Effects of Vessel and Water Temperatures on Direct Injection in Internal Combustion Rankine Cycle Engines
- 185 Downloads
This study focused on the effects of vessel and water temperatures on direct injection in internal combustion Rankine cycle engines through experimental and numerical methods. First, a study was carried out with schlieren photography using a high-speed camera for simultaneous liquid–gas diagnoses. Water was directly injected into a constant-volume vessel that provided stable boundaries. We wrote a MATLAB program to calculate spray tip penetration and cone angle from the images. For the further extension of boundary conditions, a numerical model was established and calibrated in AVL-FIRE for the thorough analysis of injection characteristics. Both experimental and numerical results indicated that injection and vessel temperatures have different effects on spray tip penetration. An increase in injected water temperature leads to shorter spray tip penetration, while the spray tip penetration increases with increasing vessel temperature. However, increased injection and vessel temperatures can both decrease the spray cone angle. Moreover, the simulation results also suggested that heat conduction is a main factor in boosting evaporation under top dead center conditions. When the internal energy of water parcels surges, these parcels evaporate immediately. These results are helpful and crucial for internal combustion engines equipped with direct water injection technology.
KeywordsInternal combustion Rankine cycle Direct water injection Water temperature Vessel temperature
Reducing environmental pollution is one of the most important research objectives for internal combustion engines. For this purpose, water injection technology has received much attention for several decades owing to its advanced effects, which not only reduce nitrogen oxide emissions and fuel consumption but also enhance thermal efficiency in both diesel and gasoline engines [1, 2]. However, inappropriate water injection is likely to quench the local flame, resulting in significant cycle-to-cycle variation and a substantial increase in hydrocarbon emissions . Because of the advantages of water injection technology, many studies have been carried out to investigate the impacts of different water injection strategies on engine performance and emissions.
Water injection technology has a positive effect on heat capacity in the cylinder, thereby lowering the peak combustion temperature. This technology  also leads to a significant gain in the knock-limiting ignition timing advance and decrease in the compression end temperature, as well as the temperature when the exhaust valve is open. Mahankali et al.  found that injecting water into the intake manifold also mitigated combustion knock, and they achieved an optimum combustion phasing based on the indicated fuel conversion efficiency with a water-to-fuel ratio of 0.6. Port water injection  could help engines operate under full load with a stoichiometric air–fuel ratio, and this resulted in up to a 5% improvement in net mean effective pressure and an improvement in thermal efficiency from 25 to 34% at 3000 rpm. Nicholls et al.  used inlet manifold water injection for control of nitrogen oxides, and experimental results showed that 10% water decreased nitrogen oxide emissions by 10–20%.
Moreover, premature ignition of the air–fuel mixture can cause severe damage to turbocharged engines influenced by high intake temperatures. To control combustion and avoid detonation damage, water was also directly injected into an engine cylinder to cool the intake charge [8, 9]. Kim et al.  considered that direct water injection showed the potential to advance spark timing and reduce brake-specific fuel consumption. Iwashiro et al.  studied direct water injection in a homogeneous charge compression ignition (HCCI) engine, and results showed that the indicated thermal efficiency was improved by about 2%. In addition, the operation range was expanded from 460 to 700 kPa, which maintained a low level of nitrogen oxide emissions. Roumeliotis et al.  determined that water injection could help control ignition timing and slow down the heat release rate in an HCCI engine. In contrast, hydrocarbon emissions increased by 20% when the water–fuel ratio was 1:1. Taghavifar et al.  studied the effects of direct water injection on a hydrogen–diesel duel fuel engine and showed that a 15% water injection ratio at 60 °C produced the best performance and that a 5% water injection ratio at 27 °C produced the lowest emissions. Recently, the condensed water injection concept  was proposed based on a stoichiometric combustion engine with a Miller cycle and cooled external exhaust gas recirculation. The results demonstrated a potential to increase efficiency from 3.3 to 3.8% in the region of minimum specific fuel consumption. The efficiency further improved by up to 16% at full-load operation.
For a deep understanding on the effect of water injection on combustion and emissions characteristics, it is necessary to investigate the injection and spray characteristics of water. Iwashiro et al.  found that distributing the water spray in the squish region was an efficient way to mitigate detonation. Emberson et al.  examined the spray cone angle and spray tip penetration of water–diesel emulsions containing 10% and 20% water (by mass). Differences in the spray cone angle and spray tip penetration demonstrated that the emulsification affected the injection process at a pressure of 50 MPa. Zhang et al.  studied the spray of water injection under high-temperature conditions using shadowgraphs, and results showed that for non-flash boiling sprays, liquid spray tip penetration had a positive correlation with the density of injection water, ambient density and injection pressure. Nishijima et al.  observed the water injection process in an optical engine and found that the water spray spread throughout the entire cylinder in only 1 ms under ambient temperature and pressure. Bedford et al.  simulated stratified direct water injection, which allowed the fuel–water percentage to be changed. They found that the liquid penetration increased approximately 35% when 23% of the fuel volume was replaced with water, due mostly to the increase in latent heat of vaporization. Van Vuuren et al.  photographed the water spray process of a urea injector with a high-speed camera and observed distinct differences when the temperature exceeded the atmospheric boiling point. Jeonghyun et al.  compared the spray characteristics of water and n-heptane in a port fuel injector. Their experimental results revealed that with increasing injection pressure, the injection quantity of water was about 21% higher than that of n-heptane. Bhagat et al.  observed a decrease in water spray tip penetration in a higher temperature air charge with a multi-hole gasoline direct injector.
However, few investigations have examined the characteristics of sprays utilized in ICRC engines, especially under top dead center (TDC) conditions. This study analyzed experimental points at different injected water and ambient temperatures in a vessel and validated a numerical model based on the experimental results for spray tip penetration. The simulation of the directed water spray process at TDC was evaluated. The results are intended to provide more details of basic water spray behavior in ICRC engines.
2 Experimental and Numerical Methods
2.1 Experimental Setup
Finally, a knife edge was placed between plane mirror 2 and the high-speed camera in the light path to record and visualize the density variations during the water spray process. In addition, a constant-volume vessel (CVV) with high ambient pressure and wide visualization windows was placed between the two concave mirrors. The CVV was a rectangular stainless-steel cube (300 mm × 300 mm × 268 mm) with a φ140-mm circular glass window in each lateral face. In addition, electrical heaters and a thermocouple were added inside the CVV to control the inner vessel temperature. A light beam transmitted through one window propagated through the CVV and exited through the opposite window.
Experimental temperature conditions
CVV temperature (°C)
Injected water temperature (°C)
2.2 Numerical Simulation
A spray is a complex three-dimensional physical event. The numerical simulation of water injection was carried out in the AVL-FIRE software for further investigation. Several equations were selected, including those for momentum, continuity, species transport, and k-epsilon turbulence. In this simulation, the KH-RT spray breakup model was used because it is suitable for the simulation of a high-pressure solid cone spray. The fluid was set as compressible, and the other parameters were set at their default values.
3 Results and Discussion
3.1 Experimental Results and Discussion
To characterize the spray process, schlieren images of high-pressure direct water injection were analyzed by calculating time-dependent spray tip penetration, spray cone angle, and mean spray speed. Results under 30 MPa injection pressure are presented here at the condition of 1 atm ambient pressure. The experiments with CVV temperatures under 100 °C were considered as low-temperature tests, and the high-temperature tests were those above 100 °C.
Viewed in another way, evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into a gaseous phase before reaching its boiling point . When the molecules of the liquid collide, they transfer energy to each other based on how they collide. When a molecule near the surface absorbs enough energy to overcome the vapor pressure, it will escape and enter the surrounding air as a gas . The internal energy of water molecules increases because of the increase in injected water temperature, and the heat conduction between the ambient air and the liquid water molecules increases when the CVV temperature increases. These two phenomena both lead to sufficient energy in the water molecules. Thus, they overcome the surface tension and viscous force, separate from the liquid surface, and enter the surrounding air as a gas . As a result, the spray cone angle decreases when the evaporation rate increases. However, the evaporation process is also influenced by the interface area between ambient air and liquid water. In one respect, the water parcels only evaporate layer by layer through the interface. In another respect, the heat conduction between ambient air and liquid water molecules is also limited by the interface area. The process of spray and evaporation reaches a dynamic equilibrium, so there is little change in the spray cone angle when both the injected water temperature and CVV temperature are high enough.
3.2 Numerical Results and Discussion
To further extend the boundary conditions, a numerical model was established and calibrated in AVL-FIRE, which was able to provide extensive data for the analysis of spray and evaporation processes.
First, to numerically reproduce the experimental water injection process, the spray tip penetration was validated at 20 °C and 160 °C ambient conditions. Second, to study the influence of injected water temperature on spray and evaporation processes, we adopted the injected water temperatures as 30 °C, 50 °C, 70 °C, 90 °C, 110 °C, 130 °C, and 160 °C, a CVV temperature as 20 °C, and an injection pressure as 30 MPa. Third, to study the influence of CVV temperature on spray and evaporation processes, we also adopted the CVV temperatures as 30 °C, 50 °C, 70 °C, 90 °C, 110 °C, 130 °C, and 160 °C, an injected water temperature as 20 °C, and an injection pressure as 30 MPa. Finally, the influence of TDC conditions on spray and evaporation processes was evaluated.
3.2.1 Influence of Injected Water Temperature on Spray Characteristics
It can be seen in Fig. 10a that the increase in injected water temperature led to decreasing spray tip penetration length. Specifically, 3 ms after the start of injection, the spray tip penetration at 160 °C injected water temperature was 31.52 mm, shorter than that at 30 °C. This trend agreed with the experimental results. As shown in Fig. 10b, the mean Sauter diameter was calculated by the local droplet size at each time step. It dropped from 54.55 to 32.51 μm when the injected water temperature increased from 30 to 160 °C. As to the variation in penetration, this can be explained by the fact that higher injected water temperature causes higher internal energy in water molecules. This leads to the water molecules on the interface between spray liquid and ambient air overcoming the surface tension and viscous force, stretching, breaking up, and separating from the liquid surface. As a consequence, the atomization and vaporization speed up, producing a shorter spray tip penetration length and a smaller mean Sauter diameter as the injected water temperature increases. It is difficult to measure evaporated mass experimentally, but it is crucial for the dynamic equilibrium of direct water injection, so it is certainly worth investigating the evaporation process by numerical simulation. To better understand the evolution of the evaporation, the evaporation rate, which is evaporated mass differentiated by time, was calculated, and the results are shown in Fig. 10c. At the same time, the percentage of evaporated mass was also calculated, which is evaporated mass divided by liquid injection mass at each time step, and these results are shown in Fig. 10d. The evaporation rate increased with increasing injected water temperature when the injected water temperature was below 100 °C, but it remained almost stable when the injected water temperature was above 100 °C. Similarly, the percentage of evaporated mass increased dramatically from 1.8% at 30 °C to 8.2% at 90 °C with the increase in injected water temperature. However, there was an opposite trend when the injected water temperature was above 100 °C: A lot of water evaporated immediately at the exit of the nozzle, and it plateaued at 10% after 2 ms. As stated above, when the injected water temperature was below 100 °C, the internal energy increased as the injected water temperature increased, and the evaporation rate also increased the percentage of evaporated mass increased. However, when the injected water temperature was above 100 °C, the fluctuation in the evaporation rate was very small. This can be explained by the fact that when the injected water temperature is above saturated steam temperature in the ambient environment, vaporization occurs violently in the initial stage of the spray process and forms a lot of bubbles. The bubbles grow and break, generating significant energy that reduces the time of spray atomization effectively. With the development of the spray process, the evaporation rate is almost the same, and the percentage of evaporated mass drops slowly and eventually stabilizes.
3.2.2 Influence of Vessel Temperature on Spray Characteristics
3.2.3 Influence of Top Dead Center Conditions on Spray Characteristics
As explained above, the previous sets of simulations were performed only under initial conditions, which were identical to the experimental conditions. However, it was meaningful to analyze the spray process under conditions similar to those in operating engines. To predict spray characteristics in ICRC engines, additional simulations were carried out to mimic the environment of an engine under full load at 1500 rpm . To reproduce TDC conditions, we specified the in-cylinder pressure as 5 MPa, temperature as 2200 °C, and air density as 7.0424 kg/m3, which corresponds to the density of dry air at this pressure and temperature.
Both experimental and simulated results showed that an increase in injected water temperature leads to shorter spray tip penetration and smaller spray cone angle. In addition, the simulation results indicated that the evaporation rate increased dramatically, by 6.4%, when the injected water temperature increased from 30 to 110 °C, but it remained roughly unchanged, at around 10%, when the injected water temperature continuously increased above 110 °C.
The results indicated that the spray tip penetration length increases with increasing CVV temperature, but the spray cone angle decreases. The CVV temperature has slight influence on the evaporation rate and percentage of evaporated mass. The evaporated mass rate increased slightly, by 1.6%, as the CVV temperature increased from 30 to 160 °C.
According to the simulation results, it can be predicted that heat conduction between ambient air and liquid water molecules is a main factor in boosting the evaporation process under TDC conditions. When the internal energy of liquid water molecules increases, these water molecules evaporate immediately.
The authors gratefully acknowledge the financial support provided by National Natural Science Foundation of China (Nos. 91441125 and 51076118).
- 1.Hountalas, D., Mavropoulos, G., Zannis, T.: Comparative evaluation of EGR, intake water addition and fuel-water emulsion as NOx reduction techniques for heavy duty diesel engines. SAE International Congress & Exhibition, Society of Automotive Engineers, pp. 509–510 (2007)Google Scholar
- 2.Miyamoto, N., Ogawa, H., Wang, J., et al.: Significant NOx reductions with direct water injection into the sub-chamber of an IDI diesel engine. In: International Congress & Exposition (1995)Google Scholar
- 3.Bhagat, M., Cung, K., Johnson, J., et al.: Experimental and numerical study of water spray injection at engine-relevant conditions. In: SAE 2013 World Congress & Exhibition (2013)Google Scholar
- 4.Netzer, C., Franken, T., Seidel, L., et al.: Numerical analysis of the impact of water injection on combustion and thermodynamics in a gasoline engine using detailed chemistry. In: WCX World Congress Experience (2018)Google Scholar
- 5.Miganakallu, N., Naber, J.D., Rao, S., et al.: Experimental investigation of water injection technique in gasoline direct injection engine. In: ASME 2017 Internal Combustion Engine Division Fall Technical Conference, pp. V001T03A013 (2017)Google Scholar
- 7.Nicholls, J.E., Ei-Messiri, I.A., Newhali, H.K.: Inlet manifold water injection for control of nitrogen oxides: theory and experiment. SAE Trans. 78(9), 70 (1969)Google Scholar
- 10.Iwashiro, Y., Tsurushima, T., Asaumi, Y., et al.: Fuel consumption improvement and operation range expansion in HCCI with direct water injection. Trans. Soc. Automot. Eng. Jpn. 33, 103–107 (2002)Google Scholar
- 14.Osman, A.: Feasibility study of a novel combustion cycle involving oxygen and water, SAE International (2009)Google Scholar
- 19.Yu, X., Wu, Z., Wang, C., et al.: Study of the combustion and emission characteristics of a quasi ICRC engine under different engine loads, SAE International (2014)Google Scholar
- 21.Zhang, Z., Kang, Z., Fu, L., et al.: Experiment on spray characteristics of water injection in thermo-atmosphere. Trans. CSICE 35(5), 443–451 (2017)Google Scholar
- 22.Nishijima, Y., Asaumi, Y., Aoyagi, Y.: Impingement spray system with direct water injection for premixed lean diesel combustion control, SAE International (2002)Google Scholar
- 23.Bedford, F., Rutland, C., Dittrich, P., et al.: Effects of direct water injection on DI diesel engine combustion, SAE International (2000)Google Scholar
- 24.van Vuuren, N., Qin, J.: High speed video measurements with water of a planar laser illuminated heated tip urea injector spray, SAE International (2013)Google Scholar
- 25.Jeonghyun, P., Kyung-hwan, L., Suhan, P.: Comparison of injection and spray characteristics of water and n-heptane in a PFI injector. J. ILASS-Korea 2017, 113 (2017)Google Scholar
- 29.Kang, Z., Fu, L., Deng, J., et al.: Experimental study of knock control in an internal combustion rankine cycle engine. J. Tongji Univ. (Nat. Sci.) (7), 1030–1036 (2017)Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.