A fuel design concept for an HCCI engine based on chemical kinetics to optimize the heat release profile and achieve robust ignition was proposed, and applied to the design of the optimal methane-based blend.
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. If the early stage of an ignition process takes place during the compression stroke, this kind of heat release profile is desirable in an HCCI engine to reduce cycle-to-cycle variation during the expansion stroke.
According to results from engine operation tests using dual-component fuels with methane as the primary component and ethane, propane, n-butane and isobutane as the secondary component, methane/ethane shows a lower COV of IMEP when CA50 is set at the same timing for the expansion stroke. Furthermore, methane/ethane also shows a lower knocking intensity when CA50 is set at the same timing, close to the knocking limit, due to its lower in-cylinder pressure rise rate.
These results suggest that methane/ethane can be the optimal methane-based blend for an HCCI engine to achieve both better fuel economy and higher performance by its robust ignition and high anti-knocking properties.
