The spray behavior of an injector nozzle is not only determined by steady-state flow conditions but is also strongly influenced by transient processes during nozzle opening and closing. These short-duration phases play a critical role in determining injection accuracy and spray stability, especially in modern engines with multiple injection events.
During the opening phase, the needle lifts rapidly from the nozzle seat, allowing fuel to enter the spray holes. At this moment, pressure and flow rate change sharply. The initial fuel jets are often unstable due to incomplete needle lift and non-uniform pressure distribution inside the nozzle. This can lead to spray fragmentation and variations in jet direction.
As the needle reaches its full lift, flow conditions stabilize and the spray becomes more uniform. However, the duration of this stable phase may be very short during small injection events. Therefore, transient effects dominate the overall spray behavior in short pulses such as pilot or post injections.
During the closing phase, the needle returns to its seat under hydraulic and spring forces. As the flow area decreases, fuel velocity through the spray holes drops rapidly. This sudden reduction in flow may cause spray collapse or formation of irregular droplets. In some cases, delayed needle closure can result in fuel dribbling, which contributes to wall wetting and increased emissions.
Transient spray behavior is strongly affected by nozzle design parameters such as needle shape, seat angle, and spray hole layout. Manufacturing tolerances and wear also influence opening and closing dynamics, leading to variation in transient spray performance among injectors.
Operating conditions such as injection pressure and fuel temperature further affect transient behavior. Higher pressure intensifies flow acceleration, while lower fuel viscosity changes flow response during rapid transitions.
In summary, transient spray behavior during nozzle opening and closing has a significant impact on injection quality. Understanding and optimizing these transient processes is essential for achieving precise fuel delivery and stable combustion performance.
