A system may include a natural gas engine; and a hydrodynamic device configured to convert mechanical energy of the natural gas engine into heat in a working fluid within the hydrodynamic device. The amount of fluid in the hydrodynamic device may be controlled by an electronically controllable valve, and the amount of fluid in the hydrodynamic device may control a resistive force of the hydrodynamic device. The system may also include a controller in communication with the natural gas engine and the hydrodynamic device, where the controller may be configured to automatically adjust the electronically controllable valve to maintain a working load on the natural gas engine at or above a threshold load.