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Although low-temperature combustion (LTC) strategies for compression-ignition engines can achieve very low emissions of nitrogen oxides (NOx) and particulate matter (PM) at high efficiency, they typically have increased emissions of other pollutants, including unburned hydrocarbons (UHC). In the current study, the equivalence ratio of mixtures near the injector are quantified under non-combusting conditions by planar laser-Rayleigh scattering (PLRS) in a constant-volume combustion chamber and by planar laser-induced fluorescence (PLIF) of a fuel tracer (toluene) in a single-cylinder direct-injection heavy-duty diesel engine at typical LTC conditions. The optical diagnostic images show that the transient ramp-down at the end of fuel injection produces a low-momentum, fuel-lean mixture in the upstream region of the jet, which persists late in the cycle.

This paper explores soot and soot-precursor formation characteristics of oxygenated fuels using experiments and numerical simulations under direct-injection diesel engine conditions. The paper strives to achieve four goals: 1)to introduce the “oxygen ratio” for accurate quantification of reactant-mixture stoichiometry for both oxygenated and non-oxygenated fuels; 2) to provide experimental results demonstrating that some oxygenates are more effective at reducing diesel soot than others; 3) to present results of numerical simulations showing that detailed chemical-kinetic models without complex fluid mechanics can capture some of the observed trends in the sooting tendencies of different oxygenated fuels; and 4) to provide further insight into the underlying mechanisms by which oxygenate structure and in-cylinder processes can affect soot formation in DI diesel engines. The oxygenates that were studied are di-butyl maleate (DBM) and tri-propylene glycol methyl ether (TPGME).

Gas ingested into the sac of a fuel injector after the injector needle valve closes is known to have crucial impacts on initial spray formation and plume growth in a following injection cycle. Yet little research has been attempted to understand the fate sac gases during pressure expansion and compression typical of an engine. This study investigated cavitation and bubble processes in the sac including the effect of chamber pressure decrease and increase consistent with an engine cycle. A single axial-hole transparent nozzle based on the Engine Combustion Network (ECN) Spray D nozzle geometry was mounted in a vessel filled with nitrogen, and the nitrogen gas pressure was cycled after the end of injection. Interior nozzle phenomena were visualized by high-speed longdistance microscopy with a nanosecond pulsed LED back-illumination. Experimental results showed that the volume of gas in the sac after the needle closes depends upon the vessel gas pressure.

This paper presents experimental results for two fuel-related topics in a diesel engine: (1) how fuel volatility affects the premixed burn and heat release rate, and (2) how ignition quality influences the soot formation. Fast evaporation of fuel may lead to more intense heat release if a higher percentage of the fuel is mixed with air to form a combustible mixture. However, if the evaporation of fuel is driven by mixing with high-temperature gases from the ambient, a high-volatility fuel will require less oxygen entrainment and mixing for complete vaporization and, consequently, may not have potential for significant heat release simply because it has vaporized. Fuel cetane number changes also cause uncertainty regarding soot formation because variable ignition delay will change levels of fuel-air mixing prior to combustion.

Methods for detection of the spatial position and timing of diesel ignition with improved accuracy are demonstrated in an optically accessible constant-volume chamber at engine-like pressure and temperature conditions. High-speed pressure measurement using multiple transducers, followed by triangulation correction for the speed of the pressure wave, permits identification of the autoignition spatial location and timing. Simultaneously, high-speed Schlieren and broadband chemiluminescence imaging provides validation of the pressure-based triangulation technique. The combined optical imaging and corrected pressure measurement techniques offer improved understanding of diesel ignition phenomenon. Schlieren imaging shows the onset of low-temperature (first-stage) heat release prior to high-temperature (second-stage) ignition. High-temperature ignition is marked by more rapid pressure rise and broadband chemiluminescence.

A full understanding and characterization of the near-field of diesel sprays is daunting because the dense spray region inhibits most diagnostics. While x-ray diagnostics permit quantification of fuel mass along a line of sight, most laboratories necessarily use simple lighting to characterize the spray spreading angle, using it as an input for CFD modeling, for example. Questions arise as to what is meant by the “boundary” of the spray since liquid fuel concentration is not easily quantified in optical imaging. In this study we seek to establish a relationship between spray boundary obtained via optical diffused backlighting and the fuel concentration derived from tomographic reconstruction of x-ray radiography. Measurements are repeated in different facilities at the same specified operating conditions on the “Spray A” fuel injector of the Engine Combustion Network, which has a nozzle diameter of 90 μm.

Diesel spray experimentation at controlled high-temperature and high-pressure conditions is intended to provide a more fundamental understanding of diesel combustion than can be achieved in engine experiments. This level of understanding is needed to develop the high-fidelity multi-scale CFD models that will be used to optimize future engine designs. Several spray chamber facilities capable of high-temperature, high-pressure conditions typical of engine combustion have been developed, but because of the uniqueness of each facility, there are uncertainties about their operation. For this paper, we describe results from comparative studies using constant-volume vessels at Sandia National Laboratories and IFP.

The major challenge of the post-processing of soot aggregates in transmission electron microscope (TEM) images is the detection of soot primary particles that have no clear boundaries, vary in size within the fractal aggregates, and often overlap with each other. In this study, we propose an automated detection code for primary particles implementing the Canny Edge Detection (CED) and Circular Hough Transform (CHT) on pre-processed TEM images for particle edge enhancement using unsharp filtering as well as image inversion and self-subtraction. The particle detection code is tested for soot TEM images obtained at various ambient and injection conditions, and from five different combustion facilities including three constant-volume combustion chambers and two diesel engines.

Multi-hole diesel fuel injectors have shown significant transients in spreading angle during injections, different than past fundamental research using single-hole injectors. We investigated the effect of a this transient spreading angle on combustion parameters such as ignition delay and lift-off length by comparing a three-hole nozzle (Spray B) and single-hole nozzle (Spray A) with holes of the same size and shape as targets for the Engine Combustion Network (ECN). With the temperature distribution for a target plume of Spray B characterized extensively in a constant-volume combustion chamber, the ignition delay and lift-off length were measured and compared. Results show that the lift-off length of Spray B increases and grows by approximately 1.5 mm after the initial stages of ignition, in an opposite trend compared to Spray A where the lift-off length decreases with time.