Computational Investigation of Heavy Fuel Feasibility in a...

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American Institute of Aeronautics and Astronautics

Computational Investigation of Heavy Fuel Feasibility in a

Gasoline Direct Injection Spark Ignition Engine

Sunil U. S. MODA1, Haibo Dong2, Hui Wan3

Department of Mechanical and Materials Engineering

Wright State University, Dayton, OH 45435

John Hoke4

Innovative Scientific Solution Inc.

Dayton, OH 45440

A computational model has been developed to explore the feasibility of heavy fuel in a

gasoline Direct Injection Spark Ignition engine. A geometrical model identical to that of the

Pontiac Solstice 2008 is developed using ANSYS Gambit 2.4. To generate high-quality

meshes and to accurately represent the piston motion, dynamic layering and local re-

meshing techniques are utilized. The computational model is selected upon validation with

gasoline fuel. A parametric study is performed on injection timing and particle size in order

to determine the engine operating characteristics using diesel fuel instead of gasoline.

Nomenclature DISI = Direct Injection Spark Ignition

K = Turbulent kinetic energy

ε = Turbulent dissipation

DPM = Discrete phase modeling

TAB = Taylor analogy breakup

PISO = Pressure implicit with splitting of operators bTDC = Before top dead center

TDC = Top dead center

aTDC = Before top dead center

CA = Crank angle

I. Introduction

IRECT injection spark ignition engines[1] are internal combustion engines with spark ignition and injection of

fuel directly into the chamber. This is a new trend of engines, in which the amount of fuel injected in to the

combustion chamber can be moderated according to the power requirement. In direct injection spark ignition engine

fuel is injected with high pressure injectors in the pressurized chamber. The fuel and air mixture is developed in the

combustion chamber and then ignited. The combustion of fuel is related to the homogeneity of fuel and air mixture

[8,10,15]. The laminar flame is developed by the spark and travels all over the cylinder to make a complete

combustion.

DISI technology has capability to burn any kind of fuel. However this technology is not being utilized for heavy

fuels which have high energy content, higher efficiency and easier to store when compared to gasoline fuel. Diesel

fuel cannot be injected directly in to a gasoline operated DISI engine with the same operating conditions as gasoline.

As the diesel is a heavy fuel it vaporizes slowly and it has a lower auto ignition time [4]. So, the gasoline operating

conditions will not suit the diesel operating conditions. To tackle this problem a parametric study is needed to find out the operating conditions suitable for diesel in a DISI engine. However, doing experimental parametric study is

1 Graduate Student 2 Assistant Professor, AIAA senior member

3 Research Scientist, AIAA member. 4 AIAA Associate Fellow

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very expensive and time taking. Once validated, Computational Fluid Dynamics (CFD) can be more time and cost

effective for understanding engine operation characteristics and finding optimal operation parameters.

To this end, we use ANSYS Fluent 12.0 to investigate the heavy fuel feasibility in Pontiac Solstice 2008 engine

which is a direct injection spark ignition engine. The strategies used in our work involve control surface modeling,

flow volume meshing, validating computational models in combustion simulation, and parametric study on injection

time and droplet size [5-7] using heavy fuel. Results demonstrate that it is feasible to use diesel fuel in DISI engine

and get a similar performance as that of gasoline fuel.

II. Methodology

A. Computational Modeling

The three dimensional models are replicated in solid works using the original geometry of the solstice engine [3].

The geometry formed in Solidworks includes the important details of the real engine. To simplify the meshing the

geometry cleanup is done in Gambit 2.4 software. A Solidworks model is imported into ANSYS Gambit and the geometry clean up is performed. The simple geometry is meshed and specific zone names and types are assigned.

The mesh is finally exported to a Fluent 12.0 supported format.

a) Solidworks model created from a

real engine b) Modeling of piston geometry

c) Meshing of working fluid

volume

Figure 1. Modeling and Meshing of a Pontiac Solstice engine.

B. Mesh Motion

The mesh motion which is the basic for all the simulations is achieved in ANSYS Fluent using the mesh

exported from Gambit. Fluent creates a dynamic mesh using the static mesh and dynamic mesh zone conditions

provided. The maximum skewness of the mesh is maintained and it is less than 0.91 for accuracy.

360 bTDC

270 bTDC

180 bTDC

(a) First stroke.

360 bTDC

180 bTDC

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180 bTDC

90 bTDC

TDC

(b) Second stroke.

Figure 2. Mesh motion and cycle terminology

The dynamic mesh created is based on the crank angle specified and effort is made to match the dynamic mesh

generation to match to the actual cylinder movement. The valve profiles help to exactly replicate the actual valve

motion in the computational environment. The size of mesh is selected by doing a grid independent study. The size

of the mesh is chosen in such a way that the accuracy is maintained and also the computational cost is reasonable.

Figure 1 shows the series of operations performed during modeling and the mesh motion achieved in Fluent. Figure

2 shows the mesh motion as well as important crank angle information.

In the current study as shown in Figure 2 (a), the first stroke or the expansion stroke starts at 360 bTDC (360 CA) and continues up to 180 bTDC (540 CA). The second stroke starts from 180 bTDC (540 CA) and continues up to

TDC (720 CA) as shown in Figure 2 (b) where the combustion occurs. The third stroke and fourth are similar to first

and second strokes respectively. The third stroke starts at TDC (720 CA) and continues up to 180 aTDC (900 CA)

which is end of expansion stroke. The last stroke or the exhaust stroke will continue from 180 aTDC (900 CA) to

360 aTDC (1080 CA).

The mesh motion is achieved in Fluent 12.0 which is the starting step of all the simulations. The four strokes of

the engine is accurately captured and the dynamic mesh generation exactly matches than actual piston movement.

C. Simulation Setup

Fluent 12.0 is used for computational analysis of direct injection spark ignition technology. The simulation set up involves mesh motion and solver settings. As explained in section II(B) the mesh motion is achieved in the Fluent

using the dynamic mesh parameters and valve profiles. Then the dynamic mesh is given the specific solver settings

based on the problem. For the study of DISI reciprocating engine the partially pre-mixed combustion model is used,

this model in Fluent provides the option of direct injection with spark. The realizable K- ε model is used for

turbulence inside the cylinder, this takes Reynolds stresses and vorticity fluctuation.

The whole simulation is an unsteady process. PISO scheme is used for gradient, standard pressure scheme is

used for faster calculation and an upwind scheme is used for better convergence for all the other parameters.

Computationally the injection setup is done using the DPM and spray modeling. The setup uses unsteady particle

tracking which tracks the injection particles inside the cylinder, TAB breakup model to simulate the behavior of

droplet break up. The particle size of the injection is fixed over a range and distributed in the range using Rossin-

Rammler diametric distribution. The spark modeling is done using 0.1 J of spark energy in the spark location. The

diameter of the spark is specified and the spherical spark is used in the simulation. Using these settings, a complete simulation is designed. All the cases in this study use same solver settings.

Although in this study various cases are simulated to check various parameters only few cases which have major

effect on tuning the engine are presented. Table 1 shows the case details used in the study.

Table 1: Summary of cases performed and the different parameters used

Fuel Used Injection start Timing Droplet size

Gasoline 270 bTDC 50 microns

Diesel 270 bTDC, 180 bTDC 50,40,30 and

20 microns

Table 1. Different cases used in the study.

TDC

180 bTDC

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In following discussion, a simulation using gasoline fuel with the same operating conditions of a real gasoline

engine is firstly conducted to get the basic knowledge of the combustion process. Simulations as described in Table

1 are then designed to check the feasibility of diesel fuel injection in the same engine.

III. Results

A. Computational analysis DISI engine with gasoline fuel.

A simulation with the gasoline fuel is performed. The simulation matches the operating conditions of the real

engine. The gasoline fuel is injected from a six plume injector at 270 bTDC in to the cylinder with a 100m/s

injection velocity and 10bTDC spark timing. As it is a simulation from a working engine a good combustion is

observed. To visualize the process contours of mass fraction of fuel, CO2 and oxygen are extracted from the

simulation. These explain the extent of combustion in the cylinder. Figure 3 pressure peak which is monitored

throughout the simulation at each and every time step.

Figure 3. Time history of average pressure obtained from the simulation of DISI gasoline fuel.

The pressure history is obtained from a simulation simulated for one cycle. The operations conditions of the

simulation are exact same as the real engine. It can be seen in Figure 3 that a comparison between the experimental

pressure peaks obtained and the CFD pressure peak obtained. A very good agreement is found. However a study of

this case is helpful to understand the whole combustion process. It is the benefit of CFD to look in to the simulation

visually to check the combustion process. In Figure 3 it can be seen that there are three points where the pressure

plot changes its behavior the three points are marked as TDC, 40 aTDC and 80 aTDC respectively. At TDC the

pressure peak due to compression is obtained as mentioned earlier the spark is 10 bTDC but the laminar flame is not

completely developed and there is no effect of combustion at pressure peak. After “1” the pressure drops due to

expansion at position “2” mentioned in Figure 3, the laminar flame is completely produced and the combustion happens in a full scale this increases the pressure peak to a maximum till position “3”. So to study combustion the

mentioned three positions are very important. Using the benefits of CFD it is possible to look in to the combustion

process visually. Figure 4 shows the pictures explaining the combustion at positions 1, 2 and 3 respectively.

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1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

360 420 480 540 600 660 720 780 840 900 960 1020 1080

Pre

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Mp

a

Crank angle in degrees

Experimental gasoline

computational gasoline

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3

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0. 30 bTDC

1. TDC

2. 40 aTDC

3. 80 aTDC

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Fuel

CO2

(a) Combustion process at position “1” (TDC).

Fuel

CO2

(b) Combustion process at position “2” (40 aTDC).

Fuel

CO2

(c) Combustion process at position “3” (80 aTDC).

Figure 4. Contours of fuel and CO2 to show the combustion process

Figure 4(a) explains the combustion process. At position “1”, the combustion process is started but the laminar

flame is not fully developed so the pressure peak at this point is purely the pressure due to compression. At position

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“2” in Figure 4(b), the laminar flame is propagated in all directions in the cylinder. This will cause increase in

pressure which is observed at position “2”. The pressure builds as the fuel in the chamber is burnt and it reaches the

maximum at the point where most of the fuel is burnt and this maximum pressure is observed at 80 aTDC which is

position “3”. The position “3” i.e. 80 bTDC as shown in Figure 4(c) is where most of the combustion process is

completed. In the future simulations 80 bTDC is taken as reference point to check the extent for combustion.

After successful completion of the gasoline fuel study in DISI engine the next step is to study the diesel fuel in the DISI engine. A similar injection strategy used for gasoline is used for diesel fuel. As the behavior of diesel fuel

in a gasoline engine is unpredictable and as there is no much data available for this study, Diesel fuel is injected in to

the DISI engine under the same operating conditions as that of gasoline. The results of this are explained in the next

section.

B. Diesel Fuel in DISI engine

As explained earlier in section III (A) diesel fuel is injected in a DISI engine with the same operating conditions

as gasoline. The injection was done at 270 bTDC. The simulation is performed and similar to the gasoline case the

pressure history is tracked for each and every time step. Figure 5 shows the pressure peak obtained from a diesel

DISI engine working with gasoline operating conditions.

Figure 5. Time history of average pressure for DISI diesel with Gasoline fuel operating conditions.

The Pressure peak observed is different from gasoline pressure peak. A huge rise in cylinder pressure is

observed at 30 bTDC in Figure 5. As the spark event is 10bTDC the combustion happened before the spark. This

explains the auto ignition of diesel. Diesel is a heavy fuel it has lower auto ignition time. As it is injected 270bTDC

it exceeds the auto ignition time which cause pre matured detonation. This means uncontrolled combustion. Using

the benefits of CFD again the detonation is visualized.

(a) Fuel contours for gasoline at 30 bTDC (b) Fuel contours of diesel at 30 bTDC

Figure 6. Difference between the fuel distribution contours and pressure time histories of gasoline and diesel.

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2

4

6

8

10

12

14

360 560 760 960

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Mp

a

Crank angle in degrees

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A position “0” is selected which is pre- ignition position at 30 bTDC where the detonation is observed and the

combustion process is visualized. 30 bTDC is the crank angle where all the fuel is supposed to be mixed with air as

the spark event is at 10 bTDC. Figure 6 shows the contours of fuel at 30 bTDC for gasoline and diesel. The diesel in

Figure 6 (b) is utilized before the spark this happens because of auto ignition. The pressure peaks obtained in figures

5 and 3 shows the difference between a good combustion and combustion with auto ignition.

C. Change in injection start time for diesel DISI engine

By injecting diesel in an engine with gasoline characteristics it detonates. The detonation is due to prolonged

exposure time of diesel. To remove the effect of auto ignition the auto ignition times are calculated for diesel at

1100K and 3.5 MPa pressure. The time that diesel can be exposed without auto ignition is calculated by the equation.

It is found that the diesel will not auto ignite for 180 degrees. So the injection start time is shifted to 180 bTDC from

270 bTDC. The simulation is performed with the changed injection spark time. Figure 8 shows the combustion

process in this simulation.

(a) Diesel fuel left over at 80 aTDC

(b) CO2 produced during combustion

Figure 7. Fuel and CO2 at position “3” (80 aTDC) showing poor combustion.

At position “3” (80 aTDC) the laminar flame is not propagated all over the cylinder and there is lots of fuel left

over. Though the fuel distribution is good because of the pocket geometry which provides good mixing capabilities

for the engine, it failed to get a good combustion to check the difference. A comparison is made in between the

extent of combustion for gasoline and diesel using the Figures 7(b) and 4(c). The difference between the amounts of

CO2 produced is clearly observed. A lot of left over fuel without combustion can be seen in Figure 7(a) which

explains the poor combustion.

(a) Gasoline

(b) Diesel

Figure 8. Difference in Vaporization of gasoline and diesel fuel at 30 bTDC (droplet size = 50 microns).

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A careful study of combustion process explains this behavior. To have a better combustion not only fuel mixing

is useful but also fuel vaporization plays a very important role. The diesel being a heavy fuel vaporizes much slower

than gasoline. Using CFD the vaporization of gasoline and diesel are compared and Figure 8 shows the difference

between the vaporization of gasoline and diesel. The poor vaporization of diesel can be clearly observed.

To improve combustion without sacrificing fuel distribution and auto ignition time the droplet size should be

reduced. For the above mentioned simulation the droplet size was 50 microns which is typical for gasoline which vaporizes relatively faster than diesel. To evaluate the best droplet size four simulations with four different droplet

sizes are designed. The details of these simulations are explained in next section.

C. Droplet size study for diesel DISI engine

Diesel evaporates much slower than gasoline and the time of injection cannot be changed as it is constrained by

the auto-ignition. So the only way to improve vaporization is to decrease the droplet size. Decrease in droplet size

increases the vaporization. Small droplets evaporate faster than the big droplets so four different droplet sizes are

considered and the vaporization is studied. In all these simulations six orifice injectors are used. Figure 9 shows the

difference in vaporization. These pictures show that all of these cases have better vaporization than diesel fuel

injection with 50 microns shown in Figure 8(b). They also show that Figure 9(c) with 20 micron droplet size has

better vaporization than 40 and 30 microns shown in Figures 9(a) and 9 (b) respectively.

(a) 40 Microns

(b) 30 Microns

(c) 20 microns

Figure 9. Difference in vaporization with change in particle size.

With decrease in particle size the amount of vaporization is decreased. For the particle size of 20 microns the

vaporization of diesel fuel is almost close to the gasoline. The 20 microns is chosen as the final particle size as it is

mechanically feasible.

Figure 10. Time history of average pressures for Solstice engine with diesel fuel.

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9

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360 460 560 660 760 860 960 1060

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computational diesel

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The pressure plots of the final simulation with six plumes, 20 micron particle size and 180 bTDC injection time

are and the diesel cycle produced almost same amount of power as gasoline. Figure 10 shows the time history of

average pressure obtained from using diesel fuel injection. Comparing with Figure 3, Figure 10 demonstrates the

similar pressure change in an engine cycle and this implies the feasibility of using diesel fuel in a DISI Solstice

engine.

IV. Conclusion

The injection in direct injection spark ignition technology is successfully optimized in such a way that the

injection system can be used for diesel fuel. The parametric study of the injection system helps in designing an

optimized injection which helps to achieve the goal of using any fuel in direct injection spark ignition technology.

The parametric study on injection time helps to eliminate auto-ignition of diesel. The multiple orifice injection

technology increases the fuel spread during injection. The decrease in droplet size causes quick vaporization larger

spread hence better mixing in the cylinder and hence good combustion. Diesel and gasoline represents the two major

kinds of fuel. The direct injection spark ignition technology is successfully applied for diesel fuel using the right

operating conditions determined by the parametric study. Similarly the capability of DISI technology to use any kind

of fuel can be tested using the similar procedure.

Acknowledgments

The authors would like to thank Dr Frederick Schauer, Timothy Smith, and William McCuddy for their support in

providing experimental data and necessary help whenever needed. This work was supported by ISSI and AFRL.

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