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HCCI

PostPosted: Tue Dec 24, 2013 8:27 am
by Prasanth
ABSTRACT

HCCI has characteristics of the two most popular forms of combustion used in IC engines: homogeneous charge spark ignition (gasoline engines) and stratified charge compression ignition (diesel engines). As in homogeneous charge spark ignition, the fuel and oxidizer are mixed together. However, rather than using an electric discharge to ignite a portion of the mixture, the concentration and temperature of the mixture are raised by compression until the entire mixture reacts spontaneously. Stratified charge compression ignition also relies on temperature increase and concentration resulting from compression, but combustion occurs at the boundary of fuel-air mixing, caused by an injection event, to initiate combustion.

The defining characteristic of HCCI is that the ignition occurs at several places at a time which makes the fuel/air mixture burn nearly simultaneously. There is no direct initiator of combustion. This makes the process inherently challenging to control. However, with advances in microprocessors and a physical understanding of the ignition process, HCCI can be controlled to achieve gasoline engine-like emissions along with diesel engine-like efficiency. In fact, HCCI engines have been shown to achieve extremely low levels of Nitrogen oxide emissions (NOx) without after treatment catalytic converter. The unburned hydrocarbon and carbon monoxide emissions are still high (due to lower peak temperatures), as in gasoline engines, and must still be treated to meet automotive emission regulations.

INTRODUCTION

HCCI has characteristics of the two most popular forms of combustion used in IC engines: homogeneous charge spark ignition (gasoline engines) and stratified charge compression ignition (diesel engines). As in homogeneous charge spark ignition, the fuel and oxidizer are mixed together. However, rather than using an electric discharge to ignite a portion of the mixture, the concentration and temperature of the mixture are raised by compression until the entire mixture reacts simultaneously. Stratified charge compression ignition also relies on temperature increase and concentration resulting from compression, but combustion occurs at the boundary of fuel-air mixing, caused by an injection event, to initiate combustion.

The defining characteristic of HCCI is that the ignition occurs at several places at a time which makes the fuel/air mixture burn nearly simultaneously. There is no direct initiator of combustion. This makes the process inherently challenging to control. However, with advances in microprocessors and a physical understanding of the ignition process, HCCI can be controlled to achieve gasoline engine-like emissions along with diesel engine-like efficiency. In fact, HCCI engines have been shown to achieve extremely low levels of Nitrogen oxide emissions (NOx) without after treatment catalytic converter. The unburned hydrocarbon and carbon monoxide emissions are still high (due to lower peak temperatures), as in gasoline engines, and must still be treated to meet automotive emission regulations.The homogeneous charge compression ignition (HCCI) engine has caught the attention of automotive and diesel engine manufacturers worldwide because of its potential to rival the high efficiency of diesel engines while keeping NOx and particulateemissions extremely low. However, researchers must overcome several technical barriers, such as controlling ignition timing, reducing unburned hydrocarbon and carbon monoxide emissions, extending operation to higher loads, and maintaining combustion stability through rapid transients.

HCCI engines can operate using a variety of fuels. In the near term, the application of HCCI to automotive engines will likely involve mixed-mode combustion in which HCCI is used at low-to-moderate loads and standard spark-ignition (SI) combustion is used at higher loads. This type of operation using standard gasoline-type fuels requires a moderate compression ratio of 10:1 to 14:1 for SI operation and significant intake heating for HCCI operation.

Automotive HCCI Engine Laboratory

The CRF's Automotive HCCI Engine Laboratory houses a versatile light-duty engine designed to allow investigations of a wide variety of issues for this type of HCCI application. The automotive-sized engine (0.63 liters/cylinder) has a 3-valve pent-roof head and is equipped with extensive optical access for the application of advanced laser-based diagnostics, including an extended cylinder with a piston-crown window and a full transparent quartz cylinder liner. The intake air system provides intake pressures up to 2 bar and heating to 250 °C. These high intake temperatures allow investigations of HCCI operation with lower compression ratios (10:1 to 12:1). Alternatively, hot residuals can be used to induce HCCI combustion using cam shafts designed to retain large amounts of combustion products.

The engine is equipped with a centrally mounted gasoline-type direct injector, a port fuel injection capability, and a fully premixed fueling system, allowing investigations of both well-mixed and stratified HCCI operation Researchers are currently using laser elastic scatter and laser-induced fluorescence imaging to study the distribution of both liquid- and vapor-phase fuel in the cylinder from the time of injection to the time of ignition. Images recorded during direct injection provide details of spray morphology, interactions between the spray and the intake air flow, and wall wetting. Images recorded during the compression stroke capture the evolution of the fuel vapor/air mixture, and provide a measure of mixture homogeneity at the time of ignition. Correlations are sought between these fuel-distribution data and simultaneously recorded combustion performance and emission data

Researchers are currently using laser elastic scatter and laser-induced fluorescence imaging to study the distribution of both liquid- and vapor-phase fuel in the cylinder from the time of injection to the time of ignition. Images recorded during direct injection provide details of spray morphology, interactions between the spray and the intake air flow, and wall wetting. Images recorded during the compression stroke capture the evolution of the fuel vapor/air mixture, and provide a measure of mixture homogeneity at the time of ignition. Correlations are sought between these fuel-distribution data and simultaneously recorded combustion performance and emission data.