Search Results

A combusting plume in an optically accessible direct-injection diesel engine was studied using simultaneous 2-D imaging of laser-induced incandescence (LII) and natural flame luminosity, as well as simultaneous 2-D imaging of LII and elastic scattering. Obtaining images simultaneously via two different techniques makes the effects of cycle-to-cycle variation identical for both images, permitting the details of the simultaneous images to be compared. Since each technique provides unique information about the combusting diesel plume, more can be learned from comparison of the simultaneous images than by any of the techniques alone. Among the insights gained from these measurements are that the combusting plume in this engine has a general pattern of high soot concentration towards the leading edge with a lower soot concentration extending upstream towards the injector. Also, the soot particles are found to be larger towards the leading edge of the plume than in the upstream region.

The combustion behavior of conventional gasoline has been numerically investigated by means of detailed chemical-kinetic modeling simulations, with particular emphasis on analyzing the chemistry of the intermediate temperature heat release (ITHR). Previous experimental work on highly boosted (up to 325 kPa absolute) HCCI combustion of gasoline (SAE 2020-01-1086) showed a steady increase in the charge temperature up to the point of hot ignition, even for conditions where the ignition point was retarded well after top dead center (TDC). Thus, sufficient energy was being released by early pre-ignition reactions resulting in temperature rise during the early part of the expansion stroke This behavior is associated with a slow pre-ignition heat release (ITHR), which is critical to keep the engine from misfiring at the very late combustion phasings required to prevent knock at high-load boosted conditions.

For lean or dilute, boosted gasoline compression-ignition engines operating in a low-temperature combustion mode, creating a partially stratified fuel charge mixture prior to auto-ignition can be beneficial for reducing the heat-release rate (HRR) and the corresponding maximum rate of pressure rise. As a result, partial fuel stratification (PFS) can be used to increase load and/or efficiency without knock (i.e. without excessive ringing). In this work, a double direct-injection (D-DI) strategy is investigated for which the majority of the fuel is injected early in the intake stroke to create a relatively well-mixed background mixture, and the remaining fuel is injected in the latter part of the compression stroke to produce greater fuel stratification prior auto-ignition. Experiments were performed in a 1-liter single-cylinder engine modified for low-temperature gasoline combustion (LTGC) research.

The objective of this investigation is to verify and characterize the influence of fuel volatility on maximum liquid-phase fuel penetration for a variety of actual Diesel fuels under realistic Diesel engine operating conditions. To do so, liquid-phase fuel penetration was measured for a total of eight Diesel fuels using laser elastic-scatter imaging. The experiments were carried out in an optically accessible Diesel engine of the “heavy-duty” size class at a representative medium speed (1200 rpm) operating condition. In addition to liquid-phase fuel penetration, ignition delay was assessed for each fuel based on pressure-derived apparent heat release rate and needle lift data. For all fuels examined, it was observed that initially the liquid fuel penetrates almost linearly with increasing crank angle until reaching a maximum characteristic length. Beyond this characteristic length, the fuel is entirely vapor phase and not just smaller fuel droplets.

This work explores the potential of partial fuel stratification to smooth HCCI heat-release rates at high load. A combination of engine experiments and multi-zone chemical-kinetics modeling was used for this. The term “partial” is introduced to emphasize that care is taken to supply fuel to all parts of the in-cylinder charge, which is essential for reaching high power output. It was found that partial fuel stratification offers good potential to achieve a staged combustion event with reduced pressure-rise rates. Therefore, partial fuel stratification has the potential to increase the high-load limits for HCCI/SCCI operation. However, for the technique to be effective the crank-angle phasing of the “hot” ignition has to be sensitive to the local ϕ. Sufficient sensitivity was observed only for fuel blends that exhibit low-temperature heat release (like diesel fuel).

Chemiluminescence imaging has been applied to investigate the naturally occurring charge stratification in an HCCI engine. This stratification slows the pressure-rise rate (PRR) during combustion, making it critical to the high-load operating limit of these engines. Experiments were conducted in a single-cylinder HCCI engine modified with windows in the combustion chamber for optical access. Using this engine, chemiluminescence images were obtained from three different view angles. These included both single-shot images with intensified CCD cameras and high-speed (20kHz) sequences with an intensified CMOS video camera. The engine was fueled with iso-octane, which has been shown to be a reasonable surrogate for gasoline and exhibits only single-stage ignition at these naturally aspirated conditions. The chemiluminescence images show that the HCCI combustion is not homogeneous but has a strong turbulent structure even when the fuel and air are fully premixed prior to intake.

Soot and fuel distributions have been studied in an optically accessible direct-injection diesel engine of the “heavy-duty” size class. Laser-induced incandescence (LII) was used to study the effects of changes in the engine speed on the in-cylinder soot distribution, and elastic (Mie) scattering and laser-induced fluorescence (LIF) were used to examine the fuel distribution. The investigation showed that, in this engine, soot is distributed throughout the cross section of the combusting region of the fuel jet for engine speeds ranging from 600 to 1800 rpm. No indication was found that soot occurs preferentially around the periphery of the plume. The LII images showed that the soot concentration decreases with increasing engine speed and injection pressure, and that the soot distribution extends much further upstream (toward the injector) at the lower engine speeds than at higher speeds.

A flamelet model is used to calculate combustion in a diesel engine, and the results are compared to experimental data available from an optically accessible, direct-injection diesel research engine. The 3∼D time-dependent Kiva-II code is used for the calculations, the standard Arrhenius combustion model being replaced by an ignition model and the coherent flame model for turbulent combustion. The ignition model is a four-step mechanism developed for heavy hydrocarbons which has been previously used for diesel combustion. The turbulent combustion model is a flamelet model developed from the basic ideas of Marble and Broadwell. This model considers local regions of the turbulent flame front as interfaces called flamelets which separate fuel and oxidizer in the case of a diffusion flame. These flamelets are accounted for by solving a transport equation for the flame surface density, i.e., the flame area per unit volume.

Two-dimensional laser-sheet imaging and high-speed cinematography have been used to examine the combustion process in a newly constructed, optically accessible, direct-injection Diesel engine of the “heavy-duty” size class. The design of this engine preserves the intake port geometry and basic dimensions of a Cummins N-series production engine. It also includes several unique features to provide considerable optical access. An extended piston with piston-crown window and a window in the cylinder head allow the processes in the combustion bowl and squish region to be observed simultaneously. Windows at the top of the cylinder wall provide orthogonal-optical access with the capability of allowing the laser sheet to enter the cylinder along the axis of the spray. Finally, this new engine incorporates a unique separating cylinder liner that permits rapid cleaning of the windows. Studies were performed at a medium speed (1200 rpm) using a Cummins closed-nozzle fuel injector.

The application of planar laser-induced Rayleigh scattering for quantitative 2-D measurements of vapor-phase fuel concentration in the main combustion zone of a direct-injection Diesel engine has been explored, developed and demonstrated. All studies were conducted in an optically accessible direct-injection Diesel engine of the “heavy-duty” size class at 1200 rpm and motored TDC conditions which were typical of the production version of this engine. First, this study verifies that beyond 27 mm from the injector all the fuel is vapor phase. This was done by investigating the Diesel jet under high magnification using 2-D elastic scatter imaging and subsequently evaluating the signal intensities from the droplets and other interfering particles (Mie scattering) and the vapor (Rayleigh scattering).

A combination of optical imaging diagnostics has been applied to the fuel jet of a direct-injection diesel engine to study the ignition and early soot formation processes. Measurements were made in an optically accessible direct-injection diesel engine of the “heavy-duty” size class at a representative medium speed (1200 rpm) operating condition. Two fuels were used, a 42.5 cetane number mixture of the diesel reference fuels and a new low-sooting fuel (needed to reduce optical attenuation at later crank angles) that closely matches both the cetane number and boiling point of the reference fuel mixture. The combustion and soot formation processes are found to be almost identical for both fuels. Ignition and early combustion were studied by imaging the natural chemiluminescence using a calibrated intensified video camera. The early soot development was investigated via luminosity imaging and simultaneous planar imaging of laser-induced incandescence (LII) and elastic scattering.

A parametric study of the liquid-phase fuel penetration of evaporating Diesel fuel jets has been conducted in a direct-injection Diesel engine using laser elastic-scatter imaging. The experiments were conducted in an optically accessible Diesel engine of the “heavy-duty” size class at a representative medium speed (1200 rpm) operating condition. The density and temperature at TDC were varied systematically by adjusting the intake temperature and pressure. At all operating conditions the measurements show that initially the liquid fuel penetrates almost linearly with increasing crank angle until reaching a maximum length. Then, the liquid-fuel penetration length remains fairly constant although fuel injection continues. At a TDC density of 16.6 kg/m3 and a temperature of about 1000 K the maximum penetration length is approximately 23 mm. However, it varies significantly as TDC conditions are changed, with the liquid-length being less at higher temperatures and at higher densities.

Laser-sheet imaging studies have considerably advanced our understanding of diesel combustion; however, the location on and nature of the flame zones within the combusting fuel jet have been largely unstudied. To address this issue, planar laser-induced fluorescence (PLIF) imaging of the OH radical has been applied to the reacting fuel jet of a direct-injection diesel engine of the “heavy-duty” size class, modified for optical access. An Nd:YAG-based laser system was used to pump the overlapping Q19 and Q28 lines of the (1,0) band of the A→X transition at 284.01 nm, while the fluorescent emission from both the (0,0) and (1,1) bands (308 to 320 nm) was imaged with an intensified video camera. This scheme allowed rejection of elastically scattered laser light, PAH fluorescence, and laser-induced incandescence. OH PLIF is shown to be an excellent diagnostic for diesel diffusion flames.

Combustion phasing is one important issue that must be addressed for HCCI operation. The intake temperature can be adjusted to achieve ignition at the desired crank angle. However, heat-transfer during induction will make the effective intake temperature different from the temperature measured in the runner. Also, depending on the engine speed and port configuration, dynamic flow effects cause various degrees of charge heating. Additionally, residuals from the previous cycle can have significant influence on the charge temperature at the beginning of the compression stroke. Finally, direct injection of fuel will influence the charge temperature since heat is needed for vaporization. This study investigates these effects in a systematic manner with a combination of experiment and cycle simulation using WAVE from Ricardo.

We have conducted a detailed numerical analysis of HCCI engine operation at low loads to investigate the sources of HC and CO emissions and the associated combustion inefficiencies. Engine performance and emissions are evaluated as fueling is reduced from typical HCCI conditions, with an equivalence ratio ϕ = 0.26 to very low loads (ϕ = 0.04). Calculations are conducted using a segregated multi-zone methodology and a detailed chemical kinetic mechanism for iso-octane with 859 chemical species. The computational results agree very well with recent experimental results. Pressure traces, heat release rates, burn duration, combustion efficiency and emissions of hydrocarbon, oxygenated hydrocarbon, and carbon monoxide are generally well predicted for the whole range of equivalence ratios. The computational model also shows where the pollutants originate within the combustion chamber, thereby explaining the changes in the HC and CO emissions as a function of equivalence ratio.

Gasoline consists of many different classes of hydrocarbons, such as paraffins, olefins, aromatics, and cycloalkanes. In this study, a surrogate gasoline reaction mechanism is developed, and it has one representative fuel constituent from each of these classes. These selected constituents are iso-octane, n-heptane, 1-pentene, toluene, and methyl-cyclohexane. The mechanism was developed in a step-wise fashion, adding submechanisms to treat each fuel component. Reactions important for low temperature oxidation (<1000K) and cross-reactions among different fuels are incorporated into the mechanism. The mechanism consists of 1328 species and 5835 reactions. A single-zone engine model is used to evaluate how well the mechanism captures autoignition behavior for conditions corresponding to homogeneous charge compression ignition (HCCI) engine operation.

Thermal stratification has the potential to reduce pressure-rise rates and allow increased power output for HCCI engines. This paper systematically examines how the amount of thermal stratification of the core of the charge has to be adjusted to avoid excessive knock as the engine speed and fueling rate are increased. This is accomplished by a combination of multi-zone chemical-kinetics modeling and engine experiments, using iso-octane as the fuel. The experiments show that, for a low-residual engine configuration, the pressure traces are self-similar during changes to the engine speed when CA50 is maintained by adjusting the intake temperature. Consequently, the absolute pressure-rise rate measured as bar/ms increases proportionally with the engine speed. As a result, the knocking (ringing) intensity increases drastically with engine speed, unless counteracted by some means.

Two methods for mitigating unacceptably high HCCI heat-release rates are investigated and compared in this combined experimental/CFD work. Retarding the combustion phasing by decreasing the intake temperature is found to have good potential for smoothing heat-release rates and reducing engine knock. There are at least three reasons for this: 1) lower combustion temperatures, 2) less pressure rise when the combustion is occurring during the expansion stroke, and 3) the natural thermal stratification increases around TDC. However, overly retarded combustion leads to unstable operation with partial-burn cycles resulting in high IMEPg variations and increased emissions. Enhanced natural thermal stratification by increased heat-transfer rates was explored by lowering the coolant temperature from 100 to 50°C. This strategy substantially decreased the heat-release rates and lowered the knocking intensity under certain conditions.

This work investigates the potential of in-cylinder thermal stratification for reducing the pressure-rise rate in HCCI engines, and the coupling between thermal stratification and combustion-phasing retard. A combination of computational and experimental results is employed. The computations were conducted using both a custom multi-zone version and the standard single-zone version of the Senkin application of the CHEMKIN III kinetics-rate code, and kinetic mechanisms for iso-octane. This study shows that the potential for extending the high-load operating limit by adjusting the thermal stratification is very large. With appropriate stratification, even a stoichiometric charge can be combusted with low pressure-rise rates, giving an output of 16 bar IMEPg for naturally aspirated operation. For more typical HCCI fueling rates (ϕ = 0.38 - 0.45), the optimal charge-temperature distribution is found to depend on both the amount of fuel and the combustion phasing.

The effects of injection timing and diluent addition on the late-combustion soot burnout in a direct-injection (DI) diesel engine have been investigated using simultaneous planar imaging of the OH-radical and soot distributions. Measurements were made in an optically accessible DI diesel engine of the heavy-duty size class at a 1680 rpm, high-load operating condition. A dual-laser, dual-camera system was used to obtain the simultaneous “single-shot” images using planar laser-induced fluorescence (PLIF) and planar laser-induced incandescence (PLII) for the OH and soot, respectively. The two laser beams were combined into overlapping laser sheets before being directed into the combustion chamber, and the optical signal was separated into the two cameras by means of an edge filter.