CAT 3410 enigine - Emission Charecteristics

Objective:
                   The objective of this project is to perform the emission characterization on a CAT3410 Engine model for two different piston profiles using converge CFD. The simulation is done on a sector geometry of two piston profiles and the results are compared. This project explains the complete case setup and the results are post-processed and explained.  

Converge:

                   CONVERGE is a revolutionary computational fluid dynamics (CFD) program that eliminates the grid generation bottleneck from the simulation process. CONVERGE was developed by engine simulation experts and is straightforward to use for both engine and non-engine simulations. Unlike many CFD programs, CONVERGE automatically generates a perfectly orthogonal, structured grid at runtime based on simple, user-defined grid control parameters. This grid generation method completely eliminates the need to manually generate a grid. In addition, CONVERGE offers many other features to expedite the setup process and to ensure that your simulations are as computationally efficient as possible.

Innovative Gridding Methods in CONVERGE:

                     Traditionally, boundary-fitted grids morph the vertices and cells in the interior of the domain to conform to the shape of the geometry. There are two significant disadvantages to using a traditional boundary fitted grid. First, whether structured or unstructured, fitting a grid to a complex geometry prevents the use of simple orthogonal grids. This, in turn, eliminates the benefits of numerical accuracy and computational efficiency associated with orthogonal grids. Second, generating a traditional boundary-fitted grid for a complex moving geometry can be time-consuming and difficult. Often the grid generation difficulties and significant time requirements are a roadblock to simulating complex moving geometries such as an internal combustion engine.
                     CONVERGE uses a different, better strategy: an innovative boundary-fitted approach eliminates the need for the computational grid to coincide with the geometry of interest. This method has two significant advantages. First, the type of grid used is chosen for computational efficiency instead of geometry. This allows the use of simple orthogonal grids, which simplifies the numerics of the solver. Second, the grid generation complexity and the time required are greatly reduced, as the complex geometry only needs to be mapped onto the underlying orthogonal grid. You are required to provide only a file containing the surface geometry represented as a closed triangulated surface. This file is easily written in Stereo Lithography (STL) format in most CAD packages. Given a proper STL file for the geometry of interest, it will take mere minutes to prepare a surface for even complex geometries. Note that this user time is not spent creating a grid, as CONVERGE performs the grid generation internally at runtime. 
                     At runtime, CONVERGE uses the given triangulated surface to cut the cells that are intersected by the surface. There are many benefits of generating the grid internally by the code at runtime rather than requiring you to generate the grid as an input to the code. Runtime grid generation allows the grid to be changed during the simulation. Possible changes include scaling the cell size of the entire domain, locally refining or coarsening during the simulation, and adaptively refining the mesh. Another major advantage of runtime grid generation is the ability of CONVERGE to regenerate the grid near moving boundaries during the simulation without any input from you. This means that setting up a case with a moving boundary is no more difficult than setting up a stationary case.

Powerful Physical Models in CONVERGE:          

                    In addition to its novel approaches to grid generation and boundary treatment, CONVERGE includes state-of-the-art numerical techniques and models for physical processes including turbulence, spray, combustion, conjugate heat transfer, and cavitation. With these models, CONVERGE can simulate a wide variety of flow problems. The models in CONVERGE have been extensively validated for internal combustion engine cases.

COMPRESSION-IGNITION ENGINE OPERATION:                                                                                 

                In compression-ignition engines, air alone is inducted into the cylinder. The fuel (in most applications a light fuel oil, though heated residual fuel is used in marine and power generation applications) is injected directly into the engine cylinder just before the combustion process is required to start. Load control is achieved by varying the amount of fuel injected each cycle; the airflow at a given engine speed is essentially unchanged. There are a great variety of CI engine designs in use in a wide range of applications-automobile, truck, locomotive, marine, power generation. Naturally aspirated engines where atmospheric air is inducted, turbocharged engines where the inlet air is compressed by an exhaust-driven turbine-compressor combination, and supercharged engines where the air is compressed by a mechanically driven pump or blower are common. Turbocharging and supercharging increase engine output by increasing the air mass flow per unit displaced volume, thereby allowing an increase in fuel flow. These methods are used, usually in larger engines, to reduce engine size and weight for given power output. Except in smaller engine sizes, the two-stroke cycle is competitive with the four-stroke cycle, in large part because, with the diesel cycle, only air is lost in the cylinder scavenging process.

             The compression ratio of diesel is much higher than typical SI engine values, and is in the range 12 to 24, depending on the type of diesel engine and whether the engine is naturally aspirated or turbocharged. The valve timings used are similar to those of SI engines. Air at close-to-atmospheric pressure is inducted during the intake stroke and then compressed to a pressure of about 4 MPa (600 lb/in2) and temperature of about 800 K (1WF) during the stroke. At about 20\" before TC, fuel injection into the engine cylinder commences. 

             The liquid fuel jet atomizes into drops and entrains air. The liquid fuel evaporates; fuel vapor then mixes with air to within combustible proportions:The air temperature and pressure are above the fuel\'s ignition point. Therefore after a short delay period, spontaneous ignition (autoignition) of parts of the nonuniform fuelair mixture initiates the combustion process, and the cylinder pressure rises above the nonfiring engine level. The flame spreads rapidly through that portion of the injected fuel which has already mixed with sufficient air to burn. As the expansion process proceeds, mixing between fuel, air, and burning gases continues, accompanied by further combustion. At full load, the mass of fuel injected is about 5 percent of the mass of air in the cylinder. Increasing levels of black smoke in the exhaust limit the amount of fuel that can be burned efficiently. The exhaust process is similar to that of the four-stroke SI engine. At the end of the exhaust stroke, the cycle starts again. In the two-stroke CI engine cycle, compression, fuel injection, combustion, and expansion processes are similar to the equivalent four-stroke cycle processes; it is the intake and exhaust pressure which are different. In loop scavenged engines both exhaust and inlet ports are at the same end of the cylinder and are uncovered as the piston approaches BC. After the exhaust ports open, the cylinder pressure falls rapidly in a blowdown process. The inlet ports then open, and once the cylinder pressure p falls below the inlet pressure pi, air flows into the cylinder. The burned gases, displaced by this fresh air, continue to flow out of the exhaust port (along with some of the fresh air). Once the ports close as the piston starts the compression stroke, compression, fuel-injection, fuel-air mixing, combustion and expansion processes Proceed as in the four-stroke CI engine cycle. The diesel fuel injection system consists of an injection pump, delivery pipes, and fuel injector nozzles. Several different types of injection pumps and nozzles are used.

effect of Air Swirl and Bowl-in-Piston Design on performance and emission characteristics:
Increasing amounts of air swirl within the cylinder are used in direct-injection diesel engines, as engine size decreases and maximum engine speed increases, to achieve adequately fast fuel-air mixing rates. In these medium-to-small size engines, the use of a bowl-in-piston combustion chamber results in substantial swirl amplification at the end of the compression process. Here, the impacts of varying air swirl on the performance and emissions characteristics of this type of DI engine are reviewed. Since air swirl is used to increase the fuel-air mixing rate, one would expect the overall duration of the combustion process to shorten as swirl increases and emissions that depend on the local fuel-air equivalence ratio to be dependent on swirl level. while higher swirl levels continue to increase fuel-air mixing rates, heat transfer increases also and eventually offsets the mixing rate gain. Particulate and CO emissions decrease as swirl increases due to more rapid fuel-air mixing. NO, emissions increase with an increasing swirl. At constant injection timing, however, about half the increase is due to the effect of injection advance relative to the optimum timing and half to the shorter combustion process.

In production engines, the various types of port design can be used to generate swirl during the induction process. Of these, 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 this swirling flow and the squish motion which occurs, As the top of the piston crown approaches the cylinder head. The squish-swirl interaction with highly reentrant bowl designs differs markedly from the interaction in non reentrant bowls. With a conventional bowl, the swirling air entering the bowl flows down to the base of the bowl, then inward and upward in a toroidal motion. In reentrant bowls the swirling air entering the bowl spreads downward and outward into the undercut region and then divides into a stream rising up the bowl sides and a stream flowing along the bowl base. Reentrant chambers generally produce higher swirl at the end of compression and maintain a high swirl level further. into the expansion stroke. Reentrant chambers usually achieve lower HC and smoke emissions and slightly lower bsfc, especially at retarded injection timings.

Flow pattern set up in a diametral plane by squish-swirl interaction in (a) conventional and (b) reentrant bowl-in-piston combustion chambers.

WORKFLOW FOR A CONVERGE CFD SIMULATION:

The workflow in CONVERGE consists of three steps:

1. Pre-processing (preparing the surface geometry and configuring the input and data files)
2. Case setup and 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 binary files saved in the output subdirectory).

Pre-processing(preparing surface geometry and flagging boundaries):

a. Preparing the surface geometry( Creating an engine sector):

Make Surface and Extract Profile
The make_surface utility in CONVERGE (also available as the Make Surface tool in CONVERGE Studio) will automatically configure the surface geometry file (e.g., surface.dat) for engine sector cases. Although you can prepare a sector geometry in a CAD program or manually in CONVERGE Studio (i.e., without using the Make Surface tool), the make_surface utility of the Make Surface tool will yield an engine sector geometry that is defect-free, boundary flagged, and periodic face-matched.

Extract Profile
The extract_profile utility can extract a bowl or head profile from almost any geometry. This utility is useful for a geometry in which the bowl or head does not have straight radial lines along which to manually copy points for the profile. To use the extract_profile utility, supply a surface.dat file. The geometry must be centered at z = 0 and any cut-plane through the geometry must form a closed loop. You will have to remove ports and valves from the geometry to form a closed loop. When you execute extract_profile, it will obtain the x and z coordinate data from points along a straight line in the bowl or head.

Here two types of piston data are provided, STL file for open w piston is provided and bowl profile for omega baseline piston is provided.

Steps for importing surface file: File>import>import STL

the window after importing STL file

Here in order to create the sector of the above surface, the Extract profile option under the make surface option is used as shown below. The engine parameters are provided as follows

Bore : 0.13716 m                  

Stroke : 0.1651 m                        

Length of connecting rod  : 0.263  m

Here the surface file whose profiles to be extracted is selected and the profile is extracted into the same window where the surface file exists.

Now a new project is opened and the extracted profile is used to generate sector geometry of the engine.

Now the surface is generated by clicking on the make surface button as shown below. For the omega piston, the bowl profile itself is provided to create the sector geometry.

Case setup and Running the simulation:

Setting up the case: 

Converge provides us with a handy wizard-like feature for setting up the case as shown below:

1. Application: IC engine application is selected.

Converge provides a special case setup for IC engine applications. Under the ic engine menu in the case setup window, the geometric specifications are provided.

The above mentioned physical parameters are provided by the CAD modeler or experimental team.

2. Materials:

Under the materials tab, the required mechanisms are selected.

The gas simulation menu is used to select the equation state and the thermodynamic properties of the gas mixture used in the simulation can be selected or imported here as shown below. 

The gas species and liquid species are already defined by importing the therm.dat and gas.dat files of the fuel used . Parcel species is defined to be diesel2 and passives are hiroy soot and NOx

Thermodynamic Data Input Files
Each gas-phase species in a CONVERGE simulation must have species-specific thermodynamic data in thermodynamic data or tabular thermodynamic data (i.e., tabular_therm.dat) file. 

Each of the gas-phase species in a CONVERGE simulation must have species-specific thermodynamic data in therm.dat or tabular_therm.dat. CONVERGE supports the NASA 7 format for the thermodynamic data file. For each gas-phase species in this file, it must contain the species name, the elemental composition of the species, the phase of the species, and temperature ranges over which a polynomial is fit to thermodynamic data.

Species Data and Reaction Mechanism - mech.dat
The reaction mechanism file (e.g., mech.dat) lists the gas-phase elements and species used in the simulation. If your simulation invokes the SAGE detailed chemistry solver, mech.dat must also include reaction data. Note that the species names and information defined in the reaction mechanism file must be consistent with the species names specified in initialize.in.

Parcel simulation:

parcel species are defined under parcel simulation tab. The Diesel2 species is selected under predefined liquids.

Default parameters are set for global transport parameters.

The reaction mechanisms are already been defined by importing mech.dat file and the specific properties are checked and found to be consistent with the therm.dat file.

3. Simulation parameters:

The run parameters for the simulation are set as shown below. This is a full hydrodynamic case setup run on transient mode.

The simulation time parameters are set for the transient mode simulation. The start time and the end time are specified as degrees for ic engine cases.

Start time: -147 deg.

End time: 135 deg.

Initial time –step: 5e-07s

Minimum time-step: 1e-08s

Maximum time- step: 2.5e-05

The solver parameters are set as shown below

PISO Algorithm
The pressure-velocity coupling in CONVERGE is achieved using a modified Pressure Implicit with Splitting of Operators (PISO) method of Issa (1986). The PISO algorithm as implemented in CONVERGE starts with a predictor step where the momentum equation is solved. After the predictor, a pressure equation is derived and solved, which leads to a correction, which is applied to the momentum equation. This process of correcting the momentum equation and re-solving can be repeated as many times as necessary to achieve the desired accuracy. After the momentum predictor and first corrector step have been completed, the other transport equations are solved in series. The PISO method was chosen for use in CONVERGE for various reasons. With only minor variations, this method can be used for solving either compressible or incompressible flows. In addition, the predictor-corrector concept allows for a semi-implicit treatment of sources and sinks: the sources and sinks can be updated at each corrector step. PISO has the advantage of respecting the hyperbolic nature of the transport equations while using the elliptic nature of the pressure equation to accelerate the communication of information through the domain.

4. Boundary conditions:

CONVERGE requires you to assign each boundary to one of the boundary types: INFLOW, OUTFLOW, WALL, PERIODIC, SYMMETRY, TWO_D, INTERFACE, or GT-SUITE. You must also set a boundary condition for each partial differential conservation equation at each boundary. Boundary conditions can be Dirichlet (i.e., specified value), given by
`phi`= f 
or Neumann (i.e., the specified value of the first-derivative), given by

`(delphi)/(delx)` = f

where f is a solved quantity (e.g., pressure, energy, velocity, or species) and f is the specified value or specified derivative on the boundary. In CONVERGE, f is generally set to
zero. Note that in some cases, you can specify other boundary conditions (e.g., slip or the law of- the-wall), but each of these boundary conditions is a special case of a Dirichlet or
Neumann boundary condition.CONVERGE reads boundary-related information from the boundary.in file.

PERIODIC Boundary Type:

PERIODIC boundaries allow you to simulate only a portion of geometry (e.g., an engine sector) to save computational resources. Sector (pie-shaped) and planar (box-shaped) PERIODIC boundaries are available in CONVERGE. You must specify PERIODIC boundaries in pairs. You can specify MOVING PERIODIC boundaries. The only type of motion allowed is translating. If a PERIODIC boundary is translating, vertices within that boundary will move according to the velocity you specify. For a stationary PERIODIC boundary, you must specify a velocity, although CONVERGE will not use this value.

5. Initial conditions and events:

This section describes the different methods by which CONVERGE can initialize physical variables (e.g., velocity, pressure, temperature, and species). This section also describes regions, which are used by CONVERGE both to allow flexibility in initialization and to connect or disconnect specific parts of the domain. The parameters discussed in this chapter are located primarily in initialize.in, events.in, and map.in.

Regions
In CONVERGE, a boundary is a collection of surface triangles. A region is a collection of one or more boundaries. (We recommend using CONVERGE Studio to assign surface triangles to boundaries and boundaries to regions.) CONVERGE uses regions to initialize variables (temperature, pressure, turbulent kinetic energy, turbulent dissipation, species, and passives), control the flow between portions of the geometry, and report simulation results.

It is important to remember that CONVERGE requires each boundary to be assigned to a single region. If you have a portion of the geometry with one set of boundary conditions but you wish to assign half of that geometry to one region and half to another region, you must create two boundaries (with identical boundary conditions). Then you can assign one boundary to each region.

6. Physical models:

Spray modeling

To calculate the spray in a simulation, CONVERGE introduces drop parcels into the domain at the injector location at a user-specified rate. Parcels represent a group of identical drops (i.e., same radius, velocity, temperature, etc.) and are used to statistically represent the entire spray field. By using the concept of drop parcels, CONVERGE significantly reduces the computational time of a simulation involving spray. Spray droplets are subject to several processes from the time of injection until the time of vaporization as shown below

Spray Model Process                       Model Options
Liquid injection                      Blob injection model, injection distribution models, variable                                                  rate-shape, hollow cone or solid cone, discharge coefficient                                                  models
Spray breakup                       KH, modified RT, modified KH-RT models, KH-ACT, LISA, TAB
Drop drag                             Spherical drag, dynamic drag models
Collision model                      O’Rourke model, NTC model
Collision outcomes model       O’Rourke, Post
Drop turbulent dispersion       O’Rourke model, TKE preserving model, LES model
Drop/wall interaction              Rebound/slide model, Wall film model, Vanish model
Evaporation model                 Multi-component vaporization

Collision and Coalescence

NTC Numerical Scheme

An alternative to the O’Rourke numerical collision scheme is the No Time Counter (NTC) method of Schmidt and Rutland (2000). The NTC method is based on techniques used in gas dynamics for Direct Simulation Monte Carlo (DSMC) calculations. This model has been shown to be faster and more accurate than O’Rourke’s model under certain conditions.
The NTC method involves stochastic (randomly determined) sub-sampling of the parcels within each cell. This potentially results in much faster collision calculations. Unlike O\'Rourke\'s method, which incurs an additional computational cost that increases with the square of the number of parcels, the NTC method has a linear cost. O\'Rourke\'s method assumes that multiple collisions can occur between parcels and that this process is governed by a Poisson distribution. However, the Poisson distribution is not correct unless collision has no consequences for the parcels. Since collisions change parcels’ velocities, size, and number, the method of repeated sampling used by the NTC method generates more accurate answers (Schmidt and Rutland, 2000). The NTC method is derived, without assumptions, from the basic probability model for the stochastic collision. The basic probability model requires that the cell size is sufficiently small such that spatial variations in spray quantities can be neglected. These assumptions are a subset of those required for deriving the O’Rourke collision mode.

Drop/Wall Interaction
CONVERGE offers three options for modeling drop/wall interaction, as described in the following sections:
· Rebound/Slide Model
· Wall Film Model
· Drop Vanish Model

Wall Film Model
CONVERGE offers a particle-based wall film for modeling the interaction of liquid drops
with solid surfaces. The model uses a hybrid approach to film modeling: some calculations
assume individual particle-based quantities, while other calculations assume film-based
quantities.

Liquid Injection
CONVERGE offers two categories of liquid injection mechanisms: injectors and nozzles. An injector is a group of nozzles that have some of the same characteristics, as described below. Each injector can have any number of nozzles, each with its own hole size, cone angle, position, and orientation.

You can configure nozzle locations and orientations using either a polar or a Cartesian coordinate system.If you select the polar coordinate system option to specify the nozzle locations, you must specify the location and orientation of each injector.

Injector Inputs

1. Fuel flow rate = 2.70167e-5 Kg/second
2. Injection start time = -9.0
3. Injection duration = 21.0
4. Fuel temperature = 341K

Nozzle Inputs
You can specify any number of nozzles for each of the injectors you define. Each nozzle has an associated geometric diameter and Sauter mean diameter of the injected spray. Each nozzle, whether it has solid cone spray or hollow cone spray, has a cone angle that specifies the full angle of the spray. Each nozzle with a hollow cone spray has a spray thickness. The nozzle locations and orientations are relative to the injector center and axis.

Nozzle positions

Nozzle diameter = 0.000259

Circular injection radius =0.0001295

Spray cone angle = 9

 

Combustion modeling:

A chemical reaction mechanism is a set of elementary reactions that describe an overall chemical reaction. The combustion of different fuels can be modeled by changing the mechanism (e.g., there are mechanisms for isooctane, gasoline, n-heptane, natural gas, etc.). SAGE calculates the reaction rates for each elementary reaction while the CFD solver solves the transport equations. Given an accurate mechanism, SAGE (in addition to AMR) can be used for modeling many combustion regimes (ignition, premixed, mixing controlled). You can use SAGE to model either constant-volume or constant-pressure combustion. Note that SAGE consistently uses CGS units for all calculations

Turbulence modeling

Turbulence significantly increases the rate of mixing of momentum, energy, and species. For a wide variety of applications, it is difficult to attain accurate CFD simulation results without including a turbulence model. Turbulence-enhanced mixing is a convective process that results from the presence of turbulent eddies in the flow. These turbulent eddies occur at many length scales. If a CFD solver does not contain a discretized domain (grid) that can resolve the smallest eddy length scales, then the solver cannot entirely account for the enhanced mixing effects of turbulence in the simulation. Currently, it is not practical to resolve all of the length scales in a typical CFD simulation, and thus turbulence models are used to account for the additional mixing.

RANS

It is normally believed (but not proven) that the Navier-Stokes equations model any kind of flow, turbulent flows included. The problem is that for very high values of Re\">Re, the resolution of NS equation is very challenging and not stable, thus a small perturbation in the parameter, initial condition, or boundary conditions may lead to a completely different solution. This problem is partially overcome by the use of the Reynolds-Averaged Navier-Stokes equations (RANS). time-averaging of the NS equations and the continuity equation for incompressible fluids, the basic equations for the averaged turbulent flow will be derived in the following. The flow field can then be described only with the help of the mean values.

The following Reynolds-Averaged Navier-Stokes (RANS) turbulence models are available in CONVERGE: Standard k-e, RNG (Renormalization Group) k-e, Rapid Distortion RNG ke (Han and Reitz, 1995), Realizable k-e, Standard k-ω 1998 (Wilcox, 1998), Standard k-ω 2006 (Wilcox, 2006), and k-ω SST.

 

7. Grid control:

CONVERGE includes several tools for controlling the grid size before and during a simulation. Grid scaling coarsens or refines the base grid size. Fixed embedding refines the grid at specified locations and times. Adaptive Mesh Refinement (AMR) automatically changes the grid based on fluctuating and moving conditions. The following sections explain CONVERGE\'s grid control techniques in detail.

Here base grid of size 1.4mm is used.

Grid Scaling
Grid scaling refers to changing the base grid size at specified times during a simulation. Grid scaling can greatly reduce runtimes by coarsening the grid during non-critical simulation times and can help capture critical flow phenomena by refining the grid at other times. For example, for an in-cylinder diesel engine simulation that includes spray and combustion modeling, the grid needs a higher resolution to ensure accurate results during spray and combustion but lower grid resolution may be sufficient during compression. Thus you direct CONVERGE to coarsen the grid during compression and refine the grid when spray begins.

Fixed Embedding
Use fixed embedding to refine the grid at specific locations in the domain where a finer resolution is critical to the accuracy of the solution. For example, when simulating sprays, you can add an area of fixed embedding by the nozzle to resolve the complex flow behavior. Fixed embedding allows the rest of the grid to remain coarse to minimize simulation time.

For each fixed embedding, you must specify an embedding scale that indicates how CONVERGE will refine the grid in that location. The embed_scale parameter, which must be a positive integer, scales the base grid sizes (dx_base, dy_base, and dz_base) according to

dx_embed = dx_base/2^embed scale.

Other types of embedding are

1. boundary embedding: when simulating flow around a valve, you may want extra resolution near the valve surface to more accurately model the flow in this section of the domain. In such cases, boundary embedding is done.
2. Sphere Embedding: The sphere is defined by its center (x_center) and radius.
3. Cylinder Embedding: The cylinder is defined by the center (x_center) and radius of one base of the cylinder, followed by the center and radius of the other base.
4. Nozzle and Injector Embedding: specifies a conical area of fixed embedding around a nozzle.
5. Box Embedding: The box is defined with its center (x_center) and the half-length of each dimension of the box (x_size).
6. Region Embedding: you could use a region embedding to refine the grid in the cylinder of an engine.

 

Adaptive Mesh Refinement
Use Adaptive Mesh Refinement (AMR) to automatically refine the grid based on fluctuating and moving conditions such as temperature or velocity. This option is useful for using a highly refined grid to accurately simulate complex phenomena such as flame propagation or high-velocity flow without unnecessarily slowing the simulation with a globally refined grid. Ideally, a good AMR algorithm will add higher grid resolution (embedding) where the flow field is most under-resolved or where the sub-grid field is the largest (i.e., where the curvature [gradient] of a specified field variable is the highest). The AMR method in CONVERGE estimates the magnitude of the sub-grid field to determine where CONVERGE will add embedding.

going to influence the mesh refinement.

8. Post variables selection:

User-defined functions are made default.

With this the case setup is complete.

Running the simulation:

        After exporting all the required input files, the simulation is run by using Cygwin.

        Cygwin is a POSIX-compatible environment that runs natively on Microsoft Windows. Its goal is to allow programs of Unix-like systems to be recompiled and run natively on Windows with minimal source code modifications by providing them with the same underlying POSIX API they would expect in those systems.

        The executable file provided by the convergent science is executed using Cygwin with the help of Microsoft Message Passing Interface (MSMPI).

        Microsoft Message Passing Interface is an implementation of the MPI-2 specification by Microsoft for use in Windows to interconnect and communicate between High-performance computing nodes.

        The output files are generated in the same folder where the input files are executed using the executable.

Post-processing(Results, plots, and inferences):

In-cylinder Pressure:

• It can be seen that the In-cylinder pressure created by Open - w piston is slightly higher than the omega piston.
• this data can be used to obtain information related to combustion progress and more importantly calculation of heat release rate.
• The open-w piston produces a slightly higher in-cylinder pressure due to the larger surface area when compared with omega piston
• the peak pressure produced by both pistons are as follows:
• Open W piston: 11.53 Mpa @ 5.89 deg CA.
• Omega Piston: 11.25 Mpa @ 12.16 deg CA.

Mean temperature:

• The mean temperature of the cylinder is higher for omega piston as seen in the above plot.
• This is because of the better combustion 
• thus omega piston provides a better combustion rate when compared to the open-w piston.
• Also, omega piston also releases high Nox emissions due to the higher temperature.
• It can be inferred from the above plot that the open-w piston produces a large number of unburnt hydrocarbons.

Heat release and integrated heat release:

• The heat release rate is defined as the rate at which the chemical energy of the fuel is released by the combustion process.
• The In-cylinder pressure vs CA plot provides the information for heat release data calculation.
• As already seen in the mean temperature vs CA plot, The above heat release plot proves that the combustion is better in the case of omega when compared to the open-w piston.
• Better combustion results in higher heat release and vice versa.
• Thus the omega piston provides better combustion when compared with the open-w piston.
• Also from the integrated heat release vs CA plot, It can be said that the engines with omega type piston have a better combustion efficiency.

Emission plots:

Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline, petrol, biodiesel blends, diesel fuel, fuel oil, or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack, or propelling nozzle. It often disperses downwind in a pattern called an exhaust plume.

The largest part of most combustion gas is nitrogen (N2), water vapor (H2O) (except with pure-carbon fuels), and carbon dioxide (CO2) (except for fuels without carbon); these are not toxic or noxious (although water vapor and carbon dioxide are greenhouse gases that contribute to global warming). A relatively small part of combustion gas is undesirable, noxious, or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (properly indicated as CxHy, but typically shown simply as \"HC\" on emissions-test slips) from unburnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot).

1. Soot (Hiroy model)

Soot is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process[citation needed] but is commonly extended to include the residual pyrolyzed fuel particles such as coal, cenospheres, charred wood, and petroleum coke that may become airborne during pyrolysis and that are more properly identified as cokes or char.

From the above plot it is clearly seen that soot is higher in open-w piston due to incomplete combustion.

2. Nitrogen monoxide:

Nox is the abbreviation of Nitrogen mono oxide which is a harmful gas. Nox formation is due to chemical processes that take place because of the high temperatures that are generated during the combustion process. The less Nox generation the better the emissions are.

NITROGEN OXIDES FORMED DURING COMBUSTION
N2O - nitrous oxide
NO - nitric oxide
NO2 - nitrogen dioxide
N = 14, O2 =16, NO = 30, NO2= 46

NOX means the sum of NO and NO 2 contents in flue gas recalculated on NO 2
NOx = NO + NO2

ZELDOVICH’s MECHANISM OF NO FORMATION
O2 +M = O + O +M  (dissociation)
Where
M is a stable molecule of high energy necessary to break the bounds of O2.
The liberated O atoms can react with N2 through a relatively slow reaction:
O +N2→ NO + N, 
the N atoms liberated in this reaction quickly react with O2
N +O2→ NO +O 
also giving NO.

• Thus from the above plot it is evident that due to the high in-cylinder temperature of omega piston, The emission of Nox is relatively high.
• Nox emissions in open-w piston is less due to less temperature.

CO and CO2 emissions:

In the complete combustion of hydrocarbons, the products are carbon dioxide, water, and unaffected nitrogen. However, with the incomplete combustion of hydrocarbons in automobiles, the products include unburned hydrocarbons, nitrogen oxides, carbon monoxide, carbon dioxide, and water. These types of auto emissions are responsible for carbon monoxide poisoning and affect global environmental trends.

 

• The above plots define the emission of CO and CO2 for both the pistons.
• Though omega piston produces better combustion the CO emission is found to be slightly higher than that of the open -w piston. This may be due to the poor air-fuel mixture.
• CO2 emissions are the result of good combustion.
• it can be seen that CO2 emissions are higher in omega piston when compared to open-w piston thus proving the better combustion.

Hydrocarbons (HC)

Hydrocarbon emissions are composed of unburned fuels as a result of insufficient temperature which occurs near the cylinder wall. At this point, the air-fuel mixture temperature is significantly less than the center of the cylinder. Hydrocarbons consist of thousands of species, such as alkanes, alkenes, and aromatics. They are normally stated in terms of equivalent CH₄ content.

• Hydrocarbon emissions of open W piston is higher compared to omega.
• HC emissions increase due to incomplete combustion.
• The major source of hydrocarbon emissions is lean air-fuel mixing. In lean mixtures, flame speeds may be too low for combustion to be completed during the power stroke, or combustion may not occur, and these conditions cause high hydrocarbon emissions.

Engine performance:

Omega bowl piston:

Work done = 3409.28 Nm

RPM = 1600

Duration = 270.171 deg

Time per Degree = 60/(360 x 1600) 

                         = 1.0417e-4‬sec/deg

Time per 240.199 deg = 270.171 x 1.0417e-4‬

                                = 0.02814 sec/cycle

Power = work done/time

Power = 3409.28/0.02814

        Power = 121.14 KW

          and Power = `(2piNT)/(\"60\")`Torque = `(\"60P\")/(2piN)`

                          =  (121.14*10^3*60)/2*pi*1600

Torque  = 723.42 NM

Open W piston:

Work done = 3028.53 Nm

RPM = 1600

Duration = 270.156 deg

Time per Degree = 60/(360 x 1600) 

                         = 1.0417e-4‬sec/deg

Time per 240.199 deg = 270.156 x 1.0417e-4‬

                                = 0.02814125 sec/cycle

Power = work done/time

Power = 3028.53/0.02814

        Power = 107.61KW

          and Power = `(2piNT)/(\"60\")`

                Torque = `(\"60P\")/(2piN)`

                          =  (107.61*10^3*60)/2*pi*1600

Torque  = 642.62 NM

Conclusions:

1. Thus the emission characteristics of open-w and omega baseline pistons were analyzed.
2. Engine parameters of both the piston types are obtained using an engine performance calculator.
3. Various plots are obtained to support the above conclusions.
4. From the above-said plots and results, It can be found that the engine parameters of omega piston are better when compared to the open-w piston.
5. Considering the engine performance, Though the carbon monoxide and NOx emissions are slightly higher in the omega piston. Omega piston is opted as the optimal piston design.