EXTENDING THE LEAN LIMIT OF METHANE-AIR MIXTURES BY OXYGEN, NITROGEN AND CARBON DIOXIDE INJECTION IN THE GAP OF THE SPARK PLUG’S ELECTRODES

An experimental investigation is carried out to investigate the feasibility of extending the lean limit for methane–air mixtures by injecting oxygen and other gases such as nitrogen and carbon dioxide in the area between spark plug electrodes. Additionally, the effect of oxygen injection on exhaust gas recirculation (EGR) is investigated. The pressure in the chamber and the equivalence ratio of the mixture are varied from 0.35 MPa to 1.03 MPa and 0.46 to 1.0, respectively. Delco 12620540 spark plug is used as the ignition source and the gas injection into spark plug gap is accomplished through a 0.159-cm OD stainless steel injection nozzle. With oxygen injection the lean limit is extended from an equivalence ratio of 0.54 to 0.46 at all pressures investigated in the present study.

Similar trend is observed with nitrogen and carbon dioxide injection in the area between the spark plug’s electrodes. This indicates that the extension of the lean limit of methane-air mixtures is mainly due to induced charge motion. From high-speed video camera the flame kernel between the electrodes in the case of oxygen, nitrogen and carbon dioxide injections is observed to be much larger than the flame kernel in the case of without injection. Thus, it appears that one of the effects of injection is to enlarge the initial flame kernel to a radius exceeding the critical radius. As a result the flame front grows and propagates, resulting in more energy transferred to the unburned mixture, enhancing the burning and reaction rates.

The effect of the oxygen injection on percent EGR represented by carbon dioxide dilution in a stoichiometric methane-air mixture is also investigated. Without oxygen injection combustion of a stoichiometric methane-air mixture is impossible to obtain with 18 % EGR, however with oxygen injection combustion is achieved even at 22 % EGR. As a result of oxygen injection the peak pressure at 22 % EGR is the same as at 15 % EGR. In contrast it is impossible to ignite the methane-air mixture with nitrogen and carbon dioxide injection, even at 10 % EGR.