This document is the statement of independent and original contribution to knowledge represented by the published works in partial fulfilment of the requirements of the University of Brighton for the degree of Doctor of Philosophy (by publication). The thesis reviews the impact of research work conducted between 1992 and 1998 on various concepts to improve the economy and emissions of gasoline engines in order to address environmental and legislative pressures. The research has a common theme, examining the dilution of the intake charge (with either recycled exhaust gas [EGR], excess air, or the two in combination) in both conventional port injected [MPI] and direct injection [G-DI] combustion systems. After establishing the current status of gasoline engine technology before the programme of research was started, the thesis concentrates on seven major pieces of research between 1992 and 1996. These explored a subsequently patented method of applying recycled exhaust gas to conventional port injected gasoline engines to improve their economy and emissions whilst staying compatible with three-way catalyst systems. Nine other studies are reviewed which took place between 1992 and 1999 covering other methods of improving gasoline engines, specifically direct injection and two-stroke operation. Together, all the studies provide a treatise on methods to improve the gasoline engine and the thesis allows a view from a broader perspective than was possible at the time each study was conducted. In particular, the review identifies a range of strategies that use elements of the research that can be used to improve economy and emissions. Four major categories of systems researched include: conventional stoichiometric MPI engines developed to tolerate high EGR rates [CCVS]; two-stroke G-DI engines; G-DI engines operating stoichiometrically with high EGR rates; and G-DI engines operating with high dilution from both excess air and EGR. The findings of the studies illustrate that although good fuel economy improvements and emissions can be obtained with EGR dilution of stoichiometric engines, the highest fuel economy improvements require lean deNOx aftertreatment [LNA] and these, in turn, require new aftertreatment technologies and preferably new fuel specifications. The development of suitable LNA and the cost of implementation of these approaches represents one of the main barriers to improving gasoline engine fuel economy and emissions.