Week 11: Project 2 - Emission characterization on a CAT3410 engine
Aim:- The Aim of this project is to simulate the combustion process in a Diesel Engine using two different piston configurations, namely omega and open w piston. It is intended to compare the performance of the engine and emission characteristic of the engine with these two configurations.
Theory:- 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.
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
Geometry Set up:-
The simulation for combustion in diesel engine was performed using sector geometry. The 360 degree geomtry of the combustion chamber was imported into CONVERGE.
Using the 'Make engine sector surface' option in Tools, The engine sectors of angle 60 degrees were created for both the omega and the open_w piston.
A bowl profile was imported after which the make surface command was executed
Case Setup:-
1.Application type:- Application type was set to Crank Angle based IC engine. The values for geometry specifications such as bore dia, rpm etc were obtained from the Engine Geometric parameters were entered as shown below.
2. Materials
- Gas Simulation:- Default values used
- Parcel Simulation- When we are modelling a simulation which has fuel spray in it, the fuel spray is represented as a Lagrangian Parcel. A langrangian based spray model is referred to as a parcel. In this case setup Diesel2 is the fuel spray.
- Global Transport Parameters:- Default value retained
- Reaction Mechanism
therm.dat and mech.dat values loaded into the profile.
- Species
Diesel2 has been added to the 'Parcel'
3. Simulation Parameters
-Run Parameters:- Simulation Mode set to Full Hydrodynamic
-Simulation time Parameters:-
Values set as shown below
- Solver Parameters- Default PISO scheme and Density based solver used.
4. Regions and Events
A region is a collection of boundaries used to differentiate different areas in the boundary. Regions are useful for a variety of reasons
- Used to specify initial conditions by regions
- Control Flow between regions (via events)
- Output results on a region by region basis
-Apply grid control techniques by regions
-Map solutions by regions
-Specify combustion models by region
-Determine time step by region specific CFL numbers.
Events control when flow is permissible between regions.
In this case set up there are four regions that have been set up, namely Cylinder region, Intake 1, Intake 2 and Exhaust regions
1) Cylinder region
- Temperature - 1360 K (Value provided in the question)
- Pressure-185731 Pa (Value provided in the question)
- Computation of Species mass concentration:- It is assumed that stoichiometric combustion of 'C_7H_16' takes place in the combustion chamber
`C_7H_16+11(O_2+3.76N_2) = 7CO_2+8H_2O+41.36N_2`
For the purpose of this analysis it was assumed that the fuel -air mixture is lean as in there is excess oxygen on the product side. This can be verified with the O2 being present in the exhaust gases
These values were inserted into species.
5.Boundary Conditions
The component of the ICE engine was classified into different region as shown below
- Piston:- Boundary Type- Wall
Wall Motion Type- Translation
Surface Movement- Moving
- Front Face
Boundary Type- Peiodic
Wall Motion Type- Stationary
Surface Movement- Rotational
-Back Face:-
Boundary Type- Periodic
Matched Boundary to front face
-Cylinder Wall:-
Boundary Type- Wall
Wall Motion Type- Stationary
Surface Movement- Fixed, Law of the Wall
Temperature Boundary Condition- Law of the wall
-Cylinder Head:-
Boundary Type- Wall
Wall Motion Type- Stationary
Surface Movement- Fixed
6. Physical Modelling
-Spray Modelling:-
Spray modelling is required because solving fluid flow equations describing spray motion directly is not computationally feasible. So a modelling approach is adopted which captures the physics of spray motion.
For liquid sprays, Converge uses a Langrangian solver to model discrete parcels and the Eulerian solver to model the continuous fluid domain. Heat, momentum and Mass transefer occur between the discrete and continuous phases via source terms in the transport equation.
Converge introduces parcels (collection of drops) into the domain at the injector. Each parcel represents a group of identical drops (same, radius, velocity, temperature). CONVERGE solves for radius, velocity on a per parcel basis.
Parcel undergo several Physical process.
- Primary Breakup
-Secondary Breakup
-Drop Drag
- Collision and coalesence
-Turbulent dispersion
-Evaporation
When fuel is injected, the liquid particles interact with the surrounding air. This interaction causes creation of a drag forces that results in propagation of disturbances. If the liquid particle is large enough it breaks into smaller particles which then break up into even smaller particles (Primary and secondary breakup). When the smaller secondary particles interact with ambient air, they undergo one or more of the other physical phenomena, namely drop drag, collision and coalesence, Turbulent dispersion and Evaporation.
For the purpose of the Full Hydro Case simulation, the following approach was adopted.
- General Setup
i) Parcel Distribution- Distribute Parcels evenly throughout cone
ii) Turbulent Dispersion - O Rourke Model (For gasoline engine, O Rourke model gives better results)
iii) Evaporation Model enabled
iv) Frossling model selected
v) Evaporation source:- Source all base parcel species selected (This implies that IC8H18 (liquid) converted to IC8H18 (gas)
vi) Temperature Discretization enabled
vii) Radius above which 1 D heat equation will be solved = 1000 m
viii) No of FV cell per spray parcel =15
ix) Thermal conductivity- Physical
-Drop Evaluation
-Film evaporation:- Default value retained
- Penetration:- Values set as shown below
-Mass Diffusivity:- Default value for Diesel and Kerosene retained
-Scaling:- Values were set as shown below
-Porus Media
- Collision Break up and modelling
i) Collision model :- NTC collision used
ii) Collision outcomes :- Post collision outcomes
iii) Use a collision mesh option enables
iv) Drop drag model- Dynamic drop drag
v) Dynamic drag values set as shown below
- Wall Interaction:-
For Wall Interaction, default values have been retained
- Injector Modelling
i) Injected Species :- Parcel Species set to C7H16 with mass fraction set to 1.0. Injection rate shape profile was imported.
- Models
Kelvin- Helmholtz model and Rayleigh- Taylor Model were selected as these are the commonly used models for solid cone sprays. The default constants were retained. Discharge coefficient set to 0.8.
The default constants control how the liquid particles breakup. The stability equation governs the process however do not capture the finer details. The constants govern how long it takes for breakup and size of the child droplets.
-Injector configuration
i) Injection temporal type - CYCLIC
ii) Cycle period - 720 degrees
ii) Start of Injection= -9 degree
ii) Injection duration = 21 degree
iii) Total injected mass:- 2.70167 e-05 kg
-Nozzles
i) Nozzle Location and orientation- Polar
ii) Injection spray type- Solid cone spray
ii) Coordinate of injectors shown below
Computation of Maximum Injection pressure:-
Peak Injection Pressure = 1097 (bar)
Peak Injection Velocity New = 471.45 (m/s)
Validation of Nozzle Locations. To ensure the newly created nozzles were inside the computational domain, the Tools-Validate Nozzle location was selected upon which the below item was displayed.
-Combustion Modelling:-
Combustion facilitates energy transfer in an engine. The Combustion model which is used for IC engines is the SAGE detailed Chemistry Solver. SAGE solver is the most predictive and accurate way to model combustion. It models ignition and laminar flame propagation quite accurately.
The SAGE model uses local conditions to calculate reaction rates based on the principle of chemical kinetics. it solves detailed chemical kinetics during combustion and determines kinetically limited phenomena such as engine knock and emissions. It reads in a reaction model in CHEMKIN format and solves ODE's to determine reaction rate. It also computes the rate of change of species concentration using the Arrehnius law. At each time step, the Chemistry solver calculates new species mass fraction immediately prior to solving transport equation. System of elementary reactions solved using CVODES.
Solving SAGE equations is a computationally slow process. In order to decrease the computational time, CONVERGE includes the following options
- Increase minimum cell temperature and minimum HC species mole fraction to reduce the number of cells in which combustion calculations are performes
- Use Start time and end time to limit SAGE to specific time intervals
-Limit SAGE to specific regions.
- Set resolve option to Only resolve if temperature changes by a specified value.
- Use analytical Jacobian to pass analytically calculated Jacobian to the solver.
- Multizone models are used to reduce number of SAGE calculations.
For the purpose of the simulation, the case was set up as shown below
The SAGE detailed chemistry solver parameters were set as follows.
The General setting values were set as shown below.
The combustion was set to start a few crank angles before ignition. The end time for combustion was set considering the opening of the exhaust valve.
- Emissions Modelling
- NOx - Thermal NOx model (Extended Zeldovich chosen)
-Hiroyasu NSC soot Model: Values set as shown below
-Phenomenological Soot model
-Detailed Soot Model
Turbulence Modelling
7. Grid control
CONVERGE uses a number of tools for controlling grid size during a simulation. Fixed embedding refines a grid at specified locations and times. Grid scaling coarsens or refines the base grid size. Adaptive Mesh Refinement automatically changes grid based on fluctuating and moving conditions.
- Base grid size was set to 0.004
- Adaptive Mesh Refinement
Adaptive mesh refinement increases grid resolution based on curvatures (second derivatives in field variables). It can be permanent or activated at specific times.It is can be activated on a region by region basis.It can be activated for both scalar and vector fields.
CONVERGE calculates the Sub grid Scalar (SGS) field as
`phi'=phi- barphi`
where `phi''= sub grid scalar field
'phi'= actual field
'barphi'= resolved field
CONVERGE uses truncated infinite series to approximate sub grid scalar field for any scalar.
where 'dx_k' is the spacing for a given rectangular cell.
For the full hydro set up two types of AMR is used
1. Velocity
Maximum no of embedding level is 3
SGS criterion is 1m/s
Whenever the curvatures exceed 1m/s, the regions would be automatically refined.
Timing control type is Permanent.
- Temperature.
Max embedding level 3.
SGS criterion 2.5 K.Whenever the curvatures exceed 2.5K, the regions would be automatically refined.
Timing control type- Cyclic
-Fixed Embedding
Fixed embedding allows to locally refine mesh in the domain where a finer detail is critical to the accuracy of the solution.
An embedding scale is specified which indicates how CONVERGE will refine the grid in that location
dx_embed= dx_base/(2^(embed scale))
For the Full Hydro setup Fixed embedding was used at the below location
1. Nozzle:-
Entity type- Nozzle
Mode - Sequential
Embed Scale = 2
Grid size at refined locations = 0.004/2^2 =0.001
2. Exhaust Valve angles
Entity type- Boundary
Boundary ID- Piston
Mode - Sequential
Embed Scale = 1
Grid size at refined locations = 0.004/2^1 =0.002
3. Cylinder Head
Entity type- Boundary ID
Boundary ID- cylinder head
Mode - Sequential
Embed Scale = 1
Grid size at refined locations = 0.004/2^1=0.002
8.Results:
i)Maximum Pressure:- In the below plot we note that omega piston has a slightly higher peak pressure value when compared to the open W piston. This could be attributed to better compression characterics of omega piston on account of piston geometry.
- Open -W -piston
- Omega Piston
ii) Mean Temperature:- It is noted that omega piston has a higher temperature when compared to open w piston. Higher temperature can be attributed to more complete combustion in the cylinder with omega piston when compared to open w piston.
- Open_W Piston
- Omega Piston
From the above animations it is observed that the temperature attained in the omega piston is higher when compared to open w piston. More effective flame spread occurs due to which effective combustion occurs in the cylinder with omega piston.
iii) Heat Release Rate:- Due to more complete combustion in the cylinder with omega piston, the magnitude of heat release rate in the piston with this configuration is marginally higher.
iv) The Integrated Heat Release Rate quantifies the heat that is generated per cycle throughout the volume. For the same reasons as stated above, this value is greater for piston with omega configuration.
Emissions
The below graphs show the plots for emissions of by products of combustion like Soot, NOx, unburnt hydrocarbons & Carbon Monoxide. From the plots above, it can be inferred that the combustion in open_w piston is incomplete. Due to this quantum of by products such as soot etc, in cylinder with open w piston configuration is higher. However for NOx, the cylinder with omega piston emits more quantity. This is due to the fact that temperatures attained are significantly higher in cylinder with omega piston. This can be verified from the below plots
i) Soot - Higher for Open_W
ii) NOx- Due to higher temperature in omega piston, quantity of NOx emitted is higher.
- Open_W piston
- Omega piston
iii) HC:- Higher for open_w piston
viii) CO- Higher for open_w piston
- Open_W piston
- Omega Piston
Power Developed by Omega and Open_W piston
1. Omega Piston
Work done= 548.985 Nm
Rotations per Minute= 1600
Rotation per Second = 26.667
Degrees per second= 26.667*360= 9600.12
Time per degree = 1/9600.12 = 1.0417 * 10e-4 sec/degrees
Time for 270.341 degrees=0.02816
Power = Work/time = 548.985/0.02816= 19.495 Kw
Torque= 60*P/(2*pi*N)= 60*19495/(2*pi*1600) = 116.35 Nm
IMEP= 224913 Pa
2. Open W piston
Work done = 481.606 NM
Rotations per Minute= 1600
Rotation per Second = 26.667
Degrees per second= 26.667*360= 9600.12
Time per degree = 1/9600.12 = 1.0417 * 10e-4 sec/degrees
Time for 270.341 degrees=0.02816
Power = Work/time = 481.606/0.02816= 17.102 Kw
Torque= 60*P/(2*pi*N)= 60*17102/(2*pi*1600) = 102.07 NM
IMEP= 197309 Pa
Cylinder with omega piston is found to develop more work and also has a higher Indicated Mean Effective Pressure. This can be attributed due to more complete combustion occuring in cylinder with omega piston configuration.
Conclusion
In this analysis two different piston profiles of Diesel engine were compared, namely the omega profile and the open w piston. The results are summarized as below.
- It was observed that more complete combustion occured in omega piston when compared to open w. As a result higher temperature was achieved.
- The unburnt hydrocarbons emitted from cylinder with omega configuration was lower when compared to piston with open w configuration
- Due to the higher temperature attained in cylinder with omega piston, NOx emissions were higher when compared to open w piston.
-Omega piston was found to develop more power and IMEP when compared to open W piston.