Week 11: Project 2 - Emission characterization on a CAT3410 engine

Emission Characterization on a CAT3410 engine

In this project, we will simulate two geometries of engine(open-v and omega) types and obtain useful performance parameters and animations showing different plots. Also, we shall characterise the emissions of NOx,soot and UHC by suitable models like Hiroy's soot model,etc.

Diesel Engine-a look

Diesel engine is an internal-combustion engine in which air is compressed to a sufficiently high temperature to ignite diesel fuel injected into the cylinder, where combustion and expansion actuate a piston. It converts the chemical energy stored in the fuel into mechanical energy, which can be used to power freight trucks, large tractors, locomotives, and marine vessels. A limited number of automobiles also are diesel-powered, as are some electric-power generator sets.

The diesel engine is an intermittent-combustion piston-cylinder device. It operates on either a two-stroke or four-stroke cycle (see figure); however, unlike the spark-ignition gasoline engine, the diesel engine induces only air into the combustion chamber on its intake stroke. Diesel engines are typically constructed with compression ratios in the range 14:1 to 22:1. Both two-stroke and four-stroke engine designs can be found among engines with bores (cylinder diameters) less than 600 mm (24 inches). Engines with bores of greater than 600 mm are almost exclusively two-stroke cycle systems.

The diesel engine gains its energy by burning fuel injected or sprayed into the compressed, hot air charge within the cylinder. The air must be heated to a temperature greater than the temperature at which the injected fuel can ignite. Fuel sprayed into air that has a temperature higher than the “auto-ignition” temperature of the fuel spontaneously reacts with the oxygen in the air and burns. Air temperatures are typically in excess of 526 °C (979 °F); however, at engine start-up, supplemental heating of the cylinders is sometimes employed, since the temperature of the air within the cylinders is determined by both the engine’s compression ratio and its current operating temperature. Diesel engines are sometimes called compression-ignition engines because initiation of combustion relies on air heated by compression rather than on an electric spark.

In a diesel engine, fuel is introduced as the piston approaches the top dead centre of its stroke. The fuel is introduced under high pressure either into a precombustion chamber or directly into the piston-cylinder combustion chamber. With the exception of small, high-speed systems, diesel engines use direct injection.

Diesel engine fuel-injection systems are typically designed to provide injection pressures in the range of 7 to 70 megapascals (1,000 to 10,000 pounds per square inch). There are, however, a few higher-pressure systems.

Precise control of fuel-injection is critical to the performance of a diesel engine. Since the entire combustion process is controlled by fuel injection, injection must begin at the correct piston position (i.e., crank angle). At first the fuel is burned in a nearly constant-volume process while the piston is near top dead centre. As the piston moves away from this position, fuel injection is continued, and the combustion process then appears as a nearly constant-pressure process.

The combustion process in a diesel engine is heterogeneous—that is, the fuel and air are not premixed prior to initiation of combustion. Consequently, rapid vaporization and mixing of fuel in air is very important to thorough burning of the injected fuel. This places much emphasis on injector nozzle design, especially in direct-injection engines.

Engine work is obtained during the power stroke. The power stroke includes both the constant-pressure process during combustion and the expansion of the hot products of combustion after fuel injection ceases.

Diesel engines are often turbocharged and aftercooled. Addition of turbocharger and aftercooler can enhance the performance of a diesel engine in terms of both power and efficiency.

The most outstanding feature of the diesel engine is its efficiency. By compressing air rather than using an air-fuel mixture, the diesel engine is not limited by the preignition problems that plague high-compression spark-ignition engines. Thus, higher compression ratios can be achieved with diesel engines than with the spark-ignition variety; commensurately, higher theoretical cycle efficiencies, when compared with the latter, can often be realized. It should be noted that for a given compression ratio the theoretical efficiency of the spark-ignition engine is greater than that of the compression-ignition engine; however, in practice it is possible to operate compression-ignition engines at compression ratios high enough to produce efficiencies greater than those attainable with spark-ignition systems. Furthermore, diesel engines do not rely on throttling the intake mixture to control power. As such, the idling and reduced-power efficiency of the diesel is far superior to that of the spark-ignition engine.

The principal drawback of diesel engines is their emission of air-pollutants. These engines typically discharge high levels of particulate matter (soot), reactive nitrogen compounds (commonly designated NO_x), and odour compared with spark-ignition engines. Consequently, in the small-engine category, consumer acceptance is low.

A diesel engine is started by driving it from some external power source until conditions have been established under which the engine can run by its own power. The simplest starting method is to admit air from a high-pressure source—about 1.7 to nearly 2.4 megapascals—to each of the cylinders in turn on their normal firing stroke. The compressed air becomes heated sufficiently to ignite the fuel. Other starting methods involve auxillary equipment and include admitting blasts of compressed air to an air-activated motor geared to rotate a large engine’s flywheel; supplying electric current to an electric starting motor, similarly geared to the engine flywheel; and applying a small gasoline engine geared to the engine flywheel. The selection of the most suitable starting method depends on the physical size of the engine to be started, the nature of the connected load, and whether or not the load can be disconnected during starting.

Spray modeling

Converge has built in models utilized for spray processes including liquid atomization, drop breakup, collision and coalescence, turbulent dispersion and drop evaporation. These models have mathematical approximations necessary to capture the physics behind the processes mentioned above and the combustion aspect as well.

In IC engines, the fuel is sprayed in the combustion chamber or near intake valves. This fuel spray can be completely simulated using CFD multiphase simulations, but doing so in a IC engine simulation would result in the simulation running for very long. This can be prevented if we statistically model the same effect of the spray instead of actually simulating it. This approximating process of fuel sprays is called Spray modelling. Any fuel spray undergoes a mechanism which has the following processes:

1) Primary Breakup: Splitting of a drop from the mainstream

2) Secondary Breakup: Splitting of a primary drop into multiple drops

3) Drop Drag: Drag caused by air on the moving drop.

4) Collision and coalescence: Collision of two or more drops and formation of the new drop.

5) Turbulent dispersion: Transport of mass, heat, or momentum within a system due to random and chaotic time-dependent motions

6) Evaporation: Evaporation of small liquid drops into vapour

The sequence of processes is easy to understand. The mainstream breaks into multiple drops by primary and secondary breakups. New drops are formed due to coalescence. The evaporation reduces the drops and dispersion spreads the drops into the region. While modelling the spray, each process is represented mathematically by using some pre-established models. These create the effect of spray without actually simulating the spray.

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.

Evaporation: The evaporation is simulated using Frossling or Chiang model. They are responsible for flagging a parcel drop as evaporated or not. They consider the size of the drop and its evaporation properties to make decisions for drops.

Penetration: The fuel spray penetration is also recorded in the software. It calculates this based on how long did the fuel travel as a liquid. The cell up to which the 0.95*(mass of liquid fuel sprayed) is seen is called the liquid penetration length.

Collision: The software contains two models: O'Rourke model and NTC model. These models simulate the collision using a collision grid. Using a collision grid creates accurate particle movement data.

Drop Drag: This is simulated using Spherical drop drag model or Dynamic drop drag model. The Spherical model calculates the drag coefficient based on assuming the shape of drop as the sphere, while the dynamic model accounts for variations in drop shape. It uses TAB model to determine the drop distortion.

Drop wall interaction: When a drop parcel would collide with the wall, it could rebound back, escape or remain on the wall, creating a film. These effects are modelled using: O'Rourke model or Kuhnke model.

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. Each injector can have any number of nozzles, each with its own hole size, cone angle, position, and orientation.

Injector models: The models used are 'Kelvin-Helmholtz model' and 'Rayleigh-Taylor model'. These models look after the primary and secondary breakup of parcel drops. The calculations are based on 'Kelvin-Helmholtz instability theory' and 'Rayleigh-Taylor instability theory'. These theories are used for surface tension instabilities of the drop, which lead to a fluid breakup.

i) General: For solid cone sprays, converge assumes the injection is conical.

• Parcel distribution: Distribute parcels evenly throughout cone.
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• Penetration: Converge writes the liquid and vapor penetration lengths at each output interval, it calculates vapor penetration length for each nozzle.

• Liquid fuel mass fraction for calculating spray penetration: 0.95

• Liquid penetration length is the distance that encompasses the calculated penetration spray mass. Converge calculates the penetrated spray mass by multiplying the mass fraction by total liquid mass in the domain.
• Liquid penetration length is not calculated along the spray axis of the nozzle but it is calculated along the injector axis
• Bin size for calculating vapor penetration: 0.001 m
• Providing bin size for calculating the vapor penetration, cell size is at least 2 times smaller than bin size, especially in the location of spray.
• Fuel vapor mass fraction for calculating vapor penetration: 0.001
• Cell, where the vapor is present, have some minimum amount of vapour and that is characterized by "fuel vapour mass fraction for calculating vapour penetration". Its value 0.001 means that particular cell content 1e-3 vapour in terms of mass fraction coma anything less than that can be considered as a cell that not containing vapour.

Collision/Breakup/Drag:

Collision model: NTC collision

Collision outcomes: Post-collision outcomes

Enable collision mesh

• Collision calculation based upon if the one parcel is going to hit the other parcel or not.
• Parcels are Lagrangian parcels have their own space and gas-phase have their own space.
• To perform collision calculation we need another mesh which is different from the original mesh provided. This is called the collision mesh

Collision calculations can be highly grid-sensitive when an underresolved the fluid-phase mesh is used. To help alleviate this issue, the collision mesh option has been implemented in CONVERGE, based on the concept of a collision mesh proposed by Hou. In a simulation without collision mesh, parcels collide only with parcels in the same grid cell. This can lead to artefacts in the spray since parcels do not collide across cell walls. This can also slow down computational time because there can be many parcels in larger cells. Using a collision mesh can eliminate both these problems. Simulations with collision mesh can much more accurately represent the spray dispersion by eliminating grid effects.

Using a collision mesh can eliminate both these problems. Simulations with collision mesh can much more accurately represent the spray dispersion by eliminating grid effects.

An alternative to the O’Rourke numerical collision scheme is the No Time Counter (NTC) method of Schmidt and Rutland. The NTC method is based on techniques used in gas dynamics for Direct Simulation Monte Carlo (DSMC) calculations. This model is 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.

Combustion modelling

Combustion Facilitates the energy transfer in an engine.

The combustion is modelled by using its reaction mechanism. There are multiple species and intermediate reactions which govern the combustion process. The reaction mechanism of real-life combustion is very large and simulating it directly would take a long time. So, the combustion is solved only for those reactions and species whose presence affects the combustion results. This is called a reduced mechanism file. These all processes of reducing mechanism and solving them is done by various solvers present in the software: SAGE, CTC/ Shell, G-Equation, CEQ, FGM, etc. In this report, SAGE solver is used, so details of SAGE are only given.

SAGE: It's a detailed chemistry solver which uses local conditions to calculate the reaction rates based on principles of Chemical kinetics. It calculates the heat release, which is used as a source term in fluid flow energy equations. The species are modelled using chemical kinetics ODEs and the values are used in species transport equations. The Solver used to solve the ODEs is CVODES.

Acceleration technique: The SAGE solver is a detailed chemistry solver, meaning that it would calculate values based on full mechanism instead of reducing it. This takes a lot of time if the calculations are done for every cell in the mesh. So, to increase the speed of solving, the solver is run using a multizone model. All the cells where combustion is going to be solved for, are grouped into bins. The bins are created based on the temperature and equivalence ratio of fuel and air. All the cells having the same temperature and equivalence ratio will have the same combustion results. So, the SAGE solver is solved once for each bin and accordingly the flow and species data is remapped to the cells. This increases the speed tremendously. The bins can also be created based on multiple variables like pressure, reaction ratio, etc.

PROCEDURE

• The procedure involves first setting up the geometry by making use of 'Make engine surface sector' tool available under tools tab.
• Here, we have to setup two different profiles-open v and omega.
• Enter the dimensions of the engine first, as shown in the picture below.
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• For the open-v 360 engine, choose extract profile from surface.dat button and enable use bowl profile and import the surface profile for v type and bowl profile for omega type, resulting in two distinct contours as shown below:
• Omega
• V-type
• Click on make surface and surface is created.
• The next step is to import the necessary input files by going to Files->Import->Import input file(s) and selecting all the .in files and click OK.
• The case setup will be automatically setup and the only change that needs to be performed is to go to boundary->piston and choose the type of motion as Piston motion instead of User defined.
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• In the materials, choose the reaction mechanism and turn it on.
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• Import the necessary ".dat" file and click OK.
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• Set the run parameters as shown below:
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• For the Evaporation source, the source specified species option was chosen.
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• Once done, export all the input files and run the setup in CYGWIN and post process the results in STUDIO and paraview and draw suitable conclusions.

RESULTS AND DISCUSSIONS

Emission modeling:-

In this project, we will discuss CONVERGE’s emissions modeling options for soot, NOx, unburned hydrocarbons, and CO. We will look at CONVERGE’s acceleration strategies that make it feasible to use even detailed soot models (which includes gas-phase chemistry for its many sub-processes, such as formation via precursor-species, coagulation, and oxidation) in complex simulations.

Fixed embedding:

Depending on the flow taking place, we may require that the flow at some region is more accurate and at some region is more approximate. This can be done by varying the mesh element sizes in the region. The embedding option in Converge CFD does this work of creating finer mesh at the mentioned location. The refinement is based on the scale and number of embed layers. If the base mesh created has cubic elements of size 'E' mm then the scale of 'n' means the elements in the refinement region will be of size E2n">E2nE2n mm. This means that if the base mesh has elements of size 4 mm then elements of scale 1, 2, 3, 4, 5 will have elements of size 2, 1, 0.5, 0.25, 0.125 mm respectively. With each element scale, there is 8 times increase in total cell count. This happens because the elements are divided in size by 2 in all directions (X, Y, Z).

Fixed embedding is provided near valve angles as the space between valve angle and valve seat is very small and base grid with 0.004 m grid size will give inaccurate results.

Fixed embedding is provided to refine the specific areas and capture the combustion details and calculate them properly.

Cylindrical shape type embedding is provided on the cylinder to make them finer as compared to the base grid.

Adaptive Mesh Refinement:

The fixed embedding setting is preferred when the refinement is needed in the complete region or complete boundary area. When the refinement is to be done based on the solution of flow equations, the refinement technique is called AMR. The AMR monitors the solution and based on the curvature (second degree of rate of change) of a variable, the mesh is refined. There are two methods to use AMR: Sub-grid scale method and Value-based method. In the value-based method, the value of a solution is specified. If the value is more/less than specified, the mesh is refined. The Sub-grid method works in the following way:

For a scalar, the sub-grid field is defined as the difference between the actual field and the resolved field or

This sub-grid can also be represented as an infinite series, but since it's not possible to evaluate the entire series, only the first term in the series is used to approximate the scale of the subgrid.

A cell is embedded if the absolute value of the sub-grid field is above a user-specified value. Conversely, a cell is released (i.e., the embedding is removed) if the absolute value of the subgrid is below 1/5th of the user-specified value. That means if the user specifies a value of 2.5, the mesh program would calculate sub-grid of the cell, based on the term shown above and if it is larger than 2.5, the cell will be refined.

NOTE: The first pic/animation under each subheading corresponds to that of open-v type and the second pic/animation corresponds to that of an omega type.

Temperature animation

Pressure Animation

Meshing animation

After 30 timesteps(say), the AMR(Adaptive Mesh Refinement) comes into play whre we see smaller subgrids of meshing appearing along the bore. During this stroke, the number of cells increase exponentially. The reason for this is to capture physics of flow injection and heating.

Mass fraction of CO animation

Emission characterization:

NOx emission animation

Diesel fuels contain more nitrogen than gasoline and hence they produce higher Nox. The above graph is generated by Zeldovich Nox Model. NOx production is low compared to however soot is higher. The critical time is when burned gas temperatures are at a maximum i.e between, the start of combustion and after a short while of the attainment of peak cylinder pressure.

Comparing the omega type with open v type, we can conclude that the omega type produces less NOx but the soot production in omega type is larger than that of v-type.

Hydrocarbon (HC)

The unburnt Hydro-Carbons result from the Hydro-Carbons present in the fuel escaping from the combustion.

The above-shown animations clearly say that the Open-W piston does not completely combust the air and fuel mixture because of the lower mean temperature inside the cylinder and also the geometry of the piston affects the process.

If we observe this thing with more detail by referring to the below plot, for the Open-W piston the UHC is higher when compared to ‘omega pistons. To achieve a more good combustion process higher cylinder temperature as well as perfect piston design is essential.

Thus we can conclude that the combustion is incomplete in the case of open-v type.

PV-plot

For an actual cycle, the shape of the PV diagram is similar to the ideal, but the area (work) enclosed by the PV diagram is always less than the ideal value. Since this simulation is performed only for closed cycle simulation, the motion of intake and exhaust plots are not available. Note that the cycle is not closed. The pressure increases during compression stroke followed by expansion of gases at high temperature along with injection of fuels.

The diesel engine differs from the gasoline-powered otto cycle, this is because of the highly compressed hot air to ignite the fuel rather than using a spark plug which is the spark ignition.

If we compare the two plots carefully, we can conclude that the maximum pressure rise is achieved in the case of omega type rather than that of open-v type, thus confirming the claim that was made in the earlier discussion. Larger pressure rise corresponds to larger temperature,which in turn causes increase in combustion efficiency.

Pressure variation:

The above graph shows variation of pressure along the crank angle as shown. Note that the pressure increases until the end of the compression stroke and then decreases gradually after power stroke. The maximum pressure is observed as 12MPa. If we compare the two plots carefully, we can conclude that the maximum pressure rise is achieved in the case of omega type rather than that of open-v type, thus confirming the claim that was made in the earlier discussion. Larger pressure rise corresponds to larger temperature,which in turn causes increase in combustion efficiency.

Volume variation

Volume variation along the crank angle is shown above. Note that this behaviour is exactly opposite of that as observed in the pressure variation. The volume decreases first during compression stroke until the beginning of the power stroke, after which the fuel gets added up thereby increasing the volume of the fluid inside the chamber. The volume variation for two different profiles are more or less similar.

Temperature variation:

The temperature increases steadily throughout the cycle, because in compression stroke, the pressure increases which causes the temperature to increase. In the power stroke, fuel is injection, which causes further rise in temperature and heat transfer and decreases during the exhaust stroke when the gas is subjected to pushing out of the engine.

As discussed earlier, larger presure rise corresponds to larger temperature rise which in turn causes better effective combustion. As seen from the two plots above, the maximum temperature in open-v type is less than 1600K but for omega type, it is ~1800K, which confirms our previous claim that the omega type engine has better combustion efficiency.

Total no.of parcel variation

The above plot shows the variation of total no of parcels. We can see that the amount of parcel is maximum when the fuel has been added(close to -20deg) and then due to combustion reaction, it reacts with air producing CO2 and other byproducts of combustion and hence we observe that the total number of parcels decreases immediately.

Thus the emission characterization of CAT3410 engine has been studied and the comparisons between two profiles of engine have been made as well.

Comparison of IMEP and Power output for both the cases

Converge provides a useful tool for getting useful engine parameters like IMEP(Indicated mean effective Pressure),Gross work etc. Go to line plotting tool and select engine calculator tool and import thermo.dat file into it as shown below

A simple calculation is performed in excel using the above known values and the plots are compared for open v type and omega type as shown below:

It can be observed that the omega type has higher IMEP and produces more power as compared to its open-v type counterpart due to the main fact that large pressure rise in omega type causes better combustion which inturn implies better efficiency and more output extracted from the gas.

Thus the Emission characterization on a CAT3410 engine is performed and characteristic plots are studied and results are tabulated and discussed.