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The paper is concerned with the effects of cyclic variation in turbulence (expressed in terms of rms turbulent velocity) on the burn rate and subsequent cyclic variation in in-cylinder pressure derived parameters. The task has been addressed by applying a thermodynamic engine modelling approach for simulations of two very different engines; a single cylinder research engine in which sources of cyclic variation other than turbulence had been minimised and a multi-cylinder production engine. The cyclic variability in the two engines had a number of similar features; the effects of turbulence variation cycle-to-cycle proved dominant in the production engine, mixture strength secondary and prior-cycle residual concentration feedback marginal.

This work is concerned with the analysis of different charge dilution strategies employed with the intention of inhibiting knock in a high output turbocharged gasoline engine. The dilution approaches considered include excess fuel, excess air and cooled external exhaust gas re-circulation (stoichiometric fuelling). Analysis was performed using a quasi-dimensional combustion model which was implemented in GT-Power as a user-defined routine. This model has been developed to provide a means of correctly predicting trends in engine performance over a range of operating conditions and providing insight into the combustion phenomena controlling these trends. From the modelling and experimental data presented, it would appear that the use of cooled externally re-circulated exhaust gases allowed fuel savings near to those achieved via excess air, but with improved combustion stability and combustion phasing closer to the optimum position.

There is worldwide interest in developing Homogeneous Charge Compression Ignition concepts for lean burn/heavily diluted, low emissions, combustion engines. Although there has been much work and considerable progress relating to autoignition onset in such engines, there has been relatively little study of the reasons for the knock which limits their operation at high power. Building on earlier work on end gas autoignition development and knock in spark ignition engines, it is suggested that the problem is rooted in the mode of autoignition. The conditions supporting the most violent “developing detonation” mode, with increasing charge dilution, bulk unburned gas temperature level and local temperature gradient are explored in relation to HCCI engines.

A novel dual seeding method has been developed to obtain full bore cyclically resolved simultaneous flame images and associated velocity fields in an optically accessed single cylinder research spark ignition engine. The technique has been used to study interaction between the propagating flame and in-cylinder gas motion. Light generated by a fast repetition rate copper vapour laser was formed into a thin light sheet, which passed horizontally through the disc shaped combustion space of a spark ignition engine having complete overhead optical access. Mie scattered light from relatively sparse and large particles (∼65μm) at successive intervals allowed flow definition by particle tracking velocimetry. Simultaneous scattering from dense small seed (∼0.22μm) was used to generate flame front images, which were digitised and analysed to quantify turbulent flame structure and development. The flame was shown to have significant effect on local unburned gas motion as well as vice versa.

Laminar and turbulent burning velocity are important parameters of a combustible mixture. Each is used in mathematical models of spark ignition engine combustion. However, reported data are inconsistent and sparse at elevated pressure and temperature. The present paper presents precise definitions and results of fundamental experiments at Leeds on both laminar and turbulent burning velocities. Reasons for the considerable differences in reported values are put forward and the often neglected effects of flame stretch rate and instabilities on burning velocities are discussed. Relevance is discussed in relation to observations of turbulent flame propagation in a research engine.

An advanced gas dynamic/chemistry interaction code, SPRINT2D, has been developed to simulate end gas autoignition and knock. This confirms that an earlier hypothesis of three distinct modes of autoignition was not an artefact of the previous numerical code. A comprehensive chemical kinetic scheme has predicted autoignition onset and demonstrated a mechanism for creating the end gas temperature gradients assumed in, as well as generated heat release rates for use in, SPRINT2D. Using the combined modelling techniques, good matches between theoretical and experimental autoignition centre growth (at up to 750,000 frames/second), particle tracking and pressure development sequence at multiple transducer sites have been obtained for “thermal explosion” and “developing detonation” autoignition events.

A range of multiple and sequential Mie scattering imaging techniques have been employed to investigate turbulent flame propagation in a relatively quiescent optically accessed two-stroke spark ignition engine. Flame structure and turbulence scales have been characterised by a number of methods. These include fractal analysis, simple flame perimeter to area ratios and techniques based on Fourier analysis of an independent stationary coordinate. From this was derived an integral scale of flame wrinkling and a parameter related to turbulent flame thickness. Fully developed values of these turbulence parameters proved independent of cyclic variation, mixture strength and (apart from increasing flame thickness) apparent flame extinction. Islands of unburned gas behind the flame front were associated with encirclement by large scale structures rather than partial quench or total quenching due to flame stretch.

A 2-D simulation of fluid dynamic and chemistry interaction following end gas autoignition has demonstrated three distinct modes of reaction, dependent upon the temperature gradient about an exothermic centre. All three modes (deflagration, developing detonation and thermal explosion) can contribute to knock; the developing detonation case, associated with intermediate temperature gradient, has been identified as the more damaging. The simulation code (LUMAD) has been used in a systematic parametric study designed to separate the complex interacting events which can lead to mixed modes in real engines. A most significant finding related to the sequential autoignition of multiple exothermic centres.

A 3-zone thermodynamic cycle model has been developed which incorporates the Leeds correlations of turbulent burning velocity. The correlations encompass both the beneficial effects of turbulence in flame wrinkling and the detrimental effects of flame strain, which can lead to partial or total flame quench. Allowance has been made for the effects of “developing turbulence”, as the initially laminar flame kernel grows and is progressively influenced by larger scales of turbulence. Available experimental cylinder pressure and flame propagation data were used to check the plausibility of the simulation code and to establish values for the various constants employed to characterize the turbulence. The program was then used to explore the effects of engine speed, mixture strength, induction pressure and turbulence levels on the development of the combustion event.

The paper is concerned with end-gas autoignition, subsequent knock severity and magnitude of induced gas velocity. An optically accessed single cylinder Yamaha two stroke engine was modified to give complete overhead optical access to the disc-shaped combustion chamber. Flame propagation and end-gas autoignition events were recorded using high speed natural light and schlieren photography; local gas motions, prior to and induced by the knock event, were determined using an oil droplet trajectory technique. Cylinder pressure was synchronously recorded at three positions around the cylinder head; one transducer's output being simultaneously displayed on the film. End gas autoignition generally developed from multiple centres. Autoignition was usually, but not invariably, followed by knock.

The nature of autoignition and knocking is investigated experimentally and theoretically in an optical engine by high speed direct light photography and laser schlieren filming. Special emphasis is devoted experimentally and theoretically to the role of exothermic centres in the end-gas in initiating knocking combustion and subsequent knock damage to the combustion chamber walls. The optical engine is a modified single cylinder ported two stroke engine equipped with a large head window for unlimited access to both the entire combustion chamber and the ring crevice region. In some experiments the formation of exothermic centres was stimulated by microscopic aluminium particles that deposited on the mirrored piston surface. The data are analysed by numerically modelling the transition from normal combustion to autoignition with a simplified 2D-code.