Effects of Injection Pressure, Injection-Rate Shape, and Heat Release on Liquid Length

The in-cylinder extent of liquid-phase fuel penetration (i.e., the liquid length) is an important parameter in combustion-chamber design because liquid lengths that are too long can lead to wall impingement and corresponding degradation of engine efficiency, emissions, and durability. Previous liquid-length measurements in constant-volume combustion chambers have shown that the liquid length is nominally independent of injection pressure, but these measurements have employed common-rail fuel systems where injection rate is approximately constant during the entire injection event, and they have been conducted under quasi-steady ambient thermodynamic conditions. The objective of the current work is to better understand the effects of injection-rate shape and injection pressure on the liquid length, including possible effects of unsteady ambient conditions in an engine. Liquid-length measurements under unsteady, non-reacting in-cylinder conditions have been reported recently, with fuel injected using a hydraulically actuated, electronically controlled unit injector (HEUI) with an unsteady injection rate. In the current work, liquid lengths using the HEUI are compared to those obtained using a common-rail injector, with the same optical setup and engine conditions, at two injection pressures. To ensure that the effects of injection rate and injection pressure are not unique to a specific fuel, measurements were made using both 2,2,4,4,6,8,8-heptamethylnonane and an ultra-low-sulfur #2 diesel fuel. To ensure minimal effects of chemical heat release, all data were acquired under essentially non-reacting conditions. Injection pressure and injection-rate shape were found to have essentially no effect on liquid length for both fuels, suggesting that results can be compared between experiments with different injection systems. In addition, the effect of heat release was evaluated using the common-rail injector by comparing liquid-length measurements obtained under non-reacting conditions to those obtained under partially reacting conditions. Results showed that combustion heat release shortens liquid length, and that the magnitude of the effect is consistent with entrainment of higher-temperature and higher-density gases due to global compressive heating.