CFD Study of Heat Transfer Reduction Using Multiple Injectors in a DCEE Concept

Abstract

Earlier studies on efficiency improvement in CI engines have suggested that heat transfer losses contribute largely to the total energy losses. Fuel impingement on the cylinder walls is typically associated with high heat transfer. This study proposes a two-injector concept to reduce heat losses and thereby improve efficiency. The two injectors are placed at the rim of the bowl to change the spray pattern. Computational simulations based on the Reynolds-Averaged Navier-Stokes approach have been performed for four different fuel injection timings in order to quantify the reduction in heat losses for the proposed concept. Two-injector concepts were compared to reference cases using only one centrally mounted injector. All simulations were performed in a double compression expansion engine (DCEE) concept using the Volvo D13 single-cylinder engine. In the DCEE, a large portion of the exhaust energy is re-used in the second expansion, thus increasing the thermodynamic efficiency. To isolate the heat losses associated with the changed spray pattern of the two-injector concept, effects of the heat release are excluded during the analysis. Results showed that the optimal injection strategy allows a decrease in the temperature close to the walls, leading to heat loss reduction up to 13 % or 2 % of the fuel energy. The residual exhaust energy was increased by 1.5 %-points with the twoinjector concept when compared to the reference case. This proved the advantage of the two-injector concept compared to conventional single injector case for the DCEE application.

Novel Geometry Reaching High Efficiency for Multiple Injector Concepts

“…No air exchange and so, no pumping losses have been taken into account in this study. More details about the methodology were reported in a previous study [12].…”

“…Another solution was proposed by Uchida et al [11] using multiple injectors. Further evaluation has been done on this concept [12] showing a heat transfer reduction of 15 % or 2 % of the fuel energy through a changed flow pattern. However, this study highlighted the issues of an omega-shaped bowl when using two injectors.…”

“…This paper focuses on a flat bowl geometry to reduce the heat transfer for multiple-injector usage. Two new geometries are proposed and evaluated against a reference geometry as well as an earlier multiple injector study [12]. This means that three different geometries were tested, geometry 2 and 3 using a flat bowl for two outer injectors and geometry 1 using a standard bowl (see Figure 3) for both one centrally mounted injector and two outer injectors.…”

“…The two outer injectors are placed just at the rim of the bowl for all geometries (see Figure 4) while spraying towards the cylinder center. This gives a changed fluid motion proven to reduce heat transfer [12]. The umbrella angles used, 145 degrees for the reference case, 150 for the two-injector case using Geometry 1 and 160 degrees for the remaining cases, are based on reaching the minimum amount of heat transfer.…”

Thermal Efficiency Comparison of Different Injector Constellations in a CI Engine

“…However, this lower flame/wall interaction was not confirmed and needs to be studied further. Other computational studies have confirmed the heat loss reduction using multiple injectors [14,15]. In these studies, CFD simulations were performed to investigate how two injectors can reduce heat losses and increase the efficiency of an HD CI engine.…”