Mixture homogeneity plays a crucial role in HCCI engine. In the present study, the mixture homogeneity was analysed by three-dimensional engine model. Combustion was studied by zero-dimensional single zone model. The engine parameters studied include speed, injector location, valve lift, and mass of fuel injected. Valve lift and injector location had less impact on mixture formation and combustion phasing compared to other parameters. Engine speed had a noticeable effect on mixture homogeneity and combustion characteristics.
Over the last decade, we have seen an exponential growth in the consumption of diesel and petrol by automobiles. This has contributed a great deal in air pollution. Hence, it is important to use the right technology to harness maximum output from the fuel and at the same time, reduce pollutions. Homogenous charged compression ignition (HCCI) engine is a mix of both conventional spark-ignition and diesel compression ignition technology. The combination of these two designs offers diesel-like high efficiency and at the same time, reduces
Xu et al. [
It can be observed that most of the previous works use multi zone/single zone zero dimensional model to study the performance of HCCI engines. These models cannot predict homogeneity of mixture formation. To study mixture formation, the three-dimensional model is the most preferred model, as it can produce more realistic results. The combustion model can be zero-dimensional model to get approximate results as the computational time increases for three dimensional modeling for combustion. This present work studies the effect of various engine parameters on homogeneity of mixture formation using three-dimensional engine model. Also, the combustion analysis was done using zero-dimensional single zone model with detailed reaction mechanism.
CONVERGE is used for the three-dimensional CFD modeling. The equations of conservation of mass, momentum, turbulence, and energy are solved. Governing equations are given elsewhere [
Control-volume-based technique was used to convert the governing equations to algebraic equations that can be solved numerically. The second-order upwind scheme was used for spatial discretization of all the governing equations. The PISO algorithm is used for pressure and velocity coupling. Pressure interpolation of the pressure values at the faces is done using momentum equation coefficients (standard pressure interpolation scheme). The double-precision segregated solver was utilized with implicit method for solving the discretized set of algebraic equations.
The present study uses three dimensional engine geometry as it can predict realistic results. The surface file was imported into CONVERGE and different boundaries faces were grouped as shown in Figure
Three dimensional engine model with various boundaries.
The specifications of the engine used for the simulations are given in Table
Engine specification.
Bore × stroke |
|
Compression ratio | 13 |
Connecting rod length | 0.18 m |
Piston geometry | Flat piston |
Injection pressure | 5 MPa |
Injection temperature | 450 K |
Port timing.
EVO bBDC | 44° |
EVC aTDC | 58° |
IVO bTDC | 64° |
IVC aBDC | 24° |
Injection parameters.
Case |
Case |
|
---|---|---|
Injection pressure | 5 MPa | 5 MPa |
Injection temperature | 335 K | 335 K |
Spray cone angle | 54° | 54° |
| ||
Pilot injection ( |
||
Mass | 1.25 mg | 0.8 mg |
Start of injection | 30 BTDC | 30 BTDC |
Duration | 1.211°CA | 0.766°CA |
| ||
Main injection ( |
||
Mass | 11.25 mg | 7.2 mg |
Start of injection | 100° ATDC | 100° ATDC |
Duration | 10.899°CA | 6.976°CA |
Pressure plot showing two injection timings.
In CONVERGE, the grid is generated run-time. Base grid size has to be specified, and there are three main ways of manipulating the grid size further. The grid is easily coarsen or refined by using the grid scaling feature which adjusts the base grid size. Secondly, the grid with a fixed or time-dependent refinements (embedding) is configured when and where it is needed. Finally, Adaptive Mesh Refinement (AMR) can be used to add grid refinement in critical areas of domain based on the solution variables. These parameters were then optimised for computational time and to attain grid independent results. Figure
Mesh slice generated during the simulation with embedding and AMR.
The various physical quantities were initialized with quantities which resemble real engine condition. The simulation was a single-cycle simulation. Hence, to get the initial conditions inside the cylinder, complete combustion of the fuel was assumed, and the resulting species mass fractions were found out using STAJAN [
In order to solve the governing transport equation, a boundary condition for each equation must be specified. The inlet valve, exhaust valve, and piston are moving wall boundaries. Cylinder, cylinder head, and ports are fixed wall boundaries. The inlet of inlet port is inflow boundary condition, where, pressure is specified. The outlet of exhaust is outflow boundary condition, where pressure is specified.
Mixture homogeneity plays an important role in HCCI engine. Mixture formation was analysed by varying speed, amount of fuel, injector location, and valve lift. The mixture formed at 20° CA BTDC in compression stroke was studied, because usually in an engine, combustion phasing happens before TDC. The simulation starts at 50°bBDC in expansion stroke. The simulation ends at 20°bTDC in compression stroke.
Two-valve lifts were considered in this study. The first valve lift had a maximum lift of 3.45 mm. Whereas, the second valve lift had a maximum lift which was 1.5 times the maximum lift of the first valve lift, that is, 5.175 mm. The first valve lift was referred as “
The clip plane used in the study.
The effect of various speeds on mixture formation is dealt with in this section. Engine speeds taken into account were 1000 rpm, 1500 rpm, and 2000 rpm. The other parameters which were kept constant includes valve lift “1.5
Contours of iso-octane mass fractions at 20°bTDC in compression for (a) 1000 rpm, (b) 1500 rpm, and (c) 2000 rpm.
It can be observed from Figure
Contours of iso-octane with velocity vector at 10°bBDC (left) and 90°aBDC (right) for (a) 1000 rpm, (b) 1500 rpm, and (c) 2000 rpm.
Mixture homogeneity is controlled by two factors, namely, direction and magnitude air velocity and fuel vaporisation. From Figure
From Figure
In this section, the effect of loading in mixture formation is studied. Two quantities of fuel were selected. They were 8 mg and 12.5 mg. The other parameters were kept constant. Engine speed was 1500 rpm, valve lift was “1.5
Contours of iso-octane mass fractions at 20°bTDC for (a) 8 mg fuel and (b) 12.5 mg fuel.
It was observed from Figure
Contours of iso-octane mass fractions with velocity vectors at 10°bBDC (left) and 90°aBDC (right), (a) 12.5 mg and (b) 8 mg.
From Figure
This section deals with the effect of injector location on mixture formation. Two injector locator locations were selected to study the effect of injector location in mixture formation. They were side injection and centre injection. The other parameters were kept constant. Valve lift was “
Contours of iso-octane formed at 20°bTDC for (a) side injection and (b) central injection.
It was observed from Figure
Contours of iso-octane mass fractions with velocity vectors at 10°bBDC (left) and 90°aBDC (right), (a) side injection and (b) central injection.
From Figure
In this section, the effect of valve lift on mixture formation is studied. The most common way of attaining negative valve overlap (NVO) is by reducing the valve lift, thereby reducing the overall opening time of the valve. Two-valve lifts considered in this study were “
Contours of iso-octane mass fractions at 20°bTDC (a) 1.5
Figure
Contours of iso-octane mass fractions with velocity vector at 10°bBDC (left) and 90°aBDC (right) for (a)
From the Figure
CHEMKIN was used for combustion analysis with zero-dimensional modeling. A reaction mechanism of gasoline with 1300 species was used [
Engine speed was varied to find its effect on the combustion phasing. The speeds considered were 1000, 1500, and 2000 rpm. The valve lift in all the cases was kept as 1.5
Heat release plots for different speeds.
It was observed from Figure
From Figure
In this section, the effect of valve lift in combustion phasing is studied. The effects of valve lift
Heat release plots for
The heat release rate curves showed negligible variations in heat release peaks. However, heat release peak for 1.5
The pressure curves shown in Figure
Heat release plots obtained for different injector locations.
The effect of injector location in combustion phasing is dealt with in this section. Two injector locations were selected for this purpose. In first case, the injector was located at the centre of the piston head; in the second configuration, the injector was located close to the intake valve at 45° inclination. The valve timing and speed were fixed as
Heat release curves are very similar for both central and angled injections as shown in Figure
In the pressure plots shown in Figure
Three-dimensional CFD engine modeling is used to study mixture formation using CONVERGE. Zero-dimensional single zone modeling with detailed reaction mechanism was done to study combustion using CHEMKIN. It was observed that 1500 rpm was able to produce more homogeneous mixture than 1000 and 2000 rpm. It was also observed that regardless of the amount of fuel injected, the mixture formed will be almost homogeneous before combustion. Central injection had more tendency to form homogeneous mixture than side injection. Simulations for different valve lifts indicated that valve lift with highest lift was able to produce more homogeneous mixture. As engine speed increased, the heat release peaks were found to be reducing. Moreover, a shift in the peak was seen with the increase in engine speed. Heat release rates for different valve lifts were found to be negligible. The central injection was found to have better combustion characteristics than side injection.
The authors wish to express their gratitude to Dr. Sunil Pandey for his valuable comments and help in performing CHEMKIN simulations.