## 2 Equipment Requirements  
Up to now there have not been many studies about the effect of the humidity in the propulsion systems of land transport, either due to lax normative in this regards or lack of proper testing facilities to carry them out. The device described in this paper has been designed to cover this gap in the literature; by developing a device that can generates and control dynamically high humidity levels even at altitude conditions with high temperature. When talking about water content in the air, we must highlight the great impact that psychometric variables pressure and temperature have over the water content in the air. The water content or specific humidity (g water/kg dry air) increases exponentially when the temperature goes up or when the pressure decreases while keeping constant the relative humidity value [30]. In Figure 1, the greenish blue area represents the specifications for the water content of the equipment. The specification has been designed given every combination of pressure, temperature, and relative humidity inside the operational range of the installation: 0.55 bar to 1.3 bar, 0 \({}^{\circ}\)C to 40 \({}^{\circ}\)C and 10% to 100%, respectively. Furthermore, the line of maximum specific humidity can be seen depicted in red, which represent 100% relative humidity at a pressure of 0.55 bar. Finally, the maximum humidity at sea level is represented in this figure by an orange line, which clearly shows how it is impossible to achieve the required humidity, in order to emulate high altitudes, just by humidifying at sea-level conditions. The most unfavorable specifications for the humidifying process, specifically 2.5 bar and 100% RH is represented by a black line in Figure 1. These operation conditions justify the selection of a bubble reactor as a humidifier instead of atomizers, which have a maximum working pressure too restrictive for this application as well as need an extensive length of piping to fully evaporate the water injected into the air stream. In addition, due to the humidity level of the most demanding point of the installation, depicted by a black cross in Figure 1 (0.55 bar, 40 \({}^{\circ}\)C and 80% RH), at the most unfavorable pressure conditions (2.5 bar) it is necessary to moisten this at a temperature of around 70 \({}^{\circ}\)C, as shown at Figure 1. The reason why 2.5 bar could be needed is to boost combustion air and to overcome pressure losses along the rest of the elements of the system in some operative conditions [16]. Figure 2 shows the location of the bubble reactor in the installation scheme (13) of Figure 2, which has been highlighted for easier identification, with the rest of the key elements responsible for carrying out the conditioning of the air. The airflow depicted in Figure 2 starts with a mechanical compressor (2) that is electrically driven, which sucks air form the ambient and increases the pressure of the installation. This mechanical compressor is protected from surge by means of a control valve (3). Then, a control valve (4) controls the temperature at point 'g' mixing cold air, cooled on the water cooler (5) and chiller (6), and hot air from the compressor outlet. Downstream, the valve (9) controls the humidity at point '1', forcing part of the airflow to pass through the bubble reactor (13). Then, the flow is expanded up to the desired pressure on control valve (15) and the variable geometry turbine (17). Additionally, control valve  
Figure 1: Psychometric conditions for the humectant process. Figure 2: Testing installation scheme. (16) regulates the air that passes through (17) to control the temperature at point '1', where the engine (21) sucks the amount of air needed and the rest flows through the dilution pipe. Lastly, the dilution flow mixes with the exhaust gases of the engine (22), gets cooled down on the exhaust cooler (23) and is sucked out of the installation with a mechanical compressor (24).