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Ionization probes built into the head gasket and uniformly distributed around the cylinder bore of a knocking, spark-ignition engine have been used to locate the autoigniting end-gas region. As normal combustion evolves after spark ignition, the ionization probes individually respond to the arrival of the propagating flame. Then, when autoignition occurs, the probes located in the end-gas region respond in rapid succession. By utilizing pressure transducer measurements to determine when autoignition occurs, the ionization probe response becomes a means to locate the end-gas region. Knowledge of the location of the last ionization probe to detect the normal flame can then be used to infer where, within the end-gas region, autoignition first occurred.

Experimental investigations of the scavenging, mixture preparation and combustion processes occurring in a blower-scavenged, direct-injected, two-stroke cycle research engine are presented. As the delivery ratio is increased, combustion performance deteriorates rapidly. We used a variety of diagnostic techniques to investigate the causes of this behavior, including pressure measurements, schlieren and Mie-scattering flow visualization, and ionization-probe determination of flame-arrival times. By comparison with premixed-charge operation of the engine, we conclude that the improved scavenging that accompanies increased delivery ratio results in cooler gas temperatures that inhibit fuel vaporization, leading to a highly nonhomogeneous fuel-air mixture at the time of combustion.

This paper presents a comparison of the autoignition tendencies for straight chain hydrocarbons and mixtures of primary reference fuels in a motored engine. Minimum initial gas temperatures required to produce autoignition were measured as a function of fuel type, engine speed, and inlet manifold pressure. In-cylinder gas pressure vs. crank angle and exhaust gas carbon monoxide concentrations were also monitored during these experiments as indicators of chemical activity. Overall, the autoignition behavior of n-pentane, n-hexane, and their equivalent octane number primary reference fuel blends was found to be dissimilar in (i) the inlet temperatures at which autoignition occurred; (ii) the amount of CO formed prior to autoignition; and (iii) the effect of engine speed on minimum inlet autoignition temperatures. Possible causes for this behavior are discussed in the paper in terms of the Negative Temperature Coefficient behavior of large hydrocarbons.

As a means of exploring the use of optical imaging techniques in the study of engine flow and combustion problems, we have applied several imaging techniques in different laboratory engines and engine flow simulators. Two single cylinder research engines were employed: a sidewall-valved engine and a square piston engine. Additional observations were made of the free jet flowing from a liquid fuel injector, the intake valve air flow in a steady flow facility, and the mixed fuel/air intake flow following the impingement of a fuel spray on the backside of a valve. We used the planar imaging of laser induced fluorescence from OH to mark the position of the propagating flame front in the engine. These results are compared to similar data obtained using a Mie scattering technique employed in an earlier study.

The process of hydrocarbon emission from an engine crevice was simulated in an operating research engine by the introduction of a small tube into the combustion chamber. This simulated crevice volume was used to determine the fate of unburned hydrocarbons that interact with the crevice. Shadowgraph photography and spontaneous Raman spectroscopy were used to determine flow patterns, temperatures, and hydrocarbon concentrations 1 mm from the tube opening. Hydrocarbon species were first detected at the tube exit late in the expansion stroke, long after the start of outflow from the simulation volume. A flame was never observed near the tube exit. Unburned hydrocarbons exiting the tube did not undergo rapid oxidation at temperatures up to 1400 Kelvins.