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

                   PORT FUEL INJECTION ENGINE (Sl8 PFI) SIMULATION

l. OBJECTIVE

1. The objective of this project is to complete the modeling of the PFI Engine 
2. Run a full-hydro transient simulation starting from a crank angle of `-520^0` to `120^0`.
3. Setup spray modeling and nozzle set up for the simulation using IC8H18 as fuel.

ll. CASE SETUP

1. Engine Parameters

Bore : 0.086 m

Stroke: 0.09 m

Connecting Rod Length: 0.18 m

 RPM: 3000

2. Regions and Initialization

(A) Region 0 - cylinder:

Pressure: 1.85731 bar

Temperature: 1360 K

Species Mass Fractions: Assuming stoichiometric combustion products

Reaction: 

`C_8H_18+12.5(O_2+3.76N_2) rarr8CO_2+9H_2O+(3.76 xx12.5N_2 )`

PRODUCTS	COEFFICIENT	M.W.	MASS	MASS FRACTION
CO2	8	44	352	0.19235
H2O	9	18	162	0.08852
N2	47	28	1316	0.71913
TOTAL			1830	1

(B) Region 1 - Intake-1 (Closer to combustion chamber)

Pressure: 1 bar

Temperature: 390 k

Species Mass Fractions:

IC8H18: 0.025508

O2: 0.20157

N2: 0.77292

(C) Region 2 - Intake-2 (Away from combustion chamber)

Pressure: 1 bar

Temperature: 370 k

Species: Air

(D) Region 3 - Exhaust

Pressure: 1.85731 bar

Temperature: 1360 K

Species Mass Fraction: Assuming stoichiometric combustion products(See Region 0)

3. Boundary Conditions

(A) Piston

Boundary Type: Wall

Wall Motion Type: Translating `rarr` Piston Motion

Wall Temperature: 450K

Region: Cylinder

(B) Liner and Cylinder Head

Boundary Type: Wall

Wall Motion Type: Stationary

Wall Temperature: 450K

Region: Cylinder

(C) Spark Plug

Boundary Type: Wall

Wall Motion Type: Stationary

Wall Temperature: 550K

Region: Cylinder

(D) Spark Plug Electrode

Boundary Type: Wall

Wall Motion Type: Stationary

Wall Temperature: 600K

Region: Cylinder

(E) Intake Ports 1&2

Boundary Type: Wall

Wall Motion Type: Stationary

Wall Temperature: 425K

Region: Intake-1 and Intake-2 respectively

(F) Exhaust Port

Boundary Type: Wall

Wall Motion Type: Stationary

Wall Temperature: 500K

Region: Exhaust

(G) Inflow

Boundary Type: Inflow

Pressure: 1 bar

Temperature: 363 k

Species: Air

Region: Intake-2

(H) Outflow

Boundary Type: Outflow

Pressure: 1 bar

Backflow Temperature: 800 k

Species: Stoichiometric composition (See Region 0)

Region: Exhaust

(I) Intake Valve Angle and Top

Boundary Type: Wall

Wall Motion Type: Translating `rarr` intake_lift.in

 

Wall Temperature: 480 K

Region: Intake-1

(J) Intake valve Bottom

Boundary Type: Wall

Wall Motion Type: Translating `rarr` intake_lift.in

 Wall Temperature: 480 K

Region: Cylinder

(K) Exhaust valve Angle and Top

Boundary Type: Wall

Wall Motion Type: Translating `rarr` exhaust_lift.in

 Wall Temperature: 525 K

Region: Exhaust

(L) Exhaust Valve Bottom

Boundary Type: Wall

Wall Motion Type: Translating `rarr` exhaust_lift.in

 Wall Temperature: 525 K

Region: Cylinder

4. Events

5. AMR

6. PARCEL SIMULATION

Select C8H18 - isooctane

7. SPRAY MODELING

From the given parameters, the fuel-injected mass is to be calculated per cycle and input in respective dialog boxes. Fuel spray always could break up into several parcels and we have to account for such distribution of vapor too while defining the spray characteristics.

Injection Parameters

1. Fuel flow rate = 7.5e-4 Kg/second
2. Injection start time = -480.0
3. Injection duration = 191.2
4. Fuel temperature = 330K

Fuel Flow rate (kg/s)	7.50E-04
RPM	3000
RPS	50
DPS(degrees/s)	18000
Time per degree	5055556E-05
Time per cycle(720 degree)	.04
Fuel injected per cycle (kg)	3.00E-05

As given in the diagram, different parcels of the spray are distributed in an even tone across the cone of dispersal. Respective models and recommended evaporation parameters are set.

Collision mesh and modeling develop detailed meshes to count for the minor changes in aftermath of parcel to parcel Collison or onto the walls of the intake chamber.

 

Our design is to incorporate 4 nozzles and designate respective alignment and cone angles for each of the nozzles.

Select edit Injector dialog box to input the following characteristics:

1. Inject species - IC8H18 with the shape trapezoidal (0,1,1,0)
2. KH and RT models with discharge coefficient 0.8
3. Circular injection radius of each nozzle = Nozzle diameter/2

The pressure at which nozzle functions can be calculated by using the tools option.

The pressure at which nozzle functions can be calculated by using the tools option.

As we simulate a gasoline engine, these pressure and velocity values make sense. For a diesel engine, the pressure would be so high, in thousands of bars, because atomizing the diesel fuel is difficult and needs to be done at high pressure.

 

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

8. SPARK SETUP

For spark setup, we need to input concentrated energy packets or source definition at specific intervals of time, right at the compression stroke for optimum combustion. For this, source/sink modeling is used.

Through experiments and analysis, it has been found out that for the first 0.5 degrees along the combustion stroke to take place, we need to cross the dielectric barrier of air using 40 MJ of energy to initiate the spark and after that energy subsidies, a 20 MJ is maintained for smooth combustion.

Spark Ignition Parameter

• Start of spark = -15 deg
• Spark duration = 10 deg
• Spark location = -0.003 0 0.0091
• Spark radius = 0.0005m

As per the required energy input, two sources are given, with one having an added 0.02 J along with another 0.02 J across a duration of 10 degrees so that they add up to 0.04 J just in the interval of -15 to -5 degrees prescribed to just one of the sources. This allows for a constant 0.02 J across the whole 10-degree duration.

 

9. COMBUSTION MODELLING

The recommended combustion models are provided along with a multizone value for faster combustion. The end time is determined by the degree to which the exhaust valve opens.

 

10. EMBEDDING

 

The larger sphere embedded for spark has been set as the scale of 3 and the smaller one is set as a scale of 5 for maximum gradient converge of temperature difference during ignition of fuel and combustion process. With that, the whole setup is complete.

 

The input files are exported to respective folders and post-processed

 

lll. PLOTS

Volume Plot

This plot defines the maximum and minimum volumes inside the cylinder during the whole simulation time period. A very important engine quantity can hence be derived using the plot itself, Compression Ratio

What is the Compression Ratio of this engine?

Compression ratio (CR) is defined as the action between the maximum and minimum volume in the cylinder.

Calculation:

Max volume = 0.00057 m^3

min volume = 5.7 x 10^-5 m^3

Therfore, CR = Max.vol/Min.vol = 0.00057/5.7 x 10^5 = 10

Hence, the Compression ratio of the engine is 10:1

Pressure Plot

The pressure plot shows that peak pressure is achieved during the compression stroke, just followed by combustion stroke. After the release in kinetic energy due to combustion, the pressure again drops to its normal value.

Heat transfer rate plots

Trap mass plot: Intake port near the combustion chamber

The trap mass is the mass of fuel present around the intake valve portion and intake port itself, not taking part in the combustion process. This might happen due to the wall collision and scatter of liquid vapor particles while they are being injected inside the cylinder. This mass is quite negligible in normal terms, but it constitutes a loss of fuel potential.

Temperature plot

 

Integrated HR Plot

The integrated heat transfer is the net amount of energy released in the engine. in this case, the maximum value of 1241 J is the output energy we get from the combustion process. Using this, we can calculate the combustion efficiency of the engine by incorporating the mass and low heating coefficient of the fuel with the integrated heat transfer. by this approach, we can get the amount of input energy given in the form of fuel and the amount of energy we got as output from the engine.

 

What is the Combustion efficiency of this engine?

Combustion efficiency is the ratio between output energy derived from the engine to input energy provided to the engine. It determines how efficiently the combustion process has occurred in the engine.

Calculation:

Energy Output = 1241 J

Mass of fuel/cycle used = 3 x 10^-5 kg

Lower heating value of fuel used, C8H18(isooctane) = 3 x 44.8 x 10^-5 x 10^6 = 1344 J

Therefore, Efficiency = Output/Input = 1241/1344 = 92.33%

The combustion efficency of the engine is 92.33%

Spray Parcel plot

 

Emission Plot

 

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

The entire PFI engine simulation takes place a long time to run since it includes several 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 it the intake and exhaust ports (solid domain with thickness) are not modeled. If the solid domains are also modeled, the simulation time would increase considerably. Due to the complex physical interaction 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.

Engine Performance Calculator

The engine performance calculator is used to analyze physical data and get meaningful results from the data simulated. It is a very useful tool to get calculate the basic quantities needed from an engine. We can navigate to the calculator from the line plot models in CONVERGE CFD, tools tab.

After that, we have to add the thermo_region data file for the cylinder and then load the engine. in the input file that consists of the bore and stroke information of the engine. This loads all important information needed for calculation and from here, we get a set of data such as:

Duration of combustion = 240.19 degrees

Work done = 468.646 N-m

IMEP (Indicated Mean Effective Pressure) = 8.96 Bar

CA10 = 6.83

CA50 = 18.46

CA90 = 3.17

There are some other data values too regarding peak pressures, heat transfer, etc. But with the given data itself, we will be able to calculate the Power and Torque of our engine analyzed.

Powe is nothing but the amount of work done per unit time.

where Time is the time of combustion

Power = work/Time

Calculation:

We know that work done = 468.646 N-m.

• Time of combustion = 240.199
• Revolutions per minute = 3000
• Revolutions per second = 3000/60 = 50
• Degrees per second = 50 x 360 = 18000
• Time of combustion = `240.199/18000` = 0.0133 s
• Power P = Work /Time = 468.646/0.0133 = 35.236.54 J/s = 35.236 KW

We also know that, Power(P) = `(2*pi.N.T)/60`

Hence, Torque = 111.78 N-m

 

What is the significance of CA10, CA50, AND CA90?

These values refer to the crank angle at which 10%, 50% and 90% of the combustion have been completed respectively. The significance of these values is that engineers can use this as a reference to reduce the time taken or crank angle at which most of the combustion takes place, as unburnt fuel is a waste of energy and also produce black soot that adds to the tailpipe exhaust pollution. The CA9 values cannot be too high as the mow crank angle it takes to combust fuel, the more likely they get used out to the exhaust permanently.

 

ANIMATION

Combustion Simulation

Flame Propagation

 

lV. CONCLUSION

We have successfully simulated a PFI Engine with recommended specifications and parameters to obtain the following results:

Compression ratio	10:1
Power(KW)	35.12
Torque (N-m)	111.78
Combustion Efficiency	92.33 %