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To cope with regulatory standards, minimizing tailpipe emissions with rapid catalyst light-off during cold-start is critical. This requires catalyst-heating operation with increased exhaust enthalpy, typically by using late post injections for retarded combustion and, therefore, increased exhaust temperature. However, retardability of post injection(s) is constrained by acceptable pollutant emissions such as unburned hydrocarbon (UHC). This study provides further insight into the mechanisms that control the formation of UHC under catalyst-heating operation in a medium-duty diesel engine, and based on the understanding, develops combustion strategies to simultaneously improve exhaust enthalpy and reduce harmful emissions. Experiments were performed with a full boiling-range diesel fuel (cetane number of 45) using an optimized five-injections strategy (2 pilots, 1 main, and 2 posts) as baseline condition.

Engine-out CO emission and fuel conversion efficiency were measured in a highly-dilute, low-temperature diesel combustion regime over a swirl ratio range of 1.44-7.12 and a wide range of injection timing. At fixed injection timing, an optimal swirl ratio for minimum CO emission and fuel consumption was found. At fixed swirl ratio, CO emission and fuel consumption generally decreased as injection timing was advanced. Moreover, a sudden decrease in CO emission was observed at early injection timings. Multi-dimensional numerical simulations, pressure-based measurements of ignition delay and apparent heat release, estimates of peak flame temperature, imaging of natural combustion luminosity and spray/wall interactions, and Laser Doppler Velocimeter (LDV) measurements of in-cylinder turbulence levels are employed to clarify the sources of the observed behavior.

Line-imaging of Raman scattered light is used to simultaneously measure the mole fractions of CO2, H2O, N2, O2, and fuel (premixed C3H8) in a modern 4-valve spark-ignition engine operating at idle. The measurement volume consists of 16 adjacent sub-volumes, each 0.27 mm in diameter × 0.91 mm long, giving a total measurement length of 14.56 mm. Measurements are made 3 mm under the centrally-located spark plug, offset 3 mm from the spark plug center towards the exhaust valves. Data are taken in 15 crank angle degree increments starting from top center before the intake stroke (-360 CAD) through top center of the compression stroke (0 CAD).

Piston-wetting effects are investigated in an optical direct-injection spark-ignition (DISI) engine. Fuel spray impingement on the piston leads to the formation of fuel films, which are visualized with a laser-induced fluorescence (LIF) imaging technique. Oxygen quenching is found to reduce the fluorescence yield from liquid gasoline. Fuel films that exist during combustion of the premixed charge ignite to create piston-top pool fires. These fires are characterized using direct flame imaging. Soot produced by the pool fires is imaged using laser elastic scattering and is found to persist throughout the exhaust stroke, implying that piston-top pool fires are a likely source of engine-out particulate emissions for DISI engines.

Appropriate spray modeling in multidimensional simulations of diesel engines is well known to affect the overall accuracy of the results. More and more accurate models are being developed to deal with drop dynamics, breakup, collisions, and vaporization/multiphase processes; the latter ones being the most computationally demanding. In fact, in parallel calculations, the droplets occupy a physical region of the in-cylinder domain, which is generally very different than the topology-driven finite-volume mesh decomposition. This makes the CPU decomposition of the spray cloud severely uneven when many CPUs are employed, yielding poor parallel performance of the spray computation. Furthermore, mesh-independent models such as collision calculations require checking of each possible droplet pair, which leads to a practically intractable O(np2/2) computational cost, np being the total number of droplets in the spray cloud, and additional overhead for parallel communications.

The objectives of this paper are to describe the ongoing projects in diesel engine combustion research at Sandia National Laboratories' Combustion Research Facility and to detail recent experimental results. The approach we are employing is to assemble experimental hardware that mimic realistic engine geometries while enabling optical access. For example, we are using multi-cylinder engine heads or one-cylinder versions of production heads mated to one-cylinder engine blocks. Optical access is then obtained through a periscope in an exhaust valve, quartz windows in the piston crown, windows in spacer plates just below the head, or quartz cylinder liners. We have three diesel engine experiments supported by the Department of Energy, Office of Heavy Vehicle Technologies: a one-cylinder version of a Cummins heavy-duty engine, a diesel simulation facility, and a one-cylinder Caterpillar engine to evaluate combustion of alternative diesel fuels.

A free piston internal combustion (IC) engine operating on high compression ratio (CR) homogeneous charge compression ignition (HCCI) combustion is being developed by Sandia National Laboratories to significantly improve the thermal efficiency and exhaust emissions relative to conventional crankshaft-driven SI and Diesel engines. A two-stroke scavenging process recharges the engine and is key to realizing the efficiency and emissions potential of the device. To ensure that the engine's performance goals can be achieved the scavenging system was configured using computational fluid dynamics (CFD), zero- and one-dimensional modeling, and single step parametric variations. A wide range of design options was investigated including the use of loop, hybrid-loop and uniflow scavenging methods, different charge delivery options, and various operating schemes. Parameters such as the intake/exhaust port arrangement, valve lift/timing, charging pressure and piston frequency were varied.

A novel direct-fabrication technique (robocasting) was used to produce periodic lattices of ceramic rods. The macrostructure is a three-dimensional mesh with controlled porosity in all dimensions but no line-of-sight pathways. These ceramic lattices can function as catalyst supports for gas combustion, and possibly self-regenerating filters for diesel particulates. Compared to the traditional two-dimensional “honeycomb” structured extrudates, the three-dimensional structures have high surface to volume ratios and highly turbulent flow. The flow behaviors of these ceramic lattices and the resulting enhancements in catalytic performance over traditional supports have been demonstrated for propane and methane combustion. Similar tests are underway for the selective catalytic reduction (SCR) of NOx. The potential utility of these structures for diesel particulate trapping will also be discussed.

Performance and emissions characteristics were measured for a part- load operating point using an optically-accessible single-cylinder gasoline research engine equipped with three different exploratory nanosecond repetitively pulse discharge (NRPD) igniters. The three igniters investigated are as follows: 1) a four-prong advanced corona ignition system (ACIS) that produces large ignition volumes from streamer discharges, 2) a barrier discharge igniter (BDI) that generates strong surface plasma along the insulator that completely encases the power electrode, and 3) a J-hook non-resistive nanosecond spark (NRNS) igniter. For select conditions, high-speed imaging (20 kHz) of excited state hydroxyl (OH*) chemiluminescence was performed to measure flame development in-cylinder. An available NRPD pulse generator was used to supply positive direct current (DC) pulses (~ 10 ns pulse width) to each igniter at a fixed 100 kHz frequency.

A set of elementary surface reactions is proposed for modeling the chemistry in a lean NOx trap during regeneration (reduction of stored NOx). The proposed reaction mechanism can account for the observed product distribution from the trap over a range of temperatures and inlet gas compositions similar to those expected for realistic operation. The mechanism includes many reactions already discussed in the literature, together with some hypothesized reactions that are required to match observations from temperature programmed reactor experiments with a commercial lean NOx trap catalyst. Preliminary results indicate that the NOx trap regeneration and byproduct formation rates can be effectively captured by using a relatively compact set of elementary reactions.

This paper describes a program to develop a miniaturized chemical laboratory (μChemLab™). This system includes multiple analysis channels each with microfabricated sample collectors/concentrators, gas chromatographic separators, and chemically selective detectors based on an array of coated surface acoustic wave devices. This development effort is currently focused on fabricating small (palm-top computer sized), lightweight, and autonomous systems that provide rapid (1 min), sensitive (1-10 ppb), and selective detection of chemical warfare agents. The small size and low power of the μChemLab™ technology make it potentially useful for monitoring of compounds such as volatile organic compounds (VOCs), ammonia, and formaldehyde in space environments.

It is generally accepted that thermal (Zeldo'vich) chemical kinetics dominate NO formation in diesel engines, so control of temperature is critical for reducing exhaust NOx emissions. Recent optical engine data revealed that when the start of injection (SOI) was retarded to very late timings, combustion luminosity decreased while exhaust NOx emissions increased, causing a “NOx bump.” This data suggested that changes in radiative heat transfer from soot may affect in-cylinder temperatures and subsequent NOx formation. In this study, soot thermometry measurements of in-cylinder temperature and radiative heat transfer were correlated with exhaust NOx to quantify the role of radiative heat transfer on in-cylinder temperatures and NOx formation. The engine was operated at low-load conditions, for which the premixed burn was a significant fraction of the total heat release.

Laser plasma ignition has been pursued by engine researchers as an alternative to electric spark-ignition systems, potentially offering benefits by avoiding quenching surfaces and extending breakdown limits at higher boost pressure and lower equivalence ratio. For this study, we demonstrate another potential benefit: the ability to control the timing of ignition with short, nanosecond pulses, thereby optimizing the type of mixture that burns in rapidly changing, stratified fuel-air mixtures. We study laser ignition at various timings during single and double injections at simulated gasoline engine conditions within a controlled, high-temperature, high-pressure vessel. Laser ignition is accomplished with a single low-energy (10 mJ), short duration (8 ns) Nd:YAG laser beam that is tightly focused (0.015 mm average measured 1/e₂ diameter) at a typical GDI spark plug location.

Video imaging has been used to investigate the evolution of liquid fuel films on combustion chamber walls during a simulated cold start of a port fuel-injected engine. The experiments were performed in a single-cylinder research engine with a production, four-valve head and a window in the piston crown. Flood-illuminated laser-induced fluorescence was used to observe the fuel films directly, and color video recording of visible emission from pool fires due to burning fuel films was used as an indirect measure of film location. The imaging techniques were applied to a comparative study of open and closed valve injection, for coolant temperatures of 20, 40 and 60 °C. In general, for all cases it is shown that fuel films form in the vicinity of the intake valve seats.

The relationship between combustion processes and engine-out soot was investigated in an optically accessible DI diesel engine using diethylene glycol diethyl ether (DGE) fuel, a viable diesel oxygenate. The high oxygen content of DGE enables operation without soot emissions at higher loads than with a hydrocarbon fuel. The high cetane number of DGE enables operation at charge-gas temperatures below those required for current diesel fuels, which may be advantageous for reducing NOx emissions. In-cylinder optical measurements of flame lift-off length and natural luminosity were obtained simultaneously with engine-out soot measurements while varying charge-gas density and temperature. The local mixture stoichiometry at the lift-off length was characterized by a parameter called the oxygen ratio that was estimated from the measured flame lift-off length using an entrainment correlation for non-reacting sprays.

This study systematically investigates the effects of various engine operating parameters on the thermal efficiency of a boosted HCCI engine, and the potential of E10 to extend the high-load limit beyond that obtained with conventional gasoline. Understanding how these parameters can be adjusted and the trade-offs involved is critical for optimizing engine operation and for determining the highest efficiencies for a given engine geometry. Data were acquired in a 0.98 liter, single-cylinder HCCI research engine with a compression-ratio of 14:1, and the engine facility was configured to allow precise control over the relevant operating parameters. The study focuses on boosted operation with intake pressures (Pin) ≥ 2 bar, but some data for Pin < 2 bar are also presented. Two fuels are considered: 1) an 87-octane gasoline, and 2) E10 (10% ethanol in this same gasoline) which has a lower autoignition reactivity for boosted operation.

The ignition and flame stabilization characteristics of two synthetic fuels, having significantly different cetane numbers, are investigated in a constant volume combustion vessel over a range of ambient conditions representative of a compression ignition engine operating at variable loads. The synthetic fuel with a cetane number of 63 (S-1) is characterized by ignition delays that are only moderately longer than n-dodecane (cetane number of 87) over a range of ambient conditions. By comparison, the synthetic fuel with a cetane number of 17 (S-2) requires temperatures approximately 300 K higher to achieve the same ignition delays. The much different ignition characteristics and operating temperature range present a scenario where the lift-off stabilization may be substantially different.

An experimental study of the glow-plug-assisted ignition and combustion of pure methanol (M100) was conducted using a modern-technology, 4-stroke, heavy-duty DI diesel engine that has been modified to provide extensive optical access into the combustion chamber. For comparison purposes, results also are presented for a two-component paraffinic diesel reference fuel with a cetane number of 45 (CN45). A 1200-rpm, moderate-load operating condition was studied. Images of direct luminosity from the combustion chamber are used along with thermodynamic analyses of cylinder pressure data to identify differences between the ignition and combustion characteristics of the two fuels. The M100 data show significant departures from the traditional diesel combustion features exhibited by CN45. Whereas CN45 readily autoignites at the conditions studied, M100 does not. The glow-plug-assisted ignition of M100 was found to be strongly dependent on glow plug (GP) temperature and proximity to a fuel jet.

For logistical reasons, the military requires that jet fuel (JP-8, F-34) be used in both jet engines and diesel engines. While JP-8-fueled diesel engines appear to operate successfully in many cases, negative impacts, including engine failures, are occasionally reported. As diesel combustion with JP-8 has not been explored in great detail, fundamental information about JP-8 fuel spray combustion is needed. In this study, we report measurements of liquid-phase penetration length, vapor penetration, and ignition delay made in an optically-accessible combustion vessel over a range of high-temperature, high-pressure operating conditions applicable to a diesel engine. Results show that the liquid-phase penetration of JP-8 is less than that of diesel, owing to the lower boiling point temperatures of JP-8. Despite the more rapid vaporization, the vapor penetration rate of JP-8 matches that of diesel and ignition does not advance.

Recently experiments were conducted on an automotive homogeneous-charge-compression-ignition (HCCI) research engine with a negative-valve-overlap (NVO) cam. In the study two sets of experiments were run. One set injected a small quantity of fuel (HPLC-grade iso-octane) during NVO in varying amounts and timings followed by a larger injection during the intake stroke. The other set of experiments was similar, but did not include an NVO injection. By comparing both sets of results researchers were able to investigate the use of NVO fuel injection to control main combustion phasing under light-load conditions. For this paper a subset of these experiments are modeled with the computational-fluid-dynamics (CFD) code KIVA3V [ 6 ] using a multi-zone combustion model. The computational domain includes the combustion chamber, and intake and exhaust valves, ports, and runners. Multiple cycles are run to minimize the influence of initial conditions on final simulated results.