**1. Introduction**

The environmental impact, in terms of global greenhouse effect and pollutant emissions, of the automotive sector is considerable. Hence, severe limitations are in force and designed for, in particular, the adoption of challenging CO2 emission targets for the next few years [1–3]. Consequently, the automotive industry is quickly adopting innovative technologies, globally aiming for a rapid increase in automotive powertrain efficiency. The leading method followed to improve powertrain efficiency is electrification, with the adoption of a progressively more significant energy storage capacity and electric propulsion system power. The final stage of this evolution is foreseen to be the battery electric vehicle, which ensures a zero CO2 local emission operation. In this scenario, the internal combustion engine will still play a significant role for many years [4] due to the complexities and costs related to the actual implementation of the electrification path. Accordingly, along with electrification, a significant evolution of the gasoline engine to improve its efficiency is mandatory. The widespread adoption of gasoline direct injection (GDI) in spray-guided or pre-chamber configurations, turbocharging coupled with downsizing and downspeeding, higher compression ratios and application of the Miller cycle seem to be the most interesting innovation lines [5,6]. Unfortunately, many of the aforementioned technologies cause a drastic increase in the knocking tendency due to the increased charge temperature before and during combustion, thus restraining the potential benefits in terms of engine efficiency. As a matter of fact, particularly for high-performance engines, the knock tendency in high load conditions is currently controlled by reducing the spark advance and by enriching

**Citation:** Postrioti, L.; Brizi, G.; Finori, G.M. Experimental Analysis of Water Pressure and Temperature Influence on Atomization and Evolution of a Port Water Injection Spray. *Appl. Sci.* **2021**, *11*, 5980. https://doi.org/10.3390/app11135980

Academic Editor: Cinzia Tornatore

Received: 31 May 2021 Accepted: 21 June 2021 Published: 27 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the fuel/air mixture, resulting in an efficiency penalty and in a restriction of the catalyst operating area with an increase of CO2, CO and HC emissions.

In this frame, water injection technology can be used as an alternative way to control the charge temperature at the end of the intake process and during the combustion phase, thus reducing the risk of abnormal combustion in downsized and highly boosted GDI engines. Water injection can potentially enable λ = 1 operation in high-load and highspeed operation, even adopting high compression ratios [7–17], thereby gaining significant benefits in terms of engine efficiency while preserving the after-treatment system efficacy in controlling exhaust emissions.

Water injection can be implemented as low-pressure injection in the intake runners (PWI), as high-pressure injection in the combustion chamber (DWI) or as direct injection of a water/gasoline emulsion. Along with the potential benefits in terms of air charge temperature control, clearly water injection has different potential drawbacks, such as a possible lubricant dilution due to the liquid's impact on the cylinder liner. Potentially catastrophic lubricant dilution can be caused by an incorrect match among the spray's global shape with the inlet duct geometry (for PWI systems) or the combustion chamber (for DWI systems) or, in general, by a slow water evaporation rate. Furthermore, the water deposition on the duct and cylinder walls limits the air charge cooling effect, increasing the water flow rate required to effectively control the knock tendency, resulting in waterto-fuel rate ratios in the range of 0.2–0.5 in full load conditions. Among the possible different schemes, port water injection (PWI) is currently considered the most attractive as a compromise between efficacy and cost, being significantly higher for direct water and water/gasoline emulsion injection systems, including the eventual water recovery technologies from the exhaust stream [18].

Since the air charge temperature control potential under water injection is related to the high latent heat of vaporization of the water, the injected water evaporation rate is a crucial factor to be considered in PWI systems' design. In order to promote water evaporation, an adequate match of the water spray characteristics with the intake system design is required, and hence, a detailed knowledge of the spray characteristics is mandatory. In particular, given the moderate air charge temperature (typically in the range of 40–60 ◦C for intercooled engines) and the short spray residence time in the inlet runner, the water spray atomization quality is crucial for obtaining complete evaporation.

Unfortunately, in the technical literature, there is a substantial lack of detailed experimental data about low-pressure water sprays' global evolution and atomization level to support PWI system design and CFD simulation. In [9], Iacobacci et al. investigated the potential of a PWI system based on PFI injectors supplied with water at 25 ◦C (Pinj = 4 bar,g), changing the water/fuel ratio to mitigate the knock tendency at full load. Cordier et al. [10] tested different water injection technologies on a single-cylinder research engine, obtaining significant efficiency improvements. For the tested PWI configuration, Pinj was varied in the range of 5–20 bar,g, but no details about the resulting spray characteristics were reported. In [12], Paltrinieri et al. experimentally investigated the application of a PWI system operated at Pinj = 7 bar,g and T w = 55 ◦C for a single-cylinder research engine with water-to-fuel ratios up to 60%. Different injector designs and positions along the inlet duct were used to explore the actuation timing effect. In this analysis, CFD simulations of the water spray evolution in the intake duct were carried out to investigate the spray–air interaction, but the effect of the spray characteristics with different operating conditions was not investigated. In other numerical analyses of water injection systems, the water spray characteristics, or even its evaporation rate, are assumed to be constant or similar to fuel sprays generated at the same injection pressure level [13,15,16].

In the present paper, a detailed experimental analysis of a low-pressure water spray is presented, discussing the effect of both the injection pressure and water temperature in the rail on the jet evolution and size characteristics. According to the current approach of the automotive industry for the definition of a PWI system's architecture, both these parameters were varied in ranges compliant with standard PFI technology to reduce the complexity and cost of key components such as injectors, rails and sensors. The injection pressure was varied from 5 bar,g to 11 bar,g, covering a pressure range explored by other Authors for PFI injectors [19]. Correspondingly, the water temperature in the rail was changed from 20 ◦C to 110 ◦C, approaching flash boiling conditions in order to promote the spray break-up and drop evaporation [20,21]. The analysis was carried out by a phase doppler anemometry (PDA) system and by a fast-shutter imaging apparatus in order to investigate both the drops' size quality and global spray characteristics obtained in a range of operating conditions. In the following sections, the experimental set-up and the test plan will be presented first, and the obtained results will be discussed.

#### **2. Materials and Methods**

The PWI injector (Bosch EV14) used for the present analysis was characterized in terms of the mean injected mass, global spray evolution, drop size and velocity. The main characteristics of the tested injector are reported in Table 1. The static flow rate was measured by the dINJ injection analyzer, a Zeuch method-type injection analyzer specifically designed to operate with low-pressure injection systems [8,22].

**Table 1.** PWI injector characteristics.


The injector under testing was fed with distilled water statically pressurized with nitrogen in the range from 5 bar,g to 11 bar,g (up to 15 bar,g only for the flow tests). Pressurized water was accumulated in a 100-cc reservoir, which was used as rail to directly feed the injector to have negligible pressure fluctuations during the injection event, according to the rules of JSAE2715. The rail structure was used as a fixture for an electric heater used to control water temperature. The feedback thermocouple was installed at the injector inlet in the same position as the Keller PAA M5 HB sensor (20 bar f.s., 50 kHz bandwidth, 1% accuracy) used to monitor the rail pressure. The injector current time history was acquired by a Pico TA189 probe (30 A, 100 kHz bandwidth) and averaged over 30 consecutive injection events. A schematic of the experimental setup is reported in Figure 1.

In each operating condition, the mean injected volume was measured by a precision balance (Radwag PS 1000/C/2, resolution of 1 mg, accuracy ±1.5 mg) during three repetitions of a 3000-shot sequence with a 10 Hz injection frequency.

In previous studies (e.g., [8,23]), the effect of the test vessel's pressure and temperature on low-pressure water sprays was investigated, concluding that the air temperature's variation from ambient to 50–60 ◦C (typical of boosted conditions with an intercooler) has negligible effects on the spray's evolution and size. On the other hand, boosted pressure levels have the similar effect of a corresponding injection pressure reduction, with the pressure differential across the injector being the main driving force affecting the spray's evolution. As a consequence, in this research, the test vessel's pressure and temperature were maintained at 1 bar,a and 25 ◦C for all the operating conditions.

In Table 2, the operating conditions used for the flow test are reported, evidencing the imposed water temperature at the injector's inlet.

The global spray evolution was investigated by a fast-shutter imaging technique, applied according to an ensemble averaging approach. The imaging apparatus was based on a pulsed Nd-Yag laser (Litron Nano L 200–20, 200 mJ/shot, shot duration 6 ns), which was used as a light source. The laser was synchronized with a fast-shutter, high-resolution CMOS camera (Dalsa Genie Nano M4020, resolution 3008 × 4112, 12-bit). According to the ensemble averaging approach, only one image per injection event was acquired at a given delay from the injection event's start (the TTL signal enabling the injector driver was used as a trigger). The statistical analysis of the spray's global development at an assigned

delay was carried out by repeating the image acquisition over a series of consecutive injection events (30 in the present work). The repetition of the aforementioned acquisition sequence at different delays from the injection start allowed for characterization of the complete spray development throughout the entire injection event. The acquired images at the different timings were analyzed off-line by means of a proprietary digital analysis procedure developed in the LabVIEW™ environment, obtaining the spray tip penetration curves and global cone angle according to the JSAE2715 prescriptions. More details about the image analysis procedure are reported in [24,25].

**Figure 1.** Schematic of the experimental setup for imaging (**a**) and PDA (**b**).

**Table 2.** Flow test plan showing the water temperature in ◦C at the injector's inlet.


The test plan used for the global spray analysis by imaging was based on a fullfactorial analysis of injection pressure Pinj levels of 5, 7 and 11 bar,g with water temperature levels of 20, 55, 90 and 110 ◦C. All spray imaging tests were carried out with an ET of 5 ms in order to ensure adequate steady flow operation for the injector and evidence the effects of both the water temperature and the injection pressure.

The effect of the water temperature and injection pressure on the drop size was evaluated using a phase doppler anemometer (Dantec Dynamics P80) along a measuring traverse composed of 18 stations. The traverse was positioned at Z = 50 mm downstream of the nozzle plate and aligned with the projection of two of the four nozzle holes on the examined plane to evidence the global spray symmetry. In Table 3, the main specifications for the PDA system used for the tests are reported.


**Table 3.** PDA system specifications.

The drop size and velocity characteristics were investigated in the operating conditions reported in Table 4. In all PDA tests, an ET of 5 ms was applied, acquiring data in a 30-ms time window after the start of the ET to capture the entire spray evolution, including its complete tail. Prescriptions from JSAE2715 were followed during the tests in terms of spray boundary detection and minimum number of samples per measuring position. The data were collected during 3000 consecutive shots, operating the injector at 8 Hz in each examined station.


**Table 4.** PDA test plan, with the water temperature measured at the injector inlet.
