Emission Characteristic of the CAT3410 Diesel engine

SITUATION

          To study the Emission characterization on a CAT3410 engine & combustion modeling in the CONVERGE studio and post-process the results in the Paraview and CYGWIN was used to run the commands for the simulation.

TASK

i) To make a cut section of the engine for two profile open-w & omega and evaluate the parameters of it.

ii) Compare the Imep and power values graphically of the engine.

Compression-Ignition Engine

 

          In CI engine the air is first injected into the cylinder. The fuel is injected directly into the engine cylinder just before the combustion starts. The compression ratio of diesel is much higher than typical SI engine and is in range of 12 to 24, depending on the type of CI engine. The liquid fuel jet atomizes into drops and entrains air. The liquid fuel evaporates into fuel vapor then mixes with the air. The air temperature & pressure are above the fuel\'s ignition point Therefore after a short delay period the ignition starts. The flame spreads rapidly through that portion of the injection fuel which has already mixed with sufficient air to burn. After the exhaust process occurs & the cycle continues.

 

Effect of different shapes of crown

One of the biggest advancements in piston technology is the use of different piston tops or crowns, the part that enters the combustion chamber and is subjected to combustion. While older piston tops were mostly flat, many now feature bowls on top that have different effects on the combustion process.

The piston bowl is primarily used in diesel engines. Diesel doesn\'t have an ignition phase, so the piston crown itself may form the combustion chamber. These engines often use pistons with differently shaped crowns, although with direct injection becoming increasingly popular, gasoline engines are starting to use them as well.

The shape of the piston bowl controls the movement of air and fuel as the piston comes up for the compression stroke (before the mix is ignited and the piston is pushed downward.) The air and fuel swirl into a vortex inside the piston bowl before combustion takes place, creating a better mixture. By affecting the air/fuel mixture, you can achieve better and more efficient combustion, which leads to more power. The bowls have a variety of different shapes — some are also designed to optimize fuel economy.

The helical ports are most effective at producing relatively uniform high swirl with the minimum loss in volumetric efficiency. The geometry of the bowl-in-piston combustion chamber governs the extent to which induction generated swirl is amplified during compression. The flow field in the bowl during fuel injection is also dependent on the interaction between the swirling flow and the squish motion which occurs. As the top of the piston crown approaches the cylinder head the fuel is injected in the crown where the mixing of fuel-air takes place depending on its shape.

       

ACTION

Workflow for a CONVERGE CFD Simulation

   1. Pre-processing(preparing the surface geometry and configuring the input and data files).

   2. Running the simulation.

   3. Post-processing(analyzing the *.out ASCII files in the Case Directory and using a visualization program to view the information in the post*.out).

 

Pre-processing 

All the shapes are symmetrical about the vertical axis. Also, there are four nozzles for fuel entry to the chamber. So it is beneficial in terms of computational effort to split the control volume into six parts for analysis purposes. So the analysis will be done on 60 deg cut out of total 360 control volume. Simulations were carried out for the selected piston shapes
at full load. In all the simulations, parameters other than piston shape were kept constant. Hence compression ratio, initial conditions, computational models, solver methodology, etc. were the same at all different configurations. The cavity volume and meshing are dependent on the piston shape hence a little difference was observed. On the emission and combustion side, temperature, NOx and SOOT have been taken into account.

Extract Surface:

You can make use of the extract_profile utility in STUDIO, in order to create a sector from a full geometry. Click on \"Extract profiles from surface.dat\" in the Make engine sector surface window. You can extract the head and bowl profiles from your full geometry and import that into the make_surface utility to create a sector.

There are some requirements before you can go ahead extracting the head and bowl profiles from your full geometry. The full geometry must be centered at z=0 and any cut plane through the geometry must form a closed loop, implying you would need to remove the valves and ports from your geometry and close the holes.

Procedure

The .stl file for the model is imported, the extract profile option under the make surface is used to create a sector of the model

                         

The extract option is used to create a sector of the engine 

   

This option is used to create different files from the geometry

The engine dimension is provided and value for the con rod is provided. Select use bowl profile & import the open w bowl profile and provide value of compression ratio as 17.5 from which the squish volume is calculated.

Open W Sector:

Open W geometry

     

Omega Sector:

     

Omega Geometry

           

Make surface is used to create the boundary automatically from the model.

               

Select on the Normal toggle option and direction on the normal must be in the direction of the flow of the mixture.

                     

Case Setup

                 

1. Application - The CONVERGE is mainly developed for running the IC engine simulation so IC Engine is selected. The parameters for the engine is provided.

                     

2. Materials - Select the parcels simulation & species.

                    i) Gas Simulation – Import the thermo.dat file and the required equation of state is selected.

     

                      ii) Parcel Simulation - The gas & liquid species are imported using the thermo.dat file & parcel are used to diesel2 and passive species to be Hiroy soot and NOx.

             

                  iii) Reaction mechanism 

                               

                   iv) Species – All the gas species are available in mech.dat file but the parcel won\'t be available. 

 

 

3. Simulation Parameter - i) Simulation mode – Full hydrodynamic solver.

                                      ii) Simulation time parameter.

         

4. Boundary Condition - The boundary conditions are set for each partial differential equation. In Dirichlet B.C the value is specified directly & in Neumann B.C the value is specified as first derivative term of the variable. The periodic boundary is used in this model since it saves computation time by solving a sector of the model. Five boundaries are used for this model.

Piston

   

Front face 

Backface

Cylinder wall

Cylinder head

5. Initial Conditions and Events

This section describes the different methods by which CONVERGE can initialize physical variables like velocity, temperature & pressure.

            i) Regions - The region is a collection of one or more boundaries. The regions are mainly used to initialize variables. Depending on the simulation & available the initial values are provided.

6. Physical models - Select RNG k-e model is chosen under the turbulence model.

                         

                               a) Spray models - i) General

Parcel distribution – Cluster parcels near the cone center. This is used when the spray ejects through the center of the nozzle.

Turbulent dispersion – O’ Rourke model. O’Rourke model in Turbulent Dispersion is for Gasoline and it used to analyze how turbulence affects droplet characteristic

Use evaporation model is enabled because to capture the rate at which the parcel radius is going to change.

Evaporation source – Source specified species. The species on evaporated gets converted into their species specified provided. The other two options are converted to the composites available and converted to the species provided.                                                                              

Max radius of ODE – 1000. The maximum radius for ODE droplet heating is given as 1000. Here outside the sphere, it is hotter and inside it is less hot due to temperature distribution. So the sphere is discretized into cells and the 2D heat equation is applied to solve it. In case of small sphere, entire sphere has same temperature and it is reasonable. But incase of bigger sphere, there will be a temperature gradient. So, outside the radius ODE is solved and inside it is not solved.

No of FV Cells – 15.

Thermal conductivity – Physical.   

Urea – No urea mode. This model is used for the simulation where exhaust flow parameters are considered for the flow.

Penetration

         

The liquid fuel penetration is calculated from the volume where there are about 97% of the total liquid fuel. The bin size for vapor penetration is used where different vapor are grouped together and the fuel vapor mass fraction value for calculating vapor penetration.

                                                 ii) Collision/ breakup – Select use a collision mesh & all the parameters are the same. Select include breakup. The parameters for the breakup must be included otherwise the fuel won\'t break up and the whole simulation won\'t be set up to a wrong.

                                                 iii) Wall interaction

                                                 Spray wall interaction – Wall film.                                                                                       Film splash model – O’ Roukee.

                                                iv) Injector – Select +injector & click on edit option & select Diesel2 & provide the mass fraction as 1. As only the Diesel is used as fuel the mass fraction is provided as 1. 

                                                 a) Models – Select kelvin Helmholtz model & Rayleigh Taylor model. These models generally used for both diesel & petrol engine & select discharge coefficient models & set the value to be 0.8. Select Diesel under set recommendation for.

                                                 b) Time/ Temp/ Mass size – Start of injection - -9.

                                                                                Injection duration – 21.

                                                                                Total inject mass – 2.7e-5.

                                                                                Total no of injected parcels – 500000.

                                                                                Injected liquid temp – 341k.

Nozzles:

     

    After all the values are provided. Select tools under injector & then spray rate preview which shows the pressure value is calculated depending on the value is provided & select tools -- > validate nozzle location to check whether all nozzles within nozzles.

                     

The pressure value for the nozzle is calculated by the values we provided, there will be only one pressure for which the fuel is ejected in order to calculate it use the option tools-->  spray rate preview to view the pressure value for our case.

b) Combustion Modelling – Select SAGE.    

                                    Max cell temp – 600k.                                                                                                      Min HC species mole fraction – 1e-8.                                                                                  Temporal type – cyclic.                                                                                                      Cyclic period – 720.                                                                                                          Start time - -10.                                                                                                              End time – 135 & under general select species name to be diesel 2. 

7. Grid control – i) Base Grid – the value of the grid is provided as 0.0014.

                        ii) Adaptive Mesh Refinement – It changes the grid based on fluctuating and moving conditions. Select the active regions as the In-cylinder region.

Velocity 

Temperature

iii)Fixed embedding - It refines the grid at specified location and duration.

                            dx_embed = dx_base/2^embed scale.

                               i) Embedding 1 – Type – injector.                                                                                                                 Mode – Cyclic.                                                                                                                 Period – 720.                                                                                                                   Scale – 4.                                                                                                                        Start time - -482.                                                                                                            End time – -286.                                                                                                              Radius – 0.002.                                                                                                                Radius – 0.004.                                                                                                               Length – 0.02.

                               ii) Embedding 2 – Type – Boundary.                                                                                                             Boundary ID – Piston.                                                                                                       Mode – Sequential.                                                                                                           Scale – 1.                                                                                                                        Embed layer – 1.                                                                                                               Start time - -20.                                                                                                               End time – 180.

                                iii) Embedding 2 – Type – Boundary.                                                                                                             Boundary ID – Cylinder head.                                                                                           Mode – Sequential.                                                                                                           Scale – 1.                                                                                                                        Embed layer – 1.                                                                                                               Start time - -20.                                                                                                               End time – 180.

Right click on the Grid control -- > Timing map.

8. Output/post-processing

Post - Variable selection 

 

Output files

Running the Simulation:

File --> export – To export the case set up for running the simulation using CYGWIN.

Mpiexec.exe -n 4 converge-2.3.26-msmpi-win-64.exe </dev/null>logfile & - The command is used to run the simulation using 4 processors & the following command is used to store the file in the name of the log file and & symbol is used to store the files in the background. Since no Nohydro simulation is done (i.e) no chemical reaction occurs or any change in the properties of the fluid so the parameters plot is done. 

Thermodynamic Plot:

1. Pressure plot

The peak pressure is achieved during the end of the compression stroke and it decreases as the combustion and expansion take place. The max pressure of Open-w piston is more than Omega piston. The maximum and minimum pressure form the above diagram is used to derive the power output and efficiency of the engine.

2. Mean Temperature plot

Due to combustion, the temperature increases. In the Open W piston, the max temperature is less than Omega piston the main reason may be due to better combustion. The emission plot of the piston can be used to obtain accurate results. 

 3. Trapped mass

The Omega piston has more trapped mass than Open W piston but other factors like heat release & combustion efficiency are higher and the difference in the value is also less which are in the range of 1e-5. The intake port region near the combustion chamber has some species mass which is trapped. Designers often focus on this trapped mass not taking place in combustion. 

4. Volume Plot

 

The compression ratio (r) of the engine is the ratio of total cylinder volume(Vt) when the piston is at the bottom dead center (BDC) to the clearance volume (Vc) when piston at top dead center (TDC).               

                                           i) Omega                                 ii) Open W

Total Cylinder Volume(Vt) = 2.46256*e-3 m3.                   2.44722*e-3 m3.

Clearance volume (Vs)     = 1.6315*e-4 m3.                      1.4782*e-4 m3.

                          C.R = i) 2.46256*e-3 / 1.6315*e-4.    ii) 2.44722*e-3 / 1.4782*e-4.

                                 = 15.09                                          16.55.

The compression ratio of Open W is more Omega by using the same parameters for the engine, so the efficiency is directly proportional to the compression ratio, the Open W is more efficient.

Liquid spray plot

This plot tells us the amount of fuel drops that are created during the spray of fuel into the combustion chamber. The drops are formed because of the vaporization (Lagrangian parcels are active in the domain. After the combustion takes place, the end of the graph shows the amount of drops that are leftover i.e, the unburnt hydrocarbons. From this data, we can know the amount of fuel involved in combustion.

Combustion Efficiency

Heat Release Plot

Integrated Heat Release Rate Plot

The heat release rate plot provides the amount of heat released per crank angle. The spike in the plot is due to some distortion which can be neglected. The integrated heat release plot gives us the amount of energy released due to the combustion process alone. This data is obtained from the integration of the heat release rate plot. The ratio of the Integrated heat release rate and total energy of the fuel mass provide the combustion efficiency of the cylinder. From the plot, it is clear that the open-w piston has a longer duration of heat release than the omega piston in which the highest heat release is obtained.

   

Integrated HR omega = 7279.46 J.

Integrated HR open W = 7212.067 J.

The difference in the total energy is 67 J which may be considered as less but the simulation is run for the single cycle. 

Calculate the mass for 67 J

The calorific value for the Diesel - 45.5 MJ/kg.

Mass of the fuel = 67/45.5*e6

                        = 1.473*e-6 kg.

This is fuel wasted for the Open W piston for 1 cycle if we consider the simulation for 1000cycles then the total fuel mass wasted is 1.473*e-3 kg, which is considerably larger value.

Open W Engine Performance  

Omega Engine Performance

                                        Omega                          Open W

Work done =                   3409.28 Nm                   3028.53 Nm.

Duration of Combustion = 270.171 deg.                 270.156 deg

Time taken for combustion = Time /deg * duration of combustion(deg).

               = i) 1.042e-4 * 270.171.               ii) 1.042e-4 * 270.156. 

               = 0.02815 sec.                                 = 0.02815 sec.

IMEP - It is the average pressure, that induced in the combustion chamber during the complete thermodynamic cycle.

                                        1.396*e6                        1.24*e6

From the pressure plot the max value is obtained in the Open W piston but the combustion occurs at the properly at the Omega piston so the average value 

Note :

RPM – 1600.                                                                                                                RPS – 26.67.                                                                                                                    Degree/ Cycle – 9601.2. (RPS*360 for conv revolution to degree)                                        Time/ degree – 1.042e-4. (1/ degree per cycle)                                                                Time/ 720 – 0.075 s. (time/degree * 720)                                                                          fuel flow rate – 3.6 e-4 kg/s                                                                                          Fuel mass/ cycle – 2.7e-5kg. (fuel flow rate * time/720).

 

Power of Engine

Power = i) Work done / Time                          ii

            = 3409.28 / 0.02815.                       = 3028.53 / 0.02815.                

            = 1,21,111.2 W.                               = 1,07,585.43 W.

            = 121.11 kW.                                   = 107.585 kW.

Torque of Engine

Torque = i) P * 60 / 2*pi*N.(N = 1600 given).  ii)

            = 121111.2 * 60 / 2*pi*1600              = 1,07,585.43* 60 / 2*pi*1600

            = 723.2 Nm.                                       = 642.43 Nm.

The Power & torque value of the Omega piston is 12.56% higher than Open W piston which is a clear indication the design is a major factor and the injector position also plays a major role.

Significance of CA 10, 50 & 90.

                     i) Omega                                 ii) Open W

CA 10(deg) = 1.008 degrees.                       0.7034 degrees.

CA 50(deg) = 12.502 degrees.                     17.834 degrees.

CA 90(deg) = 27.528 degrees.                     53.028 degrees.

The values CA10, CA50, CA90, represents the Crank Angle at the completion of 10%, 50%, 90% of combustion. From these values, we can infer the time taken for the evaporation and combustion of the fuel. If the combustion process is fast, which requires few crank angles and leads to an increase in emissions. CA10 represents the start of ignition, CA50 represents the end of fuel-air mixture combustion and at CA90 the burning of unburnt fuel is done by the propagating flame. If CA90 value is large, the crank angles are taken increases and the exhaust stroke tends to push the premature combustion products. The CA 10 the start of the ignition is same for both the piston since it is CI engine the fuel starts to burn early than the SI engine. Due to the large duration of the unburnt fuel in the open W piston, the emission values are high & temperature are also low which will have an advect effect on the engine. 

Emission Plot

The emission plot is the output obtained from one cycle of the combustion but for more accurate results each state should be run for more number of cycle and collective data must be represented

1. CO Plot

Carbon Monoxide is a poisonous gas which is resulted due to incomplete combustion of the fuel.  The oxygen available is not complete enough to the oxidation C to CO2 then CO is released. High levels of CO indicated the mixture has rich fuel content than air.

i) Open W

ii) Omega

2. CO2 Plot

The CO2 is the product of the combustion always the level of CO must be very low when compared to the CO2 which depicts the emission of CO is less & proper combustion occurs.

3. Hiroy Soot plot

Soot is a black substance that is the by-product of combustion. It comes out with exhaust gas but sometimes gets trapped inside the port, walls of the cylinder, head and piston. Its accumulation will affect the engine’s performance.

i) Open W

ii) Omega

 4. NOx plot

NOx – Nitrogen Monoxide, is a harmful gas. They are generated because of the high-temperature processes taking place inside the combustion chamber and the reactions occurring at that time. 

i) Open W 

ii) Omega

Inference from the emission 

The peak value of CO & CO2 is higher for Omega piston than the Open W piston the reason for this is due to the combustion which occurs at a shorter duration. Since the Hiroy soot for the Open W is high which shows the incomplete combustion of the fuel so the soot value is high. The NOx value is low which is due to the incomplete combustion so the temperature during the combustion reduces which causes the low value of the NOx.