Thesis (MTech. degree in Mechanical Engineering.)--Tshwane University of Technology, 2008.

The introduction of alternative fuels such as natural gas is likely to occur at an increasing rate. The dual-fuel concept allows these low cetane number fuels to be used in compression-ignition (CI, diesel) type engines. Most CI engine conversions have pre-mixed the alternative fuel with air in the intake manifold while retaining diesel injection into the cylinder for ignition. The advantage is that it is simple for practical adaptation; the disadvantage is that good substitution levels are only obtained at midload. A better solution is to inject both the alternative and diesel fuels directly into the cylinder. Here, the fuel in the end-zone is limited and the diesel, injected before the alternative, has only a conventional ignition delay. This improves the high-end performance. Modern, very high pressure diesel injectors have good turndown characteristics as well as better controllability. This improves low-end performance and hence offers an ideal platform for a dual-fuel system. Several systems already exist, mainly for large marine engines but also a few for smaller, truck-sized engines. For the latter, the key is to produce a combined injector to handle both fuels which has the smallest diameter possible so that installation is readily achieved. There exists the potential for much improvement. A combined gas/diesel injection system based on small, high pressure common-rail injectors has been tested for fluid characteristics. Spray properties have been examined experimentally in a test rig and modelled using CFD. The CFD package Fluent was used to model the direct-injection of natural gas and diesel oil simultaneously into an engine. These models were initially calibrated using high-speed photographic visualisation of the jets. Both shadowgraph and schlieren techniques were employed to identify the gas jet itself as well as mixing regions within the flow. Different orientations and staging of the jets with respect to each other were simulated. Salient features of the two fuel jets were studied to optimise the design of a dual-fuel injector for CI engines. Analysis of the fuel-air mixture strength during the injection allowed the ignition delay to be estimated and thus the best staging of the jets to be determined.

published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management

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. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1989.

Variable valve actuation remains one of the most studied technologies for diesel engines for fuel benefits, efficiency improvements and emission control. The same can be implemented on natural gas engines however presence of throttle valve in the spark ignited natural gas engine leads to different set of challenges and outcomes. In this document, focus is on GT power led analyses for a mid-range natural gas engine and the VVA strategy applied is modulation of intake valve closure timing. The simulations are run for early intake valve closure and late intake valve closure, both applied independently and run for steady state conditions. The focus is on the low torque range to study the impact of IVC modulation on throttling losses for low torque region. The simulation studies showed that IVC strategies both early as well as late IVC do benefit in terms of thermal efficiency improvements by up to 3% and reduction in brake specific fuel efficiency by up to 13%.It also showed considerable reduction in pumping loop and increase in open cycle efficiency when IVC modulation is applied. Validating the model further with real on-engine data and then calibrating the existing GT power with the on-engine data to validate the conclusions drawn would be the next set of goals for this project.<div><br></div><div>Second part of this document is focused on real life testing of soy biodiesel fueled heavy duty on-road engine with modern exhaust aftertreatment system with SCR. Soybean based biodiesel remains one of the most sought-after alternate fuel and biofuel to be used in on-road engines. Burning biodiesel leads to a cleaner exhaust compared to conventional diesel as the biofuel is oxygenated fuel leading to more complete combustion and lower amount of emission species such as CO, CO2and PM in the exhaust. The experiments discussed in this document consisted of developing torque curve envelopes and steady state tests (RMC set points). Three soy biodiesel blends were studied which included B20-20% biodiesel, B50 –50% biodiesel and B100 –100% biodiesel. NOx emissions were observed to be considerably higher for B100 at engine outlet by up to80% as well as at tailpipe outlet increased by up to380%, compared to that of conventional diesel which is attributed to the thermal mechanism of NO production. The exhaust gas temperatures were observed to be lower by up to40-degreeC while the urea dosing was considerably higher by up to83% when using biodiesel blend B100.Thisresearch paves the way to testing further using varying biodiesel blends for regulation certification trials, for tuning the diesel engines for different biodiesel blends and for developing the control strategy for the existing diesel engines to accommodate biodiesel.<br><div><br></div></div>

published_or_final_version / Public Administration / Master / Master of Public Administration

Call number: LD2668 .T4 1984 M37 / Master of Science

published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management

No description available.

published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management