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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.

So Paulo, Brazil

AbstractThe environmental legislations are challenging the engine manufacturers to redesign their products in order to attendthe 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.

IntroductionThe 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 fuelconsumption 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 prototypesgeneration, 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 theupcoming 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

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 variablesmentioned above.

An engines combustion project aims at three

specific goals:

1. Cleaner emissions2. Low fuel consumption3. High performance enginesAll 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 theexhaustion 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 injectionsystem. 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.

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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 bothtemperatures.

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 productionraise 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 fuels 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 theperformance 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 combustiondevelopment.

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 ObjectivesThe 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 ofcalibration. 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 GT-

Power 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.

Model DescriptionIt 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 theKIVA model. Figure 1 shows the build 1D model.

Figure 1 1D model for combustion simulation.

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:

The main focus of the simulation is thecombustion 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 2 xf

a xoxb exp(-EA/ (R T))

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where:

Rfis 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.

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 twoparameters: 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:

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 withvery 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 unburnthydrocarbons. As EGR reduces the overalltemperature, 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.

Results and DiscussionsThe variables that were considered in the studies

and their values are described below:

EGR Rate: 5%, 7%, 10%, 15% and 20%

Nozzle Spray Angle: 144, 146, 148 and 150 degrees

Injection Pressure: 1500, 1600, 1700, 1800 bar

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 127.4kg/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 EGRrates delays the combustion start and reduce the

BSFC.

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 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.

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.

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 theresults.

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.

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 improvementson engine design.

- The numerical results showed goodcorrelation 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 notpossible 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 3Dcombustion maps in the piston bowl region.

As further steps, the evaluation of theseresults will be done to optimize the

combustion chamber and improve

combustion kinematics.

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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., Htet 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 andexperimental, 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. Improvingthe 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 usingGenetic 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 incompression ignition engines, Sadhana, vol. 29,

India, 2004