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Close-coupled aftertreatment systems (ATS) for automotive Diesel engines composed of DOC and SCR offer a significant potential in terms of pollutant emission control capability even with the introduction of more aggressive driving cycles and rigorous limits for type-approval tests. This is particularly important for incoming certification standards where the forecast is showing a trade-off towards ultra-low NOx emissions values. As the SCR system NOx conversion capability largely relies on both the UWS mixing device and on NOx sensors used to detect the actual NH3 slip and residual NOx concentration, developing numerical simulation tools for the analysis of the actual flow pattern and species concentration over peculiar sections of the exhaust system is crucial to support the ATS development process.

A comprehensive experimental and numerical analysis of two state-of-the-art diesel AfterTreatment Systems (ATS) for automotive applications is presented in this work. Both systems, designed to fulfill Euro 6 emissions regulations standards, consist of a closed-coupled Diesel Oxidation Catalyst (DOC) followed by a Selective Catalytic Reduction (SCR) catalyst coated on a Diesel Particulate Filter (DPF), also known as SCR on Filter (SCRoF or SCRF). While the two systems feature the same Urea Water Solution (UWS) injector, major differences could be observed in the UWS mixing device, which is placed upstream of the SCRoF, whose design represents a crucial challenge due to the severe flow uniformity and compact packaging requirements.

In order to optimize gasoline direct injection combustion systems, a very accurate control of the fuel flow rate from the injector must be attained, along with appropriate spray characteristics in terms of drop sizing and jets global penetration/diffusion in the combustion chamber. Injection rate measurement is therefore one of the crucial tasks to be accomplished in order both to develop direct injection systems and to properly match them with a given combustion system. Noticeably, the hydraulic characteristics of GDI injectors should be determined according to a non-intrusive measuring approach. Unfortunately, the operation of all conventional injection analyzers requires the injection in a volume filled with liquid and the application of a significant counter-pressure downstream of the injector. This feature prevents any operation with low pressure injection systems such as PFIs.

In the present paper an innovative approach for the shot-to-shot hydraulic characterization of low pressure injection systems is experimentally assessed. The proposed methodology is an inverse application of the Zeuch’s method, which in this case is applied to a closed volume upstream the injector instead of downstream of it as in conventional injection analyzers. By this approach, the well-known constraint of having a finite volume pressurized with the injected liquid downstream the injector is circumvented. As a consequence, with the proposed instrument low pressure injectors - such as PFI, fed with gasoline or water, SCR injectors - can operate with the prescribed upstream-downstream pressure differential. Further, the injector can spray directly in atmosphere or in any ambient at arbitrary pressure and temperature conditions, allowing the simultaneous application of other diagnostics such as imaging, momentum flux measurement or sizing instruments.