A system for controlling power load on a rig engine of a wellbore rig, the system comprising: a controller for controlling said rig engine; and a sensor for sensing an exhaust temperature of said rig engine, the sensor in communic ation with the controller for providing to the controller signals indicat ive of the exhaust temperature, the arrangement being such that, in use, said controller maintains power load on said rig engine based on said exhaust temperature.