Experimental study on factors affecting lean combustion limit of S.I engine fueled with compressed natural gas and hydrogen blends

In order to study factors affecting lean combustion limit, an experimental study was carried out on a spark- ignited engine fueled with compressed natural gas and hydrogen blends. Effects of ignition timing, hydrogen fraction, engine speed, throttle opening, coolant and oil temperature were investigated. Experiments were conducted at a low and moderate level of engine load and speed with a wide range of hydrogen fraction, varying from 0 to 40 percent by volume. The results indicated that lean combustion limit could be obviously extended by adding hydrogen into compressed natural gas. In addition, leaner but more stable combustion can be acquired under higher throttle opening or lower engine speed conditions. Ignition timing was studied separately, a series of timings were adjusted around lean combustion limit under various conditions. Experiments also demonstrated that both over-advanced and over-retarded ignition could lead to a reduction in lean operating area of the engine. Further experiments on coolant and lubricant oil pointed out that lean combustion limit was positively correlated with coolant temperature; while an oil temperature increase corresponded to an initial decrease followed by an increase in lean combustion limit.