### Effects of Rail Pressure Control  
General studies describing the influence of rail pressure control on the development of spray in the combustion chamber should be also considered, in order to explain changes of different parameters observed in this investigation when managing this control variable. In Reference [11], spray tip penetration (i.e., the maximum distance of the injected spray from the injector nozzle tip measured on the axial direction) was evaluated when applying four values of injection pressure for two levels of cylinder pressure, using conventional diesel oil. Higher spray tip penetration was observed if injection pressure raised. Moreover, the higher momentum of small droplets resulted in the enlargement of the spray area. In reference [12], diesel spray was analysed in a constant volume combustion chamber, assessing that the increase of injection pressure resulted in a better atomisation of fuel droplets. As a consequence, evaporation and mixture formation were enhanced, leading to a shorter combustion duration. Large variations of injection pressure were compared in Reference [13], where a turbocharged engine (2.18 dm\({}^{3}\)) fuelled with a blend 80% diesel-20% Karanja biodiesel was tested. A Delphi fuel injection system was considered, with six-holes solenoid injectors. Increasing fuel injection pressure from 500 to 1000 bar enhanced the spray evolution, with significant lower times to reach the same penetration length, while a similar behaviour of fuel jets was observed when comparing 1000 and 1500 bar of injection pressure, even if a slightly faster evolution was apparent with the highest pressure level. The expected effects of these variations in spray development and mixture formation when applying increased levels of injection pressure to an engine are therefore a shorter ignition delay,a rise in peak pressure and maximum level of heat release and a reduced combustion duration. These results are widely reported in the literature, referring to studies on different engine types and fuels. Low displacement units fuelled with biofuels were tested in References [14; 15], while four cylinders naturally aspirated engine was considered in Reference [16]. On the other side, a single cylinder engine with a large displacement (8.8 dm\({}^{3}\)) was studied in Reference [17], applying a simple injection strategy based on one event while considering three different levels of in-cylinder gas density. A further effect of higher pressure levels for fuel injection is related to a stronger premixed combustion phase, proved by a faster rise of pressure [18] and heat release [19]. In these cases, alternative fuels and very different engine sizes were considered (natural gas in an 11.6 dm\({}^{3}\) displacement unit [18] and a B20 biodiesel blend for a naturally aspirated single cylinder engine of 0.51 dm\({}^{3}\)[19]). As a further consequence of the achievement of a better combustion process, energy and environmental parameters are also modified, with lower levels of fuel consumption and soot emissions and the increase of NO\({}_{\mathrm{X}}\) emissions. The first result is due to the shift of the combustion centre towards the TDC and to the shorter duration of combustion [14; 15; 16; 17; 18]. Soot reduction is related to the enhancement of the premixed phase of the combustion [11; 14; 15; 19]. The faster combustion process, especially in the first phase, leads to an increase of maximum pressure and temperature levels, justifying the higher NO\({}_{\mathrm{X}}\) generation [14; 15; 16; 17; 19]. As already stated, the goal of this study was to prove the capability of indicated pressure diagrams and related combustion parameters to enlarge the outcomes of measurements, extending the relevant knowledge on the influence of main engine subsystems control. The paper includes a Methodology Section, where the engine and its open control system are described, while control variables are defined. The acquisition system for pressure diagram measurements and processing procedure for RoHR calculation are presented.