Thesis (M.S. in Mechanical Engineering and Mechanical Engineer) Naval Postgraduate School, December 1998. / "December 1998." Thesis advisor(s): Knox T. Millsaps, Jr. Includes bibliographical references (p. 105-107). Also available online.

Thesis (M.S.)--University of Wisconsin--Madison, 1969. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

This study is aimed at understanding the effects of changing the upstream nozzle geometry on the development of rectangular free jets. An existing converging rectangular nozzle with an exit aspect ratio of 4 and a circular inlet (AR4 nozzle) has been used as the basic configuration for this work. The study is primarily based on the results of numerical simulations wherein the internal geometry variation is accomplished by changing the inlet aspect ratio (AR,) and the length of the converging section, expressed as a ratio with respect to the length of the nozzle (called 'converging section ratio*, CSR); all the other parameters are kept constant. The results from LDA experiments done on the AR4 nozzle are presented and used as validation data for the CPD simulations. Analyses of the numerical results help in understanding the variation of the jet spreading for different combinations of AR, and CSR. Two parameters are identified for describing the jet development: the cross-over point (XC), defined as the location downstream of the exit where the jet half-velocity-widths (B) along the major and minor axes are equal, and the difference in the half-velocity-widths at 30 nozzle equivalent diameters (Dm) from the exit (AB30), to ascertain the occurrence of axis-switching. For a given AR,, XC varies linearly with CSR; the variation of XC is non-linear with AR, for a constant CSR. The A1330 variation is non-linear with both AR, and CSR; the other variable being kept constant. The data obtained from the simulations are further used to propose two parametric models which can be used to predict the occurrence of axis-switching, within the scope of this work. The parametric models are validated and future work is proposed.

This study is aimed at understanding the effects of changing the upstream nozzle geometry on the development of rectangular free jets. An existing converging rectangular nozzle with an exit aspect ratio of 4 and a circular inlet (AR4 nozzle) has been used as the basic configuration for this work. The study is primarily based on the results of numerical simulations wherein the internal geometry variation is accomplished by changing the inlet aspect ratio (AR,) and the length of the converging section, expressed as a ratio with respect to the length of the nozzle (called 'converging section ratio*, CSR); all the other parameters are kept constant. The results from LDA experiments done on the AR4 nozzle are presented and used as validation data for the CPD simulations. Analyses of the numerical results help in understanding the variation of the jet spreading for different combinations of AR, and CSR. Two parameters are identified for describing the jet development: the cross-over point (XC), defined as the location downstream of the exit where the jet half-velocity-widths (B) along the major and minor axes are equal, and the difference in the half-velocity-widths at 30 nozzle equivalent diameters (Dm) from the exit (AB30), to ascertain the occurrence of axis-switching. For a given AR,, XC varies linearly with CSR; the variation of XC is non-linear with AR, for a constant CSR. The A1330 variation is non-linear with both AR, and CSR; the other variable being kept constant. The data obtained from the simulations are further used to propose two parametric models which can be used to predict the occurrence of axis-switching, within the scope of this work. The parametric models are validated and future work is proposed.

The high speed 2-stroke cycle engine designed for high power-to-weight ratio is relatively inefficient at part throttle. It would be advantageous to incorporate a simple method of allowing extra air to enter the cylinder prior to the fresh mixture, thus stratifying the charge and increasing the proportion of the air-fuel mixture retained in the cylinder at part loads while not deleteriously affecting the maximum power at full throttle. A series of tests, on an engine fitted with a reed valve connecting the atmosphere to the passageway leading to intake ports, were carried out with varying amounts of extra air, the results showed that power, speed, thermal efficiency, and fuel trapping efficiency gave increases at nearly all settings, but with a large excess of extra air, the air-fuel ratio through the carburetor had to be decreased to maintain stable operation. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

Thesis (M.S.)--California Polytechnic State University, 2009. / Title from PDF title page; viewed on January 12, 2010. Major professor: Patrick Lemieux, Ph.D. "Presented to the faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Mechanical Engineering." "December 2009." Includes bibliographical references (p. 46).

The design trend for light duty diesel engines is towards lower compression ratio and higher turbocharger boost. This can enable higher specific power and lower pollutant emissions to be achieved, but raises concerns that cold start operation might be adversely affected. This is investigated and quantified through the study of a modern light duty diesel engine at two compression ratios and temperatures down to -20ºC. Key indicators of cold start performance are the magnitude and cycle-to-cycle variation of indicated mean effective pressure. Initial studies were carried out at 300 rpm, a speed representative of post-first-fire conditions. Studies were then conducted at higher engine speeds representative of cold idle. The utility of different injection strategies, timings and quantities is investigated when varying test temperature and engine speed through a range of values encountered during the cold start phase of engine operation. The importance of the glow plug as a cold start aid is also investigated by varying its operating temperature and protrusion into the combustion chamber. The indicated mean effective pressure was used to assess the effects of varying input parameters, and gross heat release rate information is used to identify the phenomena responsible for desirable or undesirable characteristics. Reduction in compression ratio led to no deterioration of initial start performance from speeds just above cranking, provided an appropriate injection strategy was chosen. Higher indicated mean effective pressure was possible at low speeds using low compression ratio due to reduced losses and more complete combustion. Cycle-to-cycle variability in indicated mean effective pressure increased markedly for both compression ratios at engine speeds representative of cold idle, especially when test temperature was reduced. Stability reduction was more severe at low compression ratio. Multiple pilot injections at high compression ratio cold idle resulted in better cycle-to-cycle stability. Analysis of heat release profiles suggested that additional pilots assisted fuel mixing, a conclusion supported by a computational fluid dynamics model. Multiple pilots created a more homogeneous fuel distribution through the bowl at time of main injection. Multiple pilots could not stabilise operation at low compression ratio. Improvement in cold idle at low compression ratio was achieved by increasing glow plug temperature, which significantly increased the rate of fuel preparation. This increased the initial rate of heat release and resulted in significantly less variation in the heat release rate profiles. Small changes in glow plug protrusion rapidly degraded cold idle performance, indicating the importance of correct design.

Partially-premixed Charge Compression Ignition (PCCI) combustion is defined by increased levels of premixed charge whilst retaining control over combustion through injection timing. An experimental investigation has been carried out on a current generation DI diesel engine, equipped with High Pressure Common Rail (HPCR) fuel injection equipment and an external Exhaust Gas Recirculation (EGR) system. The aims of the investigation were to determine the constraints imposed on operating a PCCI combustion strategy with the aim of simultaneously reducing engine-out net soot and NOx emissions. The work was carried out at fully-warm steady-state conditions at engine speeds of 1500 rpm and 1800 rpm, predominantly using a single injection strategy. With a single injection the Start of Injection (SOI), fuel rail pressure, and rate of EGR have been examined with a view to realising PCCI combustion. Timing ranges of -20º to +3ºATDC, rail pressures of 500-1200 bar, and EGR rates of 0-60% have been investigated. The responses looked at have been engine-out soot, NOx, HC, and CO emissions, fuel consumption, and combustion noise. It is shown that variation of the parameters has allowed PCCI combustion to be achieved in a restricted operating region, offering improvement in the NOx-soot trade-off. This region is limited on the available test engine by oxygen availability due to the specifications of the turbocharger and EGR systems. Engine speeds up to 2000 rpm (at 2.5 bar BMEP), and loads of 4.4 bar gross IMEP (at 1500 rpm) have been found to be the limits, beyond which soot and CO emissions rise excessively. It is shown that enhancing the mixing time and intensity are both desirable in achieving PCCI combustion. The net soot reduction mechanism exploited with PCCI combustion strategies is reducing soot formation to outweigh the reduction in oxidation. Enhancing the mixing intensity by increasing injection pressure is highly effective at reducing soot output, but at the expense of brake specific fuel consumption. Increasing the mixing time can also be effective in reducing soot output, but careful parameter selection is required to avoid excessive soot output. Retarded or highly advanced injection timings are shown to reduce net soot output, but both have associated trade-offs and penalties. Retarding combustion is effective at lowering soot and NOx emissions with low associated noise, but a fuel economy penalty is paid. Advanced combustion phasing can result in large peak rates of increase of pressure, which have been shown to correlate well with combustion noise. Overall soot reductions of up to 97% were achieved, but with associated penalties. One of the most acceptable reductions of ~90% came at the cost of a 6% increase in fuel consumption, highlighting that improvements in emissions are achievable with PCCI strategies with acceptable trade-offs.

An experimental investigation of post-start cold idling behaviour has been carried out on a modern single-cylinder HPCR DI light duty diesel engine with a low compression ratio of 15.5:1 at temperatures between 10 and -20°C. The trend toward lower compression ratios from more common values of around 22:1 a few years ago has resulted in lower compression pressures and temperatures, which negatively affects cold idle operation. Improvements in cycle-by-cycle stability of indicated work output through fuel injection strategy and glow plug temperature changes have been explored. This is important to improve NVH and the consumer’s perception of vehicle quality. The key effects on heat release characteristics have been identified and the associated impact on stability discussed. High speed imaging of ignition in a combustion bomb has been used to aid interpretation of engine results. Up to four pilot injections placed in advance of the main have been used. Shorter separation between pilots and pilot-to-main improves stability independent of the number of pilot injections and extends the range of main injection timings to meet target stability of 10% or lower at -20°C. Increasing the number of pilot injections was effective in stabilising combustion at all investigated soak temperatures at fuelling levels producing indicated work required to match friction and ancillary demands. Stability can be susceptible to deterioration at moderate soak temperatures because fuelling demand is relatively low. If a high number of pilot injections are to be avoided to reduce potential wear, then increasing main injection quantity is an effective method to stabilise combustion for a lower pilot number strategy but any increase above target load has to be harnessed by additional ancillary devices. Very high glow plug temperatures of up to 1200°C were examined using a smaller diameter tip ceramic type design. Stable combustion cannot be achieved through higher glow plug temperatures alone. A temperature of 1000°C, which can be achieved using a low voltage metallic type, is adequate to stabilise combustion when combined with a triple-pilot strategy at sub-zero temperatures. The best stability is achieved using 1200°C, which can only be achieved using a more expensive ceramic type, in combination with a triple-pilot strategy producing the desirable target of ~5% or below; the effects are not mutually exclusive. At high glow plug temperatures and using three or four pilot injections, stability improved with warmer soak temperatures. At -5°C, stability was relatively poor when one or two pilots were used irrespective of glow plug temperature. A high premixed contribution to main combustion is associated with improved stability. Minimum threshold values are necessary to stabilise combustion: ~25 J/° at -20°C, ~20 J/° at -5°C and only ~10 J/° at 10°C. A higher number of pilot injections raises pilot induced combustion and improves mixture distribution. These effects subsequently increase the premixed combustion and help sustain a strong main development with less variability. This benefit is maximised when using hotter glow plug temperatures raising IMEPg magnitude and reducing variation.

The research reported in the thesis addresses questions of how engine fuel consumption and carbon dioxide emissions are can be reduced through improvements in thermal management, lubricant design, and energy recovery. The investigations are based on simulation studies using computational models and sub-models developed or revised during the work, and results provided by complementary experimental studies carried out by collaborating investigators. The brake thermal efficiency of the internal combustion engines (ICE) used in cars and light duty commercial vehicles is reduced by frictional losses. These losses vary with engine design, lubricant formulation and thermal state. They are most significant when the engine is running cold or partially warm. Over the New European Drive Cycle (NEDC), engine friction losses raise vehicle fuel consumption by several percentage points. A version of the computational model, PROMETS, has been developed and applied in studies of thermal behaviour, friction and engine lubricant to investigate the performance of a 2.0l, I4 GTDI spark ignition engine and in particular, how these influence fuel consumption over the NEDC. Core parts of PROMETS include a physics-based, empirically calibrated friction model, a cycle averaged description of gas-to-structure heat transfer and a lumped capacity description of thermal behaviour of the engine block and cylinder head. In the thesis, revisions to the description of friction and interactions between friction, local thermal conditions and lubricant are reported. It is shown that the bulk temperature of coolant rather than oil has the stronger influence on friction at the piston-liner interface, whilst bulk oil temperature more strongly influences friction in crankshaft bearings and other lower engine components. However, local oil film temperatures have a direct influence on local friction contribution. To account for this, local values of oil temperature and viscosity are used in describing local friction contributions. Implementation required an oil system model to be developed; an iterative model of the frictional dissipation within the main bearings, and a prediction of piston cooling jet heat transfer coefficients have been added to the oil circuit. Simulations of a range of scenarios and design changes are presented and analysed in the thesis. The size of the fuel savings that could potentially be made through improved thermal management has been demonstrated to be 4.5% for the engine being simulated. Model results show that of the friction contributing surfaces, the piston group is responsible for the highest levels of friction, and also exhibits the largest absolute reduction in friction as the temperature of the engine rises. The relatively low warm-up rate of the lower engine structure gives a correspondingly slow reduction in friction in crankshaft bearings from their cold start values. Measures to accelerate this reduction by raising oil temperature have limited effect unless the strong thermal links between the oil and the surrounding metal are broken. When additional heating is applied to the engine oil, only around 30% is retained to raise the oil temperature due to these thermal links.