Reactivity controlled compression ignition (RCCI) was investigated on a light-duty optical engine under different fuel stratification degrees, using multiple laser diagnostic techniques. The engine was run at a speed of 1200 rpm and under a load of about 7 bar gross IMEP. To form different fuel stratification degrees, the direct-injection timings of n-heptane were changed, while the port-injection timings of iso-octane was kept constant. The fuel/air equivalence ratio and primary reference fuel (PRF) number were quantified by the fuel-tracer planar laser-induced fluorescence (PLIF) under non-combusting condition. The results indicated that with retarding n-heptane injection timing from −90° CA ATDC (RCCI-90 case) to −10°CA ATDC (RCCI-10 case), regions of higher fuel concentration and reactivity moved downstream to the edge of combustion chamber before high-temperature heat release (HTHR) phase. Time-resolved natural combustion luminosity imaging and single-shot OH PLIF imaging indicated that RCCI-10 case presented a staged combustion process that an auto-ignition first happened in the region of high reactivity around the combustion chamber and then another auto-ignition process took place in the region of low reactivity in the central part of the combustion chamber. The staged combustion feature involved in RCCI combustion could result in lower combustion pressure-rise rate. PLIF images of formaldehyde showed that formaldehyde first formed during low-temperature heat release (LTHR) phase in the regions where n-heptane resided. With retarding n-heptane injection timings, both formaldehyde and OH PLIF images presented more stratified distribution, and the consumption of formaldehyde and formation of OH processes got slower. OH PLIF images indicated that HTHR phase of RCCI could extend to the central part of combustion chamber. In the low-load LTC conceptual model proposed by Musculus et al. (2013), no HTHR happened and UHC formed in the central part of combustion chamber. This meant that RCCI could have less UHC emission than LTC in theory.

•Spray-wall impingement of gasoline PPC was studied by multiple optical diagnostics.•Fuel-trapping effect was verified when spray-wall impingement happened.•The combustion chamber was filled with formaldehyde when misfire happened.•Flame front propagation and sequential auto-ignition coexisted in gasoline PPC.•The flame expansion speed of PPC-60 case was much higher than that in SI/SACI.Spray-wall impingement caused by early fuel injection for gasoline partially premixed combustion (PPC) can lead to low combustion efficiency and a significant rise of UHC emissions. But the influence of spray-wall impingement on the in-cylinder combustion process is not well understood. In this study, multiple optical diagnostics were applied to investigate the ignition, flame development and UHC formation of gasoline PPC with early single fuel-injection in a light-duty optical engine under low engine load. Natural combustion luminosity images and emission spectra were obtained. Planar laser-induced fluorescence (PLIF) of the fuel-tracer and formaldehyde were used to explore the fuel/air mixing and UHC formation in PPC, respectively. The results indicated that there was a fuel-injection time window (about −30° to −60° ATDC in the present study), during which the spray-impingement led to a decrease in combustion efficiency. The fuel-trapping effect in the squish region and piston crevice was shown to be the main reason because it prevented the fuel/air mixture from entering the combustion chamber. Two typical fuel injection timings of −35° (PPC-35) and −60° (PPC-60) were chosen for further study. For both cases, ignition sites first emerged in the fuel-rich regions and then the flames developed to the fuel-lean regions. The formaldehyde PLIF images revealed distinct flame front in the flame development process. For the PPC-35 case, residual formaldehyde persisted in the fuel-lean regions late during the power stroke and might become a source of UHC emissions. When misfire happened, the combustion chamber was filled with formaldehyde. For the PPC-60 case, the flame development was composed of initial flame front propagation and following sequential auto-ignition, and the flame expansion speed of the initial flame front propagation was much higher than that in SI (spark ignition) or SACI (spark assisted compression ignition) combustion. When the injection timing was further advanced (earlier than −60°), the impact of spray-wall impingement on PPC was reduced because of more time being available for fuel premixing.

The four alcoholic isomers of butanol were added into T20 diesel surrogate (80% n-heptane and 20% toluene in volume) in co-flow partially premixed flames at volumetric fractions of 20% and 40% in order to investigate the effect of butanol addition on polycyclic aromatic hydrocarbons (PAHs) and soot formation. For excluding the influence of toluene, the flame of T16 (84% n-heptane and 16% toluene in volume) and T12 (88% n-heptane and 12% toluene in volume) were also investigated to compare 20% and 40% butanol blend-flames in the same content of toluene. Laser-induced fluorescence and laser-induced incandescence were used to probe the distributions and concentrations of PAHs and soot volume fraction. A detailed n-heptane-toluene-butanols-PAH kinetic model was constructed in order to clarify the chemical effects of the different butanol-blended fuels on PAH formation. The results show that the reduced toluene content (due to butanol addition) is the dominant factor for PAH and soot reduction, compared with the base fuel of T20. The different PAH formation for the tested butanol-blended fuels is attributed to the different reaction pathways of butanol isomers. The branched-carbon-chain butanols (tertiary butanol and isobutanol) tend to produce more propargyl radicals than those of straight-carbon-chain butanols (normal butanol and secondary butanol), thus the PAH formation is higher for the branched butanols. The soot volume fractions with tertiary butanol and isobutanol addition are even higher than that without an additive in the fuel, while the addition of normal butanol and second butanol can reduce soot volume fractions further caused by its oxygenated molecular structure compared with T16 and T12. The soot formation tendency can actually be sequenced by tertiary butanol>isobutanol>secondary butanol>normal butanol at both 20% and 40% blending ratios. The concentration of four-ring PAHs is proportional to soot volume fraction, which means that four-ring PAHs can be used as the indicator of soot formation.

Gasoline partially premixed combustion (PPC) is a potential strategy to achieve high engine efficiency, as well as low NOx and soot emissions. But the in-cylinder combustion process of PPC is not well understood. In this paper, multiple optical diagnostics are applied to investigate the PPC ignition and flame development in a light-duty optical engine under single-injection condition. For the injection timing of −25 CA after top dead center (ATDC), the results indicate that the combustion process of gasoline PPC can be basically divided into four stages: 1) multiple auto-ignition kernels emerging in fuel-rich regions; 2) flame front propagation of the ignition kernels towards fuel-lean regions; 3) auto-ignition in the end-gas of fuel-lean regions; 4) a “burnout” stage in the whole combustion chamber after the main heat release process ends. The natural flame emission spectra from these four stages in PPC are analyzed. Distinct flame front propagation is verified during the early stages of the flame development process by both formaldehyde and OH planar laser-induced fluorescence (PLIF) imaging. The wide spread and late persistence of OH radicals after the main heat release process may account for the low soot emissions of gasoline PPC. The flame expansion speeds, determined by monitoring the flame fronts extracted from the combustion images, are much higher than that in SI (spark ignition) or SACI (spark-assisted compression ignition) combustion. With earlier fuel injection timing of −90 CA ATDC, the flame propagation process is less pronounced, and the sequential auto-ignition process prevails. Variation of the fuel stratification degree caused by the different fuel injection timings is responsible for this transformation in the flame development pattern for gasoline PPC.

•Systematized solubility of higher alcohols in 90%ethanol/diesel blends was achieved.•Low temperature and high water-containing break phase stable of ethanol/diesel.•Alcohols with higher carbon numbers provide a better inter-soluble capacity.•Straight chain structure shows better hydrotropy than cyclic and branched alcohols.•Hydroxy group shows a better hydrotropy than ketone group.The purpose of this study was to evaluate effects of different alcohols on the solubility of the blends between diesel and hydrous ethanol (with 10 vol.% water) at various temperatures of 5, 15, and 30 °C. Among these alcohols, n-butanol, n-hexanol, n-octanol and n-dodecanol were selected for investigating the influence of alcohol chain length on solubility, while the effect of straight and branched chain on solubility was studied via comparing with four isomers of butanol (n-butanol, 2-butanol, iso-butanol and tert-butanol). Additionally, effects of various functional groups including both hydroxy and ketone group were studied by using cyclohexanol and cyclohexanone. The blend on hydrous ethanol (10 vol.% water) and diesel fuel was acted as an analyte with different diesel blending ratios varied from 0 to 100 vol.% in 10 vol.% increments. Then the given alcohol (titrant) was gradually added into the centrifuge tube by a high-precision pipette until phase boundaries of ternary system appeared. Results show that alcohols with higher carbon numbers provide a better inter-soluble capacity, but a higher carbon-number alcohol such as n-dodecanol correlates with a lower pour point which leads to the gelling for the blends. Based on the performance of 6-carbon alcohols on phase stability, it is clearly that the straight chain structure and hydroxy group shows a better hydrotropy than cyclic structure and ketone group, respectively. For butanol isomers, straight chain butanols have better soluble capacity than that of branched structures. Taking these factors into account, n-hexanol and n-octanol can be recommended as a co-solvent additive for hydrous ethanol/diesel system due to the acceptable fuel properties and soluble performance.

Biodiesel is a type of particularly attractive alternative fuel for diesel engines. Many studies have investigated the combustion and emissions as fueling biodiesel on diesel engines and constant volume chambers. However, the understanding of the processes of biodiesel soot formation/oxidation is still limited. Therefore, in this work, high time-resolved quantitative soot measurements were carried out on a constant volume chamber by fueling soybean biodiesel. Three different ambient oxygen concentrations (21%, 16%, 10.5%) were tested at a conventional ambient temperature (1000 K) of diesel engine combustion and a lower ambient temperature (800 K). Results showed that the soot appearance was delayed at lower ambient temperatures and oxygen concentrations. At 800 K, less soot mass was observed with decreasing in oxygen concentration. However, soot mass increased with decreasing oxygen concentration as the ambient temperature reaching to 1000 K. To further illuminate the opposite trend on soot behavior in different temperature flames, a semiempirical biodiesel soot model was proposed and implemented into computational fluid dynamics (KIVA-3V, Release 2) code. Validation results showed that the proposed biodiesel soot model could successfully reproduce the entire process of soot formation and oxidation under various oxygen concentrations and ambient temperatures. With decreasing temperature, the appearance of intermediate species about soot formation/oxidation was delayed and the time-integrated mass of C2H2, soot precursors, OH radicals, and soot was reduced. The soot formation mechanism dominated soot evolution and caused a lower soot mass as the ambient temperature decreased. The formation of soot precursors presented a stronger temperature dependence than biodiesel pyrolysis. Regardless of whether the initial ambient temperature was 800 K or 1000 K, soot oxidation was significantly suppressed as the ambient oxygen concentration was reduced. However, the temperature did change the evolutionary tendency of soot formation with decreasing ambient oxygen concentrations. At 800 K, the time-integrated mass of acetylene and soot precursors and the regions of high equivalence ratios were reduced as the ambient oxygen concentration decreased; therefore, the soot formation was inhibited effectively at lower oxygen concentrations. At 1000 K, the time-integrated mass of acetylene and soot precursors and the regions of high equivalence ratios increased with the decrease of ambient oxygen concentration; therefore, the soot formation was motivated at lower oxygen concentrations. It can be concluded that soot formation transition was the responsible factor for the nonconsistent soot behavior, because of ambient oxygen dilution in conventional and low-temperature flames.

To investigate the effects of oxygenated biofuels on soot formation, oxidation, and distribution, a detailed comparative study using the forward illumination light extinction method was conducted in an optical constant volume combustion chamber. Various ambient temperatures (800 and 1000 K) and ambient oxygen concentrations (21, 16, and 10.5%) were investigated to mimic both conventional diesel combustion and low-temperature combustion conditions. Five oxygenated biofuels were used, including neat soybean biodiesel S100, neat butanol B100, and three alcohol–biodiesel blends that contained by volume 20% ethanol E20S80, 20% butanol B20S80, and 50% butanol B50S50. It is found that the composition of the biofuel has a larger effect on soot suppression efficiency at 800 K ambient temperature than that at 1000 K. Soot distribution is observed at larger distances from the injector, and less soot is located near the wall region with the additional oxygen content of biofuels at 21% ambient oxygen concentration. With a declining oxygen concentration, the soot concentration reduces at 800 K but increases at 1000 K. Soot formed in the spray jet region decreases at lower oxygen concentrations, and soot appears mainly near the wall region. Further, the soot distribution is more dispersed over a wider region at lower oxygen concentrations. B100 has shorter ignition delays at 10.5% oxygen concentration than B50S50 and S100 fuels, despite the fact that it has a lower cetane number. Therefore, the conventional correlation between ignition delay and cetane number does not hold for neat butanol at low oxygen concentrations. Soot concentrations are dramatically increased for soybean biodiesel from 800 to 1000 K at 10.5% oxygen, while such increases are not found for B50S50 and B100 fuels, indicating that proper choosing of the fuel will be very important to the high efficiency and clean low-temperature combustion.