Design and Calibration Strategies for Improving HCCI Combustion in Dual-Fuel Diesel–Methane Engines

The interest in methane is lately increased due to power-to-gas tech-nologies, through which green electricity in excess could be used to produce eas-ily storable gaseous fuels. Among engines for methane exploitation, dual-fuel piston engines is a very efficient and low impact solution. Their operation, still limited by high hydrocarbons and carbon monoxide emissions at low and knock at high loads, is characterized by many parameters. Besides the ones well recog-nized in literature, like pilot quantity and substitution rate, other parameters, like engine volumetric compression ratio, intake charge conditions, pilot injection pressure and timing, engine load and speed and exhaust gas recirculation showed an impact on engine performance and emissions. This work first describes the results of a full factorial DoE in which the effects of compression ratio, intake charge pressure, pilot injection timing and pressure, and methane flow rate effect, are evaluated and discussed on combustion devel-opment, engine performance and pollutant emission levels at the exhaust. Through ANalysis of VAriance analysis, the first and second order effects were also quantified. Moreover, the factors variation ranges leading the engine to operate in or close to HCCI combustion, i.e. guaranteeing a high conversion efficiency and low emission levels at the same time, were sought and highlighted. This suggested that not only very advanced, but also retarded injection timings, combined with high intake charge pressure determine very low levels of nitrogen oxides and maximum pressure rise rate, with little or no penalty on engine efficiency and emission levels

Advanced Combustion in Natural Gas-Fueled Engines

Current energy and emission regulations set the requirements to increase the use of natural gas in engines for transportation and power generation. The characteristics of natural gas are high octane number, less amount of carbon in the molecule, suitable to lean combustion, less ignitibility, etc. There are some advantages of using natural gas for engine combustion. First, less carbon dioxide is emitted due to its molecular characteristics. Second, higher thermal efficiency is achieved owing to the high compression ratio compared to that of gasoline engines. Natural gas has higher octane number so that knock is hard to occur even at high compression ratios. However, this becomes a disadvantage in homogeneous charge compression ignition (HCCI) engines or compression ignition engines because the initial auto-ignition is difficult to be achieved. When natural gas is used in a diesel engine, primary natural gas–air mixture is ignited with small amount of diesel fuel. It was found that under high pressure, lean conditions, and with the control of certain parameters, the end gas is auto-ignited without knock and improves the engine combustion efficiency. Recently, some new fuel ignition technologies have been developed to be applied to natural gas engines. These are the laser-assisted and plasma-assisted ignition systems with high energy and compact size