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It is well known that the addition of gaseous fuels to the intake manifold of diesel engines can have significant benefits in terms of both reducing emissions of hazardous gases and soot and improving fuel economy. Particularly, the addition of LPG has been investigated in numerous studies. Drawbacks, however, of such dual fuel strategies can be found in storage complexity and end-user inconvenience. It is for this reason that on-board refining of a single fuel (for example, diesel) could be an interesting alternative. A second-generation fuel reformer has been engineered and successfully tested. The reformer can work with both gaseous and liquid fuels and by means of partial oxidation of a rich fuel-air mix, converts these into syngas: a mixture of H₂ and CO. The process occurs as partial oxidation takes place in an adiabatic ceramic reaction chamber. High efficiency is ensured by the high temperature inside the chamber due to heat release.

High-voltage nanosecond gas discharge has been shown to be an efficient way to ignite ultra-lean fuel air mixtures in a bulk volume, thanks to its ability to produce both high temperature and radical concentration in a large discharge zone. Recently, a feasibility study has been carried out to study plasma-assisted ignition under high-pressure high-temperature conditions similar to those inside an internal combustion engine. Ignition delay times were measured during the tests, and were shown to be decreasing under high-voltage plasma excitation. The discharge allowed instant control of ignition, and specific electrode geometry designs enabled volumetric ignition even at high-pressure conditions.

Computational fluid dynamic (CFD) simulations that include realistic combustion/emissions chemistry hold the promise of significantly shortening the development time for advanced high-efficiency, low-emission engines. However, significant challenges must be overcome to realize this potential. This paper discusses these challenges in the context of diesel combustion and outlines a technical program based on the use of surrogate fuels that sufficiently emulate the chemical complexity inherent in conventional diesel fuel.

The development of surrogate mixtures that represent gasoline combustion behavior is reviewed. Combustion chemistry behavioral targets that a surrogate should accurately reproduce, particularly for emulating homogeneous charge compression ignition (HCCI) operation, are carefully identified. Both short and long term research needs to support development of more robust surrogate fuel compositions are described. Candidate component species are identified and the status of present chemical kinetic models for these components and their interactions are discussed. Recommendations are made for the initial components to be included in gasoline surrogates for near term development. Components that can be added to refine predictions and to include additional behavioral targets are identified as well. Thermodynamic, thermochemical and transport properties that require further investigation are discussed.

Controlling start of ignition in Homogenous Charge Compression Ignition (HCCI) engines remains a major challenge. Here we have investigated changes in intake charge composition and its effects on ignition delay for natural gas based HCCI engine operation. In particular, we have investigated the effects of small amounts of nitrogen dioxide (NO2) on operating characteristics. Previous research had shown that NOx presence might attenuate natural gas ignition. The hypothesized catalytic effect of NOx on methane ignition at HCCI conditions was experimentally confirmed in a custom built engine. The problem was further studied in both zero and multidimensional numerical engine simulations with detailed chemistry. The simulations were used to complete a reaction rate sensitivity analysis to elucidate the controlling chemistry, and further confirm that a significant shift in ignition phasing is produced with the addition of just several ppm by volume of NO2 or NOx (NO + NO2).

The influence of the addition of oxygenated hydrocarbons to diesel fuels has been studied, using a detailed chemical kinetic model. Resulting changes in ignition and soot precursor production have been examined. N-heptane was used as a representative diesel fuel, and methanol, ethanol, dimethyl ether, dimethoxymethane and methyl butanoate were used as oxygenated fuel additives. It was found that addition of oxygenated hydrocarbons reduced the production of soot precursors. When the overall oxygen content in the fuel reached approximately 30-40 % by mass, production of soot precursors fell effectively to zero, in agreement with experimental studies. The kinetic factors responsible for these observations are discussed.

Simulations of DI Diesel engine combustion have been performed using a modified KIVA-II package with a recently developed phenomenological soot model. The phenomenological soot model includes generic description of fuel pyrolysis, soot particle inception, coagulation, and surface growth and oxidation. The computational results are compared with experimental data from a Cummins N14 single cylinder test engine. Results of the simulations show acceptable agreement with experimental data in terms of cylinder pressure, rate of heat release, and engine-out NOx and soot emissions for a range of fuel injection timings considered. The numerical results are also post-processed to obtain time-resolved soot radiation intensity and compared with the experimental data analyzed using two-color optical pyrometry. The temperature magnitude and KL trends show favorable agreement.

A two-color Particle Image Velocimetry (PIV) technique has been applied for the first time to a firing, production, three-cylinder, two-stroke, direct-injection stratified-charge engine operated under realistic conditions. In comparison to single color PIV, two-color PIV can resolve the directional ambiguity of the velocity by cross-correlating two digitized photographic images of a particle-seeded flow field, acquired sequentially at two different light wavelengths. Such an approach is essential in complex, a priori unknown, flow fields, such as those of most I.C. engines. To gain optical access to the combustion chamber, the engine head was equipped with two optical windows in such a way that its original geometry was practically undisturbed. Although the field of view was relatively small, it covered a critical area of the combustion chamber. The measurements were made in the plane perpendicular to the engine longitudinal axis, within the crank angle range of 70 to 10 degrees BTDC.

For practical, extensive 3-D computations for engine improvements, each physical submodel needs to be the simplest that is compatible with the accuracy of all other physical submodels and of the numerics. The addition of one progress variable controlled by one Arrhenius term is shown to be adequate to reproduce Diesel ignition delay in 2-D and 3-D computations. The rest of the model is that used for years by the authors to optimize combustion in reciprocating and rotary engines with premixed and non-premixed charges, including all of its model constants. This minimal Diesel autoignition submodel reproduces well trends and magnitudes of ignition delay versus chamber temperature and pressure. As in experiments, it is found that multiple ignition sources develop in rapid succession at various locations around the fuel spray after the first ignition event.

The exciplex fluorescence technique has been used to separately visualize liquid and vapor phase fuel in engines since its development by Melton. However, as a fluorescence technique it has the potential to be quantitative and the underlying assumptions have been outlined by Melton. An initial quantitative application of the TMPD/naphthalene system, based on these assumptions, applied to a hollow-cone spray in a two-stroke engine, indicated that it substantially over-estimates the concentration of fuel vapor about TDC. The reasons for the discrepancy were investigated and it was concluded that a major factor is the effect of temperature on the photophysics of the species involved. Thus the absorption spectra of the exciplex dopants were determined at temperatures up to 700 K. These experiments showed that the increase in absorption with temperature above 500 K is responsible for the failure of the earlier calibration.

A two-color Particle Image Velocimetry (PIV) technique has been applied to a single cylinder, cup-in-head two-stroke research engine. In the two-color PIV, two wavelengths are used to successively record, at a known time separation, the positions of the particulate seeds in the flowfield. By separately interrogating the two images of different color and cross-correlating them, a two-dimensional velocity field is obtained. Since the sequence of the images is known, directional ambiguity is eliminated and two-color PIV can be used to study complex flows. The technique is here applied for the first time to an engine in the presence of combustion. The presence of combustion light complicates the application of two-color PIV because narrow band-pass laser line filters can not be used to reject it, however a suitable combination of laser power, colored glass filters and thresholding during interrogation allowed sufficiently high quality images to be obtained.

The three components of the velocity were measured by laser Doppler velocimetry at 35 locations in each of the six intake ports of a single-cylinder I.C. engine motored at 600, 900, and 1200 rpm. The intake ports were designed to impart both swirl and roll to the air. Pressure was also measured at the intake and exhaust. The detailed information is valuable mostly for computations of engine flows and for the assessment of multidimensional models. However the following trends were observed. The intake velocity is affected by resonant pressure waves. The flows in the six ports tend to be similar. The three components of the ensemble-averaged velocity generally have uniform profiles across the port area, whereas the fluctuation intensities are higher at the top of the port. All velocities tend to be higher at the beginning and end of intake.

Using an inflight simulator, a simple longitudinal decoupling concept was compared with conventional airplane characteristics for the approach and landing tasks. The decoupling system allowed the pilot to command flight path angle changes with the stick with little or no accompanying speed change; likewise, speed changes with only small accompanying flight path changes could be made with throttle only. The unique feature of the concept is that it is an open loop (that is, non-feedback) control system. Results indicate that in calm air and up to moderate levels of turbulence the decoupling system provides a substantial reduction in pilot workload. The program was supported by NASA, Langley Research Center, under Grant NSG 1234.