An organic Rankine cycle heat engine comprising: a working fluid circuit comprising: an evaporator for heating and evaporating a working fluid; a condenser for cooling and condensing the working fluid; and a positive displacement expander-generator having an inlet in fluid communication with the evaporator and an outlet in fluid communication with the condenser; the ORC heat engine further comprising: a control system coupled to the positive displacement expander-generator comprising a switch and driving means, the switch being switchable between a first state and a second state; wherein in the first state the switch is coupled to the driving means, and the positive displacement expander-generator is drivable by the driving means, and in the second state the switch is not coupled to the driving means or the driving means is switched off, and the positive displacement expander-generator is not drivable by the driving means; the control system further comprises sensing means for sensing an operating condition of the ORC heat engine and processing means for switching the switch between the first and second states in response to an input, the processing means being coupled to the sensing means and the processing means being configured to switch the switch between the first and second states when a predetermined operating condition is met; characterised in that the predetermined operating condition is the thermodynamic flow through the positive displacement expander-generator being sufficient to keep the positive displacement expander-generator rotating once the positive displacement expander-generator is disconnected from the driving means and connected to an electrical load. a working fluid circuit comprising: an evaporator for heating and evaporating a working fluid; a condenser for cooling and condensing the working fluid; and a positive displacement expander-generator having an inlet in fluid communication with the evaporator and an outlet in fluid communication with the condenser; the ORC heat engine further comprising: an evaporator for heating and evaporating a working fluid; a condenser for cooling and condensing the working fluid; and a positive displacement expander-generator having an inlet in fluid communication with the evaporator and an outlet in fluid communication with the condenser; the ORC heat engine further comprising: a control system coupled to the positive displacement expander-generator comprising a switch and driving means, the switch being switchable between a first state and a second state; wherein in the first state the switch is coupled to the driving means, and the positive displacement expander-generator is drivable by the driving means, and in the second state the switch is not coupled to the driving means or the driving means is switched off, and the positive displacement expander-generator is not drivable by the driving means; the control system further comprises sensing means for sensing an operating condition of the ORC heat engine and processing means for switching the switch between the first and second states in response to an input, the processing means being coupled to the sensing means and the processing means being configured to switch the switch between the first and second states when a predetermined operating condition is met; characterised in that the predetermined operating condition is the thermodynamic flow through the positive displacement expander-generator being sufficient to keep the positive displacement expander-generator rotating once the positive displacement expander-generator is disconnected from the driving means and connected to an electrical load.