Experimental and theoretical investigation of transient fluid dynamic phenomena in the vicinity of automotive fuel injectors

Understanding the mechanisms that lead to the atomisation and evaporation of liquid fuels is a key step towards the design of efficient and clean combustion systems. The vast majority of fuel spray research is focused on steady-state atomisation processes and based on the assumption that droplets are perfectly spherical, even though such sprays can contain a large proportion of ligaments and deformed droplets. The lack of information about transient atomisation and mixing processes (i.e. during start and end of injection) at the microscopic scale inhibits the development and validation of accurate engine simulation tools. For example, after the end of fuel injection, residual fuel present inside the injector's nozzle is discharged through inertia and capillarity. These uncontrolled fuel discharge events can present several problems. The excess fuel can undergo incomplete combustion due to its large, slow moving and often surface-bound nature. Not only does this have a negative effect on emissions and performance, but the by-products of incomplete combustion are thought to be implicated in the growth of carbonaceous deposits on the tips of fuel injectors. Accumulation of these deposits is known to lead to premature fuel injector failure that can lead to reductions in power output and engine lifetime. With modern multiple-injection strategies giving rise to an increased number of transient injection phases, post-injection discharges are increasingly problematic for modern engines