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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 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.

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.

Species analyses have been performed on engine-out and tailpipe hydrocarbon mass emissions to help understand why fuels with increasing amounts of heavy hydrocarbon constituents produce significantly higher tailpipe hydrocarbon emissions. Mass and speciated hydrocarbon emissions were acquired for a fleet of ten 1989 model year vehicles operating on twenty-six fuels of differing heavy hydrocarbon composition. These fuels formed two statistically designed matrices: one examining the effects of medium, heavy, and tail reformate and medium and heavy catalytically cracked components; and the other examining the effects of heavy paraffinic versus heavy aromatic components and the effects of the 50% distillation temperature. In this paper the fates of fuel species were traced across the engine and across the catalyst, and correlations were developed between engine-out and tailpipe hydrocarbon species emissions and fuel composition.

A detailed chemical kinetic mechanism was used to simulate the oxidation of n-butane/air mixtures in a motored engine. The modeling results were compared to species measurements obtained from the exhaust of a CFR engine and to measured critical compression ratios. Pressures, temperatures and residence times were considered that are in the range relevant to automotive engine knock. The compression ratio was varied from 6.6 to 15.5 to affect the recycle fraction and the maximum pressure and temperature of the fuel/air mixture. Engine speeds of 600 and 1600 rpm were examined which corresponded to different fuel/air residence times. The relative yields of intermediate species calculated by the model matched the measured yields generally to within a factor of two. The residual fraction derived from the previous engine cycle had a significant impact on the overall reaction rate in the current cycle.

To increase our understanding of engine knock, the cycle-to-cycle variations of knock occurrence and knock intensity in and among the individual cylinders of two multicylinder production engines run at steady speeds have been investigated. Statistical analyses, including autocorrelation analysis and cross correlation analysis between cylinders, showed that knock occurrence and intensity are random and depend solely on the conditions of each individual firing cycle. Individual cylinder knock occurrence and intensity correlated with cylinder pressure development.

A study has been made of potential methods for controlling SO4= emissions from oxidation catalyst-equipped vehicles. The methods considered included operating condition and catalyst changes, as well as the use of a vehicle trap for SO4=. Emissions of SO4= from non-catalyst cars were also measured. The only engine operating variable we found to significantly lower SO4= emission was exhaust gas O2 level. Limiting air pump use reduced SO4= emissions by factors of 5 to 7 over the FTP, and by factors of 2 to more than 10 at 96 km/h. Some increase in CO and HC emissions was observed when the greatest SO4= reductions were achieved, but it appears that properly modulated carburetion could overcome this problem. Limited excess air shows great promise as a means of minimizing SO4= emissions.