Gemischbildung von Diesel-Mischkraftstoffen bei variabler Düsengeometrie und Einspritzstrategie

  • Mixture formation of diesel fuel blends with variable nozzle geometry and variable injection strategy

Cárdenas, Maria; Kneer, Reinhold (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2014)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2014

Abstract

In addition to the continuous development and improvement of the conventional diesel combustion process, the so-called homogeneous charge compression ignition (HCCI) combustion is a promising combustion concept that intents to simultaneously reduce NOx and soot emissions. In this process a homogeneous mixture of air and fuel is burnt, in a way that the ignition starts at the same time throughout the combustion chamber. The main challenges of the HCCI combustion at high load are the control of the ignition timing and combustion and the homogenization of air, fuel and recirculated exhaust gases before the ignition begin. The use of specific fuel compositions could help to overcome these challenges. The resistance to auto-ignition of fuels with low cetane number can increase ignition delay and therefore provide sufficient time for air and fuel to mix homogenously. Also, the increase of volatility can improve the air-fuel mixture rate due to quick evaporation. Moreover, choosing a suitable nozzle geometry or a matching injection strategy are potential approaches to enhance the control and expand the operation window of HCCI combustion. The objective of this thesis is to experimentally study the influence on the mixture formation of fuel blends with low cetane number and high volatility with variable injection strategy and variable nozzle geometry in a high pressure chamber. Therefore the injection pressure and mass flow rate, macroscopic spray characteristics and quantitative parameters like droplet size and velocity are measured. The investigated fuel blends consist of a mixture of ethanol and diesel fuel and a mixture of gasoline and diesel fuel. The multiple injection strategy implemented here is a double-injection. This injection strategy simulates the interaction between a pilot injection and a main injection, a split main injection and finally a main injection and a post injection. The delay between injections will be varied for all kinds of double-injection. To investigate the influence of variable nozzle geometry on the mixture formation, an asymmetric arrangement of the injection holes was chosen in a so-called cluster configuration. Two injection holes are closely grouped with variable divergent angles between them. Using the diesel fuel blends the mixture formation was improved by varying the nozzle geometry as well as the injection strategy, which was indicated by a reduced number of droplets. Employing these strategies leads to homogenization of the air- fuel ratio of the HCCI combustion process. For varying injection strategies the measured differences between single and double injections are stronger dependent on phase shifts of shock waves generated in the injection system than on the interaction between two consecutive injections. For varying nozzle geometries, a nozzle with a 10° diverging angle between sprays provides the finest atomization of all investigated nozzles and generates an adequate spatial distribution of the fuel. This is achieved by a shorter penetration and therefore increased spray width. This can lead to reduced hydrocarbon emissions in HCCI combustion. Finally, it can be summarized that the measures considered in this work are promising to extend the operation range of the HCCI combustion process.

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