DSpace Collection:https://dspace.lboro.ac.uk/2134/58072017-02-21T23:18:45Z2017-02-21T23:18:45ZFrictionally induced, self excited vibrations in a disc brake systemNorth, M.R.https://dspace.lboro.ac.uk/2134/238682017-01-25T12:04:03Z1972-01-01T00:00:00ZTitle: Frictionally induced, self excited vibrations in a disc brake system
Authors: North, M.R.
Abstract: This work describes an investigation into the frictionally induced, self excited vibrations which occur in braking systems and
are generally known as squeal. The.work is largely theoretical, but measurements made on a rig are used to correlate the predictions of
the theory with a practical brake system.
Following an historical review, the theoretical behaviour of
a brake disc is examined and adapted to predict the approximate natural frequencies and nodal spacings of an annular disc for a range of masses added to the disc to represent the pads and caliper.
Knowing the disc behaviour, it is then possible to propose an eight degree of freedom model which describes the caliper, pads and a lumped model of the disc in the immediate vicinity of the caliper. From this model it can be shown how self excited vibrations can arise in such a system, and squeal frequencies and mode shapes can be predicted.
The effects of stiffness non-linearity in the system are then investigated and it is shown that limit cycles will occur; conditions for obtaining mode shapes at the limit cycle are defined.
An experimental rig is described and measurements made on
the rig are given in some detail. Parameter values are inserted in
the mathematical models and mode shapes and natural frequencies are computed. These are compared with the measured mode shapes and
natural frequencies to give an assessment of correlation between the theory and the actual vibrational behaviour.
Despite the simple nature of the model used to represent the brake system, and the fact that a number of parameters are only known within wide limits,.correlation between the measured mode shape and squeal frequency and the calculated mode shapes and frequencies can be made good by the choice of suitable parameter values within the defined limits. For example, a 26% reduction in the caliper stiffness altered the mode of the disc vibration and hence caused a large change in squeal frequency, but insertion of the new parameter values into the equations showed that a corresponding instability was predicted at the correct frequency.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.1972-01-01T00:00:00ZOn the combustion of premixed natural gas/gasoline dual fuel blends in SI enginesPetrakides, Sotirishttps://dspace.lboro.ac.uk/2134/235472017-02-01T11:57:26Z2016-01-01T00:00:00ZTitle: On the combustion of premixed natural gas/gasoline dual fuel blends in SI engines
Authors: Petrakides, Sotiris
Abstract: The continuous update of challenging emission legislations has renewed the interest for the use of alternative fuels. The low carbon content, the knocking resistance, and the abundance reserves, have classified natural gas as one of the most promising alternative fuels. The major constituent of natural gas is methane. Historically, the slow burning velocity of methane has been a major concern for its utilisation in energy efficient combustion applications.
As emphasized in a limited body of experimental literature, a binary blend of methane and gasoline has the potential to accelerate the combustion process in an SI engine, resulting in a faster combustion even to that of gasoline. The mechanism of such effects remains unclear. This is partially owned to the inadequate prior scientific understanding of the fundamental combustion parameters, laminar burning velocity (Su0) and Markstein length (Lb), of a gasoline-natural gas Dual Fuel (DF) blend. The value of Lb characterises the sensitivity of the flame to stretch. The flame stretch is induced by aerodynamic straining and/or flame curvature.
The current research study has therefore being concerned on understanding the combustion mechanism of premixed gasoline - natural gas DF blends both on a fundamental as well as practical SI engine level. The understanding on the contribution of Su0 and Lb to the velocity of a stretched laminar propagating flame has been extended through numerical analysis. A conceptual analysis of the laminar as compared to the SI engine combustion allowed further insights on the effect of turbulence to the mass burning rate of the base fuels.
On a fundamental level, the research contribution is made through the quantification of the response of Su0 and Lb with the ratio of methane to PRF95 (95%volliq iso-octane and 5%volliq n-heptane) in a DF blend. Methane has been used as a surrogate for natural gas and PRF95 as a surrogate for gasoline. Constant volume laminar combustion experiments have been conducted in a cylindrical vessel at equivalence ratios of 0.8, 1, 1.2, initial pressures of 2.5, 5, 10 Bar, and a constant temperature of 373 K. Methane was added to PRF95 in three different energy ratios 25%, 50% and 75%. Spherically expanding flames visualised through schlieren photography were used to derive the values of Lb and Su0. It has been concluded that for pressures relevant to SI engine operation (>5bar) and stoichiometric to lean Air Fuel Ratios (AFRs), there is a positive synergy for blending methane to PRF95 due to the convergence of Lb of the blended fuel towards that of pure gas and Su0 towards that of pure liquid.
In an SI engine environment, the research contribution is made through the characterisation and scientific understanding of the mechanism of DF combustion, and the importance of flame-stretch interactions at various engine operating conditions. Optical diagnostics have been integrated with in-cylinder pressure analysis to investigate the mechanism of flame velocity and stability with the addition of natural gas to gasoline in a DF blend, under a sweep of engine load (Manifold Absolute Pressure = 0.44, 0.51. 0.61 Bar), speed (1250, 2000, 2750 RPM) and equivalence ratio (0.8, 0.83, 1, 1.25). Consisted with the constant volume experiments, natural gas was added to gasoline in energy ratios of 25%, 50% and 75%. It has been concluded that within the flamelet combustion regime the effect of Lb is dominating the lean burn combustion process both from a flame stability and velocity prospective. The effect of Su0 on the combustion process gradually increases as the AFR shifts from stoichiometric to fuel rich values.
For stoichiometric to fuel lean mixtures, the effect of turbulence on the increase of the mass burning rate is on average 13% higher for natural gas as compared to gasoline. The higher turbulence sensitivity of natural gas is attributed to its lower Lb value.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.2016-01-01T00:00:00ZInfluence of asymmetric valve timing strategy on in-cylinder flow of the internal combustion engineButcher, Daniel S.A.https://dspace.lboro.ac.uk/2134/233272016-11-30T16:16:54Z2016-01-01T00:00:00ZTitle: Influence of asymmetric valve timing strategy on in-cylinder flow of the internal combustion engine
Authors: Butcher, Daniel S.A.
Abstract: Variable Valve Timing (VVT) presents a powerful tool in the relentless pursuit of efficiency improvements in the internal combustion engine. As the valves have such ultimate control over the gas exchange processes, extensive research effort in this area has shown how valve event timing can be manipulated to reduce engine pumping losses, fuel consumption and engine out emissions. Pumping losses may be significantly reduced by use of throttleless strategies, making use of intake valve duration for load control, while alternative cycles such as the Miller cycle allow modification of the effective compression ratio.
More recently, the value of single valve operation in part load conditions is exploited, bringing with it the concept of asymmetric valve lifts. Work in this area found the side effect of asymmetric valve operation is a significant change in the behaviour of the in-cylinder flow structures, velocities and turbulence intensity. Work presented in this thesis exploits asymmetric valve strategies to modify the in-cylinder flow conditions.
The Proper Orthogonal Decomposition (POD) is a method employed in the fluids dynamics field to facilitate the separation of coherent motion structures from the turbulence. In the presented work, the application of POD to in-cylinder flow analysis is further developed by the introduction of a novel method for identifying the POD modes representative of coherent motion and those representative of the turbulence. A POD mode correlation based technique is introduced and developed, with the resulting fields showing evidence of coherence and turbulence respectively.
Experimental tests are carried out using a full length optically accessible, single cylinder research engine equipped with a fully variable valve train (FVVT) to allow full control of both valve timing and lift. In-cylinder flow is measured through the use of Particle Image Velocimetry (PIV) at several crank angle timings during the intake stroke whilst the engine is operated under a range of asymmetric valve strategies. The exhaust valves and one intake valve have their respective schedules fixed, while the second intake valve schedule is adjusted to 80\%, 60\%, 40\%, 20\%, 0\% lift.
The resulting PIV fields are separated into coherent motion and turbulence using the developed technique, allowing for analysis of each constituent independently. The coherent element gives insight to large scale flows, often of the order of magnitude of the cylinder. These structures not only give a clear indication of the overall motion and allow assessment of flow characteristics such as swirl and tumble ratio, but the variation in the spatial location of these structures provides additional insight to the cyclic to cycle variation (CCV) of the flow, which would not otherwise be possible due to the inclusion of the turbulent data. Similarly, with the cyclic variation removed from the turbulent velocity field, a true account of the fluctuating velocity, u' and derived values such as the Turbulent Kinetic Energy (TKE) may be gained.
Results show how manipulation of a one intake valve timing can influence both the large scale motions and the turbulence intensity. By the reduction of lift, the swirl ratio is increased almost linearly as the typical counter-rotating vortex pair becomes asymmetric, before a single vortex structure is observed in the lowest lift cases. A switching mechanism between the two is identified and found to be responsible for increased levels of CCV. With the reduction in lift, TKE is observed not only to increase, but change the spatial distribution of turbulence.
Of course, the reduction in valve lift comes with the penalty of a reduced valve curtain area. However, it was identified both in literature and throughout this study that the reduction in lift did not negatively influence the engine breathing as the same trapped mass was achieved under all cases with no adjustment of manifold pressure. While literature shows both bulk motion and turbulence are key in liquid fuel break-up during the intake stroke, the mixing effects under port-injected natural gas were investigated experimentally using Laser Induced Fluorescence (LIF). The valve strategy was found to have no significant effect on the mixture distribution at the time of spark.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.2016-01-01T00:00:00ZTheoretical and experimental aerodynamic analysis for high-speed ground vehiclesFarhan, Ismail H.https://dspace.lboro.ac.uk/2134/222362016-08-08T13:24:39Z1991-01-01T00:00:00ZTitle: Theoretical and experimental aerodynamic analysis for high-speed ground vehicles
Authors: Farhan, Ismail H.
Abstract: An improved understanding of the aerodynamics of high-speed ground
vehicles can lead to significant reductions in the energy consumption required for
propulsion, an increase of vehicle cruising speed, and an increase in the safety and
comfort of passengers. To contribute to these goals, this thesis employs theoretical and
experimental techniques to investigate the air flow around a proposed geometry for a
high-speed electromagnetic suspension (EMS) train. Train motion at normal cruising
speed in still air and in crosswind conditions are studied, considering aerodynamic
forces and moments, the wake in the lee side of the train and the turbulent boundary
layer development.
The theoretical prediction work may be conveniently divided into two parts, for
inviscid flow, and with viscous effects included. In the first, a numerical technique
called the panel method has been applied to the representation of the body shape and the
prediction of the potential flow and pressure distribution. Two computer programmes
have been written, one for a single vehicle in the presence of the ground at different
yaw angles, and the second for application to two body problems, e.g. a train passing a
railway station or a train passing the central part of another train. Both programmes
have been developed in fully three-dimensional form, but are currently based purely on
the source distribution method. This limits the applicability of the method, in particular
to small angles of yaw, but useful results are still obtainable. In the second part of the
theoretical prediction work, two methods based on the momentum integral equations
for three-dimensional boundary layer flow have been developed for use with the
aforementioned potential flow analysis; these predict the development of the
three-dimensional turbulent boundary layer (i) on the central section (for the analysis of
crosswind conditions) and (ii) on the nose of the train.
The primary interest of the experimental programme was to provide qualitative
and quantitative results for comparison with the theoretical predictions as well as to give
insight into the flow behaviour around the train. The experimental tests also provided
the first results for the influence of both stationary and moving ground planes on the
EMS train. Extensive wind tunnel tests were performed on four purpose-made models
of the high-speed train to measure aerodynamic forces, moments and pressures to
establish ground effect characteristics. The experimental results demonstrated the
importance of ground clearance. Flow visualisation showed that the wake vortices
were both stronger and larger in the presence of a ground. At small yaw angles ground
clearance had little effect, but as yaw increased, larger ground clearance led for
example to substantial increase in lift and side force coefficients. The wind tunnel tests
also identified the differences between a moving and a fixed ground plane. The
measured data showed that the type of ground simulation was significant only in the
separated region.
A comparison of the results predicted using potential flow theory for an EMS
train model and the corresponding results from wind tunnel tests indicated good
agreement in regions where the flow is attached. For small yaw angles, not more than
15° , predicted pressure distributions reproduced measured behaviour. For greater
angles, the shed vorticity (associated with flow separation) has a strong effect on the
surface pressure field and this would have to be introduced into the panel method to
improve prediction.
The turbulent boundary layer calculations for the train in a crosswind condition
showed that the momentum thickness along the crosswind surface distance co-ordinate
increased slowly at the beginning of the development of the boundary layer but then
increased sharply at the side top roof on the lee side. The sharp increase is believed to
indicate a tendency for flow separation as the solution procedure exhibits signs of
failure in this region. Suggestions are made in the thesis for ways of improving both
this and other aspects of the theoretical approach.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.1991-01-01T00:00:00Z