Effects of different ammonia energy ratio on soot formation and oxidation in an ammonia diesel dual-fuel engine

Abstract

Due to environmental pollution and energy crises, zero‑carbon fuel ammonia (NH3) has attracted extensive attention as an alternative fuel for engines. In this paper, the effects of ammonia energy ratio (AER) and injection strategy on particulate emission characteristics of an ammonia diesel dual-fuel engine were examined by merging experimental and simulation results; additionally, soot formation and oxidation mechanism were investigated. Results showed that the reduction in particulate emission was substantially higher than the increase in AER. When AER increased to 60 %, the reduction in particulate mass concentration reached 97.5 %. The initial soot formation area gradually moved to the bottom of the piston bowl with increasing AER. When the piston reached the top dead center, the high-soot-concentration area was shifted to the center of the piston bowl as AER increased. The contents of acetylene (C2H2) and methyl (CH3) reduced considerably, which restricted the formation of soot precursors. With AER increasing, the contents of nitric oxides (NOx) and other nitrogen-containing species increased and reacted with CH3 and other carbon-containing species, which effectively reduced the number of C in soot formation pathway, thereby lowering particulate emissions. As AER increased, hydroxyl (OH) involved in soot formation gradually decreased, and only 14 % of OH was involved in the oxidation of n-heptane at 60 % AER, which was favorable for reducing the soot formation rate. Furthermore, OH is a substantial species in soot oxidation. The introduction of ammonia caused an increase in OH, which facilitated the removal of soot. The decrease in hydrogenium (H) hindered the hydrogen–abstraction–acetylene–addition (HACA) reaction, further limiting the soot surface growth. By optimizing the injection timing and AER, particulate emission was lowered to 4.31 × 10−5 μg/cm3, and particle size was reduced by 64.2 % when AER was 60 %, injection timing was −20° CA ATDC, and injection pressure was 60 MPa.