Diesel injector nozzle optimization for high CNG substitution in a dual-fuel heavy-duty diesel engine

Many previous studies have attempted to substitute diesel fuel with compressed natural gas (CNG) in compression-ignition engines, which can substantially reduce CO2 and soot emissions. However, the CNG substitution rate cannot be maximized due to several restrictions, most prominently the amount of diesel injection. The minimum injection quantity becomes more significant under conditions of lower torque or higher CNG substitution rate. In this study, engine experiments were performed at 0, 70, and 80% CNG substitution rates under various EGR (exhaust gas recirculation) levels in a six-cylinder, 5899-cm(3) diesel engine to determine the combustion and emission characteristics at various CNG substitution rates. The 80% CNG case exhibited the highest thermal efficiency and largest CO2 reduction with an elevated combustion noise level of 92.3 dB. The thermal efficiency increase and CO2 reduction with 80% substitution were 2.7% and 18.7%, respectively. A three-dimensional engine combustion simulation was performed to optimize the diesel injector nozzle hole size and rate-of-injection (ROI) for 80% CNG substitution, whereby the mass of diesel injection quantity was reduced to 20% that of the conventional diesel case. The computational result demonstrated that a 25% smaller diameter nozzle (150 mu m) with an advanced ROI exhibited a 3.4% increase in the gross indicated mean effective pressure.