Experimental study of the relationship between injection rate shape and Diesel ignition using a novel piezo-actuated direct-acting injector

Abstract

Injection rate shaping is one of the most attractive alternatives to multiple injection strategies; however, its implementation has been for long time impeded by limitations in the injector technology and therefore, the experimental information available in the literature about this topic is lacking.In this work, a novel prototype common-rail injector featuring direct control of the nozzle needle by means of a piezo-stack (direct-acting) allowed a fully flexible control on the nozzle needle movement and enabled the implementation of alternative injection rate shapes typologies. This state of the art injector, fitted with a 7-hole nozzle, was tested at real engine conditions studying the spatial-temporal evolution of CH* and OH* chemiluminescence intensity produced by the fuel combustion. A wide test matrix was performed in an optically accessible hot-spray test rig to understand the influence that partial needle lift and alternative injection rate shapes have on the Diesel ignitionThe results showed that alternative injection rate profiles have a substantial impact on the ignition event affecting the premixed phase of the combustion and the location where the ignition takes place. Moreover, the results proved that the modifications in the internal flow caused by the partial needle lift are reflected on the ignition timing: although partial needle lift and injection pressure have similar effects on the mass flow rate, in the first case the ignition delay is reduced, while in the second the combustion is delayed as a consequence of a different spray development. IntroductionFuel-air mixing process, combustion and emission phenomena are necessarily linked together in direct injection Diesel engines [1][2][3][4][5]. In a scenario where the global emission standards require higher engine performances in terms of combustion efficiency and emissions reduction, all the efforts by the engine community to improve the understanding of the fuel atomization, spray development and combustion are largely justified.Over the past decades, many studies have been carried out to develop a better understanding of the mixing process [6][7][8] and of the fuel ignition [9][10][11][12]. Experimental facilities, such as optically accessible engines [13] and test rig cells [11,14] combined with imaging techniques [5,15], have become the most common tools used in spray research. Throughout the years, the efforts put in place in these studies generated an accurate and deep understanding of the injection combustion event in Diesel engines.On the other hand, several activities have been performed to advance the flexibility of the fuel injection system, achieving significant improvements [2]. Most of these systems are operated with electro-hydraulic actuation, where the fuel injector is activated using either a solenoid or a piezo-stack; however, the opening of the injector itself is produced by the pressure difference at the two sides of the needle limiting the injection control to an on-off mode. As a consequence of that, multiple in...

Diesel ignition delay and lift-off length through different methodologies using a multi-hole injector

“…Experimental results by multiple authors [27,29,[31][32][33][34] have shown the e ect of oxygen concentration, ambient temperature, density and injection pressure on ignition delay. As theoretically explained by Petrucci et al [35], increasing oxygen concentration and ambient temperature accelerate oxidation reactions, resulting in a reduction in ignition delay.…”

Analysis of combustion concepts in a newly designed two-stroke high-speed direct injection compression ignition engine

“…Many strategies for further improving the mixing process and air utilization during mixing-controlled combustion are still under development, such as ultra-high injection pressure systems 7 combined with micro-hole injector nozzles, 8 multiple or pulsed injection strategies, 9 or new injection rate shaping strategies with novel direct acting piezo-actuated injectors that enables a faster and more precise control of the fuel flow through the injector nozzle compared to conventional electro-hydraulic actuated injectors. 10,11 Aside from the efforts to optimize the CDC concept, a relatively new approach known as Partially Premixed Combustion (PPC) concept has been designed to operate CI engines in between fully premixed combustion and fully diffusive combustion concepts, where low emissions can still be attained while retaining control over the combustion timing with the injection event. The injection process is advanced towards the compression stroke to be detached from the combustion event, enabling partial mixing of the mixture to avoid over-rich regions where soot is formed, whereas NO X emissions are reduced by lowering combustion temperatures by the introduction of large amounts of EGR.…”

Study of new prototype pintle injectors for diesel engine application

“…When the injector opens and the needle moves up, the mass flow through the nozzle hole depends on the area of this hole, which is constant, and the pressure in upstream section of the flow, which is controlled by the throttling effect achieved in the needle seat region. Thus, controlling the velocity at which the needle opens, it is possible to control as well the fuel injection rate [21].…”

“…Benajes et al [20] assessed experimentally the impact of injection rate shaping by implementing a device to modulate the injection pressure along the engine cycle. Other methods are based on a direct control on the needle position along the injection event [21]. A low needle lift produces a throttling effect on the needle seat, inducing a pressure drop, which results in a reduced injection rate compared to maximum needle lift conditions.…”