT. HRR shifting from a single peak in diesel mode to dual peaks in dual-fuel. BTE is higher in diesel mode but converges between fuel modes with increasing engine load. CO₂, NO X , and smoke are higher with the diesel, while CO is higher with the dual-fuel mode. Advancing DIT lengthens ignition delay, shortens combustion duration, increases NO X in both fuel modes, and decreases CO, CO 2 , and smoke with dual-fuel, whereas CO and CO 2 increase with diesel. In both fuel modes, CO 2 , NO X , and smoke increase with load, but CO decreases. The automatic diesel adjustment by MDIP influenced all results. These findings offer insights into optimizing DIT and load for better efficiency and emissions, guiding future studies, especially with NA engines containing MDIP.