Combustion characteristics of methane-diesel dual-fuel engines at light load conditions with different injection strategies

An experimental investigation has been carried out to examine the combustion process in methane-diesel dual-fuel engines under light load conditions, with different ratios of the two fuels and different injection strategies of the diesel pilot. The study has used a 2.2L turbocharged common rail compression ignition (CI) engine, converted for dual-fuel (DF) operation while retaining the ability to run on diesel fuel only. The main area of interest has been the change of heat release characteristics with methane substitution for diesel fuel, and what gives rise to these changes. The focus has been put on the light load operating conditions, where the engine suffers poor fuel economy and increased instability owing to the very weak mixture strength of the gaseous fuel and air. An effort has been made to identify the minimum size of the diesel pilot required for the ignition of the very lean methane-air mixture at near-idle low-load operating conditions. The experiments have been carried out at fully-warm steady-state conditions, at a wide range of engine speeds from 1400 to 2000 rpm at 25% engine load, with a wide range of methane substitution for diesel fuel up to 80% (energy basis). The influence of pilot injection strategy has been assessed via comparing the results of dual-fuel operation using a single pilot injection, to those obtained using pilot split-injection strategy. The maximum possible substitution ratio of methane for diesel fuel has been investigated at 5% engine load for a speed range from 1300 to 1600 rpm, and the different limitations on this ratio have been assessed. The results have demonstrated that the dominant combustion mechanism in the region away from the pilot effect is determined by the relative magnitudes of diesel pilot and gaseous fuel. Gaseous fuel equivalence ratio (CH4) has to be greater than 0.35 to sustain a flame-propagation event. For CH4 below 0.3, pilot combustion dominates. Small pilot masses (~1.6 mg/stroke) evaporate and locally intermix with the very lean gaseous fuel-air mixture, where the ensuing combustion is governed by the chemical kinetics. The minimum diesel injection mass was 0.8 mg/stroke in the test engine used. This presents a system-limitation on the maximum achievable substitution ratio of methane for diesel fuel. The maximum substitution ratio achieved was as high as 95% at 1600 rpm, but it decreased at lower engine speeds. Methane substitution beyond these limits is restricted by the operation-limitation on engine stability, as the value of coefficient of variance of the gross indicated mean effective pressure (COVIMEPgross) at this point exceeds 8%. Using larger pilot masses allows for the utilization of larger quantities of the gaseous fuel without exceeding the engine stability criterion (COVIMEPgross ~ 5%). The use of pilot-split injection in DF engines has the capacity to change the combustion mode from the conventional diesel dual fuel (DDF) combustion into a novel two-stage partially premixed dual fuel (PPDF) combustion if the timing and quantity of the pilot pre-injection are optimized. Researches on this field are still in the early stages and in need for further development. The present work has developed an optimum pilot-split injection strategy to approach PPDF combustion mode. The results demonstrated a 17% reduction in the maximum rate of pressure rise (ROPR) and a 5% increase in the brake thermal efficiency (BTE) relative to conventional DDF, while maintaining the engine stability at appropriate levels (COVIMEPgross ~2-3%). The effect of pilot pre-injection mass on PPDF combustion was studied for the first time in this work. It has been found that the pre-injection mass has to be kept above a certain limit (3.2 mg/stroke) to bring about the desirable change of the combustion mode