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The effect that thermally and compositionally stratified flowfields have on the spatial progression of iso-octane-fueled homogeneous charge compression ignition (HCCI) combustion were directly observed using highspeed chemiluminescence imaging. The stratified in-cylinder conditions were produced by independently feeding the intake valves of a four-valve engine with thermally and compositionally different mixtures of air, vaporized fuel, and argon. Results obtained under homogeneous conditions, acquired for comparison to stratified operation, showed a small natural progression of the combustion from the intake side to the exhaust side of the engine, a presumed result of natural thermal stratification created from heat transfer between the in-cylinder gases and the cylinder walls. Large differences in the spatial progression of the HCCI combustion were observed under stratified operating conditions.

Homogeneous charge compression ignition (HCCI) engines have the potential for high fuel efficiency and low NOx emissions. The major disadvantage of HCCI remains the narrow operating range. One way to extend the operating range of HCCI combustion to lower load is to inject part of the total fuel mass into the hot gas during recompression. With even lower engine load, part of the fuel can also be injected late in the main compression and ignited by a spark. The propagating flame further compresses the remaining fuel-air mixture until auto-ignition occurs (spark-assisted HCCI). In this study we investigated the effect of fuel reforming and spark assist in a gasoline engine with direct fuel injection and negative valve overlap. We performed experiments with different injection quantities and varying injection timings during recompression.

This research investigated the effects of nitric oxide (NO) on hydrocarbon (HC) emissions from a homogeneous charge spark ignition engine. Nitric oxide production inside the engine was eliminated by operating the engine on mixtures of n-butane/O2 and argon mixed from bottled gases in a custom-designed intake system. The effects of NO on HC emissions were studied by adding NO to the intake. No changes in HC emissions were measured with NO addition, although NO addition did promote autoignition chemistry. Experiments were also performed with nitrogen dilution to confirm that the argon results are applicable to normal engine operation. With nitrogen dilution there was again no effect of NO addition on HC emissions. The lack of a chemical effect of NO on HC emissions implies that a majority of the HC consumption occurs at temperatures higher than 1500 K.

The objective of this research was to develop a global correlation for hydrocarbon (HC) emissions from a homogeneous charge spark ignition engine. Engine experiments were performed with a single-cylinder engine over a wide range of speed, load, spark timing and air/fuel ratios using both n-butane and iso-octane for fuels. A global HC consumption rate correlation was developed that was able to predict measured HC emissions from both fuels to within 15 percent over all operating conditions. The results imply that the majority of the HC consumption takes place in the bulk gas at temperatures higher than 1500 K, and that for part load, low speed operating conditions, the majority of the HC consumption takes place within the cylinder before the exhaust valve opens.

The purpose of this research was to investigate the effect of fuel type and mixture composition on hydrocarbon (HC) emissions from a homogeneous charge spark ignition engine. Detailed chemical kinetic modeling indicated that at the temperatures of relevance for HC consumption in engines (T > 1500 K) a majority of the parent fuel decomposes by unimolecular thermal decomposition and that the radical pool which consumes the remaining smaller HC species is produced from the decomposition of the fuel. These results suggested that chemical kinetic interactions should exist between fuel components in a fuel mixture. Engine experiments were performed with iso-octane/toluene and n-octane/toluene fuel mixtures to determine whether kinetic interactions exist within an engine. Engine-out HC emissions exhibited a non-linear response to the amount of the paraffin in the fuel mixture and demonstrated that kinetic interactions do occur between fuel species.

A combination of engine experiments and modeling was used to investigate the effectiveness of adding di-tertiary butyl peroxide (DTBP) to gasoline to extend the light load limit in a homogeneous charge compression-ignition (HCCI) engine. The light load combustion stability limit at an engine speed of 1000 rev/min was reduced from a fueling rate of 9 mg/cycle with neat gasoline to 6.2 mg/cycle with 15% DTBP addition. A companion modeling study was performed using a three-zone, zero-dimensional engine model combined with detailed chemical kinetics. The fuel used in the model was composed of 85% iso-octane and 15% n-heptane. The model yielded trends which were similar to the experimental results. In particular, a linear relationship was found between the experimentally measured minimum fueling rate and the calculated location of maximum energy release rate for various levels of DTBP addition.

Homogeneous charge compression ignition offers the potential for significantly lower NOx emissions and up to a 20% improvement in fuel economy relative to a conventional port fuel injected spark ignition (SI) engine. The most significant challenge to developing a production viable HCCI engine is controlling the phasing of autoignition and the combustion rate across the speed and load range of the engine. This report describes an experimental and computational evaluation of controlling HCCI combustion at low loads by adding partial oxidation gas (POx), CO and H2, to the intake manifold. Experiments were performed using charge dilution obtained through conventional exhaust gas recirculation and by modified valve timings to increase the internal residuals. The experimental results showed that POx gas inhibited the low temperature energy release from n-heptane, but promoted the autoignition of isooctane.

The objective of this work was to understand the physics of combustion-generated pressure waves from Homogeneous Charge Compression Ignition combustion and the resulting audible noise that is produced. Experiments were performed with a single-cylinder engine operating in both SI and HCCI combustion modes, and comparisons were made between the pressure waves generation from the two types of combustion. Cylinder pressure oscillation amplitudes at the first circumferential mode frequency (5 to 6 kHz) generated in HCCI combustion are 5 to 10 times higher than those generated in SI knocking combustion without an undue increase in audible engine noise. Frequency analysis of the data showed that in knocking combustion a larger portion of the wave energy is contained within the higher order resonance modes. Cylinder block vibration measurements indicate that the cylinder liner significantly dissipates the wave energy below 8 kHz.

A matrix of pelleted catalysts composed of Pt, Pd, Pt co-impregnated with Pd, and Pt physically mixed with Pd supported on A l2O3 were compared with the same noble metal formulations supported on CeO2/Al2O3 for lightoff and warmed-up performance in net lean exhaust. These catalysts were tested as prepared (fresh) and following a relatively severe thermal aging treatment (cycled between net lean and net rich environment at 1000°C for 4 h). Pd showed better lightoff performance than Pt for catalyzing the oxidation of propylene, while Pt showed better lightoff and warmed-up performance than Pd for catalyzing the oxidation of propane. Having both Pt and Pd present as a result of co-impregnation or physical mixture results in good lightoff and warmed-up performance for the conversion of both types of hydrocarbons. The presence of CeO2 generally decreases lightoff performance for most of these catalysts.