The fuel used in both cases is gasoline and it was manufactured by MOL specification EN-95.The in-cylinder pressure was measured by a piezo-electric pressure transducer of the Type: Kistler 600. Top Dead Center (TDC) and engine piston speed, RPM, was determined by an optical encoder Type: Hengstler RI 32-0/1024.ER.14K. Fuel flow rate was measured using the AVL 7030 Dynamic fuel consumption measuring equipment. The fuel balance works on the gravimetric measuring principle. Fuel is supplied to the engine from a measuring vessel inside the instrument where the weight of the fuel is continuously measured. This instrument enables the highest temperature stability of the fuel conditioning system with measuring accuracy of 0.12%; including self-calibration according to ISO 9001 [8] .

Two different cases were investigated, in both cases kiva4 and kiva3vr2 were compared in order to show the numerical accuracy of the kiva4 code.

Case A: From ref. [9] , the engine used was a four valve pent-roof engine as indicted in Table 3.

Case B: Siwale et al. Performed experiments on a BASF (BadischeAnilin-und Soda Fabrik) octane rating single cylinder engine. The engine performance parameters are indicted in Table 4.

The schematic diagram of the octane rating test BASF octane rating engine used for experiments is as shown in Figure 1. The BASF octane rating engine was made to run for 20 to 30 minutes to warm up. Speed was kept constant at 600 rpm whilst the ignition timing was varied in the range of 4˚ CA BTDC to 18˚ CA BTDC, temperature and pressure values were obtained and compared with Kiva4 and Kiva3vr2. Gasoline BASF octane rating engine is a single cylinder spark ignition engine as shown below.

In this study, fluid flow simulation was carried out using the latest version of Los

Alamos National Laboratory (LANL) CFD code, KIVA-4. KIVA-4, a transient, three-dimensional, multiphase, and multi-component code for the analysis of chemically reacting flows with sprays has been under development at LANL for several years. The code uses an Arbitrary Lagrangian Eulerian (ALE) methodology on a staggered grid, and discretizes space using the finite-volume technique. The code uses an implicit time-advancement with the exception of the advective

terms that are cast in an explicit but second-order monotonicity-preserving manner. Also, the convection calculations can be sub-cycled in the desired regions to avoid restricting the time step due to Courant conditions [10] . Comparing to the earlier versions of the KIVA code, detailed evaporation model together with ability for modeling unstructured mesh were added to the KIVA-4 [11] . Kiva contains a folder kiva3d that has subroutines. The kinetic chemistry subroutine CHEM contains the chemical kinetic commands of the fuel used and sometimes it is replaced with other kinetic models [12] . To accomplish this, the user must supply an appropriate input data set, as required for each kinetic reaction. The data set is comprised of forward and backward pre-exponential factors, activation temperatures and temperature exponents, along with stoichiometric species coefficients on the left and right sides of the reaction and exponents of species concentration in both the forward and backward rates of the reaction. This is not always the case, however [13] , Table 5 indicates suggested input values for using CHEM on the gasoline fuel, and its chemical equation.

The column in Table 5 contains Itape values for the basic hydrocarbon fuel appearing in the fuel library. CF is the forward pre-exponential factor, and the stoichiometric coefficients for the fuel, O₂, CO₂, and H₂O are labeled nl, n2, n3, and n4 respectively. The concentration exponents for the fuel and oxidizer are labeled AEf and AEo. (Negative values for AEf imply that the reaction rate increases as the fuel concentration decreases.) An effective activation energy EF = 30 kcal/mole is suggested, which is equivalent to about 1.5078 × l0⁴ in KIVA input units when divided by the universal gas constant in kcal/mole degree Kelvin

[14] . Equation (1) is a single-step oxidation reaction that was used as the mechanism. This was used to remedy computational time issue; it describes the dissociation of 4 mol of C₈ H₁₇ into 32 mol of CO₂ and 34 mol of H₂O.

Chemical equation

4C₈H₁₇ + 49O₂ → 32CO₂ + 34H₂O (1)

The Case A: Kiva4 package includes a basic grid generator, K3PREP that writes a file called itape17 conforming to the specifications of the engine [4] [15] .

Pre-processing

A pre-processor is a program that processes its input data to produce output that is used as input to another program like a compiler; in this case K3PREP is that program. K3PREP is a pre-processor that generates the mesh for the given geometry in form of an output file called otape17 and that given geometry comes from a file called IPREP file that needs to be built manually. An IPREP file is a file that contains all the necessary information to generate the geometry. The geometry is built following the description of the quantities on the input data file “IPREP” in the order in which they appear using the epilogue file found in the K3PREP directory [4] [16] .

An IPREP file was used by other researchers to generate a structured mesh using kiva3vr2, the mesh is as shown below in Figure 2.

The same IPREP file was used in kiv4 to produce an unstructured mesh. In order to accomplish this, there were a few modifications or changes that had to be made in order for the file to work in kiva4. Changes made to IPREP file are indicated in Table 6.

After all the modification is done, the file is now ready to be compiled by the program K3PREP. The IPREP file should be placed in the same folder as the executed K3PREP program. On the command line the program is run with the command./K3prep. After the program has finished running, otape17 and otape11 are generated.

Itape17 to Kiva4grid Convertor

The file otape17 is renamed to itape17 and otape11 to itape11. A convertor (convertor.f) for converting kiva3vr2 mesh files to kiva4 format is provided in the main directory. It is compiled by a fortran compiler and will convert itape17 to kiva4grid [1] . itape17, itape11 and convertor.f are inserted in the folder created. On the command terminal change directory to the folder created and the command; G95-O convertor.exe convertor.f is run. Where g95 is the compiler and -O is the command to generate an exe file. Convertor.exe is produced

in the same folder. In the same directory the exe file is run with the command./Convertor.exe and Kiva4grid will be generated. Below is the generated kiva4grid unstructured mesh in Figure 3.

After all the input files are edited and generated, kiva4 is run in the command terminal with the command: ./kiva4.

Case B: Kiva4 package includes a basic grid generator, K3PREP that writes a file itape17 conforming to the specification. In order to reduce computational time, a 45˚ asymmetrical mesh was created based on the symmetry of the combustion chamber of the BASF octane rating engine-single cylinder as shown in Figure 4.

Figure 5 illustrates a summarized flow chart of generating the kiva4grid using an iprep file in k3prep and how to run kiva4.

In computational solutions of partial differential equations, meshing is a discrete

representation of the geometry that is involved in the problem. Essentially, it partitions space into elements (or cells or zones) over which the equations can be approximated. Zone boundaries can be free to create computationally best shaped zones, or they can be fixed to represent internal or external boundaries within a model. The mesh quality can be conclusively determined based on the following factor [17] .

Solution precision

A better mesh quality provides a more precise solution. For example, one can refine the mesh at certain areas of the geometry where the gradients are high, thus increasing the fidelity of solutions in the region. Also, this means that if a mesh is not sufficiently refined then the precision of the solution is more limited. Thus, mesh quality is dictated by the required precision [18] .

Viewing mesh refinement in Ensight

Ensight is a software program for visualizing, analyzing, and communicating data from computer simulations and/or experiments. In numerical analysis, adaptive mesh refinement, or AMR, is a method of adapting the accuracy of a solution within certain sensitive or turbulent regions of simulation, dynamically and during the time the solution is being calculated [18] .

Procedure

To view mesh refinement, the global attribute is turned on and the color set to black. Available parts are hidden in order to view the mesh in the fluid domain. The fluid domain is clipped and the times set to a value were the mesh refinement is able to be viewed.

Quantifying the size of the elements

Element size calculates variables such as the volume/Area/Length for 3D/2D/1D elements respectively at each element creating a scalar, element-based variable.

Procedure

The option for calculator is selected in the fluid domain on the tabs. Elesize (element size) is searched on the calculator and evaluated for selected parts.

The clip is colored with elesize and the palette changed (set range to selected part max/min).

Figure 6 shows the results for quantifying the size of the elements in Ensight.

Figure 6 shows that the sizes of single elements are smaller in some parts of the geometry and larger in other parts. For the smaller cells elesize is 0.00013 while for the largest size elesize is 0.072 as shown.

There are many more ways and functions in Ensight that are designed to give the quantification of the grid. EleMetric is another function which calculates an elements mesh metric, at each element creating a scalar, element-based variable depending upon the selected metric function. Elemetric is searched on the calculator and other parameters called metric functions appear. In order to understand what each function does, the question mark is selected on the calculator which brings up the page for the user manual that explains what elemetric is, what each function does and what are the element types it applies to. Some of the examples of the elemetric functions are element type, facecount, centroid etc.

Gradient of the velocity

Gradient of the velocity is one of the physical quantities that the solver uses to

identify which parts of the solution have to be refined. Gradient of the velocity is calculated. The designer selects the mesh configuration.

Procedure

The fluid domain is used when calculating the gradient of the quantity. The algorithm is used in each value of the cell and the true level of the neighboring cells to calculate the gradient. Selecting as a parent part 2D plane brings information that comes from the neighboring 2D cells but the solution in the solver used the 3D domain.

Grad is searched on the calculator. The scalar or vector velocity is selected and evaluated for selected parts. The clip plane is colored with GradV and the palate is edited in order to identify some features. The scale was set to logarithmic and the range was decreased from zero until it gave the required results as shown below in Figure 7.

Figure 7 shows that the solver has followed the areas where the gradient of the velocity is higher and refined those cells. The value for the highest gradient of the velocity is 1202.

A comparison is made in this section between the measured values derived from the experimental investigation and the ones calculated by kiva4 and kiva3vr2. Figure 11 shows the pressure―crank angle diagram.

Combustion is a process formed when there is a source of fuel, air (oxidizer) and heat [19] . This is the most important process taking place in spark ignition engines in which chemical energy of the fuel is transformed into internal energy of the cylinder charge. During this process, a turbulent flame propagates across

the combustion chamber and burns the premixed gasoline-air mixture. The fuel is injected at 5˚ ATDC during the intake stroke when the piston is moving from top dead center to bottom dead center. The piston moves from bottom dead center to top dead center. The air-fuel mixture is compressed isentropically through the compression ratio, in this case 10:1. Heat is added to the working fluid during compression process at constant volume. The temperature and pressure slowly increases as the piston moves from bottom dead center to top dead center. Combustion occurs when the temperature and pressure of the air-fuel mixture is high enough to ignite at 4˚ CA, 8˚ CA, 10˚ CA and 18˚ CA before top dead center. This is called the ignition phase at 356˚ CA as seen from Figures 11(a)-(d). When combustion takes place, the piston moves from TDC to BDC with a greater amount of force and work is done by the system. The peak temperature is above 2000 K and the peak pressure is above 20 bars. Kva4 results are closer to the experiment than kiva3vr2. Figures 11(a)-(d) shows a variation of pressure with time during the combustion of gasoline-air mixture where the pressure is in bars and the time is in crank angle degrees [CAD]. The initial pressure Presi and temperature Tempi are 1.929802218e+6 Pa and 400 K. The calculations begin at 60 degs btdc and finishes at 540 degs atdc. The simulation results in kiva4 agree with the experimental results. The average pressure is 19.56 bars in kiva4 and 18.2115 bars in kiva3vr2 while for the experiment the pressure is 18.8756 bar. Kiva3vr2 generates slightly lower pressure results than Kiva4 due to the changes made in kiva4.

After testing Kiva4’s performance and accuracy, simulations are performed based

on the input parameters of the experimental cases for the ignition timing of 4˚ CA, 8˚ CA, 10˚ CA and 18˚ CA BTDC at 600 rpm. When the calculations begin at 60 degs BTDC, the response of the pressure and temperature is slow and uniform. During the compression stroke at 180 CAD the pressure and temperature in the cylinder begins to build up until it is high enough for combustion to occur. The ignition timing in crank angle degrees (CAD) is (a) 356 CAD, (b) 352 CAD, (c) 350 CAD and (d) 342 CAD as shown in Figure 12. Table 7 indicates the initial conditions used in kiva4.

When Combustion occurs, the volume of the burned gasoline ? air mixture expands. The expansion of the burning mixture starts at the center and travels in the outward direction towards the cylindrical vessel wall. This movement created a combustion wave that causes a rise in pressure (Lewis & Elbe, 1987) [20] . However, as the combustion process passes over 350 CAD, the pressure and temperature starts to increase exponentially until it reaches its peak pressure and temperature. The average temperature and pressure decreases while the peak temperature and pressure increases as shown in Figure 12 (a) 20.37 bars, 2075.7 K, (b) 21.13 bars 2101.5 K, (c) 21.71 bars, 2163.8 K and (d) 23.47 bars, 2166.0 K for the ignition timing 4˚ CA, 8˚ CA, 10˚ CA and 18˚ CA respectively. As the combustion wave propagates and echoed back once it hit the cylinder wall, the rapid movement interacts with the burning flame front. The small disturbance over the flame front increases the rate of combustion in a way that the unburned mixture is compressed towards the flame front.

The results show that increasing spark advance increases the peak temperature and peak pressure in the combustion chamber. Figure 13 and Figure 14 show how temperature and pressure increases with increase in spark advance. Best performance will be achieved when the greatest portion of the combustion takes place near top dead center. If the spark is not advanced enough, the piston will already be moving down when much of the combustion takes place. In this case, we lose the ability to expand this portion of the gas through the full range, decreasing performance. If ignition is too advanced, too much of the gas will burn while the piston is still rising. As a result, the work that must be done to compress this gas will decrease the total work produced [6] [21] . Figure 13 and Figure 14 show the peak temperature and pressure at different ignition time. It is expected that peak temperature and pressure should increase, thereby increasing power and torque. After peak temperature and pressure increases, it then drops off as the spark continues to advance further. Increasing the spark advance does not mean that the performance of an SI engine increases. An excessive increase in the ignition timing may result in detonation when using fuels with a low octane rating. The performance of an SI engine highly depends on ignition timing, and its optimum value should be determined for each SI engine in order to achieve higher combustion efficiency. The peak temperatures at 4˚ CA, 8˚ CA, 10˚ CA and 18˚ CA BTDC are 2075.7 K, 2101.5 K, 2163.8 K and 2166.0 K respectively. The peak pressures at of 4˚ CA, 8˚ CA, 10˚ CA and 18˚ CA BTDC are 20.37 bars, 21.13 bars, 21.73 bars and 23.47 bars respectively.