FULL SCALE COMBUSTION MODELLING OF A PORT FUEL INJECTION ENGINE

                                                                    FULL SCALE COMBUSTION MODELLING OF A PORT FUEL INJECTION ENGINE

 

1. SURFACE PREPARATION AND BOUNDARY FLAGGING

1.1. OBJECTIVES

• Remove geometrical errors in the provided geometry.
• Flagging the various components of the PFI engine. (Boundary Flagging)

 

1.2. GEOMETRY PROVIDED

There exists several types of errors in the geometry of the IC engine provided. These errors may affect the smooth running of the simulation and in some cases not allow the simulation to run at all. Therefore it is of utmost importance to remove such errors before doing anything else while setting up the simulation.

 

1.3. LIST OF POSSIBLE ERRORS

1. Intersections
2. Non-manifold Problems
3. Open Edges
4. Normal Orientation
5. Overlapping Triangles

 

1.4. EXAMPLE OF EACH TYPE OF ERROR AND ITS RECTIFICATION TECHNIQUE

 

1.4.1. INTERSECTIONS

ERROR

One triangle passes through another triangle in the imported geometry.

 

 

RECTIFICATION TECHNIQUE

Perform boundary flagging to identify the intersecting triangles only. Then move or translate the intersecting triangles along the normal axis to gegt a geometry free from intersection as shown below.

 

1.4.2. NON-MANIFOLD PROBLEMS

ERROR

More than 2 triangles share a common edge.

 

RECTIFICATION TECHNIQUE

Delete conflicting triangles. Then fill the newly generated empty area with triangle patching.

 

1.4.3. OPRN EDGES

ERROR

Occurance of holes in the surface geometry.

 

RECTIFICATION TECHNIQUE

Patch with triangles wherever there exists a hole.

1.4.4. NORMAL ORIENTATION

ERROR

The normals for all the triangles should point towards the fluid domain, i.e., inwards. The Diagnosis tool can identify Normal Orientation issues by considering the directions of normals of the neighboring triangles, since they need to be pointed in the same

     

 

RECTIFICATION TECHNIQUE

CONVERGE Studio provides the tool which can invert the normals of all the connected triangles propagating from one

 

1.4.5. OVERLAPPING TRIANGLES

ERROR

This problem arises when there are overlapping triangles in the geometry. This problem is not easily visible, but the Diagnosis tool of CONVERGE Studio can effectively identify overlapping triangles in the geometry.     

 

RECTIFICATION TECHNIQUE

 

 

After resolving the errors in the geometry, run the diagonose tool once more to double check for any remaining errors.

 

 

1.5. BOUNDARY FLAGGING

Proper boundary flagging is essential to apply realistic boundary conditions and assign them to proper regions in the computational domain. The boundary flagging is performed based on the guidelines provided by CONVERGE CFD to model a proper crank-angle based IC Engine simulation in the software.

 

The flagged boundaries are given below:

 

• It can be observed that the inlet port is divided into two portions. This is to help with the fuel injection. As the fuel is sprayed closer to the inlet valve. This segregation helps to initialse the fuel neat the intake valve.

 

 

• The inlet and exhaust valves are flagged into three portions. This is so that they can be classified into different regions in order to accomodate the formation of disconnect triangles for specified events.

 

 

• The spark plug terminal is the point where the combustion begins.

 

• Overall the flagged regions are classified into four regions namely:

1. Intake Region Outer
2. Intake Region Outer
3. Cylinder Region
4. Exhaust Region

This can be seen the figure below:

 

• It can be seen in the figure that the intake region has two portions, the outer and inner. The fuel can now be initialsed closer to the intake valve as described above.
• The intake and exhaust valve parts are classified as, the top and angle of inlet valve being a part of the Intake Region, the top and angle of exhaust valve being a part of Exhaust Region and the bottom portions of both valves are a part of the cylinder region.
• This helps the formation of disconnect triangles near the angles of the valves and formation of a divider between the two regions.

 

1.6. SUMMARY OF SURFACE PREPARATION AND BOUNDARY FLAGGING

1. A diagnosis is carried out and all the surface faults are detected.
2. Boundary flagging is carried out.
3. Geometry cleanup is performed and all the open surfaces are patched.
4. Inlet valve is translated  to remove intersection with the cylinder head.
5. The flagged boundaries are classified into 4 regions.

 

 

2. SIMULATION SETUP

2.1. BASIC PARAMETERS AND INFORMATION

• The IC Engine simulation setup is selected for this case.
• Air is chosen in materials and the Parcel Modelling, Reaction Mechanism and Species files are also invoked. the therm.dat file is imported into the gas file for the thermodynamic data. 
• The Reaction mechanism used is Jia PRF Mechanism is used as the fuel used is gasoline.
• `IC_8H_18` (Iso-Octane) is chosen for parcel simulation and is also selected in the species file. 
• The simulation is run from -520 degrees crank angle to 120 degrees crank angle. 

 

2.2.VALVE MOVEMENT DEFINITION

The movement of the valves is characterized by inlet and exhaust lift profiles. The are set using the lift files for inlet and exhaust. The following image gives the detailed information about the lift profiles:

Here the x-axis shows the crank angle and the y-axis shows the lift in meters.

 

2.3. SPECIES CONCENTRATION

The combustion reaction equation for the current setup is as follows:

`C_8H_18+12.5(O_2+3.76N_2)→8CO_2+9H_2O+47N_2`

 

The species concentration at the outflow is converted to mass fraction so that it can be set as input in converge.

Species	Moles	Molecular Weight	Mass	Mass Fraction
CO2	8	44	352	0.192349727
N2	47	28	1316	0.719125683
H2O	9	18	162	0.08852459
Total	64		1830	1

 

2.4. INITIAL CONDITIONS AND EVENTS

 

EVENTS:

Events are defined between regions in order to control the interaction between the regions. For the current study, Two types of events are invoked.

• The cyclic event for the inlet and exhaust valves. 
• The permanent event between the two intake ports ensures that the two portions of the port seperated into regions are interacting at all times.

 

Type of Event	Region 1	Region 2	Event	Timing
Cyclic	Cylinder	Intake Region Inner	Valve	intake_lift.in
Cyclic	Cylinder	Exhaust	Valve	exhaust_lift.in
Permanent	Intake Region Inner	Intake Region Outer	Open	N.A.

 

2.5. BOUNDARY CONDITIONS

 

2.6. PHYSICAL MODELS

2.6.1. SPRAY MODELLING

BASIC DESCRIPTION

• Spray is introduced into the system in the form of Lagrangian parcels. Each parcel consists of droplets that have the same properties. Additional source terms are introduced into the governing equations in order to define the interaction between the Eulerian phase(existing computational mesh) and the Lagrangian Phase. 
• There can be different types of interactions between these two phases such as the primary breakup, secondary break up, drop, drag, collision and coalescence and evaporation and all these effects are modelled in spray modelling.
• The spray can be represented in the form of two types of cones, the first where the particles are clustered at the cone centre and the particles are evenly distributed along the spray cone. The second type of representation is used in the current case.
• The O'Rourke model is used to model turbulent dissipation and the frossling model is used for evaporation. All the reactant species are assumed to evaporate into their pure gaseous forms.
• Due to the temperature of the region, when a fuel is sprayed the droplets tend to keep evaporating. As a result as the time passes the penetration of the liquid into the domain reduces and attains a steady state value. This is called the spray penetration length and is calculated based on the fulfillment of three conditions.
• The fraction of the total mass of the parcels, in this case is chosen to be 95%, the bin size for the mesh which is chosen as 0.001m and finally the fuel vapor mass fraction which is chosen as 0.001.
• Collision Model: NTC Collision with Post collision Outcomes. Collision Mesh is used.
• Drag Model: Drop Drag model is used.
• Wall Interaction: The wall film is formation is chosen with the O'Rourke model for Film splash modelling.

 

INJECTION PARAMETERS

• Parcel Species: IC8H18(Iso-Octane)
• Start of Injection: -480deg
• Injection Duration: 191.2 deg
• Injection Mass: 3e-5kg (Based on 720 deg with a 3000 rpm piston)
• Temperature: 330K

 

NOZZLE POSITIONS

• An injector with 4 nozzles is used. The nozzle positions and directions are depicted in the figure below:

• Kelvin-Helmoltz and Rayleigh-Taylor models are used.
• The rate of injection is in form of a trapezoid as shown in the figure below:-

• Nozzle Diameter: 250e-6 m.
• Circular Injection Radius: 150e-6 m.

 

2.6.2. COMBUSTION MODELLING

• SAGE chemistry solver is used. 
• The Zeldovich Nox model and Hiroyasu Soot model are also selected.
• Start Time:-17 deg
• End Time:130 deg
• Temporal Type: Cyclic
• Fuel Species: `IC_8H_18`

 

2.6.3. TURBULENCE MODELLING

• RNG k-epsilon model is utilized for the current simulation.

 

2.6.4. SOURCE/SINK MODELLING

• During the ignition of the fuel, for the generation of the spark, higher energy is required during the breakdown phase as compared to the propagation of the spark. Hence higher energy is provided during this phase, by modelling two source terms. The details of the sources given below:

SOURCE	ENERGY	DURATION	LOCATION
Source 1	0.02 J	-15 to 10 deg	(-0.003,0,0.0091)
Source 2	0.02 J	-15 to -14.5 deg	(-0.003,0,0.0091)

• The location and geometry of the source terms are identical, both being spheres of radius 0.0005m. For the duration -15 to 14.5 degrees of the crank angle the energy of both the source terms gets added and thus provides sufficient energy for the breakdown.

 

2.7. MESHING

The base grid used is 0.004m. Embedding is applied in specific Regions in orger to refine the Mesh and provide a Proper seed value to the SGS parameter during the application of AMR.

 

2.7.1. FIXED EMBEDDING

• The boundary embedding provided at the valve angles is required as the pressure and velocities vary at the opening and closing of the valve. The cylindrical embedding is provided to get a finer mesh in the combustion zone in the cylinder region. This refinement provides a better seed value for the AMR.
• The spherical embedding is provided for the source terms modelled in order to get a better refinement at the region of generation of the spark and its propogation. This also helps with providing better refinement using the temperature AMR.
• The injector embedding is used at the injector as the spray parcels come in from here and this is required for better application of velocity AMR at the intake port.
• The following figure highlights the embedded regions:

 

2.7.2. AMR (Adaptive Mesh Refinement)
A velocity based AMR and a temperature based AMR is applied at 2 different reggions. The details of the same are as follows:

NAME	REGION	TYPE	LEVEL	DURATION
Group 1	Cylinder Intake	Velocity	3	Permanent
Group 2	Cylinder Intake Exhaust	Temperature	3	Cyclic (-17 deg to 135 deg)

The following figure is the engine time diagram indicating the various engine processes based on crank angle.

 

The entire case is set-up and the files are exported and the noHydro case is run to check the formation of the mesh as well as to check the valve movements have been set up correctly. Once the No Hydro case is verified, a full hydro case is run and the results are processed.

 

3. RESULTS AND DISCUSSIONS

3.1. DYNAMIC MOTION OF THE VALVES AND THE MESH for NO HYDRO CASE

• The embedding along the valve angles can be seen to be moving along with the valves.
• The valve movement is coordinated with the crank angle. 
• It can be seen that the engine is on its exhaust stroke at the start of the animation. The exhaust valve closes and the inlet valves open.

 

3.2. INJECTED SPRAY PARTICLES

The fuel is modelled in the form of spray and is injected into the Engine towards the start of the Intake stroke just before the opening of the Inlet valves. The spray particles dispersed throughout the combustion chamber and are coloured with the velocity in the following clip.

 

3.3. MEAN TEMPERATURE AND PRESSURE DISTRIBUTION IN THE CYLINDER REGION

 

 

The peak pressure and temperature can be seen to be near 0 to 35 degrees after which showing a slow decrease in both the quantities. This also verified with the fact that all 90% of the combustion has taken place before 35 degrees of the crank angle and the expansion stroke has begun.

 

3.4. CALCULATION OF COMPRESSION RATIO

The above plot indicates the volumes in the cylinder region.

`CR=V_max/V_min=0.000574/(5.762*10^-5)=9.96`

 

3.5. NEED FOR WALL HEAT TRANSFER MODEL

• The entire SI8-PFI engine simulation takes a long time to run since it includes a number of physical phenomena interacting with each other- fluid flow, turbulence, injection, spark ignition, combustion, wall motions etc. 
• Also, for the simulation only the fluid domain is modeled and the outer walls of the cylinder or the intake and exhaust ports (solid domains with thickness) are not modeled. If the solid domains are also modeled, the simulation time would increase considerably. Due to the complex physical interactions happening inside the regions and the absence of physical properties for the walls, it is not ideal to calculate the wall temperatures based on CFD alone. 
• Hence, wall heat transfer models are used to predict the wall temperatures which are influenced by the turbulent mixing, reaction mechanisms, flame kernel hitting the walls etc.

 

3.6. COMBUSTION EFFICIENCY CALCULATION

For calculating the combustion efficiency of the current engine, we take aid of the Heat release rate and the total heat released rate curves. Total heat released is also called integrated heat released.

`eta_(C)=(IHR)/(m*LHV_(fuel))`

where: IHR=1241.1646 J (from plot); LHV of Iso-Ocatane=44MJ/K;  m=3*10^-5 Kg.

Therefore, `eta_C=(1241.1646)/(3*10^-5*44*10^6)=94.03%`

 

3.7. CALCULATING ENGINE PERFORMANCE, POWER AND TORQUE

Engine performance calculator is utilized to calculate the various performance parameters of the given engine. The calculated parameters are as follows:

Parameter	Value
Crank Duration (deg)	240.199
Work (N-m)	486.656
IEMP (Pa)	896428
CA10 (deg)	6.8317
CA50 (deg)	18.4623
CA90 (deg)	31.701

 

POWER

Revolutions per minute = 3000

Revolutions per second = 3000/60=50

Degrees per second = 50*360=18000

Time of Combustion = `240.199^0/(18000^0/s)``=0.01334 s`

W=486.656 N-m (from Engine Calculator)

`P=W/t=486.656/0.01334=36480.95 W.`

 

TORQUE

`P=(2*pi*NT)/60 impliesT=(60P)/(2*pi*N)`

`T=(60*36480.95)/(2*pi*3000)=116.181N-m` (approx.)

 

3.8. SIGNIFICANCE OF CA10, CA50 AND CA90

CA10, CA50 and CA90 represent the crank angles at which 10%, 50% and 90% of the combustion is completed. CA10 is generally considered as the starting point of combustion and CA90 is considered as the end point of the combustion. CA50 is the mid point of the combustion which denotes the point of maximum heat release rate from the combustion.

 

3.9. EMISSIONS FROM THE ENGINE

• The emissions from the engine can be seen in the plots and the plot showing the formation of soot has been magnified. 
• The emission consists of carbon dioxide, carbon monoxide and unburnt hydrocarbons. These are formed due to incomplete combustion of the fuel. 
• Nox and soot are also formed. Nox is formed due to the reactions of nitrogen.

 

4. CONCLUSION

• No Hydro simulation is carried out in order to reduce computation time while checking if any modifications need to be made in the mesh generation and also to check if the valve lift profiles have been implemented correctly.
• The full hydro simulation is then carried out and the various profiles of temperature and spray parcel distribution are observed. Various Engine performance parameters are also calculated.