2.5 Conclusion
An experimental study on the Diesel spray characteristics was performed with an advanced direct injection system (common rail injection system). High speed photography was employed to record the transient high injection pressure Diesel sprays in a rapid compression machine (Proteus). For the study of the spray, the images taken with a high speed video camera were processed and analysed. The results include the analysis of the time delay, the initial spray hesitation, the spray characteristics at the end of the injection and the spray tip penetration of different types of nozzle.
The effects of the phase Doppler anemometry setting parameters were studied and tailored to allow the measurements of dense Diesel sprays. The effects of injection and cylinder pressures and cylinder temperature on the droplet behaviour were using phase Doppler anemometry.
This investigation found the following:
A study of the PDA system parameters (laser power, photomultiplier voltage and measurement volume size) is required to tailor the system to the measurement of dense sprays.
The photomultiplier voltage, the laser power and the size of measurement volume all show significant effects on measured droplet diameters.
The droplet velocity and diameter profiles were shown to be dependent upon the position in the spray (radially and axially), the injection and incylinder pressures.
An increase in mean droplet velocity was seen when the injection pressure or axial distance from the nozzle was increased.
Increasing the cylinder pressure reduced the droplet velocities.
Increasing the injection and cylinder pressures improved the spray atomisation.
3. Analysis of mixture formation and flame development of diesel combustion
Mixture formation plays as a key element on burning process that strongly affects the exhaust emissions such as nitrogen oxide (NOx) and Particulate Matter (PM). The reductions of emissions can be achieved with improvement throughout the mixing of fuel and air behavior. Measurements were made in an optically-accessible rapid compression machine (RCM) with intended to simulate the actual diesel combustion related phenomena. The diesel combustion was simulated with the RCM which is equipped with the Denso single-shot common-rail fuel injection system, capable of a maximum injection pressure up to 160 [MPa]. Diesel engine compression process could be reproduced within the wide range of ambient temperature, ambient density, swirl velocity, equivalence ratio and fuel injection pressure. The mixture formation and combustion images were captured by the high speed camera. Analysis of combustion characteristics and observations of optical visualization of images reveal that the mixture formation exhibit influences to the ignition process and flame development. Therefore, the examination of the first stage of mixture formation is very important consideration due to the fuel-air premixing process linked with the combustion characteristics. Furthermore, the observation of a systematic control of mixture formation with experimental apparatus enables us to achieve considerable improvements of combustion process and would present the information for fundamental understanding in terms of reduced fuel consumption and exhaust emissions.
In diesel engines, combustion progresses by nature heterogeneous. Diesel spray spontaneous ignites within short period after fuel injection. The diesel engine has undergone continues improvements through the development of engines technologies especially in controlling the combustion process in order to reduce the NOx and PM levels and also to tackle the fuel economy vehicle. The most important issue in diesel combustion is achieving sufficient rapid mixing between the injected fuel and the air in cylinder prior to ignition. In this research, the new combustion concept based on the characteristics of diesel ignition and combustion is investigated focusing on fuel-air mixing with changing ambient condition.
The oxidation reactions at the end of endothermic period depend on the physical process such as air entrainment, the breakup of the jet spray, and droplets evaporation.
This study investigated diesel combustion fundamentally using a rapid compression machine (RCM). A constant volume chamber with displacement of 1701.4 cm3 was used to simulate actual phenomenon inside the combustion chamber with changing design parameter such as ambient condition, air motion, injection strategies and variable nozzle concept. Experimental parameters were ambient temperature and oxygen concentration, ambient density, swirl velocity, injection pressure, pilot injection and injection nozzle specifications such as nozzle hole-diameter and number of holes. Along with these parameters, a better comprehension of combustible mixtured, auto-ignition and combustion process is also needed for the optimization of diesel engines. In consequence, experiment used a rapid compression machine together with the schlieren photography and direct photography methods. The detail behavior of mixture formation during ignition delay period was investigated using the schlieren photography system with a high speed digital video camera. This method can capture spray evaporation, spray interference and mixture formation clearly with real images. Ignition process and flame development were investigated by direct photography method using a light sensitive high-speed color digital video camera. The sensitive camera could clearly capture spray ignition with extremely dark flame and observed flame development with the mixture of dark and bright flames. In this research, the observation and advanced monitoring of mixture formation and combustion flames plays an important role in-depth understanding of the fuel-air premixing, combustion process and exhaust emissions as shown in Fig. 1. The graphic shows that the flow parametric study can be used to investigate the relation between mixture formation and burning process that can give valuable information to improve and optimize the combustion process. Finally, the in-cylinder pressure data are analyzed to obtain the apparent heat release rate. The images of mixture formation and flame development are analyzed together with the heat release and expected to provide very accurate information prior to the combustion process and exhaust emissions.
Figure 3.1: Influence of the mixture formation on evolution of combustion process