Optimization of combustion chamber geometry and operating conditions for compression ignition engine fueled with pre-blended gasoline-diesel fuel

Highlights

• Pre-blended gasoline-diesel fuel was used with direct injection system.

• KIVA-CHEMKIN code modeled dual-fuel fuel spray and combustion processes with discrete multi-component model.

• The characteristics of Combustion and emission on pre-blended fuel was investigated with various fuel reactivities.

• Optimization of combustion chamber shape improved combustion performance of the gasoline-diesel blended fuel engine.

Abstract

In this study, experiments and numerical simulations were used to improve the fuel efficiency of compression ignition engine using a gasoline-diesel blended fuel and an optimization technology. The blended fuel is directly injected into the cylinder with various blending ratios. Combustion and emission characteristics were investigated to explore the effects of gasoline ratio on fuel blend. The present study showed that the advantages of gasoline-diesel blended fuel, high thermal efficiency and low emission, were maximized using the numerical optimization method. The ignition delay and maximum pressure rise rate increased with the proportion of gasoline. As the gasoline fraction increased, the combustion duration and the indicated mean effective pressure decreased. The homogeneity of the fuel-air mixture was improved due to longer ignition delay. Soot emission was significantly reduced up to 90% compared to that of conventional diesel. The nitrogen oxides emissions of the blended fuel increased slightly when the start of injection was retarded toward top dead center. For the numerical study, KIVA-CHEMKIN multi-dimensional CFD code was used to model the combustion and emission characteristics of gasoline-diesel blended fuel. The micro genetic algorithm coupled with the KIVA-CHEMKIN code were used to optimize the combustion chamber shape and operating conditions to improve the combustion performance of the blended fuel engine. The optimized chamber geometry enhanced the fuel efficiency, for a level of nitrogen oxides similar to that of conventional diesel over a variety of operation ranges.

Introduction

Emissions of internal combustion engines must be reduced due to emission regulations. Compression Ignition (CI) engines are widely used for their efficient fuel consumption, but CI combustion is progressed in locally rich mixture and high temperature region. This is why their soot and NO_x emissions are higher than those of spark ignition engines. To reduce exhaust emissions, Homogeneous charge compression-ignition (HCCI) technology, in which the charge is premixed before being compression ignited, is used to achieve both high efficiencies and low NO_x and soot emissions [1], [2]. The combustion phasing, ignition timing and cylinder pressure rate rise for HCCI concept is difficult to control [3]. To control the combustion of HCCI, Kokjohn et al. [4] used reactivity controlled compression ignition (RCCI) by varying fuel reactivity. The variation in fuel reactivity was conducted by in-cylinder fuel blending using port fuel injection of gasoline and early cycle direct injection of diesel. Yang et al. [5] also investigated the effect of RCCI combustion and the blended-fuel low temperature combustion (LTC) mode with gasoline and diesel. Blended fuel LTC mode had higher fuel concentration than dual fuel highly premixed charge combustion (HPCC). For this reason, the combustion in HPCC mode was less complete than that of LTC mode and had a lower combustion efficiency.

Chao et al. [6] compared the combustion of gasoline homogeneous charge induced ignition (HCII) and gasoline/diesel blend fuels (GDBF). As the gasoline fraction increased, the improved fuel-air mixing in both HCII and GDBF reduced soot emissions by over 90%. This demonstrated that HCII and GDBF have a benefit compared to the diesel CI combustion. Benajes et al. [7] carried out to analyze mixing air-fuel and auto-ignition processes in RCCI combustion conditions, using a fuel blend of gasoline and diesel. In order to analyze air-fuel mixing process in detail, a 1-D spray model was used. The ignition delay increased and the mixture formation time was extended when the diesel/gasoline ratio was reduced. A reduction in NO_x and soot emissions was achieved using RCCI combustion.

Cha et al. [8] conducted an experiment and a modeling study for gasoline-diesel dual-fuel engines. Numerical calculations were performed using KIVA code coupled with the primary reference fuels (PRFs) mechanism. Soot and nitrogen oxides were reduced by suppressing the formation of locally rich mixtures. Ma et al. [9] investigated diesel injection strategies for high efficiency and low emissions of gasoline/diesel dual fuel in a modified single-cylinder diesel engine. The gasoline/diesel dual-fuel combustion mode used port fuel injection for gasoline and direct injection of diesel fuel. Injection parameters were optimized about the first and second injection timing. Injected diesel mass was split into the two injections. The maximum indicated mean effective pressure was increased by the late start of the injection.

This study optimizes an engine for high fuel efficiency via numerical simulation. Numerical investigations related to a direct–injection CI engine fueled with gasoline–diesel pre-blended fuel have not been considered as much as the RCCI concept because spray simulations with more than two types of fuel simultaneously are challenging. In the RCCI method, it is assumed that gasoline is distributed homogeneously over the cylinder before the injection of diesel fuel. This assumption means that the computational fluid dynamics (CFD) program does not have to simulate gasoline spray behavior in the computational domain. Therefore, many researchers have developed new concepts to approach multiple fuels during spray simulation. However, gasoline–diesel pre-blended research in a direct injection compression ignition (DICI) engine is rarely considered using numerical methods. Li et al. [10] investigated gasoline–diesel blended fuel in a DI diesel engine numerically. They used four blended ratios from pure diesel (0% gasoline) to 40% gasoline with diesel fuel. The coupled KIVA4–CHEMKIN code was used to study the effect of blended fuel on combustion and emissions characteristics. Their results showed that a high blending ratio increased engine performance and NO emission under high load conditions although the combustion was deteriorated by gasoline–diesel blended fuel under low load conditions. Thoo et al. [11] studied the effect of gasoline content on blended gasoline–diesel fuel on the ignition delay in a CHEMKIN simulation. They separated the factors of the ignition delay as physical and chemical delays. By the kinetic reaction modeling, the chemistry properties of gasoline fuel highly affected the long ignition delay period up to 3 °Crank Angle (C.A.) more than physical properties. The KIVA code was implemented to describe the spray and mixture formation processes for gasoline–diesel blended fuel in Jeon’s research [12]. The discrete multi–component (DMC) model enabled the modeling of blended fuel using a direction injection system. They concluded that the gasoline fuel decreased inhomogeneity during mixture formation, which resulted in higher performance and lower soot emissions compared to conventional diesel fuel. The numerical approaches allowed the optimization research of combustion chamber and operation conditions. The various optimization tools were used to optimize the engine system coupled with CFD programs. Shi and Reitz [13] introduced a non-parametric regression analysis tool for their optimization study to achieve low emissions and high fuel economy in a heavy-duty diesel engine. They used bowl geometry, spray targeting and swirl ratio conditions as optimization parameters. The Pareto front was employed to evaluate the optimal solutions, which satisfied multi-objectives. Among a number of solutions, they found a shallow and side bowl design to achieve low emissions and improved fuel consumption under high and low load conditions. Park [14] developed the auto mesh generator code to optimize bowl geometry using Bezier curves, which allowed various geometry types such as open-crater, re-entrant, deep bowl and shallow bowl. They used five parameters to vary bowl shapes and these parameters assigned optimization variables in optimization tools. Park conducted the optimization of combustion chamber geometry for stoichiometric diesel combustion and dimethyl ether [15] in CI engines. The optimized bowl geometries allowed the reduction of engine emissions and better engine performances compared to original combustion chambers. In addition, Yaliwal et al. [16] studied the effects of combustion chamber geometry on engine performance using two bowl types experimentally. They optimized combustion chamber shape and nozzle type, evaluating the engine performance in terms of brake thermal efficiency, specific fuel consumption and exhaust gas temperature. Although it could be challenge to conduct optimization process with experimental approach, they concluded that the chamber type and nozzle highly affected combustion process. Also, optimal chamber configuration and nozzle type for a dual fuel engine is associated with the chamber type and nozzle.

According to the previous investigations, the fuel reactivity has positive effect on improving high efficiency in CI engine. The purpose of this study was to explore the combustion and emission characteristics of the pre-blended gasoline-diesel fuel and to expand the practical availability pre-blended fuels in DICI engines with experimental and numerical methodologies. To investigate the effect on variation of fuel reactivity, the experimental research in the various pre-blended fuel ratio is conducted using the direct injection compression ignition engine. Furthermore, theoretical approach expanded research into areas that cannot be studied by experimental methods. Many numerical studies have shown the spray and combustion phenomena in the combustion chamber in terms of equivalence ratio, temperature and emission formation. In the present study, an advanced optimization technique was introduced to improve the CI engine performance for an engine fueled with gasoline–diesel blend.