## 1 Introduction
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 \({}^{\circ}\)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], [PERSON] et al. investigated the potential of a PWI system based on PFI injectors supplied with water at 25 \({}^{\circ}\)C (P\({}_{\rm inj}\) = 4 bar.g), changing the water/fuel ratio to mitigate the knock tendency at full load. [PERSON] et al. [10] tested different water injection technologies on a single-cylinder research engine, obtaining significant efficiency improvements. For the tested PWI configuration, P\({}_{\rm inj}\) was varied in the range of 5-20 bar,g, but no details about the resulting spray characteristics were reported. In [12], [PERSON] et al. experimentally investigated the application of a PWI system operated at P\({}_{\rm inj}\) = 7 bar,g and T\({}_{\rm W}\) = 55 \({}^{\circ}\)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 \({}^{\circ}\)C to 110 \({}^{\circ}\)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.