The Thermal Lag Engine (TLE) was proposed by Tailer in 1993 and its mechanical simplicity makes it an appealing technology to access renewable sources of energy or recover waste heat. This engine has a single moving part, the piston, which delivers power and acts as the prime mover of the working fluid. The TLE consists of a hot space with a high heat transfer capacity and a cold space with limited heat transfer capabilities. This engine is still not well understood. The paper presents a two control volume model for the TLE. In order to validate the model it is parameterized to describe a TLE built by Organ. The simulations of Organ's TLE experiments when compared to his experimental results have a correlation coefficient of 0.92 and are significantly correlated within the 95% confidence interval. The different characteristics of Organ's TLE are discussed based on the simulation results. Conclusions about the required characteristics of the heat transfer regimes and energy fluxes for the TLE are made. It is discussed that the key to a more efficient TLE will result from a careful design of the gas path and the cold heat exchanger, so that more of the enthalpy shuttled into the cold space can be converted into work. (C) 2013 Elsevier Ltd. All rights reserved.