Effect of water injection on the knock, combustion, and emissions of a direct injection gasoline engine

Fuel 268 (2020) 117376

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Full Length Article

Effect of water injection on the knock, combustion, and emissions of a direct injection gasoline engine

T

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Aqian Lia, Zhaolei Zhenga, , Tao Pengb a b

Key Laboratory of Low-grade Energy Utilization Technologies and System, Ministry of Education, Chongqing University, Chongqing 400044, China China National Heavy Duty Truck Croup Chongqing Fuel System Co., LTD, Chongqing 400044, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Water injection Direct injection gasoline engine Knock Emission Combustion

A turbocharged downsizing spark ignition (SI) engine cooperating with in-cylinder direct injection technology is one of the most effective ways to improve the fuel economy and to reduce the emissions of gasoline engines, but knock combustion limits the application and development of downsizing of SI engines in practice. In this research, a numerical simulation method was used to study the feasibility of in-cylinder direct water injection technology to weaken the knock tendency of a turbocharged direct injection gasoline (GDI) engine and improve its combustion emission performance. First, the knock of a certain type of turbocharged direct injection gasoline engine was induced by increasing the spark timing, thereby determining the position at which the end mixture was spontaneously ignited. Then, at a given water injection moment, the influence of the amount of water injection on the knock and emissions of the turbocharged direct injection gasoline engine was investigated. The results show that the knock intensity gradually decreased with the increase of the water injection quality. For the cyclic work, the amount of circulating work decreased with the increase of the water injection quality. Water injection is beneficial for reducing the emissions of nitrogen oxides (NOX), carbon monoxide (CO), and unburned hydrocarbons (UHC). However, the soot emissions will increase as the amount of water injection increases.

1. Introduction Vehicles powered by internal combustion engines account for more than 95% of total vehicles. In order to reduce oil consumption and emissions, efficient and clean engine technology must be developed. From a global perspective, spark ignition engines are still the main source of power, and the share of gasoline-powered passenger cars is about 98%. Improving the efficiency of gasoline engines and reducing emissions is a big problem that is being faced by engine researchers. Currently, a turbocharged downsizing SI engine cooperating with incylinder direct injection technology has the greatest potential to solve the above-mentioned problems [1–3]. However, from the current point of view, there are few engines with a unit power exceeding 110 kW/L in the gasoline engine market. The reason for this is that the turbocharged downsizing technology increases the thermal load in the cylinder and increases the pressure in the cylinder. This causes the intensification of the abnormal combustion phenomenon of SI engines-knock. Therefore, knock is the main limitation for the further performance improvement and downsizing of SI engines [4–7]. In order to suppress knock, Ricardo proposed water injection technology in the 1930s. This technology was applied to racing cars and

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buses. Later, due to the emergence of intercoolers, researchers’ attention on water injection technology gradually disappeared. However, the cooling effect of an intercooler on the intake air could meet the demand for the development of a turbocharged downsizing SI engine. At present, water injection technology is receiving the attention of researchers. Compared with an intercooler, water injection technology can absorb a large amount of heat and reduce the temperature in a cylinder, thereby replacing the method of enriching the mixture and improving fuel economy. At the same time, due to the reduction of the maximum temperature in the cylinder, the generation of NOX is reduced, and the heat transfer of the wall surface can be reduced. The injected water directly absorbs the heat released by the combustion in the cylinder, reduces the combustion temperature in the cylinder, reduces heat exchange loss in the cylinder, and it can suppress knock and improve effective heat efficiency [8–10]. There are three ways to apply water injection technology to an engine: First, the inlet/intake pipe injecting water, second, water being injected in the cylinder, and third, fuel-water emulsified mixing and reinjection. Inlet water injection dominates current research [11]. The main advantage of the inlet water injection compared to the other two injection methods is that the engine structure is hardly changed [12].

Corresponding author. E-mail address: [email protected] (Z. Zheng).

https://doi.org/10.1016/j.fuel.2020.117376 Received 13 October 2019; Received in revised form 27 December 2019; Accepted 10 February 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.

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Although the water injection cooling effect of the inlet can increase the volumetric efficiency, if the amount of water injected is large, the liquid water or the vapor generated by the evaporation still occupies a part of the space, affecting the volumetric efficiency. Therefore, the water injection method of the inlet is limited by the maximum amount of water injected [13–16]. In addition to the cooling effect, which is similar to the water injection method of the inlet, the direct water injection method in the cylinder has other advantages. The water is injected directly into the cylinder, which can flexibly adjust the amount of water injected and the time of injection without affecting the volumetric efficiency, and its cooling effect is better than the cooling effect of the inlet water injection [17–19]. For the third method, the emulsification and mixing of fuel and water is a complex process. There is still a lot of work to be done in the selection of the emulsifiers and the stability of the emulsion fuels. Compared with gasoline-water emulsion fuel, the application of direct water injection in the cylinder is more convenient. Therefore, the method of direct water spray in the cylinder was selected in this research [20,21]. Boretti [22] studied the effect of inlet water injection on a directinjection spark plug ignition supercharged engine through simulation. The conclusions were that inlet water injection could improve the charge efficiency, reduce the tendency to knock, control the temperature of the exhaust turbine, and increase the thermal efficiency peak and output torque under partial load. Direct water injection could increase fuel conversion efficiency more than water injection in the inlet. Wei et al. [23] used three-dimensional computational fluid dynamics (CFD) to study the combustion and emission performance of water injection in a cylinder for the low-load condition of a direct injection gasoline engine. The results showed that by optimizing the amount of water injection, not only could better engine performance be achieved, but also NOx emissions and soot emissions could be reduced. Gadallah et al. [24] studied the influence of in-cylinder water injection on the discharge performance of a direct-injection hydrogen engine. The research showed that the effect of injecting water into the cylinder during the expansion stroke had little effect on the NOx emissions. The main reason for this was that a large amount of the NOX was generated before injection. The effect of the water injection on the compression stroke of the emissions was better. The amount of water injected and the time of injection had a great influence on NOX emissions. Hoppe et al. [25] explored the potential of direct water injection in a cylinder to reduce the knock tendency and improve the efficiency of a direct injection gasoline engine. The results showed that during a Miller cycle and exhaust gas recirculation (EGR) conditions, the efficiency of a direct injection gasoline engine increased by 3.3–3.8%. Using water injection technology under lean burn conditions, the efficiency of a direct injection gasoline engine increased by 4.5%. Hoppe et al. did not analyze the reasons for the direct injection of water in the cylinder to improve the efficiency of the direct injection gasoline engine. The effect of water injection on the knock was also not clearly stated. As can be seen from the foregoing discussion, direct water injection in a cylinder has great potential for reducing the knock tendency, improving engine performance, improving fuel economy, and reducing emissions. However, direct water injection technology in a cylinder is used less under high load conditions for a turbocharged downsizing SI engine. Specifically, the physical chemistry mechanism of water on combustion is not clear. The laws governing the effects of specific water injection parameters on the knock and emissions of a turbocharged downsizing SI engine are not known. Therefore, further research on direct water injection technology in a cylinder is needed. In this research, a numerical simulation method was used to study the effect of water injection technology on the suppression of the knock, combustion, and emission performance under high speed and high load conditions. The injected water directly absorbed the heat released by the combustion in the cylinder and reduced the combustion temperature in the cylinder. This could reduce the heat exchange loss in the

Fig. 1. Geometry model of the engine.

cylinder, suppress knock, and improve thermal efficiency. The main purpose of this research was to determine the appropriate ratio of water injection for suppressing knock, improving thermal efficiency, and exploring the effects of water injection on emissions. 2. Verification of the numerical models In this research, a numerical model was established for the D20T direct injection gasoline engine and the numerical model was verified with the corresponding experimental data. The experimental data came from the Changan Automobile (Group) Co. 2.1. Geometric model and calculation model of the direct injection gasoline engine 2.1.1. Geometric model and operating conditions of the direct injection gasoline engine The geometric model, basic parameters, and operating conditions of the GDI engine are shown in Fig. 1, Table 1, and Table 2, respectively. 2.1.2. Selection of the numerical model submodel In the numerical simulation of the engine, each submodel needed to be determined, such as the turbulence model, combustion model, spray model, etc. The flow in the engine cylinder was very complicated due to not only fluid flow, but also chemical reactions as well as heat and mass transfer. In order to truly reflect the flow of fluids and chemical reactions in the cylinder, a suitable selection of models was required. The turbulence model chosen in this paper was the RNG k-ε twoequation model. Krishna et al. [26] used STAR-CD software to explore the in-cylinder flow field of a single-cylinder two-stroke engine using Table 1 Basic parameters of the GDI of the engine.

2

Parameter

Numerical Value

Number of cylinders Bore diameter/mm Stroke/mm Compression ratio Link length/mm Displacement/L

1 86 86 9.5 142.8 0.5

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Table 2 Operating condition of the GDI of the engine. Parameter

Numerical Value

Rotating speed Fuel injection moment Fuel injection duration Fuel injection Ignition moment Mean effective pressure

5500 r/min −337.99 °CA 160.908 °CA 82.31 mg −11 °CA 17.53 bar

Table 3 Choice of numerical model. Model

Setting

Turbulence model Fuel fracture model Collision model Fuel wall model Combustion model Nitrogen oxide model Soot model

K-ε double equation model RT-KH (ReyleigtTaylor-Kehrin Helmholz) fracture model NTC(No Time Counter) collision model Wall film model SAGE model Extended Zeldovich model Hiroyasu model

Fig. 3. Comparison of the experimental and simulated average pressure results.

Table5 Basic parameters of the engine.

Table 4 Initial condition. Parameter

Numerical Value

Cylinder temperature/K Cylinder pressure/Pa Composition of various substances in the cylinder Inlet temperature/K Inlet pressure/Pa Exhaust channel temperature/K Exhaust pressure/Pa Turbulent energy/m2/s2 Stimulating energy dissipation/m2/s2

1099.0 276871.0 O2, N2 (Quality score:0.23, 0.77) 316.0 173196.74 1029.0 220310.7 1.0 100.0

Parameter

Value

Number of cylinders Cylinder bore/mm Stroke/mm Compression ratio Link length/mm Intake valve opening timing/°CA Intake valve closing timing/°CA Exhaust valve opening timing/°CA Exhaust valve closing timing/°CA

1 95 114 17 255 225 15 15 225

combustion model in this research was the one-component mechanism. This mechanism was the SAGE combustion model coupled with the IC8H18 (isooctane) proposed by Jia Ming [28]. Compared with other reaction mechanisms, this mechanism can react better to the in-cylinder combustion reaction. The selection of each model is shown in Table 3. 2.1.3. Initial and boundary conditions The initial conditions and boundary condition values were calculated from experiments and one-dimensional GT-Power software. The initial conditions are shown in Table 4. The boundary conditions were: The top temperature of the piston was 585 K, the wall temperature of the combustion chamber was 550 K, the temperature of the spark plug was 1050 K, the static pressure of the exhaust port was 101325 Pa, and similar parameters. 2.2. Numerical simulation model verification The software had adaptive encryption to encrypt or coarsen the mesh at the specified time and space, which could greatly save computing time. The grid size was not uniform. A coarse grid was used for the intake and exhaust channels. Fine mesh was used for the intake and exhaust valves, pistons, spark plugs, cylinder heads, etc. Adaptive encryption was applied to the flow and temperature of the entire cylinder. When the temperature and pressure gradient in the cylinder area exceeded the limit value, the area automatically encrypted the grid. To make the numerical simulation model accurately reflect the actual operating conditions of the direct injection gasoline engine, the numerical simulation results were compared with the experiment. Knock is prone to occur under high load conditions, so high speed and full throttle operating conditions were selected for the numerical simulation and experimental conditions. To obtain the initial conditions that could not be accurately given by the experiment, the one-

Fig. 2. Number of Calculation model meshes.

different turbulence models (standard two-equation model, Chen twoequation model, and RNG k-ε two-equation model), and compared the results with the in-cylinder velocity field measured by particle image velocimetry (PIV). The results showed that the RNG k-ɛ two-equation model was closer to the experimental measurements than the other two models. Pomraning et al. [27] believe that the RNG k-ɛ two-equation model coupled with a detailed chemical reaction in the numerical simulation of an internal combustion engines could effectively predict the turbulent flow field in a cylinder. The unique SAGE combustion model in the software can adapt to various reaction mechanisms well, and the calculation speed is fast. The 3

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Fig. 4. Experiment and simulated average pressures under different water injection quality (equivalent ratio = 1, compression ration = 17).

(a) Average pressures in the cylinder at different spark timings

(b) Average temperatures in the cylinder at different spark timings

Fig. 5. Average pressures and temperatures in the cylinder at different spark timings (n = 5500 rpm, equivalence ratio = 1.1).

for primary reference fuel (PRF) oxidation. In view of the successful exploration of the improvement of a skeletal model on PRF and isooctane, the semidecoupling methodology presented was considered reliable and generally applicable for different alkane fuels. However, for a skeletal chemical model, due to the omission of some intermediate reaction processes, it might perform well in one reactor while performing poorly in another. This might cause the simulation and experimental values to differ slightly. In other research studies, the comparison with three engine experiments showed that the simulated pressure trace was in excellent agreement with the experiment [28]. In addition, the experimental results were accidental. Due to various uncertainties, the simulation results and experimental results might not be exactly the same. The difference between simulation results and experimental results is within 5%, so it can be considered that the calculation model was realistic and it could simulate the operating conditions of the direct injection gasoline engine. Based on the above pressure verification, a set of experimental and simulation comparison picture of the average pressure in the engine cylinder under different water injection qualities is also added. The parameters of the engine are shown in the table below. Water injection quality is 50 mg and 90 mg, respectively. The equivalent ratio is 1 (Table5). It can be seen from the Fig. 4 that the experimental and simulated

Table 6 Engine operating conditions. Parameter

Numerical Value

Rotating speed/r/min Ignition moment/°CA Equivalent ratio

5500 −21 1.1 ± 0.01

dimensional simulation software GT-Power was used to simulate the working state of the direct injection gasoline engine in order to obtain accurate initial conditions. The simulation calculation started at −366 °CA and the termination time was 150 °CA. Fig. 2 shows the curve of the number of grids in the calculation model. As can be seen from the figure, the grid peak was about 2.6 million grids. Fig. 3 is a comparison of the experimental cylinder pressure curve and the numerical simulation pressure curve. It can be clearly seen from the figure that the experimental values were very close to the results of the 3D simulation software. The simulation results were generally consistent with the pressure traces in the experiment, with few differences. In the mechanism of the combustion model in this research, to develop skeletal chemical kinetic models, a semidecoupling methodology was presented and applied in order to construct an enhanced skeletal model

4

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approached the top dead center (TDC) and the engine power increased. The final ignition advance was selected at −21 °CA. As can be seen from the following content, when the spark timing was −21 °CA, knock occurred in the cylinder without injecting water. The operating parameters of the direct injection gasoline engine are shown in Table 6. For the strength of the knock trend, a parameter needed to be defined to measure the intensity of the knock. This parameter was defined as [30]:

KI =

1 N

N

∑ PPmax,n 1

In the formula, KI indicates the intensity of the knock. The cause of the knock was that the flame that formed near the spark plug propagated to the end mixture for a longer time than the end mixture formed the center of the flame and ignited. Therefore, the vicinity of the wall surface of the combustion chamber, which was far away from the spark plug, was more prone to knock [7,32–35]. The monitoring points were all placed in the vicinity of the wall of the combustion chamber and away from the spark plug, as shown in Fig. 6. 3.2. Analysis of the spontaneous combustion of the end mixture and determination of the knock position

Fig. 6. Schematic diagram of monitoring points.

values of the average pressure in the cylinder are basically the same under different water injection quality. Their differences are small, so it can be considered that the calculation model can effectively simulate the operating conditions of gasoline engines.

It can be determined from the knock theory that knock is caused by a sharp increase in the local pressure caused by the spontaneous combustion of the end mixture, and the knock position can be determined according to the change of the temperature field in the cylinder or the change of free radicals of some of the components [36–39]. Simona et al. [40] used chemiluminescence to detect the free radicals HCO, OH, and CH that were generated during the knock of a single-cylinder visualized spark plug ignition gasoline engine. The results showed that the occurrence of free radical HCO in the end combustible mixture often marked the starting point of the knock, and then the HCO radical decreased and the OH radical increased. According to Simona's theory, it is only necessary to know the distribution of HCO radicals in a cylinder to determine where the knock occurs. The pressure field, HCO radical distribution, and OH radical distribution at the time of knock in this research are shown in Fig. 7. It can be seen from Fig. 7(a) that when the knocking occurred, the area with the highest pressure was concentrated in the monitoring points of No. 4, No. 1, and No. 2, wherein the pressure peak exceeded 15 MPa, and the possibility of knock was the largest in this part. The OH was concentrated in the area near the spark plug and monitoring point area 3, which indicates that the area near the spark plug and monitoring point area No. 3 had become burned areas. At the same time, a large amount of HCO appeared near the OH region. The appearance of HCO marked the beginning of the low-temperature reaction. After the HCO region appeared, the end combustible mixture was about to undergo knock combustion. In the HCO radical distribution map, it can be found that during the process of gradually decreasing the HCO concentration for the monitoring points of No. 4 and No. 1, the pressure at the monitoring points of No. 4 and No. 1 sharply increased. This shows that knock had occurred in the monitoring points of No. 4 and No. 1. The results of the numerical simulation were consistent with the Simona experimental results. That is, when knock occurred, HCO radicals appeared, and then HCO radicals gradually decreased and OH radicals appeared in large quantities. The velocity field in the cylinder during the knock process is shown in Fig. 8. Before the occurrence of knocking, the flow velocity in the cylinder was generally below 75 m/s, and the flow field was essentially symmetrically distributed with respect to the horizontal axis. The monitoring point area of No. 4 was the squeezing zone, and there were a large amount of high temperature and high-pressure gas flows to the monitoring point No. 4. After the knock occurred, the velocity field in

3. Numerical simulation of the knock of a direct injection gasoline engine 3.1. Determination of the knock conditions The calculation and verification conditions in this research are high speed and high load conditions. According to the relevant literature [29–31], when the equivalent ratio of the mixture in the cylinder was between 0.9 and 1.1, the knock tendency was obvious, and the tendency of knock was the largest. Under the experimental conditions, the equivalent ratio was 1.1, so the equivalence ratio was selected as 1.1 in the numerical simulation process. The spark timing of the engine under experimental conditions was −11 °CA, at which time the engine had the best power and economy without knock. In order to further induce the occurrence of knock of the direct injection gasoline engine, the initial conditions, boundary conditions, fuel injection amount, and similar parameters given above were maintained, and the spark timing was increased to −21 °CA. To explore the relationship between the spark timing and the knock trend, three different spark timings were set: −11 °CA, −16 °CA, and −21 °CA. Fig. 5(a) shows the average in-cylinder pressure corresponding to different ignition timings. As the spark timing increased, the average pressure peak in the cylinder rose and the time to reach the pressure peak gradually decreased, indicating that the rate of change in the pressure increased and the tendency to knock increased. Fig. 5(b) shows the average temperature in the cylinder at different spark timings. It can be seen from the figure that the average temperature was consistent with the change of the average pressure in the cylinder. With the increase of the spark timing, the average temperature in the cylinder rose before the crank angle of 30 °CA, the possibility of the spontaneous combustion of the end mixture increased, and the tendency to knock increased. It can also be seen that as the spark timing increased, the knock tendency increased. In addition, the spark timing was appropriately increased and the maximum combustion pressure and temperature of the gas in the cylinder increased. The position where the maximum combustion pressure and temperature appeared gradually 5

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1.50

1.40

1.30

1.20

1.10 (e+007)

(Pa)

(a) Pressure map

2.0

1.5

1.0

0.5

0.0 (e-006)

(b) HCO quality score

2.4

1.8

1.2

0.6

0.0 (e-003)

(c) OH quality score Fig. 7. Field of each specified parameter in the cylinder (n = 5500 rpm, spark timing = −21 °CA).

the cylinder changed drastically. First, the fluid flow rate in monitoring point area 4 increased sharply and propagated outward rapidly (the maximum value exceeded 150 m/s), indicating that the area had a spontaneous combustion reaction, and the pressure and temperature rose. Then the flow field velocity in the monitoring point area of No. 2 and No. 1 also increased significantly. This indicated that knocking occurred in the monitoring point areas of No. 2 and No. 1 at this time. When the ignition timing was −21 °CA, the numerical simulation of

the in-cylinder average pressure curve and the pressure curve of each monitoring point were as shown in Fig. 9. It can be clearly seen from the figure that the pressure at monitoring point No. 4 was the largest, and the pressure at the monitoring points No. 3 and No. 5 was the smallest. The main reason for the dramatic change in pressure at monitoring point 4 was that the spark plug was located in the center of the combustion chamber and the flame traveled the same distance. However, after the spark plug was ignited, the flame traveled slowly 6

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0

37

75

112

150 (m/s)

Fig. 8. Velocity field of knocking combustion (n = 5500 rpm, spark timing = −21 °CA).

toward monitoring point No. 4, and the flame took longer to reach the area near monitoring point No. 4, so it had enough time to spontaneously ignite. The peak-to-peak values of each monitoring point are shown in Fig. 10. It can be intuitively seen that the peak-to-peak value of monitoring point No. 4 was the largest, and the knock intensity KI = 7.01 could be calculated. When the knock strength KI exceeded 2, it indicated that knocking had occurred in the cylinder of the direct injection gasoline engine. The knock area was mainly concentrated in the monitoring points of No. 1 and No. 4. In order to reduce knocking in this area, water needed to be injected into the two areas in order to effectively reduce the in-cylinder knock intensity.

time. Based on the equivalent ratio distribution cloud picture, the equivalent ratio cloud picture under different water injection ratios were essentially the same. Direct water injection in the cylinder had almost no effect on the equivalence ratio distribution, which was also explained by the fact that in the case of limited water injection, direct water injection in the cylinder could not improve the mixed state of the fuel and air and the turbulent flow state. At the same time, it can also be seen from the cloud picture of the equivalent ratio that the fuel distribution was more uniform (the equivalent ratio was between 0.8 and 1.2). However, the equivalent ratio in the area near the injector was higher than that in other areas (see Fig. 12). 4.2. The inhibition law of different water injection ratios for the knock

4. Inhibition of different water injection ratios on the knock under high load conditions

Fig. 13 shows the pressure curves of eight monitoring points for different water injection rates. It can be seen from the figure that as the amount of water injection increased, the pressure at each monitoring point decreased continuously and the monitoring point pressure remained almost constant within a certain crank angle. The monitoring point pressure curve fluctuated sharply when there was no water injection, and the pressure curve fluctuated slightly when the water injection amount was 25%. This indicates that the knock was completely suppressed as the amount of water injection increased. When the water injection amount was 5%, the pressure fluctuations of the monitoring points No. 3, No. 4, and No. 5 were effectively suppressed, while the pressure fluctuations of other monitoring points seemed to be only slightly affected. When the water injection amount was 5%, the water droplets were mainly concentrated in the left area of the cylinder. That is, the monitoring points of No. 3, No. 4, and No. 5, so the occurrence of knock in other areas could not be effectively suppressed. When the amount of water injection was 10%, the water droplets were partially distributed in the right side area. The pressure fluctuations in each monitoring point area were partially suppressed. When the amount of water injection exceeded 10%, the pressure fluctuation was slight, and the peak pressure was below 12 MPa. At this time, no knock had occurred in the cylinder. In the case of a lower amount of water injection,

When the ignition timing was −21 °CA, knock had occurred in the cylinder, so the spark timing was set to −21 °CA. At the time of the −80 °CA water injection, the distribution of water droplets in the cylinder was the most uniform, and it had the greatest impact on the pressure fluctuation of monitoring point 4. At this time, the possibility of suppressing knock was the largest, so the water injection timing was determined to be −80 °CA. The operating conditions of the direct injection gasoline engine are shown in Table 7. Considering the structure of the combustion chamber and the shape of the piston, the water injector and the fuel injector were arranged in the same plane. The schematic diagram of the water injection position is shown in Fig. 11. 4.1. Equivalent ratio distribution diagram of different water injection quantities before the ignition time Different water injection ratios may have affected the equivalence ratio of the mixed gas in the cylinder before ignition. Therefore, it was necessary to explore the distribution of the equivalence ratio in the cylinder under different water injection conditions before the ignition 7

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Fig. 9. Average pressure curve and pressure curve at each monitoring point (n = 5500 rpm, spark timing = −21 °CA).

it was difficult to decrease the pressure peak-to-peak value by increasing the amount of water injection. When the water injection amount was 25%, the peak-to-peak pressure of the monitoring point was below 0.5 MPa. Fig. 15 shows the knock intensities for different water injection rates. Each point in the figure represents the knock intensity for a different water injection ratio. It can be seen from the figure that as the water injection increased, the knock intensity decreased gradually and the decrease speed decreased. In combination with Fig. 13, it can be seen that although increasing the amount of water injection could effectively reduce the knock intensity, the increase of the amount of water injection caused the pressure in the

it was possible to effectively suppress knock. The pressure peak-to-peak value of each monitoring point is shown in Fig. 14. Each point on the polyline represents the maximum value of the pressure of a certain monitoring point in one of the crank angles for its water injection ratio. It can also be seen from the figure that the pressure peak-to-peak values of the monitoring points No. 1, No. 4, and No. 6 were higher than those of other monitoring points, indicating that the monitoring points of No. 1, No. 4, and No. 6 were more prone to knocking. When the amount of water injection increased from 0% to 10%, the peak-to-peak pressure of the monitoring point was significantly reduced. When the amount of water injection exceeded 15%,

8

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Fig. 9. (continued)

Fig. 11. Schematic diagram of water injection position.

Fig. 10. Peak-to-peak pressure of the monitoring point.

of the combustion, the direct injection period of the direct injection gasoline engine was extended, thereby reducing the efficiency of the direct injection gasoline engine. It can be seen that the knock could increase the efficiency of the direct injection gasoline engine to a certain extent. As shown in Fig. 17, the direct water injection in the cylinder had no effect on the cumulative heat release in general, and the cumulative heat release amount of the different water injection amounts was about 2900 J. The effect of the water injection on the instantaneous heat release rate shows that the more water was injected, the longer the time to reach the peak value of the cumulative heat release, and the effect on the total heat release was not large. In addition, the temperature of the in-cylinder mixture was high and the amount of water injection was small (up to 25% of the fuel mass), and the temperature reduction of the in-cylinder mixture was limited. Therefore, the low amount of water injection did not affect the cumulative heat release. As can be seen from Fig. 18, as the amount of water injection increased, the average temperature in the cylinder gradually decreased. An increase in the amount of water injection causes an increase in the amount of heat absorbed by the water in the cylinder. Fig. 19 shows the average pressure curve of the cylinder for different water injection rates. As can be seen from the figure, the smaller the amount of water injected was, the larger the average pressure in the cylinder was and the earlier the peak appeared. As can be seen from the analysis of Figs. 18 and 19, when the amount of water injection was small, the peak appeared earlier. At that time, the closer the piston was to the TDC, the smaller the internal volume of the cylinder was, the faster the heat

Table 7 Operating conditions of the GDI engine. Parameter

Numerical Value

Rotating speed/r/min Ignition moment/°CA Equivalent ratio Water injection moment

5500 −21 1.1 ± 0.01 80 °CA before top dead center 0, 5%, 10%, 15%, 20%, 25%

Water injection ratio/

mass water massIC 8H 18

Fuel injection/mg Water injection pressure/MPa Water injection temperature/K

82.31 15 333.15

cylinder to decrease. This was not conducive to the improvement of the efficiency of the direct injection gasoline engine, so it was necessary to select an appropriate amount of water injection. 4.3. The effect of different water injection ratios on the combustion Fig. 16 shows the instantaneous heat release rates for different water injection conditions. As can be seen from the figure, as the amount of water injection increased, the peak value of the instantaneous heat release rate gradually decreased and the peak value shifted with the back. The in-cylinder water injection reduced the temperature of the combustible mixture, reduced the flame propagation speed, caused the instantaneous heat release rate peak to shift back, and increased the combustion duration. Due to the increase in the duration 9

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0.4

Equivalent ratio 0.6 0.8 1

1.2

0

5%

10%

15%

20%

25%

Fig. 12. Equivalent ratio distribution diagram of different water injection quantities before ignition timing (n = 5500 rpm, spark timing = −21 °CA).

equivalent ratio, temperature, and pressure in the cylinder could affect the ignition delay time to a certain extent [41–44].Although the effect of water injection on the equivalence ratio was small, water injection would significantly reduce the temperature and pressure in the cylinder. Under high temperature conditions, the increase in pressure had a significant effect on reducing the ignition delay time. As the temperature increased, the overall reaction rate increased and the ignition delay time decreased. Water injection reduced the temperature and pressure in the cylinder, which caused an increase in the ignition delay time. Fig. 21 (a) shows the P-V plot for different water injection conditions. As the amount of water injection increased, the area enclosed by the closed curve gradually decreased, indicating that the functional force was continuously reduced. In order to quantitatively analyze the functional force for different water injection amounts, the closed curve in Fig. 21(a) was solved by integrating the area. The amount of work performed under different injection amounts is shown in Fig. 21(b). It can be seen from Fig. 21(b) that as the amount of water injection increased, the amount of work done in the cycle was gradually reduced, and the amount of work done in the cycle was between 1175 J and 1245 J. When the amount of water injection was between 5% and 10%, the amount of water injection had a lower effect on the amount of work done in the cycle. The main reason for this was that when the water injection was 5% or 10%, the instantaneous heat release rate curve and the average temperature curve in the cylinder were close. Therefore, the pressure curve in the cylinder was not much different. Although the knock intensity was the lowest when the water injection was 25%, the cycle work was too low. Therefore, 25% of the water injection was not the best choice. Under the experimental conditions, the cycle work was 1184.57 J.

release rate was, and the higher the average pressure in the cylinder was. The higher the average pressure was, the stronger the thermal motion of the molecules in the cylinder was, the higher the intramolecular energy was, and the higher the temperature inside the cylinder was. The average temperature in the cylinder was affected not only by the heat absorbed by the water in the cylinder but also by the slow burning rate of the combustible mixture due to the water injection in the cylinder. It can also be seen from Fig. 19 that after 35 °CA, as the amount of water injection increased, the average pressure in the cylinder also gradually increased. The main reason for this was that the water absorbed heat in the cylinder and evaporated into water vapor, which increased the pressure inside the cylinder. The pressure increase was determined by the water quality in the cylinder. However, due to the small amount of water injected, the average pressure difference between the cylinders was small. Based on the change of the average pressure curve in the cylinder, direct water injection in the cylinder could make the average pressure change in the cylinder more stable, which was beneficial for stabilizing the torque of the direct injection gasoline engine. The ignition delay time is defined as the time from the ignition moment to the moment when the pressure in the cylinder starts to depart from the pure compression line. The ignition delay time at different water injection ratios is shown in Fig. 20. It can be seen that as the amount of water injection increases, the ignition delay time increases accordingly. It can be seen that as the amount of water injection increases, the ignition delay time increases accordingly. It can be seen from Figs. 18 and 19 that as the water injection amount increases, the average temperature and pressure in the cylinder gradually decrease, which is the main reason for the increase in ignition delay time. The

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Fig. 13. Pressure change curves of each monitoring point for different water injection qualities (n = 5500 rpm, spark timing = −21 °CA).

torque. In order to maximize the water injection benefit and reduce the dilution effect that the water injection may have on the lubricant, many water injection parameters must be carefully controlled, such as water injection pressure, injection timing and number of injections. The structure of water sprays is influenced by, among other factors, engine speed, injection timing, in-cylinder flow motion, pressure, and temperature. All these parameters can affect atomization and mixing of the fuel in substantial ways leading to potential cycle-to-cycle variation in

When the water injection amount was 10%, the knock was completely suppressed and the cycle work was 1229.71 J, which was higher than the experimental condition of 45.14 J. Under high-speed and high-load conditions, the direct injection gasoline engine efficiency increased by 3.81% when the water injection rate was 10%. Even if the water injection was 20%, the efficiency of the direct injection gasoline engine increased by 0.068%. At the same time, due to the water injection in the cylinder, the average pressure curve changed more gently, which was conducive to the stable output of the direct injection gasoline engine 11

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Fig. 13. (continued)

Fig. 16. Instantaneous heat release rate for different water spray qualities.

Fig. 14. Monitoring point pressure Ppmax.

Fig. 17. Cumulative heat releases for different water injection qualities. Fig. 15. Knock intensities for different water injection qualities.

cylinder. The water injection parameters are combined with the operating parameters of the gasoline engine to achieve multi-parameter optimization and reduce the dilution of the lubricant.

combustion performance [45–47]. By adjusting these parameters, the distribution of Water in the cylinder is adjusted to ensure that water does not contact the cylinder wall surface or that as little water as possible hits the wall surface, and to achieve the purpose of suppressing knock on the premise of the least water into the cylinder. We are currently studying the effects of water injection timing, temperature, and pressure on suppressing knocking and water distribution in the

4.4. The effect of different water injection ratios on the emissions 4.4.1. The effect of different water injection ratios on the NOx and soot Fig. 22 shows the NOx mass change curve and the final NOx 12

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reaction was reversible, the reverse reaction played a dominant role in the process of the piston moving down, so the amount of NOX was reduced. When the amount of water injection was 0, the peak of NOX production was the highest, but at 60 °CA, the amount of NOX produced was slightly lower than the amount of NOX produced when the amount of water injection was 5%. It can be seen from Fig. 22(b) that when the amount of water injection increased from 0 to 5%, the amount of water injection slightly increased. When the amount of water injection increased from 5% to 25%, the final mass of NOX was gradually reduced. The variation curve of the soot in the cylinder is shown in Fig. 23(a). The soot model used a two-step reaction model proposed by Hiroyasu, which used two overall reactions to describe the formation and oxidation of soot. The formation of soot included the formation and oxidation of soot. In the early stage of combustion, as the flame propagated, the fuel burned and decomposed, a large amount of soot was generated, and then the soot was oxidized and it gradually decreased. From Fig. 23 (a), it can be found that with the increase of the water injection, the peak of the soot quality change curve gradually moved backward, which was caused by the slower burning rate caused by water injection. Fig. 23(b) shows the final production of soot for different water injection ratios. With the increase of the amount of water injection, the quality of soot production first decreased and then gradually increased. The amount of soot produced when the amount of water injected was 25% was slightly higher than the amount of soot produced when water was not injected. When the amount of water injection was less than 15%, the water injection in the cylinder could reduce soot emissions. When the water injection ratio was in the range of 5–15%, the soot generation quality was lower than when there was no water injection. Based on the peak of the curve of the soot production quality, when the water injection ratio was in the range of 5–15%, the peak of the soot gradually increased with the increase of the water injection amount. When the water injection ratio was in the range of 15–25%, the soot peak gradually decreased as the water injection quality increased. From the point of view of the final generation quality, when the water injection ratio was in the range of 5–25%, as the water injection quality increased, the soot emission increased. By comparing the changes in the oxygen content for the water injection ratios of 0% and 5% in the cylinder, it could be considered that the soot oxidation occupied a dominant role in the final soot formation quality. The distribution of oxygen with 0% and 5% water injection at 16 °CA–31 °CA is shown in Fig. 24. From Fig. 24, it can be clearly found that at the same time, the oxygen concentration in the cylinder when the water injection amount was 5% was higher than that when no water was sprayed. At 20 °CA, the soot content in the cylinder was close when the water injection ratio was 0% and 5%, and the oxidation rate of the soot with a water injection amount of 5% was higher than that without water injection. The main reason for this was that the oxygen concentration was directly proportional to the soot oxidation. The oxygen content when the water injection amount was 5% was higher than that when the water was not injected. From the perspective of the oxygen concentration distribution, oxygen was mainly concentrated in the area where the equivalence ratio was less than 1 before the ignition time. In the entire area of the cylinder, the equivalence ratio was more than 1, so the oxygen reaction could be complete and the content of soot oxide was limited. This led to higher soot emissions overall. However, the water injection could indeed reduce soot emissions to a certain extent. When the water injection ratio was in the range of 5–25%, as the water injection quality increased, the temperature in the cylinder decreased, resulting in an increase in the total amount of soot produced.

Fig. 18. Average temperatures in the cylinder for different water injection qualities.

Fig. 19. Average pressures in the cylinder under different water injection qualities.

Fig. 20. Ignition delay time at different water injection ratios.

production quality in the cylinder under different water injection rates. It can be clearly seen from Fig. 22(a) that the NOX mass first increased gradually and then gradually decreased, which was determined by the NOX generation mechanism. The NOX was rapidly generated in the high temperature zone. As the piston moved to the top dead center the temperature in the cylinder decreased. Since the NOX generation

4.4.2. Effect of different water injections on UHC and CO emissions The mass change curves of the unburned hydrocarbons for different water injection amounts are shown in Fig. 25(a). In the initial combustion stage after ignition of the spark plug, the fuel in the cylinder is not significantly reduced. As the flame gradually spread outward from the spark plug, the unburned fuel began to fall rapidly. As the amount 13

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(b) Cycle power

(a) P-V diagram

Fig. 21. Comparison of the amount of work done for different water injection qualities (n = 5500 rpm, spark timing = −21 °CA).

(a) NOX mass change curve in-cylinder

(b) The final quality of NOX production

Fig. 22. Comparison of NOX emissions for different water injection qualities (n = 5500 rpm,spark timing = −21 °CA).

(a) Soot mass change curve in-cylinder

(b) The final quality of soot production

Fig. 23. Soot mass change curve in the cylinder (n = 5500 rpm,spark timing = −21 °CA).

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0.000e+0

Water injection

1.250e-002

6.250e-003

2.500e-002

1.875e-002

ratio

0%

5%

16°CA

21°CA

26°CA

31°CA

Fig. 24. Oxygen quality score distributions (n = 5500 rpm,spark timing = −21 °CA).

(a) Unburned hydrocarbons change curve

(b) Unburned hydrocarbon emissions

Fig. 25. Unburned hydrocarbon change curves for different water injection qualities.

unburned hydrocarbons. When water was injected into the cylinder, the unburned hydrocarbon emissions were significantly lower than the hydrocarbon emissions when the water was not injected. This indicates that the direct water injection in the cylinder had a certain effect on reducing the unburned hydrocarbon emissions. The temperature during combustion had a great effect on UHC. In

of water injection increased, the instantaneous heat release rate decreased and the burning speed slowed down. Therefore, the rate of decline of the unburned hydrocarbon fuels became slower. In terms of the overall emissions of the unburned hydrocarbons, the unburned hydrocarbon emissions were still at a low level, although the combustible mixture was relatively rich. Fig. 25(b) shows the emissions of 15

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Water injection ratio

1800

temperature 2050 2300

2550

2800

(K)

0

5%

16°CA

21°CA

26°CA

31°CA

Fig. 26. Temperature field in the cylinder (n = 5500 rpm, spark timing = −21 °CA).

injection ratios, the distribution of water in the cylinder during combustion may have been different. This would have had a different degree of influence on the temperature reduction rate in the high-temperature area of the cylinder. Therefore, the total amount of UHC generated was different for different water injection ratios. In general, direct water injection in the cylinder would reduce the total UHC emissions. It can be seen from Fig. 27 that when the water was directly injected in the cylinder, the amount of CO generated was significantly lower than that of the pure gasoline. The process of CO formation included direct oxidation of the hydrocarbon fuel to CO and CO2 reduction to form of CO. It can also be intuitively found from Fig. 27 that the CO decline trend was exactly the same under different water injected conditions. At this stage, the CO was mainly oxidized to CO2. Since the oxidation of CO was essentially the same under different water injection conditions, the peak value of the CO mass in the cylinder determined its final production. The main source of CO in the cylinder was the decomposition reaction of the formaldehyde (CH2O) at the high temperature. The CH2O interacted with the OH to form HCO radicals, and the HCO radicals were further converted into CO. The water injection in the cylinder reduced the average temperature in the cylinder, which was not conducive to the conversion of CH2O to CO2. This effectively inhibited the formation of CO2. In summary, the in-cylinder water injection could effectively reduce CO2 emissions.

Fig. 27. Curve of CO mass changes for different water injection qualities.

order to clarify the reason why the water injection ratio was 5% lower than the total amount of UHC generated without water injection, the temperature field in the cylinder in these two cases needed to be analyzed. Fig. 26 shows the temperature field when the crank angle was in the range of 16 °CA–31 °CA. It can be seen from the figure that at 16 °CA, the temperature field without water injection was higher than the temperature field with 5% water injection. However, at 31 °CA, when the water injection ratio was 5%, the high temperature area above 2550 K was more than the high temperature area above 2550 K without water injection. The temperature drop rate in the high-temperature region became slower, which was beneficial to the UHC being further oxidized in the high-temperature environment of the expansion stroke and the exhaust stroke. Therefore, when the water injection ratio was 5%, the temperature in the high temperature area decreased more slowly than it did when the water was not injected. This was an important reason why the water injection ratio was 5% lower than the total amount of UHC generated without water injection. When the water spray ratio was in the range of 10–25%, due to different water

5. Conclusion 1. The most severe changes in the monitoring point pressure were the monitoring point areas No. 1, No. 2, and No. 4, in which the peak pressure of monitoring point No. 4 exceeded 23 MPa, indicating that this area was the most prone to knock. Water injection in the cylinder could effectively reduce the temperature and pressure in the cylinder, thus effectively suppressing knock. 2. Under different water injection conditions, the peak value of the instantaneous heat release rate decreased with the increase of the water injection quality. However, the cumulative heat releases were essentially the same, both around 2900 J. The average temperature

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and pressure in the cylinder decreased as the water injection ratio increased. For cyclic work, as the amount of water injection increased, the amount of work done in the cycle was gradually reduced, and the speed of the decrease was slow at first and then became rapid. When the ignition timing was −21 °CA and the water injection amount was 10%, the cycle work efficiency was 3.81% higher than that under the experimental conditions. The synergistic effect of the spark timing and the direct water injection in the cylinder had the potential to increase the efficiency of the direct injection gasoline engine. 3. When the amount of water injection was between 5% and 25%, the NOX emissions would gradually decrease as the amount of water injection increased. The NOX emissions were slightly lower than the 5% water injection NOX emissions when no water was injected. The main reason for this was that when the water injection was 5%, the temperature in the cylinder was higher than when the water was not injected. When the water injection was between 5% and 25%, the soot emissions increased gradually with the increase of the water injection quality. When the water injection was 5%, the soot emissions were lower than the soot emissions of the pure gasoline. The unburned hydrocarbon emissions for different water injection qualities were all below 0.00225 mg. The CO emission when the water was not injected was higher than the CO emission when the water was directly injected in the cylinder, and the direct water injection in the cylinder could reduce the CO emission.

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