A Numerical Study on the Effects of Exhaust Gas Recirculation Temperature on Controlling Combustion and Emissions of a Diesel Engine running on HCCI Combustion Mode

Homogenous charge compression ignition (HCCI) mode of combustion is an alternative combustion strategy for conventional diesel engine that offers the potential to high power output with significantly reduced exhaust emissions of NOx and Particulate matter [1,2]. HCCI engines encounter low temperatures during the combustion, producing high levels of emissions of HC and CO [3]. In HCCI engine, the fuel and air are mixed homogenously before the start of combustion. The homogenous mixture auto-ignites as a result of the temperature increase during the compression stroke and burns volumetrically in a faster process giving a parallel energy release throughout the entire combustion zone. HCCI combustion is a chemical kinetic combustion process and is influenced by various factors like incylinder pressure, temperature, fuel characteristics and composition of the charge within the cylinder. HCCI needs no centralized combustion initiation and correspondingly suffers lack of control of start of combustion [4]. Researchers have reported various strategies for the control of combustion phasing by adjusting the compressed gas temperature so that the charge mixture auto-ignites at the desired crank angle such as variable compression ratio [5], variable valve timing [6], and intake air heating [7]. The HCCI combustion and emissions are significantly affected by the initial temperature of the mixture [8]. The CO emissions can be reduced by modifying the key oxidation reaction rate constant. Exhaust gas recirculation (EGR) is widely used to reduce the NOx emissions and is considered as the basic method to control the combustion phasing and burn rate in HCCI combustion engines. The auto-ignition and simultaneous combustion nature of HCCI engine limit the combustion Abstract


Introduction
Homogenous charge compression ignition (HCCI) mode of combustion is an alternative combustion strategy for conventional diesel engine that offers the potential to high power output with significantly reduced exhaust emissions of NO x and Particulate matter [1,2]. HCCI engines encounter low temperatures during the combustion, producing high levels of emissions of HC and CO [3]. In HCCI engine, the fuel and air are mixed homogenously before the start of combustion. The homogenous mixture auto-ignites as a result of the temperature increase during the compression stroke and burns volumetrically in a faster process giving a parallel energy release throughout the entire combustion zone. HCCI combustion is a chemical kinetic combustion process and is influenced by various factors like incylinder pressure, temperature, fuel characteristics and composition of the charge within the cylinder. HCCI needs no centralized combustion initiation and correspondingly suffers lack of control of start of combustion [4]. Researchers have reported various strategies for the control of combustion phasing by adjusting the compressed gas temperature so that the charge mixture auto-ignites at the desired crank angle such as variable compression ratio [5], variable valve timing [6], and intake air heating [7]. The HCCI combustion and emissions are significantly affected by the initial temperature of the mixture [8]. The CO emissions can be reduced by modifying the key oxidation reaction rate constant. Exhaust gas recirculation (EGR) is widely used to reduce the NO x emissions and is considered as the basic method to control the combustion phasing and burn rate in HCCI combustion engines. The auto-ignition and simultaneous combustion nature of HCCI engine limit the combustion

Abstract
In this study a comprehensive study is carried out numerically on a single cylinder four-stroke Diesel engine operating in homogenous charge compression ignition (HCCI) mode of combustion for the effects of exhaust gas recirculation (EGR) temperature and percentage on the combustion and emission characteristics. An advanced version of ANSYS IC Engine FORTE coupled with highly efficient and detailed pre-defined industry standard chemical kinetics CHEMKIN is used to solve the chemical reaction mechanism and species thermodynamic data. The analysis was carried out at three different EGR temperatures of 363K, 404K and 513K for 10%, 20% , 30%, 40% and 50% EGR each. The results predicted that the combustion ignition timing is advanced by increasing the EGR temperature. It was found that the combustion timing was advanced by 3 degree crank angle by increasing the temperature of EGR from 363 K to 404 K and the heat release rate was reduced by 163.85J/degree of crank angle. The effect of low EGR temperature is predominant at higher percentages of EGR. It was also found that the CO and UHC levels nearly kept constant with an increase in EGR temperature the NO x levels increase linearly with an increase in EGR temperature. The HCCI combustion in diesel engine can be controlled by adjusting the temperature and mass percentage of exhaust gas recirculation while retaining lower NO x emissions and very little increase in CO and unburnt hydrocarbons.
Keywords: CHEMKIN; EGR; HCCI; IC Engine FORTE; NOx; Thermal effects towards leaner air-fuel mixture and the exhaust gas recirculation (EGR) can be extended significantly which increases the heat capacity and lowers the heating value of the mixture resulting in lower peak pressure and temperature during the combustion, thus reducing NO x emissions [9][10][11]. The effect of EGR technique on combustion and emission characteristics using modified first law heat release for HCCI engine running with n-heptane/natural gas reported by Fathia et al. [12] indicated reduced mean in-cylinder temperature and rate of heat transfer as well as increased ignition delay and prolonged combustion duration. The results also revealed reduction in NO x emissions along with increase in CO and unburnt hydrocarbons. Voshtani et al. [13] investigated the effects of dilution, thermodynamics and chemical kinetics on the start of combustion timing in HCCI engine fuelled with natural gas and reformed gas blend. It was found that the chemical kinetics had strong effect on the start of combustion timing than other factors. Results also showed that H 2 O in the combustion mixture affects significantly, while CO has negligible effect. Zheng et al. [14] studied the effect of EGR on combustion characteristics of a HCCI engine fuelled with di-methyl ether using a multidimensional CFD coupled with chemical kinetic model. It was revealed that the main combustion occurred near TDC by utilizing reformed exhaust gas recirculation. The NO x emissions were reduced and there was an increase in HC and CO emissions when EGR was employed.
The EGR technique has a number of effects on the combustion process and emissions [15]. Firstly the thermal effects are related to the increase in inlet temperature of mixture which affects the volumetric efficiency and increase the specific heat of air-fuel mixture due to the presence of tri-atomic molecules of CO 2 and H 2 O. Secondly the chemical effect which is related to dissociation of species during combustion. The third effect of EGR is related dilution, which is the reduction in availability of oxygen for combustion process. Based on the literature survey the cooled EGR has a significant effect on improving the performance and emissions of a diesel engine. This work is based on the evaluation of the important issue to investigate the effects of exhaust gas temperature on the thermal and emission characteristics of an HCCI diesel engine for different EGR rates.

Simulation Software
The simulation software used in this study is IC Engine FORTE 18.2 version developed by ANSYS. The software couples a highly efficient and detailed pre-defined industry standard chemical kinetics CHEMKIN which solves the chemical reaction mechanism and species thermodynamic data on the basis of Arrhenius type correlation and the CFD code is used to solve the liquid spray, turbulent gas dynamics and other transport equations.

Computational Modeling and assumptions
The software uses full Reynolds-averaged Navier-Stokes equations with the Re-Normalization Group (RNG) k-ϵ model to describe the flow field. The combustion is initiated by auto-ignition in HCCI engines and the movement of charge inside the combustion chamber and the combustion chamber geometry is having very little effect on the performance of combustion [16]. In this study a single-cylinder diesel engine geometry is transformed into a sector of 60 o angle with periodic boundary conditions applied at the periodic faces of the sector as shown in The simulation is carried for EGR temperatures of 363K, 404K and 513K for EGR mass percentages of 10%, 20%, 30%, 40% and 50% each to determine the effect of the temperature of EGR along with different mass percentages on the thermal and emission characteristics of a single cylinder diesel engine running on HCCI mode. A numerical grid shown in Fig. 2 containing 218364 cells was adopted to model the combustion chamber sector geometry after accomplishing the grid-independent results The various engine geometry parameters are given in Table 1.

Model validation
The simulation results for the HCCI diesel engine with no EGR were compared and validated with the experimental data from for the similar engine specifications and imposing the same initial and boundary conditions.

Results and discussions
A combustion simulation has been carried out to determine the effects of exhaust gas recirculation temperature on the combustion and emission characteristics of a single cylinder four stroke diesel engine running on HCCI mode of combustion for intake exhaust gas recirculation temperatures of 363K, 404K and 513K for each EGR mass per-centages of 10%, 20%, 30%, 40% and 50%. The temperature of the entire cylinder charge depends upon the temperature and percentage of the EGR and the temperature of the air-fuel mixture. An increase in the temperature of EGR results in an increase in temperature of the entire cylinder charge. Therefore, the ignition timing of HCCI combustion can be adjusted by tuning the temperature and quantity of the EGR. Exhaust gas consists of many species, which includes the main components of burnt gases and partially burnt gases. In this study only the main components like CO2, H2O, N2 and O2 are considered. The results obtained have been discussed as under: Fig 3 and 4 show the effect of EGR temperature on the cylinder pressure and temperature for different EGR mass percentages respectively. A higher temperature of EGR results in the increase of thermal energy of the charge, which helps the fuel to overcome its activation energy and the pre-ignition chemical reactions are improved. The combustion ignition delay is dependent on pre-ignition chemical reactions and reduces if the engine inlet temperature is increased. It can be seen that higher the temperature of EGR, higher will be the temperature of air-fuel and EGR gas mixture, therefore the earlier ignition. The reduced ignition delay leads to an increase of peak cylinder pressure with increasing EGR mass percentage [17,18]. The EGR has a significant effect on cylinder temperature due to its serious effect on ignition delay. The starting time of sharp increase in cylinder pressure and temperature advances as quantity and temperature of EGR is increased. The dilution effect of EGR species results in large reductions in cylinder temperatures.  Fig 5 shows the apparent heat release rate with varying EGR temperatures for different EGR mass percentages. It can be seen that the higher temperature of EGR advances the combustion timing and the species inside the EGR dilute the combustible cylinder charge and influence both combustion ignition delays and heat release rate. With increase in EGR temperature, ignition delay is reduced and the combustion process advances and the duration of combustion increases resulting in decreased peak apparent heat release rates.

Emission effects
Exhaust gas recirculation is a promised technique to reduce the NO x formation in diesel engines. Figure 6  It is revealed from the results that the formation of NO x is dependent on the temperature and availability of oxygen within the cylinder charge. In case of high EGR temperature the charge temperatures are significantly higher and lead to an increased formation of NO. The effect of EGR temperature is more predominant at increased levels of EGR percentages. It can be concluded that EGR temperature has two different effects on the HCCI combustion in diesel enginesthermal and chemical effects. The thermal effects are due to the increased temperature of cylinder charge due to high temperature EGR which plays a significant role to overcome the activation energy and thereby advances the ignition timing and the various chemical species present in EGR have the dilution effects which affect both the combustion delays and apparent heat release rates. It was found that the combustion timing was advanced by 3degree crank angle by increasing the temperature of EGR from 363 K to 404 K and the heat release rate was reduced by 163.85J/degree of crank angle. The effect of increased temperature of EGR at higher EGR percentages has very little effects NOx emissions. The NO2 levels increased from 1.16928 g/kg of fuel to 1.53692 g/kg of fuel when the temperature was increased from 363 K to 404 K and 2.14301g/kg of fuel at 513 K. The reduction of oxygen concentration due to EGR and increase in EGR temperature affect both CO and UHC formation. It is demonstrated in the present study that the combustion phasing can be controlled by adjusting the EGR temperature and EGR mass percentage while retaining the benefits of low NO x without significantly increasing the CO and UHC emissions.