The Effects of Intake Charge Stratification on HCCI Combustion

Abstract

A laboratory was constructed to investigate what effects could be generated in an HCCI engine by altering the properties of the engine?s intake charge. The principle changes investigated were altering the spatial distribution of the fuel in the intake and adding thermal gradients to the charge. The primary goal of the work was to develop a method to separate the effects of the spatial stratification of the charge from the effects of the thermal gradients in the charge and to study both independently. The laboratory included faculties for measuring the exhaust emissions, recording cylinder pressure, and monitoring and controlling the many engine inlet and outlet parameters. Auto-ignition state was achieved by externally adding thermal energy to the intake charge utilizing heaters wrapped around the intake piping, this allowed us to generate an intake charge with a very uniform temperature distribution unlike an engine that uses exhaust rebreathing or recompression strategies. Intake temperatures in excess of 350 �C were achievable in this laboratory. Two fueling systems were developed for this work. One was located over two meters away from the inlet to enable thorough mixing and a homogeneous mixture. The other utilized the traditional fueling port to introduce a concentrated shot of fuel and keep it separated from the air as long as possible in order to minimize mixing. In order to separate the thermal and the spatial effects the fuel was heated up to the same temperature as the bulk of the inlet air. The goal was to ensure that the charge in the cylinder started the compression process with a known and controlled temperature. The level of stratification was controlled by manipulating the timing of the injection of the fuel. The results of these experiments validated the experimental setup and confirmed that consistent and repeatable results could be obtained. A series of experiments performed with spatial stratification of the fuel, as well as thermal stratification in the charge, showed that increasing thermal stratification shifted the window of acceptable intake temperatures, over which combustion could be achieved, to higher temperatures. The engine also emitted higher levels of NOx and CO as the stratification increased, but all other indicators were constant when reviewed as a function of the combustion efficiency. In this setup the premixed fueling system had the lowest window of intake air temperatures and generated the lowest pollutant emissions. A set of experiments conducted with only stratification of the fuel showed that the engine operated over the same set of intake air temperatures for both fueling methods and for all injection timings investigated. In these experiments the engine still emitted higher amounts of NOx and CO compared to the premixed system, but these emissions were lower in this setup than they were in the setup that also introduced thermal stratification. The spatial stratification did not alter the engine?s behavior, in terms of cylinder pressure. Over a range of speeds, air/fuel ratios, and fueling rates the effects of fuel stratification, generated in the intake system, only caused slight increases in the emissions of NOx and CO. While empirical evidence leads to the conclusion that stratification of the fuel does exist inside the cylinder, the results do not provide any evidence as to what level of stratification is actually generated. However, an optically accessible engine has been constructed and installed in the laboratory and will be used, in the near future, to quantify the level of stratification generated.