Browsing by Author "Meiring, Pierre Andre."

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The Analysis of computer systems for performance optimisation.
(1987) Meiring, Pierre Andre.; Nattrass, Henry Lee.
The project investigated the problem of performance optimisation of computer systems at the systems level. It was ascertained that no generally accepted technique for approaching this problem exist. A theoretical approach was thus developed which describes the system, the workload and the performance in terms of matrices which are deduced from measured data. An attempt is then made to verify this theory by applying it to a real system in a controlled environment. A dummy workload is used and measurements are made on the computer system for various configurations. The results thus obtained are compared with the expected trends in system performance and conclusions are drawn which appear to verify the validity of the theory proposed.
Automated processor for optimizing tractor operation.
(1991) Lyne, Peter William Liversedge.; Meiring, Pierre Andre.
The agricultural tractor is designed as a general purpose machine and consequently, does not perform all its tasks at maximum efficiency. Various methods of increasing the field performance of these vehicles have been studied. Traction is one of the main factors limiting the field performance of the modern tractor. The process of developing traction has therefore been investigated by many researchers and although this study has resulted in a better understanding of the mechanics, it has not to any great extent assisted the operator to optimize performance in the field. It was concluded that in order to solve the problem the operator required a control system to maintain the dynamic load and inflation pressure at optimum levels. Work was carried out to develop and evaluate such a system using the Single Wheel Traction Research Vehicle at the USDA's National Soil Dynamics Laboratory in Auburn, Alabama, USA. A computer management system was developed to control the dynamic load, net traction and inflation pressure of the test tyre. During a simulated field operation the system was programmed to cycle the tyre over its operating range of dynamic load and inflation pressure while monitoring tractive efficiency. A tractive efficiency response surface was computed for the particular condition and the surface searched for the dynamic load and inflation pressure levels which resulted in maximum tractive efficiency. The tyre was then controlled and operated at maximum tractive efficiency. Evaluation showed that within the operating range of the tyre, tractive efficiency varied considerably with dynamic load, inflation pressure, net traction and soil condition. The results indicated that a considerable advantage could be obtained by using such an arrangement on a tractor. The system would automatically maximize the tractive efficiency of the tractor under the particular field conditions and with the particular implement being used. Implements could be ballasted and the hitch system used to control the weight transfer to ensure maximum tractive efficiency. Systems such as these would result in a significant improvement in the field performance of the machine and a reduction in management time required to optimize the performance of the tractor implement combination.
Combustion stress in compression-ignition engines.
(1989) Taylor, Andrew Bruce.; Meiring, Pierre Andre.
South Africa produces alternative fuels from a number of different sources. The properties of a fuel are known to affect the nature of combustion in compression-ignition engines significantly, and have occasionally resulted in engine failures. Combustion analyses have been conducted on a wide range of fuels and combustion has been thoroughly quantified. However, the role played by the different combustion variables in failures was not known. The result was that it was not possible to predict the implications of variations in the nature of combustion. There was thus a need to investigate the relative role of combustion variables in the failure of engines. The mechanisms of combustion and engine failure were studied. All the variables required to determine combustion and engine durability were measured simultaneously. This research required the development of a complete engine research facility as well as specialized transducers. Fast response surface thermocouples were designed and constructed in order to monitor transient surface temperatures. Heat transfer rates were then calculated with the aid of Fourier analysis. Dynamic stresses were monitored by strain-gauges applied to the engine. A special high speed data acquisition system was developed. An existing heat release model was modified and used to calculate combustion rates. A comprehensive finite element model was developed to calculate piston temperatures and stresses. The role of each combustion variable in stress and durability was investigated by statistical analysis. The results successfully identified the causes of combustion related engine failures. The primary cause of engine failure was found to be thermal loading. The principal cause of any variation in thermal loading and thus engine durability was maximum cylinder pressure. The life of the engine was proved to be determined almost entirely by peak cylinder pressure. The role of the rate of pressure rise was proved to be insignificant. All the implications of variations in the nature of combustion can now be determined accurately. It will thus be possible to optimise engine modifications and fuel properties before validation by durability testing.
A diagnostic quasi-dimensional model of heat transfer and combustion in compression-ignition engines.
(1989) Hansen, Alan Christopher.; Meiring, Pierre Andre.
Investigations into the combustion of alternative fuels in compression-ignition engines in South Africa have underlined the inadequacies of existing zero-dimensional combustion models. The major aspect of concern in these models was the computation of heat transfer which had been singled out by a number of researchers as the leading cause of inaccuracies in heat release computations. The main objective of this research was to develop a combustion model that was less empirically based than the existing zerodimensional models for use in evaluating the combustion and resulting thermal stresses generated by alternative fuels. in diesel engines. Particular attention was paid to the development of a spatial and temporal model of convective heat transfer that was based on gas flow characteristics and to the introduction of a radiation heat transfer model that made use of fuel properties and fuel-air ratio. The combustion process was divided into two zones representing burnt and unburnt constituents and the resulting temperatures in each zone were used in the calculations of convective and radiative heat transfer. The complete model was formulated in such a way that it could be applied with the aid of a micro-computer. Calibration and verification of the gas flow sub-models which involved the squish, swirl and turbulence components necessitated the use of published data. Good agreement for the squish and swirl components was obtained between the present model and the experimental data from three engines, two with a bowl-in-piston and the other with a flat piston. These gas flow components dominated the gas velocities in the combustion chamber and provided a reliable foundation for the calculation of convective heat transfer. In spite of the well documented difficulties of characterising turbulence, after calibration the model generated turbulence levels with acceptable trends and magnitudes. Tests were carried out on a naturally aspirated ADE 236 engine involving the measurement of cylinder pressure and heat flux at a single point. Motored engine data were used to verify the convective heat transfer rates and to ascertain the effects of soot deposition on the heat flux probe. Close correlation between predicted and measured heat flux was achieved after accounting for the effects of chamber geometry at the probe site. Soot deposition on the probe caused a significant attenuation of the heat flux within a short period of the engine running under fired conditions. The results from fired engine tests showed that the two zone combustion model was providing plausible trends in the burnt and unburnt zone temperatures and that the model generated combined heat transfer rates which were credible not only on a global basis but also in terms of point predictions in the combustion chamber. The results also highlighted the considerable variation in heat transfer that could occur from one point in the chamber to another. Such variations added considerable weight to the objective of moving away from a zero-dimensional model to a quasi-dimensional type where predictions could be made on a more localised rather than global basis. It was concluded that the model was a definite improvement over zero-dimensional models and competed favourably with existing quasi-dimensional models with advantages in both simplicity and accuracy.