Liquefaction is one approach for increasing the density of natural gas and hydrogen to make them practical motor vehicle fuels. Productive on-vehicle utilization of the “cold energy” invested in these fuels by the liquefaction process is investigated. This “cold energy” enthalpy change is approximately 1.7% and 3.2% of the lower heating value of LNG and LH2, respectively. Direct utilization for truck cargo cooling and engine charge densification has been demonstrated. Indirect utilization involves power generation to drive vehicle auxiliary equipment. The maximum power is limited by the thermodynamic availability, which can exceed the “cold energy” enthalpy change. A system to drive an LNG vehicle high-pressure fuel pump without engine attachments or parasitic power consumption is described. Ethylene in a Rankine cycle receives heat from the engine coolant, direct drives the reciprocating pump, and rejects heat to the LNG. Bench test fixture results demonstrate approximately 0.45 hp per Lbm/min of equivalent LNG flow using a simple but inefficient full-admission cycle.