Interactions between charge conditioning, knock and spark-ignition engine architecture

There are currently many factors motivating car manufacturers to reduce the tailpipe CO2 emissions from their products. One of the major routes to achieving reduced CO2 emissions in spark-ignition 4-stroke engines is to ‘downsize’ the swept volume which, among other advantages, reduces the proportion of fuel energy expended on pumping losses. The full-load performance deficit caused by reducing the swept volume of the engine is normally recovered by pressure charging. One of the limits to pressure charging is combustion knock, which is the unintended autoignition of the last portion of gas to burn in the combustion chamber after combustion has been initiated. This thesis presents results from investigations into a number of methods for suppressing knock, including (1) tests where the density of the intake air is closely controlled and the effect of charge air temperature is isolated, (2) where the latent heat of vaporization of a fuel is used to reduce the outlet temperature of a supercharger, and (3) where the engine architecture is configured to minimize exhaust gas residual carryover to the benefit of stronger knock resistance. Extensive comparison of this resulting engine architecture is made with published data on other strategies to reduce the effect of the knock limit on engine performance and efficiency. Several such strategies, including cooled EGR, were then investigated to see how much further engine efficiency (in terms of brake specific fuel consumption) could be improved if they are adopted on an engine architecture which has already been configured with best knock limit performance in mind. Within the limits tested, it was found that if the charge air density is fixed then the relationship between knock-limited spark advance and air temperature is linear. This methodology has not been found in the literature and is believed to be unique, with important ramifications for the design of future spark-ignition engine charging systems. It was also found that through a combination of an optimized direct-injection combustion system, an exhaust manifold integrated into the cylinder head, and a 3-cylinder configuration, an engine with extremely high full-load thermal efficiency can be created. This is because these characteristics are all synergistic. Against the baseline of such an engine, other technologies such as excess air operation and the use of cooled EGR are shown to offer little improvement. When operating a pressure-charged engine on alcohol fuel, it was found that there exists a maximum proportion of fuel that can be introduced before the supercharger beyond which there is no benefit to charge temperature reduction by introducing more. Strategies for reducing the amount of time when such a system operates were developed in order to minimize difficulties in applying such a strategy to a practical road vehicle. Finally, a new strategy for beneficially employing the latent heat of vaporization of the fuel in engines employing cooled EGR by injecting a proportion of the fuel charge directly into the EGR gas is proposed. This novel approach arose from the findings of the research into pre-supercharger fuel introduction and cooled EGR

Design of an advanced air path test stand for steady and transient evaluation

Different air systems such as turbochargers (TC), hybrid boosting, turbo compounding and exhaust gas recirculation (EGR) are increasingly used to improve the thermal efficiency of internal combustion engines (ICE). One dimensional (1D) gas dynamic codes supports their development and integration by modelling the engine and air systems and reducing testing time. However, this approach currently relies on steady flow characteristic maps which are inaccurate for simulating transient engine conditions. This is a key weakness of using gas-stand measured maps in engine simulations. Performing TC mapping on an engine would in principle solve this problem, however engine-based mapping is limited by the engine operating range and on these facilities, high-precision measurements are challenging. In addition, simple turbocharging can no longer be constrained to an individual TC supplying boost air to an engine. Instead, modern downsized engines require air-path system making use of multiple components including TCs, mechanical superchargers, electrically driven compressors (EDCs), EGR paths and control valves. Thus studying multiple air systems requires an experimental test facility to understand how they work in synergy. This is also useful in developing empirical models to minimize test time. Therefore the aim of this paper is to present a novel experimental facility that is flexibly designed for evaluating air systems individually and also at the system level representing a complicated air path both in steady and transient condition. The advanced test facility is built around a 2.2 l diesel engine to test the above air systems which can isolate the thermal and load transients from engine pulsating flows. Removing the flow pulsation allows study of the system characteristics in a steady state. Several examples of component and system level tests including a two-stage air path comprising of a VGT (variable geometry turbine) TC and a 48V EDC with typical operating condition (provided by 1D modeling) are discussed.</p

Comparison of 1-D Modelling Approaches for Wankel Engine Performance Simulation and Initial Study of the Direct Injection Limitations

Recent interest in the possible use of Wankel engines as range extenders for electric vehicles has prompted renewed investigations into the concept. While not presently used in the automotive industry, the type is well established in the unmanned aerial vehicles industry, and several innovative approaches to sealing and cooling have recently been developed which may result in improved performance for ground vehicle applications.One such UAV engine is the 225CS, a 225 cc/chamber single-rotor engine manufactured by Advanced Innovative Engineering (UK) Ltd. To be able to analyse the parameters, opportunities and limitations of this type of engine a model was created in the new dedicated Wankel modelling environment of AVL BOOST. For comparison a second model was created using the established method of modelling Wankel engines by specifying an ‘equivalent’ 3-cylinder 4-stroke reciprocating engine. The output from both of these models was evaluated using engine test data supplied by Advanced Innovative Engineering (UK) Ltd. The model created in the dedicated Wankel environment was found to fit the experimental data more closely.The model was then used to evaluate the impact on performance and fuel economy of applying direct injection to a Wankel rotary engine. This potential is because the nozzle can be situated in the cold side of the trochoid housing, taking advantage of the longer intake phase of the Wankel in turn permitting lower delivery pressures (the intake ‘stroke’ having 270 degrees of eccentric shaft rotation vs. 180 degrees for the reciprocating engine), plus the fact that the injector can be shielded from combustion pressure and hot burned gases. As it was found to be more accurate, the dedicated Wankel model was used to analyse the interrelationships between injector position, injection pressure and engine speed.Although a number of assumptions were required, and these will affect the accuracy of the model, the results provide a reasonable preliminary assessment of the feasibility of applying direct injection to the 225CS engine. A notable finding was that injection pressures of approximately 4.5 bar should be sufficient to supply fuel at all engine speeds and that the optimum position for the injector (for maximum fuel injection) corresponded to a position defined by the rotor apex tip at 597 degrees of eccentric shaft rotation after top dead centre firing. The advantage of both the injection pressure and injector location suggests a less complex fuel system design (compared to equivalent reciprocating systems) is possible at a reduced cost

A comparison of the flow fields generated for spark and controlled auto-ignition

Valve timing strategies aimed at producing internal exhaust gas re-circulation in a conventional spark ignition, SI, engine have recently demonstrated the ability to initiate controlled auto-ignition, CAI. Essentially the exhaust valves close early, to trap a quantity of hot exhaust gases in-cylinder, and the fresh air-fuel charge is induced late into the cylinder and then mixing takes place. As a logical first step to understanding the fluid mechanics, the effects of the standard and modified valve timings on the in-cylinder flow fields under motored conditions were investigated. Laser Doppler anemometry has been applied to an optical engine that replicates the engine geometry and different valve cam timings. The cycle averaged time history mean and RMS velocity profiles for the axial and radial velocity components in three axial planes were measured throughout the inlet and compression stroke. The turbulent mixing for the two cases are described in terms of the flow field maps of the velocity vectors, vorticity and turbulence kinetic energy and the integrated tumble ratio as a function of crankangle

Homogeneous Charge Compression Ignition combustion and fuel composition

Homogeneous charge compression ignition, HCCI, combustion has potentials to deliver high efficiency and negligible cycle-to-cycle variations, while keeps NOx and particulate emissions at very low levels in comparison with conventional SI and CI combustion concepts. Since HCCI combustion is an auto ignited combustion, fuel structure has direct impact on its auto-ignition performance. In this research, by mixing iso-octane and n-heptane, the auto-ignition nature of fuels with different research octane number, RON, were simulated and analysed using a single-zone engine combustion model with detailed chemical kinetics and convective heat transfer loss. The effects of internally recirculated engine exhaust gas, IEGR, as a potential control strategy was also calculated

The effect of oxygenate fuels on PN emissions from a highly boosted GDI engine

Gasoline Direct Injection (GDI) engines are increasingly available in the market. Such engines are known to emit more Particulate Matter (PM) than their port-fuel injected predecessors. There is also a widespread use of oxygenate fuels in the market, up to blends of E85, and their impact on PN emissions is widely studied. However the impact of oxygenate fuels on PN emissions from downsized, and hence highly-boosted engines is not known. In this work, PN emissions from a highly boosted engine capable of running at up to 35 bar Brake Mean Effective Pressure (BMEP) have been measured from a baseline gasoline and three different oxygenate fuels (E20, E85, and GEM – a blend of gasoline, ethanol, and methanol) using a DMS500. The engine has been run at four different operating points, and a number of engine parameters relevant to highly-boosted engines (such as EGR, exhaust back pressure, and lambda) have been tested – the PN emissions and size distributions have been measured from all of these. The results show that the oxygenate content of the fuel has a very large impact on its PN emissions, with E85 giving low levels of PN emissions across the operating range, and GEM giving very low and extremely high levels of PN emissions depending on operating point. These results have been analysed and related back to key fuel properties

Axion Radiation from Strings

This paper revisits the problem of the string decay contribution to the axion cosmological energy density. We show that this contribution is proportional to the average relative increase when axion strings decay of a certain quantity $N_{\rm ax}$ which we define. We carry out numerical simulations of the evolution and decay of circular and non-circular string loops, of bent strings with ends held fixed, and of vortex-antivortex pairs in two dimensions. In the case of string loops and of vortex-antivortex pairs, $N_{\rm ax}$ decreases by approximately 20%. In the case of bent strings, $N_{\rm ax}$ remains constant or increases slightly. Our results imply that the string decay contribution to the axion energy density is of the same order of magnitude as the well-understood contribution from vacuum realignment.Comment: 29 pages, 10 figure

Generalizations of normal ordering and applications to quantization in classical backgrounds

A nonlocal method of extracting the positive (or the negative) frequency part of a field, based on knowledge of a 2-point function, leads to certain natural generalizations of the normal ordering of quantum fields in classical gravitational and electromagnetic backgrounds and illuminates the origin of the recently discovered nonlocalities related to a local description of particles. A local description of particle creation by gravitational backgrounds is given, with emphasis on the case of black-hole evaporation. The formalism reveals a previously hidden relation between various definitions of the particle current and those of the energy-momentum tensor. The implications to particle creation by classical backgrounds, as well as to the relation between vacuum energy, dark matter, and cosmological constant, are discussed.Comment: 17 pages, revised, title shortened, to appear in Gen. Rel. Gra

Academic freedom: in justification of a universal ideal

This paper examines the justification for, and benefits of, academic freedom to academics, students, universities and the world at large. The paper surveys the development of the concept of academic freedom within Europe, more especially the impact of the reforms at the University of Berlin instigated by Wilhelm von Humboldt. Following from this, the paper examines the reasons why the various facets of academic freedom are important and why the principle should continue to be supported