Effect of Water Injection on SI Engine Performance and Emissions

Osama H. Ghazala, Gabriel Borowskib a Applied Science Private University, Department of Mechanical Engineering, Amman, Jordan, e-mail: [email protected]. b Lublin University of Technology, Faculty of Environmental Engineering, Lublin, Poland, e-mail: [email protected]

Abstract

The internal combustion engine operating on fossil fuel is one the main sources of air pollution. Therefore, the major challenge faced by automobile manufacturers is to reduce the engine emissions and increase their efficiency. Water injection technique is a good technology to reduce the engine emissions, due to its ability to decrease the combustion temperature inside the cylinder by absorbing high amount of heat released from combustion. In this paper, the effect of water injection on the gasoline engine performance and emissions was investigated theoretically. A four cylinder SI engine model was built using GT-Power professional software. The gasoline fuel was injected directly to the cylinder and the water was injected into the intake manifold with different mass flow rate. The calculated engine outputs were: cylinder pressure and temperature, brake torque, brake power, mean effective pressure, thermal efficiency, carbon monoxide, hydrocarbon, and nitrogen oxide emissions. The engine knock analysis also was performed. The results show that the increased water mass flow rate resulted in improved engine performance and decreased emissions compared to neat gasoline fuel. In addition, the injected water decreased the in-cylinder temperature causing engine parts cooling resulting in knock prevention and engine durability improvement.

Keywords: water injection, emissions, engine modeling, engine knock

1. INTRODUCTION The production of an internal combustion engine with a high performance and low emissions was investigated before decades by researchers. Recently, the low temperature combustion (LTC) technique considered as a one of the promising technologies to reduce in-cylinder temperature and engine emissions. Water injection technique is a good technology to reduce engine emissions due to its ability to decrease the combustion temperature inside the cylinder by absorbing high amounts of the heat released from combustion. However, the water injection technology noticeably reduces the cylinder temperature due to its high heat of vaporization, resulting in lower engine emissions and knock ability. In addition, the cooling of the engine parts increased the engine lifespan and prevented the load shock inside the engine. Many researchers have investigated the effect of water engine on the IC engine due to improved engine efficiency and reduced emissions [1-9]. The effect of water injection on the engine knock resistance was studied by Breda et al. [10]. They concluded that when water and methanol were injected to the intake manifold, the knock resistance increased. Boretti [11] studied the effect of water on a turbocharged engine. He observed a reduction in the intake gas temperature, resulting in improved engine power and fuel efficiency. Totala [12] observed a reduction of carbon monoxide (CO) and hydrocarbon (HC) emissions when a water/methanol mixture was introduced to the gasoline engine. Bernie et al. [13] confirmed theoretically the effect of water/fuel mixture percentage on the engine knock resistance. The effect of intake manifold water injection was performed by Tauzia et al. [14]. The extension of the ignition delay compound with NOx reduction was observed when water was added to the mixture. Tesfa et al. [15] investigated the effect of water injection in an IC engine operating on biodiesel fuel. They observed a significant reduction of nitrogen oxide (NOx) with engine performance deterioration. Subramanian [16] analyzed the effect of water injection on engine emissions. The goal of this work was to investigate the effect of water mass flow rate on the emissions, performance and engine knock as well. A four cylinder SI engine model was built and simulated. The water was injected to the intake manifold with various mass flow rate ranging from 0.5 g·s-1 to 1.5 g·s-1. The gasoline fuel was injected directly to the combustion chamber. The engine speed was constant for all runs, amounting to 2000 rpm, and the engine was operated at full load. The model was validated using the data available from the literature and a good agreement was obtained.

2. MODEL DESCRIPTION A four cylinder direct injection spark ignition engine model is presented in figure 1. The gasoline fuel was injected to the cylinder with a constant mass flow rate of 6.5 g·s-1. The water was injected into the intake manifold with mass flow rate varying from 0.5 to 1.5 g·s-1 and temperature of 300 [K]. The neat gasoline fuel combustion was also investigated in order to compare the gasoline/water injection parameters. The engine speed was 2000 rpm and it was kept constant for all simulation runs. The air/fuel ratio was kept constant at 12.5.

Figure 1. Four cylinder engine model using GT-Power code

The GT-Power program solves the conservation equations using a 1-dimmensional model. The Woschni model was used to calculate the heat transfer in the combustion chamber. The model used the SIWiebe combustion model, which imposes the burn rate for SI engine using the Wiebe function. However, the Wiebe constants should be determined experimentally. The engine specifications are presented in table (1). The engine initial conditions are presented in table (2). The operation conditions are illustrated in table (3). The model was validated using the data available from the literature. A good agreement was obtained.

Table 1. Engine geometry Parameter Unit Value Table 2. Engine initial conditions Bore mm 85 Parameter Unit Value Stroke mm 87 Initial pressure bar 1

Conn. Rod Length mm 180 Initial temperature K 300

Piston Pin Offset mm 0 Head temperature K 575

Number of Cylinders 4 Piston temperature K 575 Compression Ratio 10 Cylinder temperature K 400 Bore/Stroke 0.97 IVC CA -105 IVO CA 308 EVC CA 385 EVO CA 128 Table 3. Engine operating conditions Properties Units Oper. Cond Engine speed RPM 2000 Combustion Start [CA] -20 Injection Start [CA] 365 Vol. Eff Ref. Pressure [bar] 1 Vol. Eff Ref. Temperature [K] 300 Mean Piston Velocity [m/s] 5.6

2.1. Conservation Equations

The conservation equations solved by GT-Power code are presented below [19].

푑푚 Continuity: = ∑ 푚̇ (1) 푑푡 푏표푢푛푑푟푖푒푠 푑(푚푒) 푑푉 Energy: = −푝 + ∑ (푚퐻) − ℎ퐴 (푇̇ − 푇 ) (2) 푑푡 푑푡 푏표푢푛푑푖푒푠 푠 푓푙푢푖푑 푤푎푙푙 푑(휌퐻푉) 푑푝 ̇ Enthalpy: = ∑ (푚̇ 퐻) + 푉 − ℎ퐴 (푇 − 푇 ) (3) 푑푡 푏표푢푛푑푖푒푠 푑푡 푠 푓푙푢푖푑 푤푎푙푙 휌푢|푢|푑푥퐴̇ 1 푑푚̇ 푑푝퐴+∑(푚̇ 푢)−4퐶푓 −퐾푝[ 휌푢|푢|]퐴 Momentum: = 2 퐷 2 (4) 푑푡 푑푥

2.2. Heat transfer model

The heat transfer model used in the simulation was the Woschni model which calculates the heat transfer according to the following equation [19]

0.8 A.p 0.8  T .V.( p  p )  h   B.U  C. soc motor  (5) 0.55 0.2  piston  T Dcyl  psoc. Vsoc 

Where A, B, C are Woschni coefficients. Thus the heat transfer per unit area of cylinder wall is defined as:

dQ  h (T  T ) (6) F gas wall

Where: dQ /F = heat transfer per unit area [W/m2]

3. RESULTS AND DISCUSSION

3.1. The effect of water injection on engine performance

Figure 2 shows the effect of water injection on the engine torque. As shown, the engine torque increased slong with the mass of the water injected. The engine power also improved when the water mass increased, as illustrated in figure 3. The water mass positively affects the engine effective pressure and thermal efficiency, compared to neat gasoline fuel, as seen in figures 4 and 5. This is due to the increased working fluid mass caused by water droplets evaporation. In general, the engine performance improved with water injection, compared to the dry engine combustion. In addition, the mean effective pressure is affected by the spark timing. When water mass increased, the advanced timing should be increased as well and consequently the engine effective pressure should be improved. Moreover, the brake effective pressure will increase along with the water mass due to knock mitigation caused by the introduced water. Another reason for the engine power improvement could be the combustion of the mixtureunder stoichiometric conditions caused by the exhaust gases temperature reduction due to the injected water. 270

250

m] - 230 210 190

Brake torque Brake [N 170 150 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Water mass flow rate [g/s]

Figure 2. Effect of water injection on engine torque 55

Brake power Brake [kW] 35

30 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Water mass flow rate [g/s]

Figure 3. Effect of water injection on engine power 17

Mean efffective Meanefffective pressure [bar] 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Water mass flow rate [g/s]

Figure 4. Effect of water injection on engine pressure 33

Brake thermal Brake efficiency [%] 29 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Water mass flw rate [g/s]

Figure 5. Effect of water injection on engine efficiency

3.2. The effect of water injection on engine emissions The extended Zeldovich model was used to calculate the NOx emissions. The equations are shown below [19]:

O + N2 = NO + N (7) N + O2 = NO + O (8) N + OH = NO + H (9)

The NO amount decreased when the water mass flow rate increased, as illustrated in figure 6. The increase of the injected water amount resulted in better control of engine emissions. This observation is in agreement with Mathur et al. [17] who performed a reduction of NOx emissions as the water flow rate increased. Further, Adnan et al. [18] reported that the increased water injection duration resulted in elevated water amount in the mixture and, consequently, reduced

NOx emission. Moreover, when the water mass increased, the maximum cylinder temperature decreased, resulting in lower NOx emission. The reduction of NOx emissions is about 50% for 1.5 g·s-1 water mass, compared to neat gasoline fuel. Figure 8 illustrates the hydrocarbons concentration versus the water mass flow rate. As can be seen, the increased water amount resulted in decreased HC emissions due to extension of burn duration by 0-50% of fuel mass. The effect of water mass on the CO and CO2 emissions is presented in figures 7 and 9. As shown, the CO and

CO2 concentration decreased when the water flow mass increased. The CO reduction is about 50% when the water flow mass is 1.5 g·s-1, compared to dry combustion. In general, we can conclude from the analysis that the engine emissions decreased significantly as the water added to the mixture. 0.1

0.09

0.08

0.07

0.06 NO concentration[ppm] 0.05 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Water mass flow rate g/s]

Figure 6. Effect of water injection on nitrogen oxide emissions 105000

103000

101000

99000

concentration[ppm} 97000

2 2 CO 95000 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Water mass flow rate [g/s]

Figure 7. Effect of water injection on carbon dioxide emissions 280

270

260

250 HCconcentratiion [ppm] 240 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Water mass flow rate [g/s]

Figure 8. Effect of water injection on hydrocarbons emissions 49000

44000

39000

34000 CO concentration CO [ppm]

29000 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Water mass flow rate [g/s]

Figure 9. Effect of water injection on carbon monoxide emissions

3.3 Engine knock

Several knock models have been proposed by the GT-Power code. The Douaud & Eyzat knock model was used in this simulation. The knock occurrence can be predicted using the knock index parameter. It is defined as a crank angle-dependent quantity, forming the following equation [19]:

푉 −60000 퐼 (훼) 퐾퐼 = 10000 ∗ 푀 ∗ 푢(훼) 푇퐷퐶 푒푥푝 [ ] ∗ 푚푎푥(0, 1 − (1 − ∅(훼)) 푎푣푒 (10) 푉(훼) 푇(훼) 퐼푘−푟푒푓퐼푘−푐표푟푟

Where: KI = Knock index, M = Knock Index Multiplier, u = percentage of cylinder mass unburned, VTDC = cylinder volume at top dead center, V = cylinder volume, T = bulk unburned gas temperature (K), Φ = equivalence ratio of the unburned zone, Iave = induction time integral, averaged over all end gas zones, IK-ref = Reference Induction Time Integral (1.0 for Douaud&Eyzat and Kinetics-Fit), IK-corr = induction time integral correlation factor (1.0 for Douaud&Eyzat, Kinetics-Fit and Franzke). When knock does not occur, the knock index becomes zero. Otherwise, the value of the knock index will be reported. In addition, when knock occurs, the knock probability is reported as 100% and when knock does not occur, the probability is 0%. As shown in figure 10, the knock index is reported for neat gasoline fuel and for water flow mass 0.5 g·s-1. This means that the knock will probably occur during the engine run with these parameters. The increase of water flow mass resulted in decreased pressure rise rate and, consequently, reduced engine knock probability, engine vibration and increased engine stability. 200

150

100

50 Knock index [KI]

0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Water mass flow rate [g/s] Figure 10. Effect of water injection on engine pressure

CONCLUSIONS

As a result of the investigations, the following conclusions were drawn: 1. When injected water mass increased to the optimum value, the engine power and thermal efficiency increased significantly compared to neat gasoline fuel combustion. 2. In general, the engine emissions decreased as the water mass increased. This is due to decrease the cylinder temperature caused by water evaporation. 3. The water injection technique can improve the engine knock resistance due to a decrease pressure rise rate. This could decrease the engine vibration and increase the engine durability. 4. The water injected mass should be limited to the optimum value for each engine operating conditions. Further addition of the water amount resulted in deteriorated engine performance and emissions. Moreover, the increase of the water amount causes engine wear and damage due to water condensation inside the cylinder. Furthermore, the high water amount leads to quenching of the combustion flame and, consequently, poor combustion efficiency.

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