Numerical Analysis of Syngas Enhanced Mixed Mode Combustion in a Conventional Diesel Engine

A numerical study is performed to investigate the syngas enhanced mixed mode combustion in a conventional diesel engine. The syngas is produced from diesel reforming by recovering the exhaust energy, and induced back into the engine through the intake system together with the fresh air, and it is ignited by the injected diesel fuel. The computational package KIVA 3V II with appropriate modifications is used to predict different in cylinder processes during an engine cycle. By recovering the exhaust energy, syngas enhanced combustion can increase the thermal efficiency and also achieve a cleaner combustion because of the nature of the syngas. Simulation has been conducted to compare the engine performance of the syngas enhanced combustion with the diesel only combustion. The effects of EGR rate and injection timing have been studied and the optimized engine operation condition is given by performing the numerical calculations. The numerical results show that syngas enhanced mixed mode combustion can reduce the engine emissions, such as soot and NOx, dramatically, and by adjusting the injection timing properly, a higher thermal efficiency can also be achieved. Corresponding author: cflee@illinois.edu INTRODUCTION As global energy crisis and environmental issues get worse and worse, researchers have paid more attention to increase the internal combustion engine efficiency and reduce the emissions. Synthesis gas (syngas), as one of the various renewable energy sources, offers a promising solution to the stringent future emission limits for vehicles [1]. Syngas is mainly composed of hydrogen and carbon monoxide. Because of the high antiknock behavior of H2 and CO mixtures, syngas can be used in internal combustion engines as fuels [2, 3]. However, the hydrogen content in syngas tends to increase the combustion temperature and NOX emissions under stoichiometric condition [4]. Therefore, syngas is more appropriate for lean-burn application [5]. Lean combustion has great potential to reach higher thermal efficiency and lower NOX and soot emissions [6-8], and a great benefit of H2 addition to the gaseous fuels is that the lean limit of gas operation will be extended without entering the lean misfire region [5]. A number of studies have been conducted to investigate the use of syngas in internal combustion engines [9-11], but most of them focused on spark ignition (SI) engines. However, under high load condition, syngas fuels are not suitable for SI engine operations: the fluctuation of syngas compositions may result in unstable combustion; and the energy density is relatively low compared to gasoline [1]. The more efficient way to utilize syngas as a fuel for power generation is to use syngas in compression ignition dual-fuel engines that operate under lean-burn condition, using a pilot injection of diesel fuel [12]. Since the auto ignition temperature for syngas is very high, it is necessary to use pilot diesel fuel to initiate syngas combustion. Some experiments have been performed to study the performances and emissions of syngas powered dualfuel engines. Tomita et al. [13] conducted experiments in a micro-pilot ignition dual-fuel engine fueled with supercharged syngas, and investigated the combustion characteristics and performance of the syngas fuel. They reported that a premixed flame of syngas-air mixture develops from multiple flame kernels produced by the ignition of diesel fuel. Also, by increasing the hydrogen content of the syngas, the lean limit of the mixture is enhanced, and the engine can even operate at an equivalence ratio as low as 0.45 with stable combustion and high efficiency. Roy et al. [1416] studied the effect of injection timing on the occurrence of knocking, and also the effect of hydrogen content and EGR on the performance and exhaust emissions of a dual-fuel engine. They concluded that knocking combustion occurred when the injection timing was advanced more than 13.5 BTDC, and a higher hydrogen content would increase the engine power and the enhance the leaner operation. Azimov et al. [1, 17] undertook a detailed investigation of twostage heat release combustion with syngas and nature gas, and concluded that the second stage of heat release is due to the autoignition of the gaseous fuel mixture in the end-gas region. Compared to the extensive experimental studies of syngas fueled dual-fuel engines, the modeling and simulation studies performed on syngas combustion in IC engines are rather limited [18, 19]. Papagiannakis et al. [18] developed a numerical model to simulate the performance and exhaust emissions of an SI engine operating on syngas. In this study, a two-zone combustion was adopted, whereas the syngas burning rate depends on its entrainment rate into the burning zone, and it is controlled by the velocity of the flame front that forms around the area of the burning zone and then spreads inside the combustion chamber. For chemical reactions, all the species were assumed to be in chemical equilibrium and equilibrium equations were solved to get the species concentrations. Gamino and Aguillon [19] performed syngas combustion model in an IC engine by solving the conservation equations of mass and energy and reaction rates for different species. Three step homogeneous reactions were used for the syngas combustion. One of the limitations of the previous numerical studies is that they performed syngas enhanced dual fuel engine simulations ignited by spark plugs instead of diesel pilot injection which can give higher energy density and thermal efficiency. Therefore, the objective of this study is to develop a syngas induced engine combustion model with diesel fuel injected as ignition sources. The premixed syngas air mixture is introduced through the intake port, and the mixture gas is ignited by the pilot diesel injection. The rest of the diesel fuel is injected during the main injection. The effects of EGR rate and pilot injection timing are tested to optimize the dual fuel engine operations. MATHEMATICAL FORMULATIONS The objective of this research is to model the syngas enhanced mixed mode combustion in a conventional diesel engine numerically. The computational package KIVA 3V release 2 [20, 21], with appropriate modifications which will be described below is used to predict different processes during an engine cycle. The computational package is capable of conducting threedimensional modeling of the evaporation, mixing, ignition, combustion and pollutants formation processes, each of which is handled by a separate submodel within the program. The most important advantage of KIVA over other simulation packages is that it is an open source package, meaning that the user can modify the source code to fit one’s need. KIVA GOVERNING EQUATIONS The governing equations that the KIVA solves can be found in Amsden et al. [21]. The code maintains a fuel library listing the thermo-physical properties of thirty-seven fuels, listing the vapor pressure, density, enthalpy, latent heat of evaporation, surface tension and viscosity for each of the fuels. The KIVA code comprises two main sections: spray calculation and combustion calculations. The spray calculation predicts and describes the spray process: droplet breakup, collision and evaporation. The combustion portion predicts and describes the combustion process: ignition, combustion and emission. The code, basically, solves the conservation of species, momentum and energy equations [21]: ∂ρi ∂t + ·( ) ρi u → = · ( ) ρi Dxi + ρ̇i‚ comb + ρ̇i‚ spray δi (1)