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Soot Formation of Different Diesel-Fuels Investigated by Chemical Luminescence and Laser Induced Incandescence
Abstract
Differences in thermo-physical parameters of fuels have high impact on the ignition, combustion and emission. Pure rapeseed FAME and diesel fuel with a cetane number of 60 have been compared to reference fuel. In an optical accessible vessel the fuels have been injected in order to investigate the spray, the ignition and soot formation. The high cetane number fuel showed similar behavior in spray phase to the reference fuel but the FAME fuel is more present at all operating points due to low volatile fuel components. The ignition and combustion process was investigated via chemical luminescence (CL) and laser induced incandescence (LII). In engine investigations a reduced ignition delay is detected in case of high cetane-number. The more sensitive optical techniques show differences in the combustion process. The ignition behavior of the reference fuel and the increased cetane number fuel were similar until the cetane increaser of the high cetane fuel came into effect. The FAME showed a shortened ignition delay but was found to be slower in combustion propagation. For the LII investigation a Nd:YAG pulsed laser with a wavelength of 532 nm was used. The light sheet excited the soot particles up to temperatures where the black body radiation is shifted towards the UV-spectra. The soot formation of the reference fuel is equal to the soot formation of the high cetane number fuel until the cetane increaser affected the flame and increased soot production. The soot production of the FAME is advanced in time but overall significantly lower.
... 30,31 The technique has been applied for the understanding of soot fundamentals in flames, [32][33][34] as well as in combustion vessels. 35,36 LII has also been used to study combustion in engines operated under conditions pertinent to the LTC and PPC modes. Crank angle-resolved LII measurements have been utilized to study the effect of in-cylinder soot oxidation via the recording of soot particle sizes in an optical high-speed diesel engine operated under relative early injection timings. ...
This work employs a combination of pressure trace analysis, high-speed optical measurements and laser-based techniques for the assessment of the effects of various post-injection schemes on the soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine was operated under a multiple injection scheme of two pilot and one main injection, typical of a partially premixed combustion mode, at the lower end of the load and engine speed range (ca 2.0 bar IMEP at 1200 r/min). Experiments considering the influence of the post-injection fuel amount (up to 15% of the total fuel quantity per cycle) and the post-injection timing within the expansion stroke (5, 10 and 15 CAD aTDC), under a constant total fuel mass per cycle, have been conducted. Findings were analysed via means of pressure trace and apparent rate of heat transfer analyses, as well as a series of optical diagnostic techniques, namely, high-speed flame natural luminosity imaging, CH*, and OH* line-of-sight chemiluminescence, as well as planar laser-induced incandescence measurements at 31 and 50 CAD aTDC. The combination of post-injection fuel amount and timing has substantial effects on charge reactivity and soot oxidation potential. The analysis reveals that an amount of fuel (7% of the total fuel mass per cycle) injected more than 10 CAD after the main combustion event leads to higher levels of soot emissions, while a larger amount of fuel (15% of the total fuel mass) injected 5 CAD after the main combustion event appears to have a beneficial effect on the soot oxidation processes. Overall, results indicate that a post-injection scheme close to the main combustion phasing could reduce soot levels and improve engine performance, that is, higher IMEP levels at the same fuel consumption rates, although it could increase engine noise.
A way to reduce net CO2 emission and circumvent the high particle emissions from compression ignition (CI) engines, while retaining high efficiency, is by utilizing lighter alcohols in the partially premixed combustion (PPC) process. Methanol and ethanol have a very short carbon chain, and form less soot/particulate matter (PM) during combustion compared to diesel fuel. This study analyzes and compares the characteristics of PM emissions from methanol, ethanol and diesel in terms of soot mass concentration, number concentration and particle size distribution in one cylinder of a six cylinder Scania D13 heavy duty (HD) engine under two operating loads; 6 bar and 10 bar gross mean indicated effective pressure (IMEPG). An electrostatic precipitator (ESP) was used to sample PM on copper grids for transmission electron microscopy (TEM) and energy dispersive X-ray analysis. Also, new and used lubrication oil together with methanol and diesel were analyzed for their sulphur and metal content.
Nucleation mode and the majority of accumulation mode particles from methanol and ethanol combustion, showed mainly Ca, S, P and Zn in the energy dispersive X-ray spectrometry (EDX) analysis and were hypothesized to be emitted mainly from the lubrication oil rather than the combustion of the fuel itself. From diesel combustion, the accumulation mode particles were more abundant in comparison with the alcohols and PM/soot emissions measured with the photoacoustic technique were 3 to 10 times higher than for the alcohols. There were also fewer nucleation mode particles present; although they consisted of the same four abovementioned elements. Utilizing alcohols in CI engines have a great advantage regarding PM, particle number emissions and efficiency. However, the resulting nucleation mode particles need to be reduced to avoid future health concerns.
In the present study, the effects of the neat biodiesel (rapeseed methyl ester, RME) fueled diesel engine with the use of EGR on the particle size distribution were examined. The combustion of REM significantly improves the engine smoke and total particle mass but increases both NOx and particle concentration with low aerodynamic diameters (<0.091 μm) when compared to the diesel (ultralow sulfur diesel, ULSD) fueled engine. Although the particle size and mass distribution were not affected significantly by the different EGR additions, the particle total number and mass were increased considerably for both fuels. For the RME fueled engine, the EGR addition reduces the particles in the lowest aerodynamic diameter measured (0.046 μm). The use of EGR better suits the RME combustion, as apart from resulting in the higher NOx reduction, it maintained the smoke (soot, particulate matter) at relatively low levels. The results are also confirming that it is challenging to reduce simultaneously total particle mass without increasing the number of particles at low aerodynamic diameters. Furthermore, the lower RME calorific value compared to ULSD results in increased fuel consumption, although the engine efficiency was not noticeably affected.
The effects of mineral diesel fuel, gas-to-liquid fuel, rapeseed methyl ester, neat soybean and neat rapeseed oil on injection, combustion, efficiency and pollutant emissions have been studied on a compression ignition heavy duty engine operated near full load and equipped with a combined exhaust gas aftertreatment system (oxidation catalyst, particle filter, selective catalytic NOx reduction). In a first step, the engine calibration was kept constant for all fuels which led to differences in engine torque for the different fuels. In a second step, the injection duration was modified so that all fuels led to the same engine torque. In a third step, the engine was recalibrated in order to keep the NOx emissions at an equal level for all fuels (injection pressure, injection timing, EGR rate). The experiments show that the critical NOx emissions were higher (even behind the exhaust gas aftertreatment systems) for oxygenated fuels in case of the engine not being recalibrated for the fuel. GTL and the oxygenated fuels show lower emissions for some pollutants and higher efficiency after recalibration to equal NOx levels.
Simultaneous laser-induced previous termincandescence (LII) and laser-induced scattering (LIS) were applied to investigate soot formation and distribution in a single cylinder rapid compression machine. The fuel used was a low sulfur reference diesel fuel with 0.04% volume 2-ethylhexyl nitrate. LII images were acquired at time intervals of 1 CA throughout the soot formation period, for a range of injection pressures up to 160 MPa, and in-cylinder pressures (ICP) up to 9 MPa. The data collected shows that although cycle-to-cycle variations in soot production were observed, the LII signal intensities converged to a constant value when sufficient cycles were averaged. The amount of soot produced was not significantly affected by changes in in-cylinder pressure. Soot was observed distributed in definite clusters, which were linked to slugs of fuel caused by oscillations in the injector needle. The highest injection pressure exhibited lower soot productions and more homogeneous soot distributions within the flame. Despite diffusion flames lasting longer with lower injection pressure, it appeared that the extended oxidation time was insufficient to oxidize the excess production of soot. In addition, soot particles were detected closer to the nozzle tip with higher injection pressures. The recording of LII sequences at high temporal resolutions has shown that three distinct phases in soot formation can be observed. First, high soot formation rates are observed before the establishment of the diffusion flame. Second, a reduced soot formation rate is apparent from the start of diffusion flame until the end of injection. Finally, high soot oxidation rates occur after the end of injection and for the duration of the flame.