Combustion Optimization of a Diesel Engine with EGR system using 1D and 3D simulation tools Vinicius Peixoto, Celso Argachoy, Ivan Trindade, Marcelo Airoldi Department of Engine Design Engineering, MWM International Diesel Engines of South America Ltd. São Paulo, Brazil

Abstract The environmental legislations are challenging the engine manufacturers to redesign their products in order to attend the new requirements. A variety of solutions are being implemented to achieve cleaner emissions and reduce the fuel consumption. The recirculation of the exhaust gases (EGR), re-burning a part of them is being widely used by the automotive industry. The scope of this work is to discuss and simulate the EGR influence on the emissions and engine performance. processes and their influence on engine and emissions performance were described by 1D simulation and detailed by 3D CFD. DoE tools were used to find the most appropriate design and the compromise relation between the variables mentioned above. An engine’s combustion project aims at three specific goals: 1. Cleaner emissions 2. Low fuel consumption 3. High performance engines All of them are requirements for a competitive engine and to obey the new environmental regulations imposed by the new emissions standards (EPA 010 and EURO V). These objectives can point to different directions. To minimize the NOx emission it is necessary to delay the injection, however the same delay can imply in a higher concentration of soot on the exhaustion gases, being necessary to obtain a compromised solution among them. A design option is the use of Exhaust Gas Recirculation (EGR), commonly applied on Diesel engines to improve the emissions (cleaner emissions). In order to comply with the emissions standards, the rates of EGR are rising rapidly, changing the behavior of the combustion and requiring studies to observe more properly the effect of these gases on the combustion performance. For instance, the EUROV emissions standard will be institutionalized in Brazil and to attend this emission standard a higher rate of EGR needs to be imposed. The EGR system does not affect the injection system. However, it influences the combustion because of the presence of burned fuel mixed with the air from the inlet system. The first and most basic EGR effect is the increase on the inlet air ratio after mixing with the EGR rate raise, and the consequence is a lower air density and reduction of the air mass trapped inside the cylinder. Even if the engine has an EGR cooler, this impact on the temperature is observed.

Introduction The narrower environmental restrictions are the current challenge that Diesel engine manufacturers are facing in the last years. The demand on cleaner emissions (both for NOx and soot), the fuel consumption reduction and the customer claim for improved engine performance are variables that should be matched to attend the legislation and market requirements. The search for these problems solution is a target that needs to be continuously reached, as the limits for emissions are year-by-year being updated and becoming more and more severe. Also, fossil fuel saving is a must for the future trends due to the reduction of its global reserves, the high cost associated with new fuel sources research and the greenhouse effect. Besides the external demands, the automotive industry is on a process to reduce costs and improve its efficacy. Time-to-market reduction, less validation tests and prototypes generation, smarter solutions and more robust design are only some of the achievements that are being sought. On this way, the use of numerical simulation is a powerful and significant tool. In order to reduce emission levels, some external engine features can be applied, such as EGR or after-treatment systems. The optimization of the piston bowl and injector design may also bring significant improvements on NOx and soot reduction. Piston bowl profile, injector nozzle diameter and angle, injector position on combustion chamber, and calibration variables (injection start, fuel mass, etc..) are some of the parameters that can be set for this purpose. The larger use of EGR system is a trend for the upcoming Diesel engines as it is very effective to mitigate the NOx levels, reducing the flame temperature. On the other side, soot levels are increased due to poor oxygen offer. The purpose of this paper is to describe an optimization study of the combustion design (intake and exhaust system, fuel system and piston bowl) for engines with EGR. The desirable design solution shall result in soot level reduction in order to compensate the effects of the gas recirculation. The understanding of combustion

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According to reference [1] and [3], the EGR is an additional diluent to the unburned gas (therefore having the dilution effect over the O2) diminishing the peak temperature inside the cylinder and the temperature of the flame, once it increases the heat capacity of the mixture (this is mainly a result of the mass trapped increasing inside the cylinder). The decrease observed in the peak temperature is also due to the fact that the H2O and CO2 suffer dissociation by an endothermic process, absorbing part of the heat released by combustion and having impact on both temperatures. As a consequence of the lower peak temperature, the formation of NOx decreases. According to [1], the main reduction in NOx emission provoked by EGR is observed with an EGR rate of 10 to 25%. Although the EGR system lowers the NOx formation, it also has a downside effect of reducing the combustion rate which implies in a combustion with a higher degree of instability. The reduction in the burn rate results in higher emissions of hydrocarbon. Still on the emissions topic, there is a soot production raise and this effect generates a higher heat rejection due to a substantial increase of the flame radiation emission, therefore there is a decrease of the flame temperature. Due to the dilution of O2 from the inlet air, the EGR system also delays all the process related to combustion, such as: Ignition Delay/Start and diffusion in general [3]. The increased ignition delay implies in a higher premixed combustion part, and, without EGR, the NOx emissions will probably increase. However, in an engine equipped with an EGR system, the combustion in the premixed part is higher and the rate of heat release premixed peak lowers, resulting in a reduction of NOx emissions. Another implication of the EGR rate is in the Brake Specific Fuel Consumption (BSFC) and mean exhaust temperature. Both suffer a decrease with higher EGR rates. According to [1], there are three specific reasons for the decrease in the BSFC: the less heat losses to the wall because the burned gas temperature decreases significantly, the pumping work is reduced as EGR increases and “a reduction in the degree of dissociation in the high-temperature burned gases which allows more fuel’s chemical energy to be converted into sensible energy near TDC” [1]. Just as important as the EGR rate, the injection system strategy is fundamental to achieve the performance and emission targets. The pressure in which the fuel is injected affects its distribution in the cylinder and consequently the combustion and its rate. The high pressure gradient is a requirement for Direct Injection engines. This high injection pressure gradient implies in a high speed flux, which is responsible for the atomization of the fuel entering the combustion chamber. The higher the pressure, the

finer the atomization and more complete is the combustion, implying in a minor quantity of soot produced. Besides the combustion, there are two other physical processes that are fundamental for a homogeneous distribution: the fuel evaporation and its diffusion throughout the chamber. The fuel evaporation and diffusion occur more rapidly with a higher level of atomization. Once these processes are fundamental for a complete burning, the atomization is a key factor for the combustion development. Concluding, the atomization and the consequential distribution of the fuel are fundamental for a complete combustion and have strong influence in the engine performance. These parameters are fundamental in the conception of an engine and need to be optimized. The main goal of this paper is to optimize these variables and observe the effect of each of them in the engine performance. Specific Objectives The first optimization step is to evaluate the injection strategy. And for more detailed studies, future steps involve the optimization of the combustion chamber, by simulating different bowl profiles. The main objective is to use a 1D tool and a 3D CFD tool to optimize the injection system, looking at the resulting combustion and the products/parameters of this combustion. To obtain the optimized solution, the following simulation software were used: GT-Power (from Gamma Technologies) and KIVA 3V (Winsconsin Engine Research Center). KIVA is a 3D CFD solver focused on the description of the flow inside the cylinder and of the combustion. The GT-Power models the engines air system and engine performance. In this case, GT-Power was mainly used to define boundary conditions and a few of the injection parameters. Although GT-Power can predict combustion, it is not so accurate as KIVA-3V, which is a software dedicated to in-cylinder flow and can give more reliable predictions and results. The KIVA code has many parameters to be determined, so another objective was creating a methodology to work with KIVA, specially focused on the parameters that have the necessity of calibration. The development of a methodology is based on software and scientific phenomenon knowledge and is very useful for future works; once it decreases the time spent in calibrating the model. For these analyses, KIVA was used through the GTPower interface, once the KIVA routine was adapted in GT-Power. Therefore, for the user, all the parameters were defined in the same interface.

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The use of these software allows more accurate predictions related to emission and fuel consumption. The optimization is achieved through a Design of Experiment (DoE), which allows the definition of a range of values for each significant variable that will be simulated. De DoE is used in order to obtain the most appropriated result with the intention of implementing the solution in the engine being designed. The range values have some restrictions, mainly because of the construction and design requirements. The optimized variables that were chosen are: EGR rate (Exhaust Gas Recirculation), Spray angle, and injection pressure. These parameters have significant impact in the combustion and, therefore, an optimized solution is a key factor in the conception of an engine. For a comparison base, the first step was determining a baseline model which will be the reference for the other cases simulated in the DoE. Another goal is to verify the effect of the injection and EGR have in the combustion and performance of the engine. As well as the objectives described above, it will be verified the viability of the use of these software in the design of Direct Injection (DI) Engines, more specifically if the predictions can be used to amore solid and reliable project.

In order to keep the same burning conditions, the intake fresh air mass flow was corrected for each EGR rate case. So, with the EGR rate increase, the total mass flow is also raising, but keeping the same A/F ratio from the baseline. The KIVA code is base in the calculation of the fluid dynamic equation conservation as follows:

Model Description It was built a 1 cylinder engine model using the 1D simulation. As the main objective of these simulations is to evaluate the combustion process, it was considered in the model only the intake and the exhaust ports and valves and the cylinder as well. The GTPower interface was used to impose the boundary conditions and to input the flow data on the KIVA model. Figure 1 shows the build 1D model.

The main focus of the simulation is the combustion processes modeling. According to the KIVA code, its species calculations are based on an 8-step chain-branching reaction scheme. The chemical reactions are: RH + O2  2R* R*  R* + P + heat R*  R* + B R*  R* + Q R* + Q  R* + B B  2R* R*  Linear Termination 2R*  Quadratic Termination where: RH is a hydrocarbon fuel (CnH2n) R* is a radical formed from the fuel B is a branching agent Q is an intermediate species The chemical kinetic of each reaction is modeled by the Arrhenius equation, parallel to the fluid dynamics conservation laws (mass, momentum and energy conservation laws.. As an example, the fuel burning is shown below: Rf = A · ñ2 · xfa · xoxb · exp(-EA/ (R · T))

Figure 1 – 1D model for combustion simulation.

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where:

Results and Discussions The variables that were considered in the studies and their values are described below:

Rf is the consumption rate of unburned fuel; A, a, b, EA are constants; R is the universal gas constants; xf and xox are mass fractions of unburned fuel and oxidizer mass respectively.

EGR Rate: 5%, 7%, 10%, 15% and 20% Nozzle Spray Angle: 144, 146, 148 and 150 degrees Injection Pressure: 1500, 1600, 1700, 1800 bar

This combustion model is focused on high temperature chemical kinetics and is started when the ignition models/condition is achieved. The ignition model depends mainly of two parameters: the temperature inside the cell and the species concentration in the cell. When the temperature and the composition of the mixture achieve adequate values, the combustion model takes over and the combustion model (reaction + Arrhenius equation) takes over When EGR is used, it is possible to verify its effects through the Arrhenius equation. Two variables from the equation are reduced, the O2 concentration and cell temperature, lowering the burning and auto-ignition rates. KIVA considers three mechanisms of NOx formation. The first one is originated from the nitrogen presence in the fuel, and the second comes from the reaction between nitrogen from the air with free hydrocarbons (called prompt NO). However, the most significant one is the Thermal NO, being the reaction ruled by the temperature. As known, EGR reduces the overall temperature and the oxygen concentration, playing the major role on this mechanism. The chemical reactions in this case are:

The baseline case is composed by: 5% EGR Rate, 146 degrees of nozzle spray angle and 1600 bar of injection pressure. The fresh air mass flow is 127.4 kg/hr and will be kept constant for all cases. The obtained numerical results for this case that will be used for comparison are: NOx Level: 214 ppm HC Level: 21.15 ppm Brake Power: 38.6 hp Considering the baseline and modifying only the EGR rate, it is possible to verify the effect of this parameter on the combustion delay and on BSFC. As expected and mentioned by [1] and [3], higher EGR rates delays the combustion start and reduce the BSFC.

N2 + O NO + O N + O2 NO +O N + OH NO + H Regarding the Hydrocarbons, one of its formation mechanisms is known as overleaning. When the fuel is sprayed, it is formed a layer with very high fuel/air rate at the spray cone border. The fuel on this layer does not auto-ignite, and sometimes, it is not burned. A solution is to avoid the formation of the low A/F rate layer as much as possible, keeping the combustion delay low. However, EGR has the opposite effect, increasing the HC formation. Quenching is another HC formation mechanism that is considered by KIVA and is really affected by EGR. The temperature drops on the combustion chamber boundaries and results in unburnt hydrocarbons. As EGR reduces the overall temperature, HC formation is favored. Two other HC formation mechanisms considered by KIVA are undermixing and overfuelling, but they are not so affected by the EGR rate.

Figure 2 – EGR influence on Combustion Delay

Figure 3 – EGR influence on BSFC

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Improving this study about the parameter influence, it was modified not only the EGR rate, but also the fuel injection pressure. Whatever is the injection pressure value, the BSFC behavior is the same observed for the baseline when the EGR rate is increased. As showed in figure 3, higher the EGR rate, lower the BSFC. However, keeping the same EGR rate, the injection pressure showed very low or none influence on the fuel consumption (fig.4).

Figure 6 – Influence on NOx Concentration The same kind of studies was done keeping the fuel injection pressure from the baseline, but varying the EGR rate and the nozzle spray angle. However, keeping the same EGR rate and changing the spray angle, it was not possible to observe any trend on the results. This fact can be explained as the nozzle spray angle has a strong direct relation with the piston bowl geometry. The most appropriate way to evaluate this parameter is to compare different bowls and define what is the best spray angle for each.

Figure 4 – Influence on BSFC As mentioned on literature and as expected from engine tests experience, the EGR rate increase tends to reduce the soot levels (indirectly given by HC concentration). It is also known that the same effect can be achieved by increasing the injection pressure. As showed in figure 5, these both conditions were obtained in the numerical simulation.

Conclusion and further steps - The use of numerical simulation is a powerful tool to better understand the phenomena involved on the combustion. The possibility in visualizing effects that are not so easy to observe in engine tests can contribute to outperforming improvements on engine design. - The numerical results showed good correlation within the experimental data that was used to calibrate the model and, all the trends when varying the input parameters are within expected behaviors. - With the developed model, it was not possible to evaluate what is the nozzle spray angle influence on emissions and on engine performance results. This issue needs to be further investigated. - The KIVA output also presents the 3D combustion maps in the piston bowl region. As further steps, the evaluation of these results will be done to optimize the combustion chamber and improve combustion kinematics.

Figure 5 – Influence on HC Concentration On the other hand, the inverse effect is expected on the NOx Concentration. Higher the EGR rate and the injection pressure, higher the NOx. These was also observed in the simulation, as showed in figure 6.

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References and Bibliography [1] Heywood, J .B., Internal Combustion Engines Fundamentals, McGraw Hill Book Co., 1988 [2] Bosch, Automotive Handbook, SAE, 2004 6th Edition [3] Maiboom A., Tauzia H., Hétet J.-F., Cormerais M., Tounsi M., Jaine T., Blanchin S., “Various Effects of EGR on combustion and emission on an automotive DI Diesel Engine: numerical and experimental”, SAE paper 2007-01-1834, 2007. [4] KIVA Reference Manual [5] N. Ladommatos, S. M. Abdelhalim and H. Zhao, Z. Hu.”Effects of EGR on Heat Release in Diesel Combustion”, SAE paper 980184, 1998 [6] Ki-Doo Kim and Dong-Hun Kim. “Improving the NOx-BSFC Trade Off of a Turbocharged Large Diesel Engine Using Performance Simulation”, [7] Hamosfakidis, V. and Reitz, R.D.; “Optimization of a Hydrocarbon Fuel Ignition Model using Genetic Algorithms”, Engine Research Center, University of Wisconsin-Madison [8] Agrawal A.K. et al., “Effect of EGR on the exhaust gas temperature and exhaust opacity in compression ignition engines”, Sadhana, vol. 29, India, 2004

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