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NO formation during direct-injection (DI) diesel combustion has been investigated using planar laser-induced fluorescence (PLIF) imaging. Measurements were made at a typical medium-speed operating condition in a heavy-duty size-class engine modified for optical access. By combining a unique laser system with a particular spectroscopic scheme, single-shot NO images were obtained at realistic operating conditions with negligible O2 interference. Temporal sequences of NO PLIF images are presented along with corresponding images of combined elastic scattering and natural luminosity. These images show the location and timing of the NO formation relative to the other components of the reacting fuel jet. In addition, total NO formation was examined by integrating the NO PLIF signal over a large fraction of the combustion-chamber volume.

A phenomenological description, or “conceptual model,” of how direct-injection (DI) diesel combustion occurs has been derived from laser-sheet imaging and other recent optical data. To provide background, the most relevant of the recent imaging data of the author and co-workers are presented and discussed, as are the relationships between the various imaging measurements. Where appropriate, other supporting data from the literature is also discussed. Then, this combined information is summarized in a series of idealized schematics that depict the combustion process for a typical, modern-diesel-engine condition. The schematics incorporate virtually all of the information provided by our recent imaging data including: liquid- and vapor-fuel zones, fuel/air mixing, autoignition, reaction zones, and soot distributions.

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.

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.

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.

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 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.

An experimental study has been conducted to characterize NO and soot evolution in an optically-accessible D.I. diesel engine with a square combustion chamber. Two-dimensional laser-induced fluorescence was used to characterize NO evolution. Soot evolution was characterized by two-dimensional laser-induced incandescence (LII) and Mie scattering techniques as well as direct photography of the flame luminosity. The engine operating parameters were set to provide optimum conditions for NO imaging. Attenuation of the UV beam proved to be the major obstacle in obtaining NO images. Therefore, oxygen was added to the intake air charge in order to reduce the optical density of the combustion medium. The NO images showed that the NO formation started almost immediately after ignition and ceased no later than 40 degrees ATDC. No soot images could be obtained by the laser-induced incandescence or Mie scattering methods before 20 degrees ATDC since the soot concentration was very low.

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.

Flame kernel growth and cylinder pressure data were simultaneously obtained from an optically-accessible, square piston, SI engine. Flame kernel growth was measured using simultaneous, orthogonal, Schlieren photography, while cylinder pressure was measured using a piezoelectric pressure transducer. The data were analyzed on a cycle-resolved basis to determine the correlation between cyclic fluctuations in flame kernel growth and cylinder pressure. The engine was operated at 875 RPM with premixed, prevaporized, stoichiometric isooctane in air. The engine, designed with ported intake and exhaust, was fired every tenth cycle to ensure complete scavenging. Tests were conducted with and without nitrogen dilution, while ignition timing was fixed at 25° BTDC. With 0% dilution the percent variation in the maximum cylinder pressure was 8.5%, while with 10% dilution the percent variation increased to 14%.

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.

Enhancement of methanol combustion in a direct injected Diesel engine using catalytically coated glow plugs was examined for platinum and palladium catalysts and compared to a non-catalytic baseline case. Experiments were performed for 6 and 10 brake Kilowatts (bKW) at 2500 rpm. Comparisons were made based on combustion, performance, and emissions including carbon monoxide (CO), oxides of nitrogen (NOx), unburned hydrocarbons (UHC), unburned methanol (UBM), and aldehydes. Results show a decrease in glow plug temperature of 100 K is achievable using platinum catalysts, and 150 K for palladium. Furthermore, the palladium catalyst was found to provide better combustion characteristics than the platinum catalyst. Also, the use of both catalysts produced lower aldehyde emissions, and the palladium reduced NOx emissions as well. However, unburned methanol increased for both catalytic glow plugs with respect to the non-catalytic case.

Laser-induced incandescence (LII) has been explored as a diagnostic for qualitative two-dimensional imaging of the in-cylinder soot distribution in a diesel engine. Advantages of LII over elastic-scatter soot imaging techniques include no interfering signals from liquid fuel droplets, easy rejection of laser light scattered by in-cylinder surfaces, and the signal intensity being proportional to the soot volume fraction. LII images were obtained in a 2.3-liter, single cylinder, direct-injection diesel engine, modified for optical access. To minimize laser sheet and signal attenuation (which can affect almost any planar imaging technique applied to diesel engine combustion), a low-sooting fuel was used whose vaporization and combustion characteristics are typical of standard diesel fuels. Temporal and spatial sequences of LII images were made which show the extent of the soot distribution within the optically accessible portion the combusting spray plume.

The effect of chromium oxide (Cr2O3) and partially stabilized zirconia (PSZ) surfaces on the gas-phase ignition of hydrocarbon fuels has been studied in a high pressure chemical flow reactor under low heat-loss Diesel engine simulated conditions. Results are presented for propane and n-hexane fuels at 6 and 10 atmospheres and are compared to previously reported results obtained under similar conditions with platinum surfaces. Thermal enhancement of the gas-phase reactions, as evidenced by the catalytic production of CO and CO2, was found to be negligible for both Cr2O3 and PSZ, where as significant thermal enhancement was observed with platinum. Chemical enhancement of the gas-phase ignition reactions, as evidenced by an increase in the production of intermediate hydrocarbons, was observed under fuel rich conditions with both Cr2O3 and PSZ. Oxygen was shown to be essential in that no change in the intermediate hydrocarbon concentration was observed under pyrolysis conditions.

The potential for catalytically enhanced ignition in low-heat rejection Diesel engines has been experimentally studied under engine simulated conditions in a high pressure chemical flow reactor. Results are presented for propane oxidation on platinum at 6 and 10 atmospheres, at temperatures from 800K to 1050K, and at equivalence ratios from 0.5 to 4.0. For turbulent transport rates which are typical of those in an engine, as much as 20% of the fuel was found to react on the catalyst before the onset of the gas-phase ignition reactions. Depending on the adiabaticity of the combustion chamber walls, this could lead to significant thermal enhancement of the gas-phase ignition process. Evidence of chemical enhancement was also observed, at 10 atm under very fuel rich conditions, in terms of a change in the concentration and distribution of the hydrocarbon intermediate species. Possible mechanisms for the observed chemical enhancement due to surface generated species are discussed.

The interaction of turbocharger characteristics with engine design and performance parameters is presented. Turbocharger matching requires a compromise among the following variables: combustion air-fuel ratio, turbocharger characteristics, peak cylinder pressures, engine speed range capability, transient response, and overall package efficiency. The mechanisms of interactions of the above variables are reviewed to highlight the turbocharged engine designers problems. The impact of turbocharger performance limits on overall engine characteristics is reviewed with special attention focused on what limitations point out as the turbomachinery designers' job for the future.

The authors discuss the design and development of a single cylinder research engine which was used for combustion chamber development and for providing test data for cycle analysis. Two basic criteria for the IH engine were reproducible performance and freedom from mechanical problems. Three major designs were developed to satisfy different experimental requirements, with the ultimate goal being the achievement of advanced combustion systems beyond present limits and reduction in the cost and time required for these developments.