Optimization of the Operating Conditions of a Dual CI Engine Fueled with Methanol

Among the new low temperature combustion modes, Reactivity-Controlled Compression Ignition (RCCI) offers a low NOx-soot trade off (keeping a relatively high engine efficiency). Also, RCCI permits the introduction of a renewable fuel with a lower CO₂ direct emission such as short-chains alcohols. For this work, methanol and diesel fuel were used as low and high reactivity fuels, respectively. A 1.3 L single-cylinder engine, with a cylinder volume usual in medium- and heavy-duty truck and bus engines, was used in this work. The engine was operated at an engine speed of 1600 rpm and 25% load (representing one of the 13-mode test on medium duty trucks), which results in an indicated mean effective pressure of 5.2 bar. The effects of methanol substitution ratio (MSR) at 20 and 35% on performance and pollutant emissions was investigated and compared to conventional diesel combustion (CDC). The main target of the work is to find an optimum point according to a defined objective function for each MSR through a statistical methodology. The results indicated that dual-fuel combustion improves NOx emissions up to six times compared to CDC mode (both in their respective optimal regions) mainly due to high EGR rate and the cooling effect of methanol. However, the thermal efficiency was slightly lower under dual-fuel combustion. As the objective function was defined to balance the indicated thermal efficiency and the nitrogen oxide emission, a decrease in the objective function as observed for 35%, but it increased for 20%, suggesting the existence of a neutral methanol ratio.