A Fundamental Investigation On Engine Combustion Fueled With Methanol For High Efficiency And Low Emissions

High efficiency and clean combustion technology of internal-combustion engine has been a hotspot in international engine field all along. New engine combustion modes are presented aiming at future ultra-low and even zero emission regulation. At the same time, the development of new clean alternative fuel has also becomes a hotspot of research in many countries. In this paper, new high-efficiency and clean combustion modes of alternative fuel methanol are exploringly investigated. Based on DME and methanol dual-fuel method achieved by DME generation from a part of methanol, two kinds of combustion types are innovatively presented to achieve high efficiency and clean combustion of methanol in full load. One is HCCI combustion of methanol/DME dual-fuel port injection; another is the dual-fuel compound combustion with port injection of DME and direct injection of methanol.HCCI combustion of methanol/DME dual-fuel port injection can extend the operating range to middle-high load level of original non-pressurized diesel engine, keeping very low NOX emissions, but having low combustion efficiency and thermal efficiency, high HC and CO emissions at low load. In addition, adjustable region of methanol/DME concentration at middle-high load is narrow so that combustion control difficulty increases. EGR can enlarge the controlled region at middle-high load, but can't extend maximum load range. At low load, adopting pure DME HCCI combustion with high EGR rate can improve thermal efficiency. In middle-high load, adopting large DME percentage and high EGR rate can improve thermal efficiency of HCCI and decrease HC and CO emissions.To the dual-fuel compound combustion with port injection of DME and direct injection of methanol, various combustion modes, which represent different combustion characteristics and emissions characteristics, can be achieved by controlling different injection timings of methanol. High temperature diffusion combustion mode should be adopted to attain relatively high thermal efficiency under low methanol concentration and low load. With the increase of methanol concentration, the crank angle corresponding to high temperature diffusion combustion mode advances, and combustion controlled region of early injection becomes narrower. By adopting late injection, combustion process can be well controlled, knocking combustion can be avoided under high methanol concentration and large load, and so engine load range can be further extended. CFD simulation shows that, for late injection case, there is intensive concentration stratified near combustion chamber wall. And high temperature combustion region, which is relatively narrow, is distributed in compression clearance and near combustion chamber wall. Modeling study of varied parameters indicates that faster injection rate can shorten combustion duration under high methanol concentration for late injection.Based on the above study, this paper presents a control strategy for high efficiency and clean combustion of engine fueled with methanol in full load. At low load, pure DME HCCI combustion with high EGR rate or high temperature diffusion combustion mode of methanol should be adopted to attain relatively high thermal efficiency. In middle-high load, HCCI combustion of methanol / DME dual-fuel port injection should be adopted to achieve clean combustion with high efficiency and low NOX emissions. At high load, the dual-fuel compound combustion with port injection of DME and in-cylinder direct injection of methanol achieved by late injection should be adopted to ensure high power output. This control strategy, which provides theoretical and experimental basis for high-efficiency and clean utilization of methanol in engine, has a useful reference value.