Soot Formation In Diesel Engines Research Articles

Solid particulate mass and number from ducted fuel injection in an optically accessible diesel engine in skip-fired operation

Ducted fuel injection (DFI) is a novel combustion strategy that has been shown to significantly attenuate soot formation in diesel engines. While previous studies have used optical diagnostics and optical filter smoke number methods to show that DFI reduces in-cylinder soot formation and engine-out soot emissions, respectively, this is the first study to measure solid particle number (PN) emissions in addition to particle mass (PM). Furthermore, this study quantitatively evaluates the use of transient particle instruments for measuring particles from skip-fired operation in an optical single cylinder research engine (SCRE). Engine-out PN was measured using an engine exhaust particle sizer following a catalytic stripper, and PM was measured using a photoacoustic analyzer. The study improves on earlier preliminary emissions studies by clearly showing that DFI reduces overall PM by 76%–79% and PN for particles larger than 23 nm by 77% relative to conventional diesel combustion at a 1200-rpm, 13.3-bar gross indicated mean effective pressure operating condition. The degree of engine-out PM reduction with DFI was similar across both particulate measurement instruments used in the work. Through the use of bimodal distribution fitting, DFI was also shown to reduce the geometric mean diameter of accumulation mode particles by 26%, similar to the effects of increased injection pressure in conventional diesel combustion systems. This work clearly shows the significant solid particulate matter reductions enabled by DFI while also demonstrating that engine-out PN can be accurately measured from an optical SCRE operating in a skip-fired mode. Based on these results, it is believed that DFI has the potential to enable fuel savings when implemented in multi-cylinder engines, both by lowering the required frequency of active diesel particulate filter regeneration, and by reducing the backpressure imposed by exhaust filtration systems.

Design of a Production Process for Poly(oxymethylene) Dimethyl Ethers from Dimethyl Ether and Trioxane

AbstractPoly(oxymethylene) dimethyl ethers (OME) are promising synthetic fuels. When compared to fossil diesel fuel, OME reduce the soot formation in diesel engines. OME can be produced from the C1 platform syngas via different routes. This work investigates an OME production process via dimethyl ether and trioxane. The process is simulated and optimized using process simulation with varying model depth. As no experimental data are available on the chemical equilibrium of the reaction of dimethyl ether and trioxane, chemical equilibrium constants are partly estimated from formation data of the reactants.

DESIGN AND CONTROL OF POLY(OXYMETHYLENE) DIMETHYL ETHERS PRODUCTION PROCESS DIRECTLY FROM FORMALDEHYDE AND METHANOL IN AQUEOUS SOLUTIONS

Poly(oxymethylene) dimethyl ethers (OME) are widely used in the reduction of soot formation in diesel engines, and also as physical solvents for absorbing carbon dioxide from natural gas. In this work, a new production process of OME is designed and rigorously studied. In the conventional process, multiple process sections are required to first separately produce trioxane and methylal, and then to react of these two intermediates to produce OME. In this paper, the plant-wide process via a much simply direct synthesis of OME from formaldehyde and methanol in aqueous solutions is newly developed. The overall process includes an upstream section and a downstream product purification section. After the steady-state simulation of the process is established, the process flowsheet is optimized by minimizing total annual cost (TAC). The dynamics and control of the proposed OME production process is also investigated in this study. A proper control strategy is developed for the optimized process to reject the process disturbances.

From methanol to the oxygenated diesel fuel poly(oxymethylene) dimethyl ether: An assessment of the production costs

Poly(oxymethylene) dimethyl ethers (OME) are diesel fuel components, which can be produced from methanol, a global future platform chemical. OME reduce the soot formation in diesel engines and their addition to low quality refinery fractions increases the total refinery output. In this work, a benchmark process chain for OME production from methanol is described, which is based on the concepts of Burger et al. (2010). The OME are synthesized from trioxane and methylal, which are produced in intermediate process steps from methanol via the formaldehyde route. In each process step, the process chain uses state-of-the-art reaction- and distillation technology and is therefore scalable to large capacities. The economy of the process chain is assessed by analyzing the influence of the price of methanol and the investment costs, for which crude estimates are given, on the production costs of the OME. It is shown that OME production is competitive with conventional diesel fuel production.

Effect of biodiesel saturation on soot formation in diesel engines

To understand soot formation in a diesel engine fueled with different biodiesels, a numerical study was performed using the KIVA-3V code, combined with a multi-step phenomenological soot model. The simulations were used to predict differences in soot formation for three various biodiesel feedstock types. Good agreements on soot emissions were achieved in comparisons of engine experiments and simulations at various engine operating conditions. The experimental data and simulated results showed that the degree of saturation and the oxygen content of biodiesel fuels are the major factors responsible for biodiesel soot production. The reduction of soot mass concentration for biodiesel is achieved due to the suppressed soot formation process and improved oxidation rate compared with diesel. It is observed that the acetylene generated in the pyrolysis of biodiesel is proportional to the content of unsaturated fatty acid methyl ester (the number of CC double bonds). Among the three different biodiesel fuels, the lowest soot tendency was found for the Jatropha Methyl Ester because of its lowest amount of unsaturated alkyl esters through both numerical modeling and diesel engine experiments.

Is Carbon Black a Suitable Model Colloidal Substrate for Diesel Soot?

Soot formation in diesel engines is known to cause premature engine wear. Unfortunately, genuine diesel soot is expensive to generate, so carbon blacks are often used as diesel soot mimics. Herein, the suitability of a commercial carbon black (Regal 250R) as a surrogate for diesel soot dispersed in engine base oil is examined in the presence of two commonly used polymeric lubricant additives. The particle size, morphology, and surface composition of both substrates are assessed using BET surface area analysis, TEM, and XPS. The extent of adsorption of a poly(ethylene-co-propylene) (dOCP) statistical copolymer or a polystyrene-block-poly(ethylene-co-propylene) (PS-PEP) diblock copolymer onto carbon black or diesel soot from n-dodecane is compared indirectly using a supernatant depletion assay technique via UV spectroscopy. Thermogravimetric analysis is also used to directly determine the extent of copolymer adsorption. Degrees of dispersion are examined using optical microscopy, TEM, and analytical centrifugation. SAXS studies reveal some structural differences between carbon black and diesel soot particles. The mean radius of gyration determined for the latter is significantly smaller than that calculated for the former, and in the absence of any copolymer, diesel soot suspended in n-dodecane forms relatively loose mass fractals compared to carbon black. SAXS provides evidence for copolymer adsorption and indicates that addition of either copolymer transforms the initially compact agglomerates into relatively loose aggregates. Addition of dOCP or PS-PEP does not significantly affect the structure of the carbon black primary particles, with similar results being observed for diesel soot. In favorable cases, remarkably similar data can be obtained for carbon black and diesel soot when using dOCP and PS-PEP as copolymer dispersants. However, it is not difficult to identify simple copolymer-particle-solvent combinations for which substantial differences can be observed. Such observations are most likely the result of dissimilar surface chemistries, which can profoundly affect the colloidal stability.

Effects of mixture inhomogeneity and combustion temperature on soot surface activity and soot formation in diesel engines

The soot surface growth plays significant role on the soot mass accumulation, which starts with H (hydrogen) atom abstraction forming activated soot surface sites, and is followed by the acetylene addition process. In this study, the effect of the mixture inhomogeneity and combustion temperature on the soot surface activity and soot formation was investigated by developing a new multi-step phenomenological (MSP) soot model of diesel engines. A new detailed soot surface growth mechanism was proposed by correlation analysis of combustion parameters with soot formation. The inhomogeneity coefficient of soot surface activity αCH and the specific rate of soot surface growth RCH were derived to highlight the effect of inhomogeneity of mixture and combustion temperature on soot formation. The predicted diesel engine-out soot agreed well with experimental findings in wide ranges of combustion conditions. In the case of lower engine load with single fuel injection and higher EGR (exhaust gas recirculation) rate, it had quiet homogeneous mixtures before ignition when the combustion temperature dominated the soot surface activity. At medium engine load with multi-pulse fuel injections, it got mixture slightly stratified before ignition and revealed that the mixture inhomogeneity became more dominated on soot surface activity than the combustion temperature. An increased soot surface activity led to increased soot emission. Under the full engine loads with single fuel injection but quite high boost pressure over 0.4 MPa, it led to the combustion conditions of higher mixture density and higher mixture heat capacity, which benefits the mixture homogeneity. The decay rate of soot surface activity became lower due to the decreased combustion temperature and the soot surface activity decreased due to improved mixture homogeneity. In addition, the lowered intake oxygen concentration due to usage of EGR played a role to lower the specific rate of soot surface growth RCH, but to increase the soot surface activity αCH.

An investigation of mixture formation and in-cylinder combustion processes in direct injection diesel engines using group-hole nozzles

Mixture properties of the transient fuel spray injected by group-hole nozzles were quantitatively studied in a constant volume chamber via the laser absorption scattering (LAS) technique, and compared with conventional single-hole nozzles. Specific areas investigated included non-evaporating and evaporating ambient conditions, and the conditions of free spray and spray impinging on a flat wall. The particular emphasis was on the effect of one of the key parameters of the group-hole nozzle structure, namely, the interval between orifices. Subsequently, in-cylinder ignition and combustion processes were investigated by direct flame visualization in a single-cylinder optical research engine using the group-hole nozzle and the standard multi-hole nozzle, together with heat release analysis. Results show that the group-hole nozzle can produce a smaller Sauter mean diameter (SMD) of droplets under the non-evaporating condition. Under the evaporating condition, for the free spray cases, there exists a trade-off between the fuel evaporation and the spray penetration in the application of the group-hole nozzle. With the increase in interval between orifices, the ratio of evaporation increases, but the spray tip penetration decreases. For the spray impinging on a flat wall, the group-hole nozzle can enhance the spray tip penetrations to some extent compared with the single-hole nozzles. Flame images taken in a direct injection optical engine show that the enhancement of the fuel atomization and evaporation by the group-hole nozzle may contribute to a decrease in overall flame luminosity level, suggesting reduced soot formation in diesel engines.

Structural group analysis for soot reduction tendency of oxygenated fuels

Oxygenated additives are known to reduce soot formation in diesel engines. Numerous studies, both experimental and numerical, have reported that the reduction of particulate emissions depends on the molecular structure of the additives. In this paper, a structural group contribution approach is proposed to interpret experimental observations on the effect of oxygenated additives on the sooting propensities of hydrocarbon fuels. The statistically based method makes it possible to distinguish between chemical effects caused by the presence of oxygenated groups in the fuel mixture and mere dilution of the original fuel by the additive. The analysis was carried out on several experimental databases encompassing both premixed and nonpremixed configurations that include a new extensive set of smoke point measurements for mixtures of a given fuel with several oxygenated molecules. The current approach unifies the conclusions on the relative efficiency of the various oxygenated functionalities such as alcohols, esters, ethers, and carbonyl groups and provides a potential explanation for the seemingly contradictory trends exhibited by some raw experimental data.