A turbo-charged, inter-cooled governed speed diesel engine directly coupled to an alternator for electrical generation is adapted for dual-fuel operation and mounted on a trailer which also supports the engine's fuel sources including diesel fuel in a conventional tank. The second fuel is producer gas generated in a down draft continuously operating producer of the moving-packed-bed vertical flow reactor type with co-current gas flow. Solid waste such as crop residues, e.g., corn cobs, is used as fuel stock for the producer and is conveyed automatically to the producer from a bin on the trailer. Before delivery to the air intake of the diesel engine the producer gas is conditioned by passing it through a cyclone and filter to remove solid particulate matter and a cooler-condenser for cooling the gas and condensing out tars. The diesel governor and fuel pump assembly is modified only by the application of a fuel rack stop for maintaining a pilot flow of diesel fuel and the engine control system is such that the response of the diesel injection system to changes in engine load is normal in general; however, automatic controls are provided for monitoring diesel pump fuel rack position and engine load so that after engine loading is increased a throttle valve in the gas supply to the engine is progressively and relatively slowly opened tending to increase engine speed so that the diesel injection rate is conventionally cut back correspondingly until an equilibrium condition is reached in which engine fuel demand in excess of the pilot minimum diesel fuel flow required to maintain combustion is provided by producer gas. The response of the system to a decrease of engine loading when the engine is operating at minimum diesel fuel flow (that is with the fuel rack stop effective) is to reduce fuel supply rapidly and appropriately by partially closing the gas throttle valve. Automatic safety shutdown provisions include, in at least one shutdown mode, delayed shutting off diesel fuel flow following producer gas shutoff so that the engine and exhaust system are purged of producer gas so as to reduce explosion hazards.