Ignition control of methane fueled homogeneous charge compression ignition engines using additives

Homogeneous charge compression ignition is a new combustion technology that may develop as an alternative to diesel engines with high efficiency and low NOx and particulate matter emissions. In this paper, the effect of additives such as dimethyl ether (DME), formaldehyde (CH2O) and hydrogen peroxide (H2O2) for the control of ignition in natural-gas HCCI engines have been investigated numerically by adopting a single-zone zero-dimensional model. The chemical kinetic mechanism incorporated the GRI-3.0 mechanism that considers 53 species and 325 reactions together with the DME reaction scheme consisting of 79 species and 351 reactions. To simulate HCCI engine cycles, a variable volume computation has been performed by including a piston motion into the SENKIN code at a fixed equivalence ratio of 0.3 and initial mixture pressure of 1.5 bar. It was found that an additive-free mixture did not ignite for the intake temperature of 400 K. A mixture containing a small quantity of additives at the same temperature was ignited. For a fixed quantity of additive, it was found that H2O2 addition was effective in advancing the ignition timing as compared to the other two additives. It was found that the percentage of additives required to achieve a near TDC ignition timing increases linearly with the increase in the engine speed while decreases with the increase in the equivalence ratio with the superiority of H2O2. Furthermore, the addition of even 7% by volume of H2O2 could ignite a mixture at an intake temperature of 350 K, while at least the fractions of 12.5% and 35% by volume were needed for DME and CH2O, respectively. It was also found that the mass fraction of NO with CH2O addition was less than that with H2O2 addition. At the same time, however, a near TDC ignition timing resulted in a similar amount of NO for both additives. Overall, the enhanced reactivity of CH4 in the presence of small amounts of additives could be used in HCCI engines fueled with methane to alleviate the high intake temperature requirements.