A new form of variable geometry, extended expansion, four-stroke internal combustion engine is described that is unable to employ squish over the full operating range. Accordingly a prediction was made of the influence of squish on the production of kinetic energy available for turbulence generation in a representative spark-ignition engine. It was found that the contribution of kinetic energy useable during the combustion process was commonly in the region of about 6% of that due to induction. It was, therefore, deduced that for typical spark-ignition engines the major benefit derived from the provision of squish surfaces is the resultant compactness of the combustion zone. Several techniques to counter the penalty resulting from the elimination of squish were reviewed. It was found that the most promising device was a turbulator incorporating low-pressure, compressed air, injection into the cylinder at the end of induction.MOST MODERN SPARK-IGNITION ENGINES employ squish, a feature of combustion-zone geometry often assumed to contribute, significantly, to turbulence occurring during the combustion process. The notion that squish is an important contributor to combustion turbulence arose many years ago with the introduction of a side-valve engine configuration featuring a compact combustion chamber located, in the cylinder head, over the valves and extending over only a portion of the cylinder bore instead of encompassing the whole cylinder-bore area as was customary with earlier engines. The compact combustion system was labelled a “turbulent head” to distinguish it from the earlier form implying, also, an increase in combustion-zone turbulence (1)*.Since a recently proposed form of Otto-Atkinson type, extended expansion, engine cannot employ squish under all operating conditions it is particularly important to investigate, in detail, the importance of squish both from the viewpoint of the contributions of squish to combustion-zone turbulence and to combustion-zone compactness. The configurations of greatest interest for such a study are not, of course, side-valve engines but rather overhead-valve units with, and without the provision of squish. Firstly the variable geometry extended expansion, Otto-Atkinson engine concept will be introduced briefly since it is this concept which has lead to a focus on the importance or otherwise of squish and the possibility of the need for alternative systems to compensate for the absence of squish in the proposed form of Otto-Atkinson engine.