The past twenty years has seen an explosion in our knowledge of engine processes, steadily improving engine power density and efficiency, major reductions in exhaust emissions, and a substantial increase in engine sophistication and complexity. This paper explains how engineering analysis has played a major enabling role in realizing these improvements in spark-ignition engine performance. Examples are given of the many different types of analysis tool in areas such as combustion, emissions, stress analysis, system dynamics, and fluid flow that have been found useful in resolving different engine development and design problems and opportunities. The significant improvements achieved in engine fuel consumption, power density, and emissions control are then reviewed. It is argued, however, that the improvements in urban air quality do not correspond to the reductions achieved in vehicle exhaust emissions. Our current understanding of the link between vehicle emissions and air quality does not explain this discrepancy. What matters is low enough in-use emission, and future regulations do not adequately focus on this essential requirement. An available energy analysis of the four-stroke spark-ignition engine operating cycle is used to identify where opportunities for further increases in efficiency and power are to be found. Approaches that would improve combustion efficiency, reduce heat losses, increase expansion stroke work, reduce pumping work, and decrease friction are discussed. It is concluded that many analysis tools are now available to identify more precisely how large these opportunities are, and how best they might be realized. The potential of various modifications to the four-stroke cycle SI engine cycle, and alternative spark-ignition and diesel cycles, are reviewed. Finally, it is argued that relative to Europe and Japan, the United States lacks a sufficiently broad and organized research effort designed to support the exploration and development of these opportunities.

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