In the scenario you described, where a gas inside a cylindrical tube exerts pressure on pistons causing them to accelerate, there is a conversion of thermal energy to mechanical energy. However, it is important to note that the decrease in entropy of the system requires considering the entire system, including its surroundings.
As the gas exerts pressure on the pistons and causes them to move, the gas itself loses some of its thermal energy, resulting in a decrease in the internal energy of the gas. This energy is then transferred to the pistons, which convert it into mechanical energy as they accelerate.
While the internal energy of the gas decreases, it is crucial to consider the overall entropy change of the system, including the surroundings. The decrease in entropy within the gas itself is offset by an increase in the entropy of the surroundings. This is due to the dissipation of heat to the surroundings and the generation of waste heat during the energy conversion process.
The second law of thermodynamics states that the total entropy of an isolated system (or the system and its surroundings) tends to increase or, at best, remain constant. In this case, even though the thermal energy is being converted to mechanical energy, the overall entropy of the system and its surroundings will not decrease.