Characterization of reaction zone growth in an optically accessible heavy-duty diesel/methane dual-fuel engine

Abstract

The performance of dual-fuel engines in terms of fuel conversion efficiency and unburned hydrocarbon emission is strongly influenced by the turbulent flame propagation through the premixed natural gas. To improve dual-fuel engine design and provide validation data for numerical models, the fuel conversion process must be better characterized, specifically the reaction zone growth rate. In this work, high-speed imaging of OH*-chemiluminescence is performed in an optically accessible 2 L engine operated with port-injected CH₄ and direct-injected diesel for different diesel and CH₄ fueling rates and pilot injection pressures (P_pilot). The cumulative histogram time series is introduced for directly comparing high-speed optical data of dual-fuel combustion with simultaneously measured apparent heat release rate. The cumulative histogram time series diagram is also used to evaluate a “global” reaction zone speed, S_RZ,g, based on OH*-chemiluminescence images. The S_RZ,g calculation normalizes the reaction zone area growth rate by the perimeter of the reaction zone to determine the velocity scale, while a “local” reaction zone speed, S_RZ,l, is based on the local displacement of the reaction zone boundary per unit time. From the distribution of S_RZ,l for each image frame, a previously proposed metric for determining the transition from pilot autoignition based on apparent heat release rate was validated and used to evaluate a single mean flame propagation speed, ${\overline{S}}_{\mathrm{FP}}$ . Using these metrics, it was noted that increasing ϕ_CH4 from 0.40 to 0.69 results in an increase in ${\overline{S}}_{\mathrm{FP}}$ from 4 to 8 m/s and 8 to 14 m/s for pilot injection pressures of 300 and 1300 bar, respectively. The spatial distribution of S_RZ,l also indicates that autoignition of the pilot jets is not simultaneous (arising from asymmetric injector geometry) and leads to an overlap of the autoignition and flame propagation processes. This is not considered in the conventional conceptual model of dual-fuel combustion and impacts calculation of ${\overline{S}}_{\mathrm{FP}}$ for the small diesel injections commonly used for dual-fuel engines.