Spark Ignition - Port Fuel Injection engine simulation Part 1 - Surface preparation, Boundary Flagging, Initial setup and no-hydro simulation

In this project spark ignition engine with port fuel injection (SI8-PFI) is simulated using Converge CFD.

The full process of the project is split into two parts:

1) Initial setup and no hydro simulation.

2) Final setup and hydro simulation.

First, the surface of the geometry is prepared for CFD simulation, this is done by checking the geometry for intersections, nonmanifold problems, open edges, normal orientation error and isolated triangles and fixing them. These errors can be found by using the diagnosis dock. Upon importing the SI8-PFI geometry to Converge and running diagnosis, the geometry is found to have all the errors except the isolated triangles error (all the errors are highlighted using pink lines) .

Open edges error:

Open edges error are fixed either by stitching the vertices or by patching the free edge loops. Here all open edge errors were patched using the free loop method. These error are represented by yellow boundary lines. One example of the open edge error in the geometry is attached shown below:

Nonmanifold error:

In Converge, it is a rule that every edge should be shared by only two triangles unless there is an interface. If more than two triangles share the same edge and it is not an interface it is a nonmanifold error. In this geometry nonmanifold error is due to overlapping of geometry triangles, this error was resolved by deleting the overlapping triangles and patching the open edges. Example of nonmanifold error found in the geometry is shown below:

Intersection error:

Intersection error is caused by two surfaces/faces intersecting with each other. Here, in this geometry the intake port valves were found to be intersecting with the the cylinder head of the geometry. To resolve this issue the intake valves were translated by a small distance such that there is no longer any intersection. A intersection error of the geometry is shown below:

Normal orientation error:

If the normal vectors of the triangles that make up the geometry do not point in the direction of fluid, it results in the normal orientation error. In some places of this geometry the normals were found to be pointing outside where there is no fluid present. The error was rectified by using the transform normal option by propogating change from a single triangle. The errors found are shown below:

Secondly, the boundaries are flagged and regions are assigned. For flagging boundaries, boundary fence and angle selection methods are used. Caution should be taken in flagging the boundaries as improper flagging could result in intersection of disconnect triangles with other boundaries. Converge automatically creates disconnect triangles in areas where two regions meet but it is advisable to create disconnect triangles to check for intersection as the intersection created by them won't be found while running the diagnosis. The disconnect triangles created should be deleted before running the simulation. The flagged boundaries are higlighted in red and shown below:

Piston:

As only the fluid domain is modelled here, the boundary highlighted here is the surface of the piston and it moves along the liner during the simulation. The image shows piston using the bottom view of the geometry.

Liner:

The image is taken from the side view of the geometry.

Head (Cylinder head):

Precaution should be taken in flagging this boundary as improper flagging of this boundary will cause intersection of disconnect triangles in the intake port side and increases the length of disconnect triangles on the exhaust port side. The image shows cylinder head from the top view of the geometry.

Intake (Inflow boundary):

Inflow boundary is at the end of the intake port boundary. This is a side view of the geometry.

Exhaust outflow:

Exhaust outflow boundary is at the end of the exhaust port and the image shows the region from side view.

The intake port are divided into: intake port and intake port B. This division will enable assigning different regions to different parts of the port. Since different regions can be assigned with different initial conditions the intake port near to the cylinder region i.e., intake port B is initialised with some amount of fuel. All the intake ports are shown from the top.

Intake port B:

Intake port B is the intake port nearer to the cylinder head.

Intake port:

This intake port is away from the cylinder region.

Exhaust port:

The exhaust port as seen from the top view.

All these spark plug views are shown from bottom view with liner and piston hidden away.

Spark plug view 1:

Spark plug view 2:

Spark plug electrode:

The exhaust valves and the intake valves are split into three boundaries. This is because of the following reasons:

1) If the valve angles and valve bottoms were one boundary. The disconnect triangle formed will underpredict the mass flow of the fuel.

2) Embedding of the valve angles are essential so that the change in pressure and velocity at the gap between the ports and the valves are accurately calculated during the opening of the valves and also due to the fact that the area near valve angles are the smallest. If the valve angles and valve tops were one boundary. Embedding of the valve angles cannot be achieved separately instead valve tops also need to be embedded leading to the requirement of much higher computational power.

Exhaust valves cross section image:

Exhaust valves top:

Exhaust valves angle:

Exhaust valves bottom:

Intake valves cross section:

Intake valves top:

Intake valves angle:

Intake valves bottom:

Assigning Regions:

Now the boundary are grouped into four regions namely: intake system, intake system 2, exhaust system and cylinder regions. The reason for splitting up the boundaries into seperate regions is that each region can be given its own intital conditions.

Intake system region: This region contains the intake (inflow) and the intake port.

Intake system region 2: The intake port B, intake valve top and intake valve angle boundaries come under this region.

Exhaust system region: The exhaust port, exhaust valve top, exhaust valve angle and exhaust outflow belong to this region.

Cylinder region: Head, piston, liner, spark plug, spark plug electrode, intake valve bottom and exhaust valve bottom are of cylinder region.

Note that intake valve bottom and exhaust valve bottom are included in the cylinder region but intake valve angle and exhaust valve angle are not. This is because if the valve angles are also assigned into cylinder region, the disconnect triangles formed will be longer and will result in less mass flow being calculated than actual.

Before simulating using no hydro option intial setup must be done.

IC engine application type is selected with the following parameters considered for the engine:

1. Bore =0.086 m
2. Stroke = 0.09 m
3. Connecting rod length = 0.18 m
4. RPM = 3000

Pistion surface ID: Piston

Head ID: Head

Liner ID: Liner

Since the engine being simulated runs on gasoline, Jia PRF (Primary reference fuel) mechanism is utilised and corresponding thermodynamic data is imported into the gas simulations tab. Under run parameters no hydrodynamic simulation is to be selected. The purpose of the no hydrodynamic simulation is to check whether the motion of the valves are working perfectly and mesh generation is without errors.For the simulation time parameters for the no hydrodynamic simulations:

1. Start time : -480 deg (only for no hydro simulation)
2. end time : 240 deg (only for no hydro simulation)
3. minimum time step: 1e-08s
4. maximum time step: 2.5e-05s
5. initial time step: 5e-07s

Other values are set as default values.

Next, the boundary conditions are set up:

1. Piston temperature - Translating wall boundary following piston motion with wall temperature 450K
2. Liner temperature - Stationary wall with temperature 450K
3. Head temperature - Stationary wall with temperature 450K
4. Spark Plug temperature - Stationary wall with temperature 550K
5. Spark Plug electrode temperature - Stationary wall with temperature 600K
6. Exhaust port - Stationary wall with temperature 500K
7. Exhaut outflow - Outflow boundary with pressure 1 bar and temperature 800k with exhaust species concentration having 0.71913 mass fraction of N2, 0.19235 of CO2 and 0.088525 of H2O.
8. Exhaust valve top temperature - Translating wall boundary following exhaust lift profle with wall temperature 525K
9. Exhaust valve angle temperature - Translating wall boundary following exhaust lift profle with wall temperature 525K
10. Exhaust valve bottom temperature - Translating wall boundary following exhaust lift profle with wall temperature 525K
11. Intake port - Stationary wall with temperature 425K
12. Intake port B - Stationary wall with temperature 425K
13. Intake - Inflow boundary with pressure 1 bar and temperature 363K and inflow species air
14. Intake valve top temperature - Translating wall boundary following intake lift profle with wall temperature 480K
15. Intake valve angle temperature - Translating wall boundary following intake lift profle with wall temperature 480K
16. Intake valve bottom temperature - Translating wall boundary following intake lift profle with wall temperature 480K

Usually the intake port and exhaust port profiles are obtained from the design engineer and the minimum lift is 2e-4m. The exhaust species are obatined by balancing the equation of complete combustion of iso-ocatane fuel as shown below:

`C_8H_18 + 12.5(O_2+3.76N_2) rarr 8CO_2+9H_2O+(3.76xx12.5)N_2`

Initial conditions of the regions:

1. Intake port system 2 (closer to combustion chamber): This regions have the following species: 0.025508 mass fraction of iC8H18, 0.20157 mass fraction of O2 and N2 of mass fraction 0.77292. The region temperature is 390k with 1 bar pressure.
2. Intake port system (away from combustion chamber): This region contains Air at temperature 370K and pressure 1 bar
3. Cylinder :This region has a pressure of 1.85731 bar at temperature 1360K with species pulled from the exhaust outflow.
4. Exhaust region: This region has a pressure of 1.03737 bar at temperature 1215K with species pulled from the exhaust outflow.

After this Events should be set up:

The cylic period in the events mentioned below is 720 degrees.

Event 1: Cyclic Valve type event connecting cylinder region to the intake system 2 region using intake lift profile.

Event 2: Cyclic Valve type event connecting cylinder region to the exhaust system region using exhaust lift profile.

Event 3: Cyclic Open type event connecting intake system to the intake system 2 region with start degree 0 degree (signifies it is always open).

For turbulence modelling RNG k-epsilon model is selected. The base grid is selected to have 4mm with permenant fixed embedding applied to the intake valve angle and exhaust valve angle. The fixed embedding for intake valve angles are given a scale of 3 with 2 embed layers and the fixed embedding for exhaust valve angles are given a scale of 2 with 2 embed layers.

Additionally another permanant cylinder type fixed embedding is applied over the cylinder region extending beyond the length of the cylinder region both above and below with a scale of 2 (embedding 1). Converge automatically ignores the places where there is no geometry while applying the fixed embedding.

Embedding 1:

Now the no hydro simulation is run and the animation of it is attached below:

This is the first part of simulation that is initial set up and no hydro simulation.