Effect of spark ignition location on the turbulent jet ignition characteristics in a lean burning natural gas engine

Abstract

The Turbulent Jet Ignition is an effective concept to achieve stable lean burning for natural gas engines due to the multiple ignition sources, high ignition energy, and fast combustion rate. A variation of the ignition location has a non-negligible effect on the ignition performance of the TJI system. The present work aims to provide more details on this effect by numerical simulations. Both factors of the additional fuel supply to the pre-chamber and the in-cylinder flow field are taken into consideration in this study. A numerical model is built based on a lean burning natural gas engine and validated by experimental results. Five different spark ignition sources are equally arranged on the vertical axis of the pre-chamber, with different distances from the connecting orifices. Simulations are carried out under the same initial and boundary conditions except for the location of the ignition source. Combustion pressure, in-cylinder flow field, fuel mass fraction distribution, and heat release rate are analyzed to study the in-cylinder ignition and combustion process. The results show that a rotational flow and a non-uniform fuel distribution are formed in the pre-chamber during the compression stroke. The turbulent jet characteristics are significantly influenced by the coupling of two factors: the combustion rate inside the pre-chamber as well as the flame propagation distance from the ignition source to the connecting orifices. Rapid combustion rate and shorter flame propagation distance both lead to the earlier ejection of cold jets and hot jets. Among five ignition sources, the one located closest to the connecting orifices generates earlier hot jets with the highest mean velocity. The jets are more effective to ignite the lean mixture and could decrease the combustion duration of the main chamber.

Effect of spark timing on the pre-chamber jet ignition in a lean-burn natural gas engine

“…Figure 9 shows the methane mass fraction, flow velocity, and turbulent kinetic energy (TKE) contours on plane A at spark timing for the cases of ST1 and ST9. It can be seen from Figure 9 that a swirl flow around the horizontal axis is formed in the pre-chamber under the action of the compression flow from the main chamber (as demonstrated in the authors’ previous work 24 ). Due to the inflow of the main chamber lean mixture and the short mixing time, the distribution of the mixture concentration, flow velocity, and TKE in the pre-chamber is not homogeneous.…”

“…One is composed of low-temperature unburned gas, which is defined as a cold jet, and the other is composed of high-temperature flame and combustion product, which is defined as a hot jet. 24 As shown in Figure 16, the first zero-crossing point of the velocity curve from negative to positive indicates that the gas in the pre-chamber starts to flow into the main chamber, which is the low-temperature cold jet. When the temperature at the outlet of the orifice rises rapidly, the hot jet begins to spray into the main chamber.…”

“…Table 2 shows the specifications of the adopted sub-models in the simulation. 24 The Renormalization Group (RNG) k-e turbulence model is selected to calculate in-cylinder turbulent flow in our work considering sufficient computational accuracy and low cost. 32 RNG k-e turbulence model is a Reynolds-Averaged Navier-Stokes turbulence model, which is suitable for simulating flows with high strain rate behavior and a large degree of streamline bending.…”

“…Therefore, the above factors should be taken into account in the optimization of the spark location. 24–26…”

“…Therefore, the above factors should be taken into account in the optimization of the spark location. [24][25][26] From the above literature review, it can be known that the combustion heat release rate of the prechamber is the most critical factor for turbulent jet characteristics. The mixture concentration, temperature, as well as turbulence quantities in the pre-chamber at spark timing would affect the pre-chamber combustion and later govern the main chamber combustion.…”