Chemical Kinetics Study on Ignition Characteristics of Natural-Gas Components: Fuel Design Concept for Robust Ignition in Compression Ignition Engines

摘要

Ignition process chemistry of each single-component of natural gas, methane, ethane, propane, n-butane and isobutane, was analyzed using detailed chemical kinetic computations. Ethane exhibits low ignitability, close to that of methane, when the initial temperature is below 800 K, but higher ignitability, close to those of propane, n-butane and isobutane, when the initial temperature is above 1100 K. Furthermore, ethane shows a higher heat release rate during the late stage of the ignition process. This is because H_2O_2 is accumulated with a higher concentration within the lower temperature range and promotes the OH formation rate of H_2O_2(+M)=OH+OH(+M) in the higher temperature range. These ignition characteristics are reflected in those of methane-based dual-component fuels with the sub component of ethane. In a homogeneous charge compression ignition (HCCI) engine, misfire and partial combustion occur easily because the heat release rate during the ignition process after low-temperature OH chain branching and also during the ignition process bypassing low-temperature OH chain branching, becomes lower than the rate of decrease in internal energy during the expansion stroke. Partial combustion causes cycle-to-cycle variation in combustion as well as high HC emissions. Therefore, the heat release profile, which is slow in the early stage of ignition process, but more rapid than those of other fuels in the late stage, can reduce cycle-to-cycle variation in combustion. This type of profile is similar to that of CH4/C2H6 for an HCCI engine targeting CH_4-based fuels.