Conventional economic growth theory assumes that technological progress is exogenous and that resource consumption is a consequence, not a cause, of growth. The reality is different and more complex. A 'growth engine' is a positive feedback loop involving declining costs of inputs and increasing demand for lower priced outputs, which then drives costs down further, thanks to economies of scale and learning effects. In a competitive environment prices follow. The most important 'growth engine' of the first industrial revolution was dependent on coal and steam power. The feedback operated through rapidly declining fossil fuel and mechanical power costs. The advent of electric power, in growing quantities and declining cost, has triggered the development of a whole range of new products and industries, including electric light, radio and television, moving pictures, and the whole modern information sector. The purpose of this paper is to reformulate the idea of the 'growth engine' in terms of the service provided by energy inputs, namely,useful work', defined as the product of energy (exergy) inputs multiplied by a conversion efficiency. We attempt here to reconstruct the useful work performed in the US economy during the twentieth century. Some economic implications are indicated very briefly.