Week 10: Project 1 - FULL HYDRO case set up (PFI)

Aim

To setup a full hydrodynamic case of PFI and perform simulation to determine the engine characteristics

1. Introduction

Using the boundary flagging and mesh motion obtained from the no-hydro case, this project involves spray and combustion modeling to perform a full hydrodynamic case of Port Fuel Injection system. The case is simulated using a supercomputer and the output files are post-processed for engine characteristics.

2. Engine Parameters

• Engine Geometric Parameters

Bore =0.086 m

Stroke = 0.09 m

Connecting rod length = 0.18 m

RPM = 3000

 Run parameters

Simulation mode: No Hydro

Simulation time parameters

Start time: -520 deg

End time: 120 deg

 

• Boundary conditions

Piston temperature - 450K

Liner temperature - 450K

Head temperature - 450K

Spark Plug temperature - 550K

Spark Plug electrode temperature - 600K

Exhaust ports temperature - 500K

Exhaust outflow - 1 bar

Exhaust outflow temperature - 800k

Exhaust species concentration - Calculate the stoichiometric condition

Exhaust valve top temperature - 525K

Exhaust valve angle temperature - 525K

Exhaust valve bottom temperature - 525K

Intake port - 1 temperature 425K

Intake port - 2 temperature 425K

Inflow pressure - 1 bar total pressure

Inflow temperature - 363K

Inflow species - Air

Intake valve top temperature - 480K

Intake valve angle temperature - 480K

Intake valve bottom temperature - 480K

 

• Initial conditions

Intake port - 1 (closer to combustion chamber)

ic8h18 - 0.025508

o2 - 0.20157

n2 - 0.77292

Temperature - 390k

pressure - 1 bar

Intake port - 2 (away from combustion chamber)

Air

Temperature - 370K

Pressure - 1 bar

Cylinder

Pressure - 1.85731 bar

Temperature - 1360K

Stoichiometric composition

 

• Injection Parameters

Fuel flow rate = 7.5e-4 Kg/second

Injection start time = -480.0

Injection duration = 191.2

Fuel temperature = 330K

 

• Nozzle positions

Nozzle diameter = 250 micro-meter

Circular injection radius = Nozzle radius

Spray cone angle = 10

• Nozzle 0

Center 0.0823357 0.00100001 0.07019

Align Vector -0.732501 0.210489 -0.647408

• Nozzle 1

Center 0.0823357 -0.00099999 0.07019

Align Vector -0.732501 -0.210489 -0.647408

• Nozzle 2

Center 0.0823357 -0.0004 0.07019

Align Vector -0.5 -0.2 -0.647408

• Nozzle 3

Center 0.0823357 0.0003 0.07019

Align Vector -0.5 0.2 -0.647408

 

• Spark Ignition Parameters

Start of spark = -15 deg

Spark duration = 10 deg

Spark location = -0.003 0 0.0091

Spark radius = 0.0005m

 

3. Objective

With the inputs provided in the challenge description, find out the following:

• What is the compression ratio of this engine?
• Why do we need a wall heat transfer model? Why can't we predict the wall temperature from the CFD simulation?
• Calculate the combustion efficiency of this engine
• Use the engine performance calculator, to determine the power and torque for this engine.
• What is the significance of ca10, ca50 and ca90?

4. Setup

4.1 Spray modeling

Materials -> Parcel simulation -> Pre-defined parcels-> IC8H18

We select IC8H18 because its characteristics are similar to that of gasoline

Species-> Parcel- > Add IC8H18

Physical models-> Spray modeling

4.1.1 Parameters of spray modeling

Parcel distribution: Evenly throughout the cone

Turbulent dispersion: O’Rourke model

Injector-> Add injector with IC8H18 parcel with mass fraction = 1.0

4.1.2 Rate shape

Rate shape is given to effectively control the flow rate of fuel injected into the combustion chamber. The graph shows the normalized profile using which Converge determines the mass flow rate for the given conditions of injection duration and mass of fuel injected. 

4.1.3 Spray rate preview

It is to be noted that the given set of inputs do not include the injection pressure which is the most important parameter in design of an injector. This is because the given set of inputs can be used to calculate the injection pressure and using the spray rate preview tool in Converge, the below data can be obtained:

4.1.4 Injector models

Injector model-> Kelvin Helmholtz and Rayleigh- Taylor model (RT)

Set recommendations for-> Gasoline PFI

Injector temperature-> 330 K

4.1.5 Injector fuel mass calculation

The mass flow rate of injector is specified as kg/sec. However, during the crank angle rotation of 720 deg, the injection occurs only for 191.2 deg. 

Considering engine speed = 3000 rpm = 50 rps = 18000 deg/s

For 1 deg rotation, time taken = 5.55 e-5 sec

For 720 deg, time taken = 0.04 sec

Given, Injector fuel rate = 7.5e-4 Kg/second

Therefore, total mass of fuel injected in one cycle = 3 e-5 kg

 

The default location of the injector is set up at the origin. For the given case, we set up the nozzles at intake region -1 as shown in the figure below using the nozzle positions given in the data sheet.

4.1.6 Fixed embedding for Injector

Entity-> Injector

Scale-> 4 ; Inner radius: 0.001 and outer radius: 0.002 m ; Length = 0.02 m

5. Combustion model

Combustion model-> SAGE combustion model with multi-zone model

Combustion start time is just before sparking. Based on given data, start time= -17 deg

Combustion end time is the time when exhaust valves are opened, end time= 130 deg

6. Spark Ignition

Physical models-> Source/sink modeling

Add two energy sources with sphere shape. We use two energy sources to map the below profile of spark ignition.

Source-1 duration: -15 to 14.5 deg

Source-2 duration: -15 to 5 deg

Black line refers to the experimental profile. To simulate this, we break it down and approximate it represented by the red curve.

7. Fixed embedding and AMR

To minimize numerical diffusion in the source terms, we add fixed embedding of spherical entity with scale-5 embedded with a larger sphere with scale-3 over the sources.

Velocity AMR and Temperature AMR in intake and cylinder regions are applied.

8. Timing map

The timing map show the entire case setup timeline with events, embedding and mesh refinement duration. This can be used to verify the case setup in accordance with the given engine parameters.

 

9. Animation

9.1 Mesh motion

9.2 Parcel

The liquid spray drops in the cylinder region plot against the crank angle, as shown below, shows that the parcels are created at the start of the injection. These liquid droplets atomize during the compression stroke and are ignited using the spark.

At the end of the combustion stroke, it can be noted that there is no liquid spray drops in the cylinder region.

However, in the intake region close to the intake valves, there is residual fuel accumulation at the end of the intake stroke. This is captured in the plot below. The mass of IC8H18 at the end of the intake stroke is approximately 3% of the injected fuel.

9.3 Temperature

The mean temperature plot signifies the effect on combustion on the temperature. This data can be used to perform conjugate heat transfer analysis by constraining the case with boundary condition equivalent to the temperature profile obtained.

9.4 Pressure

The pressure plot indicates the combustion start and finish. Also, the peak pressure inside the cylinder region is found to be 3.88 MPa.

9.5 Emissions

The graph above shows the emissions in the cylinder region during the periodic cycle. This data can be used to benchmark the engine and design modifications and their impact on emissions can be iteratively determined.

9.6.1 What is the compression ratio of this engine?

Dataset	Y min	Min location	Y max	Max location
Volume (m^3)	5.70294e-05	-360.221	0.00057422	-176.239

From the graph, min volume = 5.70294 e-05 m^3 and max volume = 5.7422 e-04 m^3

Compression ratio = (Vs + Vc)/ Vc

Vs, the stroke volume is the difference between the maximum and minimum volume

Vc, the clearance volume is the minimum volume at the Top dead centre

Vs = 5.12872E-04 m^3 and Vc = 5.74E-05 m^3

Therefore compression ratio = 9.93

9.6.2 Why do we need a wall heat transfer model? Why can't we predict the wall temperature from the CFD simulation?

In IC engine analysis, the mass, momentum and energy conservation equations in the regions are solved using the turbulence and combustion model. As we are more interested in the fluid region and not the solid region, we treat the walls as adiabatic. This helps in arriving at a convergent solution using less computational resources without affecting the combustion characteristics. It is to be noted that heat equations are found to converge much faster than that of the solids, and treating walls as adiabatic helps us save time.

For studying the wall temperature, CHT analysis is performed by initializing the wall material with suitable heat transfer coefficient. Using this, the heat loss from through the walls is determined and the combustion efficiency calculated again.

9.6.3 Calculate the combustion efficiency of this engine

The heat release rate captures the energy that is released during the combustion. Using the integrated heat release, the total energy released during the combustion process can be determined.

Total energy from combustion process = Max value of Integrated HR curve = 1241.16 J

Energy content of the fuel = Mass of fuel x Lower Heating value of IC8H18

LHV of IC8H18 = 44.8 MJ/kg, Mass of fuel = 3e-5 kg

Combustion efficiency = Total energy from combustion process / Energy content = 92.4%

9.6.4 Use the engine performance calculator, to determine the power and torque for this engine

Tools -> Engine performance calculator

Work done = 486.64 N-m

Engine speed = 3000 rpm = 18000 deg/sec

Combustion duration = 240.199 deg

Total time = 0.01334sec

Power = Work done/Time

Power = 35.13 KW

We know, Power (P) = (2*π*N*T)/60

Therefore, torque T = P * 60 / (2*π*N) = 111.879 N-m

9.6.5 What is the significance of ca10, ca50 and ca90?

CA10 represents the crank angle at which 10% of the combustion is completed. Similarly, CA50 and CA90 gives the crank angle at which 50% and 90% of combustion is completed.

CA10 denotes the start of the combustion or ignition of fuel. The difference between the start of fuel injection and CA10 gives the ignition delay. CA50 denotes the end of premixed combustion and start of diffusion combustion of fuel in the chamber. CA90 denotes the end of the combustion and the difference between CA90 and CA10 gives the combustion duration. These values are also useful in validating the results with the experimental values.