Abstract
The effects of narrow-throat pre-chamber geometry on the main chamber combustion were investigated on a heavy-duty optical engine fueled with methane. Simultaneous dual formaldehyde PLIF imaging and OH* chemiluminescence imaging were applied to characterize the early-stage formaldehyde jet and flame jet discharge process. The formaldehyde jet velocity on the jet boundary was quantified by dual formaldehyde PLIF for the first time. Three narrow-throat pre-chambers with two rows of orifices on the nozzle and with different pre-chamber volume and inner throat diameter were studied under the same pre-chamber to main chamber fuel ratio and global excess air ratio of 2.0. The results indicate that the narrow-throat pre-chamber performance is not very sensitive to the pre-chamber volume. The pre-chamber with a larger volume produces only slightly higher pressure buildup in the pre-chamber (ΔP) and similar main chamber combustion phasing and engine efficiency. The inner throat diameter is the key dimension in the narrow-throat pre-chamber design. A larger inner throat diameter produces a smaller peak ΔP and a slower jet penetration, resulting in longer combustion duration and lower engine efficiency. The formaldehyde PLIF shows that the main chamber combustion can be generally classified into two stages: the initial flame ignition and the following flame propagation. The maximum local formaldehyde jet boundary velocity of the narrow-throat pre-chamber with an inner throat diameter of 3.3 mm is up to about 280 m/s and it decreases dramatically when the inner throat diameter is increased to 5.3 mm. The flame jet characteristics of lower and upper orifices can be significantly different due to the local pressure difference between them in the pre-chamber throat. A smaller inner throat diameter results in a larger pressure difference between the lower and upper row orifices and could lead to weak upper-row flame jets. The narrow-throat pre-chamber design with an optimized inner throat diameter can produce highΔP, fast jet penetration, and short combustion duration, which favor the lean limit operation and high engine efficiency.