Soot Formation within Conventional and Low Temperature Diesel Combustion

The soot formation processes of conventional and low temperature diesel combustion were investigated by a newly developed Forward Illumination Light Extinction (FILE) soot measurement technique in a constant volume spray chamber. The FILE technique has the capability to obtain two-dimensional time-resolved quantitative soot measurements. For diesel combustion at conventional ambient condition, it was found that soot generation begins shortly after the beginning of premixed combustion and increases rapidly as the flame penetrate into the chamber. Soot oxidation becomes a dominant factor after the total soot amount reaches the peak at the end of injection. The total soot mass had a peak value about 1% of total fuel mass, which demonstrates that only a small fraction of hydrocarbon fragments are converted into soot. Combustion at 800K ambient temperature and 21% to 15% ambient oxygen concentration had much lower soot generation than conventional diesel combustion. Even lower ambient oxygen concentration or other ways to promote better near wall mixing could further lower the flame temperature to have sootless diesel combustion. Corresponding author Introduction Particulate matter emission from diesel engine has to be dramatically reduced in order to meet more stringent emission regulations. Understanding the incylinder soot generation and oxidation processes is very important to reduce the engine-out soot emission. Recently, low temperature diesel combustion or HCCI-like diesel combustion has attracted more attention due to its ability to simultaneously reduce NOx and soot emission [1, 2]. To understand how soot is formed at conventional diesel combustion environment and how soot mostly disappears at low temperature combustion region thus becomes great interest. Over the past decade, laser diagnostics has been utilized to investigate the in-cylinder diesel combustion. New insights and understandings of diesel combustion along with soot formation process have been achieved for the conventional diesel combustion and was proposed as a concept model [3, 4]. However, further understanding diesel combustion and soot formation is limited by the available soot measurement techniques. Laser Induced Incandescence (LII), for example, has difficulty to obtain quantitative measurement [5]. Single point light extinction has been applied due its quantitative soot measurement capability and its simplicity [6, 7]. However, it is tedious to obtain 2D soot distribution using 1D point measurement and optical setup of light extinction can not be simply expanded from 1D to 2D since only qualitative results can be obtained due to the Schlieren effect [8, 9, 10]. To obtain 2D quantitative capability, a new Forward Illumination Light Extinction (FILE) method was developed by the authors [11-14]. The new optic setup not only gets rid of the Schliren effect but only requires only one window to study confined combustion, not two in-line windows. The accuracy of FILE was verified by comparing the measurement of laminar ethylene diffusion flame with 1D light extinction measurement [11-14]. FILE has been used to demonstrate the ambient effects [12], fuel effects [13, 14] on soot formation in a constant volume spray chamber. In this paper, the combustion and soot formation with high ambient temperature, 1000K, is studied to illustrate how soot is generated and oxidized during the conventional diesel combustion process. In comparison, combustions and soot evolutions at low ambient temperature (800K) with different exhaust gas recirculation (EGR) levels (21%, 18% and 15% ambient oxygen concentration) are investigated to explore the potential of low temperature combustion. A constant-volume spray chamber was utilized to provide stable and controllable high-temperature and high-pressure environment for studying diesel combustion. The 800K ambient temperature corresponds to the early injection before top dead center in a diesel engine and early injection strategy is widely adopted for low temperature combustion. The FILE soot measurement and natural flame emission measurement were two combustion diagnostics methods adopted in this research. Constant-Volume Combustion Chamber Setup This study was conducted in a constant-volume spray chamber that is able to simulate the combustion condition inside a diesel engine. Simulation of the compression stroke was achieved by burning a lean premixed combustible mixture, which was also adopted in other constant-volume combustion chambers [6]. The chamber was designed for optimum optical access and fitted with a Caterpillar HEUI diesel injector. A schematic of the spray chamber is shown in Figure 1. More information is provided in the reference [13, 15]. The chamber design allows a maximum operating pressure of 18 MPa and a maximum chamber gas density up to 30 kg/m. The chamber has a bore of 110 mm and a height of 65 mm. The end window on the top of the chamber was used for current study and one of the six spray jets was examined. Figure 1. Constant-volume optical spray chamber. The injector is mounted in the head with chambers for hydraulic oil and fuel. The injector points upward and the spray plumes emit out of the six orifices with 20° ascension angles measured from the head surface. The chamber is heated with cartridge electric heaters to simulate engine-wall temperatures of 380K and to prevent water condensation on the windows. Gases were mixed and filled into the chamber to reach a density of 15 kg/m for the test. By controlling the mixture composition, the burned mixture can have different oxygen concentration to simulate EGR. A spark ignited the mixture and brought rapid increases in pressure and temperature inside the chamber. The spray injection began when the pressure of the cooling burned mixture reached the preset condition corresponding to the testing ambient temperature. The operating conditions for the tests are summarized in Table 1. The ultra low sulfur ECD diesel fuel was provided by BP. Combusiton Diagnostics Techniques The new Forward Illumination Light Extinction (FILE) method was developed and verified by the authors [11-14]. It was demonstrated that FILE technique has the ability to provide a 2D timeresolved quantitative soot measurement while using only one optical window. Unlike the back illumination light extinction method, where two parallel windows are required [9, 10], the FILE technique has a light diffuser placed behind the flame plume, so that the light source and camera can be located at the same side of the flame through one window, as shown in Figure 2. For the forward illumination technique the light passes through the soot cloud twice instead of once as shown in Figure 3. Additionally, the light is modified by the light diffuser besides the extinction due to soot appearance. However, it was verified that the change of reflected light intensity is only resulted from the presence of soot similar to Lambert-Beer’s Law [1114]: − = L ext dx K I I 2