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

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

Search for Higgs Bosons in e+e- Collisions at 183 GeV

The data collected by the OPAL experiment at sqrts=183 GeV were used to search for Higgs bosons which are predicted by the Standard Model and various extensions, such as general models with two Higgs field doublets and the Minimal Supersymmetric Standard Model (MSSM). The data correspond to an integrated luminosity of approximately 54pb-1. None of the searches for neutral and charged Higgs bosons have revealed an excess of events beyond the expected background. This negative outcome, in combination with similar results from searches at lower energies, leads to new limits for the Higgs boson masses and other model parameters. In particular, the 95% confidence level lower limit for the mass of the Standard Model Higgs boson is 88.3 GeV. Charged Higgs bosons can be excluded for masses up to 59.5 GeV. In the MSSM, mh > 70.5 GeV and mA > 72.0 GeV are obtained for tan{beta}>1, no and maximal scalar top mixing and soft SUSY-breaking masses of 1 TeV. The range 0.8 < tanb < 1.9 is excluded for minimal scalar top mixing and m{top} < 175 GeV. More general scans of the MSSM parameter space are also considered.Comment: 49 pages. LaTeX, including 33 eps figures, submitted to European Physical Journal

A Measurement of the Product Branching Ratio f(b->Lambda_b).BR(Lambda_b->Lambda X) in Z0 Decays

The product branching ratio, f(b->Lambda_b).BR(Lambda_b->Lambda X), where Lambda_b denotes any weakly-decaying b-baryon, has been measured using the OPAL detector at LEP. Lambda_b are selected by the presence of energetic Lambda particles in bottom events tagged by the presence of displaced secondary vertices. A fit to the momenta of the Lambda particles separates signal from B meson and fragmentation backgrounds. The measured product branching ratio is f(b->Lambda_b).BR(Lambda_b->Lambda X) = (2.67+-0.38(stat)+0.67-0.60(sys))% Combined with a previous OPAL measurement, one obtains f(b->Lambda_b).BR(Lambda_b->Lambda X) = (3.50+-0.32(stat)+-0.35(sys))%.Comment: 16 pages, LaTeX, 3 eps figs included, submitted to the European Physical Journal

Measurement of the Michel Parameters in Leptonic Tau Decays

The Michel parameters of the leptonic tau decays are measured using the OPAL detector at LEP. The Michel parameters are extracted from the energy spectra of the charged decay leptons and from their energy-energy correlations. A new method involving a global likelihood fit of Monte Carlo generated events with complete detector simulation and background treatment has been applied to the data recorded at center-of-mass energies close to sqrt(s) = M(Z) corresponding to an integrated luminosity of 155 pb-1 during the years 1990 to 1995. If e-mu universality is assumed and inferring the tau polarization from neutral current data, the measured Michel parameters are extracted. Limits on non-standard coupling constants and on the masses of new gauge bosons are obtained. The results are in agreement with the V-A prediction of the Standard Model.Comment: 32 pages, LaTeX, 9 eps figures included, submitted to the European Physical Journal

A Study of One-Prong Tau Decays with a Charged Kaon

From an analysis of the ionisation energy loss of charged particles selected from 110326 e+e- -> tau+tau- candidates recorded by the OPAL detector at e+e- centre-of-mass energies near the Z0 resonance, we determine the one-prong tau decay branching ratios: Br(tau- -> nu_tau K- >=0h0) = 1.528 +- 0.039 +- 0.040 % Br(tau- -> nu_tau K-) = 0.658 +- 0.024 +- 0.029 % where the h0 notation refers to a pi0, an eta, a K^0_S, or a K^0_L, and where the first uncertainty is statistical and the second is systematic.From an analysis of the ionisation energy loss of charged particles selected from 110326 e+e- -> tau+tau- candidates recorded by the OPAL detector at e+e- centre-of-mass energies near the Z0 resonance, we determine the one-prong tau decay branching ratios: Br(tau- -> nu_tau K- >=0h0) = 1.528 +- 0.039 +- 0.040 % Br(tau- -> nu_tau K-) = 0.658 +- 0.024 +- 0.029 % where the h0 notation refers to a pi0, an eta, a K^0_S, or a K^0_L, and where the first uncertainty is statistical and the second is systematic