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Particulate emissions from homogeneous charge compression ignition (HCCI) engines are often considered as negligible and the measurement of particulate matter (PM) with HCCI combustion systems has been rare. An earlier publication in the literature and the authors' own recently published work suggest that PM emissions from gasoline direct injection (DI) HCCI engines should not be neglected. It has been shown that PM emissions from HCCI engines, although generally lower, can be similar to spark ignition (SI) levels for certain engine conditions, especially in the accumulation mode. The present work shows that although injection timing is effective for control of HCCI, it can have a significant impact on PM emissions. Engine speed also appears to have an impact on PM emissions for the HCCI combustion mode. Typically for HCCI operating at 1500 r/min, the total mass and total number of the PM emissions could be up to six times higher when the fuel is injected during the recompression period compared with the case for injection during the inlet valve open period. Similarly, when the engine was operated in SI mode, an attempt to inject fuel close to induction top dead centre (TDC) made the PM emission noticeably higher, reaching unacceptable levels. The split injection strategy for HCCI mode has been investigated for its effect on PM emissions. It has been found that the PM number concentration for split injection lies between the maximum concentration measured for very early injections (near TDC at recompression) and the minimum concentration measured for the latest injections used (at crank angles near to the inlet valve maximum opening time), with the magnitude of number density varying approximately from 0.2 to 1.2 million/cm3. It is also observed that for split injection the number of particulates in nucleation mode varies less, while the number in accumulation mode level varies within a limit of one order of magnitude; both are dependent on the injection strategy. The shape of the PM distribution is such that the nucleation mode exceeds the number concentration of 1×107 (d N d log Dp) and the accumulation mode falls parabolically to approximately 1×106. For comparison, it can be said that the PM number concentration measured in the authors' engine is approximately one order of magnitude lower in accumulation mode than the level for a typical modern diesel engine.

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This collection of nineteen previously published essays reflects the research interests of Professor Stone since the publication of his Mathematics in the Social Sciences and Other Essays.Essay I employs an argument favouring the usefulness of mathematical models of the economy in prediction and planning to introduce a condensed summary account of the Cambridge Growth Model with the aid of flow diagrams. Essay II introduces a simple dynamic aggregative model to illustrate the potential contribution of control theory to the problems of economic stabilization whilst essay III employs input-output analysis to work out the implications of specific proposals for export expansion or import savings in an economy characterized by full employment and balance of payments difficulties.Essays IV and V deal with a taxonomy of the problems of consistency in multi-sector models and the application of the method of adjusting conditioned observations by the method of least squares to the problem of making consistent input-output projections incorporating explicit reference to price movements, respectively.Essay VI presents an application of econometric technique to the analysis of seasonal adjustment while essay VII generalizes a theorem of Frisch and Waugh on regression analysis.Essays VIII, IX and X form a group on the econometrics of consumer behaviour. The first extends Stone's linear expenditure system for demand projections; the second surveys British studies on consumers' wants and expenditures; and the third presents new calculations with a model of aggregate spending on British data.Essays XI to XVI constitute a group of studies on national accounting. The first two essays provide background material on the development of national accounting conventions and explain recent efforts to incorporate national and sector balance sheets into the national accounting structure. Subsequent essays in this group provide illustrations of the use of social accounting matrices in building planning models and an adaptation of input-output technique to the building of financial models based on the revised system of national accounts.Essays XVII, XVIII and XIX form another group on socio-demographic systems. The first essay brings together ideas on the application of systems building and control theory to educational planning, whereas the last two essays present the conceptual basis of a population accounting matrix and a preliminary estimate of a population accounting matrix for the school-age population of England and Wales, respectively.The collection of independent essays into a volume must suffer from a certain degree of lack in continuity and overlap. However, this volume counteracts such a tendency to a considerable extent in that (1) the essays all relate to an implicit common concern with the problems of building empirical planning models, (2) the invididual essays are grouped according to various themes, (3) the notation is kept remarkably consistent throughout and (4) references and notes are made in individual essays to relevan...