Abstract
Common rail systems have been widely used in diesel engines due to the stricter emission regulations. The advances in injector technology and ultrahigh injection pressure greatly promote the development of multiple-injection strategy, leading to the shorter injection duration and more variable injection rate shape, which makes the mixing process more significant for the formation of pollutant emission. In order to study the mixing process of diesel sprays under variable injection rate shapes and find the optimized injection strategy, a one-dimensional spray model was modified in this paper. The model was validated by the measured spray penetrations based on shadowgraphy experiments with the varying injection rate. The simulations were performed with five injection rate shapes, triangle, ramping-up, ramping-down, rectangle and trapezoid. Their spray penetrations, entrainment rates and equivalence ratios along spray axial distance are compared. The potentials of multiple-injection and gas-jet after end-of-injection (EOI) to improve mixing process and emission reduction are discussed finally. The results indicated that ramping-up injection rate obtains the highest entrainment rate after EOI, and it needs 1.5 times of injection duration for the entrainment wave to arrive at the spray tip. For the other four injection rates, the sprays can be treated as a steady-like state, needing twice of injection duration from EOI to the time the entrainment wave reaches the spray tip. The multiple-injection with proper injection rate shape enhanced the entrainment rate, and the gas-jet after EOI affected the mixture distribution and entrainment rate in spray tail under ramping-down injection rate.
Highlights
With the implementation of more stringent emission regulations, such as US Tier 3 and post-Euro6, diesel engines are facing unprecedented challenges to reduce their adverse environmental impact in terms of pollutant and greenhouse emissions
Even for the ammonia injection rather than the diesel fuel injection, Lamas et al [18] found that the varying injection rate shapes, including the rectangular, triangular, or parabolic, took distinct propensities to NOx reduction in a hydrogen-diesel engine by a computational fluid dynamics (CFD) model
12, 4221 triangle, and ramping-down injection rate shapes have the same maximum injection rate, their spray tip penetrations evolve with different growth rates in the early injection duration, they all gradually tend to the same penetration value and slope over time
Summary
With the implementation of more stringent emission regulations, such as US Tier 3 and post-Euro. The advanced innovation of piezoelectric injectors makes a significant contribution to the extremely fast needle response time [14,15], meaning it is easer for diesel engines to adopt highly defined injection rate shapes to improve their performance In this way, the varying injection rate shape plays a more and more important role in the fuel injection of diesel engine, which deserves in-depth study. Even for the ammonia injection rather than the diesel fuel injection, Lamas et al [18] found that the varying injection rate shapes, including the rectangular, triangular, or parabolic, took distinct propensities to NOx reduction in a hydrogen-diesel engine by a CFD model It can be summarized justifiably from these researches that the varying injection rate shape has a potential to optimize the engine combustion and emission characteristics. This paper investigated the effect of the gas-jet after EOI on mixture distribution and emission reduction under the ramping-down injection rate shape
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