The aim of the paper is to demonstrate that a regenerative Otto cycle with adiabatic or polytropic expansion can achieve improved performance over traditional Otto engines, even exceeding the Carnot factor. Thus, the work deals with a novel regenerative Otto based internal combustion engine which differs from the conventional Otto thermal cycles in that the process of heat conversion into mechanical work is performed obeying a polytropic path function instead of the conventional adiabatic expansion without regeneration. Design characteristics concern the fact that combustion at constant volume is carried out undergoing large air excess so that the top combustion temperature is significantly lower than in conventional Otto cycles and consequently NOx emissions are neglected. Furthermore, during the polytropic expansion based path function, heat is absorbed by being submitted to a controlled heat flow rate, to achieve the desired polytropic expansion. The analysis of the regenerative Otto based on polytropic expansion is presented and results are compared with a regenerative Otto based on the adiabatic expansion and CF. The results show that a relevant advantage of the proposed regenerative Otto with polytropic expansion over the regenerative Otto cycle with adiabatic expansion involves performance enhancement within a wide range of combustion pressures, temperatures and regeneration capacities. Thus, thermal efficiency and specific work as function of the top combustion pressure ranges are of 71.95-58.43% and 143.5-173.6 kJ/kg respectively, when combustion pressures vary between 105 kPa and 200 kPa and CF is 60.8% (lower than the thermal efficiency). The successful results involving a compact engine structure, technically and economically viable, promises a new generation of efficient reciprocating internal combustion engines operating at relatively low temperatures. Crown Copyright (C) 2016 Published by Elsevier Ltd. All rights reserved.