CFD simulation of Spark ignited port fuel Injected Gasoline Engine
Aim: To perform the three-dimensional Numerical simulation for a port fuel injected, spark ignited Gasoline engine
Pre-Analysis-
1) Importing the geometry and checking for surface errors:
The three-dimensional CAD geometry of the engine is imported and checked for surface errors. Most of the intersection errors were resolved by slightly moving the intake valve downwards. The open edges are patched up using the patch tool in Converge.
2) Boundary flagging & creating regions
The various parts of the engine are flagged accordingly.
The valves are flagged into three different boundaries to ease up the process of creating regions. They are divided into the valve top, valve bottom and valve angle.
The intake manifold is divided into two different regions to give accurate initial conditions. Since all the fuel injected doesn\'t flow into the engine, some of the fuel remains in the intake manifold itself. Thus the intake manifold is divided into two regions, namely intake1 and intake 2.
Ring Triangles:
The ring triangles are created to make sure that only the valves are moved and the manifold is not deformed.
Disconnect Triangles:
The disconnect triangles are created during the running time to control the fluid flow between two different regions. The intake and exhaust valves are not open all the time during the simulation.
3) Case setup:
IC Engine Parameters and setting up of Compression Ratio:
The compression ratio of the engine can be set up using the compression ratio tool. The boundaries around the combustion chamber are selected to calculate the total cylinder volume and then the moving boundary is selected to get the desired compression ratio as follows:
Simulation files:
The PRF(Primary reference fuel) mechanism which combines the oxidation of Iso-octane and n-heptane is used for the simulation.
Simulation Parameters:
The simulation was run for one complete engine cycle (720-degree crank angle(CA)).
Boundary conditions:
The boundary conditions for various parts of the engine are given as user input. The valve profiles with minimum lift value are given for intake and exhaust valves respectively.
Regions and initialization:
The following four regions are created and assigned to the boundaries:
1) Intake port 1
2) Intake port 2
3) In-cylinder
4) Exhaust
The intake port is divided into two different regions to give better initial conditions as explained before:
Creating-Events:
A permanent event(Continuous flow) is created for Intake port 1 and intake port 2 regions. A cyclic event(Non-continuous flow) is created for the flow between in-cylinder, exhaust and intake port regions.
Physical models:
Combustion and Turbulence Modeling
The SAGE detailed chemical kinetics solver is used for the combustion modeling. Converge includes the multizone model which accelerates the chemistry calculations by grouping together similar computational cells and then invoking the chemistry solver once per group rather than once per cell. In the multizone model, at any given time t, each cell is at some thermodynamic state. Based on the thermodynamic states of the cells, CONVERGE groups the cells into zones. The thermodynamic state of each zone is based on the average temperature and composition of all the cells in that zone. Converge invokes the chemistry solver once on each zone. The RNG-K-EPSILON model was used for turbulence modeling. The various turbulence constants values are used based on the recommended values from CONVERGE.
Spray Modelling:
The Kelvin-Helmholtz model and Rayleigh-Taylor model are used for spray modeling. An injector with four nozzles is used to spray the fuel.
Source Modelling:
There are two sources used in the source modeling. Initially, two sources are used after which only one source is used.
Grid Control:
Adaptive mesh refinement and fixed embedding techniques are used to achieve the desired meshing size with greater accuracy.
Results and Discussion:
The combustion efficiency of the engine:
The combustion efficiency gives the percentage of fuel burned inside the engine. This parameter is very important as it gives you an insight about HC emissions and work output of the engine. When the combustion efficiency is very low, work output automatically goes down.
Combustion Efficiency= Net Heat released/Mass of the fuel*lower_heating value of the fuel
The calculated value for the combustion efficiency of the engine is 95.2 %.
Engine performance parameters:
The engine performance parameters are calculated using the engine performance tool available in Converge. The thermo file(contains all pressure rise and heat release data) for the combustion region is imported into the engine performance calculator which further calculates all the performance data as shown below.
Indicated power:
The Gross indicated power of the engine is the power calculated by taking the work done during the compression and expansion strokes of the engine, which is available at the beginning of power stroke. Some of this power is used to overcome frictional losses(mainly between cylinder liner and the cylinder). The power that\'s available at the crankshaft is called brake power.
Indicated power= brake power + frictional power
The calculated indicated power of the engine at 3000 RPM is 35 KW.
The Indicated torque can be calculated by using the indicated power.
The calculated indicated torque of the engine at 3000 RPM is 111 N-m
Importance of CA 10, CA 50 and CA 90:
CA 10 (Degrees in crank angle for 10 % cumulative heat release)
CA 50 (Degrees in crank angle for 50 % cumulative heat release)
CA 90 (Degrees in crank angle for 90 % cumulative heat release)
These parameters are very important because they give the duration(time taken in terms of crank angle degrees) for different percentage of total heat release.
CA 10 is considered as the combustion duration from the initiation of spark to a small measurable fraction of charge(10 %) burned. This is helpful in finding the ignition lag of the engine.
CA 50 is the crank angle required to burn the bulk(50 %) of the charge. This is the stage where the flame propagates throughout the charge. If CA 50 is high then flame velocity is low, which leads to low Thermal efficiency as we always need higher pressure exactly at TDC(Top dead center).
CA 90 is the crank angle for 90 % of the fuel burned. This gives the overall duration of the burning process. Lower the CA90, the higher the power output of the engine, as the piston expands at a considerably higher pressure.
CA 10= 6.83717 CAD
CA 50= 18.4623 CAD
CA 90= 31.701 CAD
Why wall heat transfer models are used? Why not calculate wall temperature from CFD calculations itself?
Thermal wall function models are used in CFD to calculate wall temperature. Because of the assumptions made in the model, it underpredicts wall heat losses compared to experimental measurements.
The assumptions made includes:
A1: Flow is parallel to the chamber walls with negligible flow normal to the walls
A2: Quasi-steady flow with negligible transients
A3: Constant density and transport properties in the boundary layer
A4: High Reynolds number flow, very thin laminar shear layer
A5: Zero pressure gradients along the flow
A6: No combustion and chemically inert mixture
None of the following assumptions can be met with engines. The flow is neither one dimensional nor steady. Assumption three is perfectly invalid, because of large thermal gradients within the boundary layer.
Hence, we need non-equilibrium models to predict wall temperatures accurately.
Some of them include Angleberger model, Werner Wengle model, etc.
Three-dimensional flow patterns inside the engine:
Beginning of compression stroke:
The above image depicts the velocity distribution at the beginning of compression stroke. You can see how a part of fuel remains in the Intake manifold.
Flame propagation and Combustion:
The above image shows the velocity distribution during combustion. You can see how the flame is propagating during combustion.
Pressure distribution during combustion:
The above image shows the pressure distribution in the combustion chamber. The pressure is uniform meaning the charge is homogeneous and the localized variation of equivalence ratio is less.
Temperature distribution during combustion:
The above figure depicts the variation of temperature in the combustion chamber.