In contrast, no conceivable future engine technology could achieve the same overall efficiency increase for a single occupancy vehicle. Engine efficiency would need to be 100 per cent, which is a thermodynamic impossibility. It should also be noted that there is only limited potential for efficiency improvements in power supply technologies, some of which, such as gas combined cycle and integrated gasification combined cycle (IGCC) plants, operate not far from their theoretical limits. Similarly, the so-called cumulative degree of perfection is high for diesel oil and natural gas, compared with other materials, implying only modest room for improvements in this area (Szargut, 1988).