Theoretical thermodynamic analysis reveals that, when a fixed amount of heat energy is added into an Otto cycle the thermal efficiency of that cycle can be substantially improved by increasing the expansion ratio while keeping the compression ratio unchanged to achieve a greater net work output. As such, to maximise the cycle work output, the exhaust gas is allowed to expand to atmospheric pressure within the power machinery itself. With this approach, the pressure versus specific volume diagram of this modified cycle at exhaust valve opening is thus minimised and hence waste heat recovery/utilisation such as the implementation of turbocharging system can be eliminated.This paper presents the development and design considerations of a double helical screw internal combustion engine. The engine consists of two pairs of helical screw type, positive displacement machines in which one pair performs the operation of compressing the working fluid, while the other performs the operation of expansion. The machines, being separate, could be designed in such a way that the effective gas expansion ratio is greater than the compression ratio.