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Technical Paper

Velocity Measurements in the Wall Boundary Layer of a Spark-Ignited Research Engine

Laser Doppler velocimetry has been used to measure velocity and turbulence intensity profiles in the wall boundary layer of a spark-ignited homogeneous-charge research engine. By using a toroidal contoured engine head it was possible to bring the laser probe volume to within 60 μm of the wall. Two different levels of engine swirl were used to vary the flow Reynolds number. For the high swirl case under motored operation the boundary layer thickness was less than 200 μm, and the turbulence intensity increased as the wall was approached. With low swirl the 700-1000 μm thick boundary layer had a velocity profile that was nearly laminar in shape, and there was no increase in turbulence intensity near the wall. When the engine was fired the boundary layer thickness increased for both levels of swirl.
Technical Paper

Compression-Ignited Homogeneous Charge Combustion

Experimentally obtained energy release results, a semi-empirical ignition model, and an empirical energy release equation developed during this research were used to evaluate the combustion of compression-ignited homogeneous mixtures of fuel, air, and exhaust products in a CFR engine. A systematic study was carried out to evaluate the response of compression-ignited homogeneous charge (CIHC) combustion to changes in operating parameters with emphasis being placed on the phenomena involved rather than the detailed chemical kinetics. This systematic study revealed that the response of the combustion process to changes in operating parameters can be explained in terms of known chemical kinetics, and that through the proper use of temperature and species concentrations the oxidation kinetics of hydrocarbon fuels can be sufficiently controlled to allow an engine to be operated in a compression-ignited homogeneous charge combustion mode.
Technical Paper

A Computational Investigation into the Cool Flame Region in HCCI Combustion

Multi-dimensional computational efforts using comprehensive and skeletal kinetics have been made to investigate the cool flame region in HCCI combustion. The work was done in parallel to an experimental study that showed the impact of the negative temperature coefficient and the cool flame on the start of combustion using different fuels, which is now the focus of the simulation work. Experiments in a single cylinder CFR research engine with n-butane and a primary reference fuel with an octane number of 70 (PRF 70) were modeled. A comparison of the pressure and heat release traces of the experimental and computational results shows the difficulties in predicting the heat release in the cool flame region. The behavior of the driving radicals for two-stage ignition is studied and is compared to the behavior for a single-ignition from the literature. Model results show that PRF 70 exhibits more pronounced cool flame heat release than n-butane.
Technical Paper

Comparison of HCCI Operating Ranges for Combinations of Intake Temperature, Engine Speed and Fuel Composition

A series of engine experiments have been performed to explore the impact intake temperature, engine speed and fuel composition on the HCCI operating range of a CFR engine. The experimental matrix covers a range of engine speeds 600 - 2000 RPM), intake temperatures (300 K - 400 K), and four different fuels. Three of the fuels had different chemical composition but had equivalent research octane numbers of 91.8. The fourth fuel, a blend of primary reference fuels had a research octane number of 70. The acceptable HCCI operating range of the engine was defined through two criteria; the rate of pressure rise needed to be less than 10 MPa per crank angle and the covariance of the indicated mean effective pressure needed to be less than 10 percent. Using these limits the HCCI operating range for the engine was evaluated for the experimental matrix. Data for emissions, and fuel consumption as well as in-cylinder pressure were recorded.
Technical Paper

An Investigation Into the Effect of Fuel Composition on HCCI Combustion Characteristics

A single cylinder CFR research engine has been run in HCCI combustion mode for a range of temperatures and fuel compositions. The data indicate that the best HCCI operation, as measured by a combination of successful combustion with low ISFC, occurs at or near the rich limit of operation. Analysis of the pressure and heat release histories indicated the presence, or absence, and impact of the fuel's NTC ignition behavior on establishing successful HCCI operation. The auto-ignition trends observed were in complete agreement with previous results found in the literature. Furthermore, analysis of the importance of the fuel's octane sensitivity, through assessment of an octane index, successfully explained the changes in the fuels auto-ignition tendency with changes in engine operating conditions.
Journal Article

Multiple-Event Fuel Injection Investigations in a Highly-Dilute Diesel Low Temperature Combustion Regime

The objective of this research is a detailed investigation of multiple injections in a highly-dilute diesel low temperature combustion (LTC) regime. This research concentrates on understanding the performance and emissions benefits of multiple injections via experiments and simulations in a 0.48L signal cylinder light-duty engine operating at 2000 r/min and 5.5 bar IMEP. Controlled experiments in the single-cylinder engine are then combined with three computational tools, namely heat release analysis of measured cylinder pressure, a phenomenological spray model using in-cylinder thermodynamics [1], and KIVA-3V Chemkin CFD computations recently tested at LTC conditions [2]. This study examines the effects of fuel split distribution, injection event timing, rail pressure, and boost pressure which are each explored within a defined operation range in LTC.