An organic Rankine-cycle system (ORCS) operating on the heat from the exhaust gases of a truck diesel engine is described. The addition of an ORCS to a long haul diesel truck can improve the fuel economy by 15% over a typical duty cycle. This represents a potential reduction of 1.8 billion gal/yr (120,000 barrels/day) in near-term transportation fuel requirements. Emission levels will also be reduced by an amount equal to the gain in efficiency. At current diesel fuel cost, the potential fuel savings also provide an economic incentive to the fleet operator to invest in the added cost of a bottoming cycle system. Results from tests conducted with a Mack 676 diesel engine and an organic Rankine bottoming cycle (not specifically designed for this application) are presented. At the peak power condition, 36 additional horsepower was produced, a gain of 13% in power without additional fuel.