The Rankine cycle for mobile applications has been studied for years as possible waste heat recovery method.
Mack trucks developed an organic Rankine cycle for a long haul heavy duty application in 1976 which recovered heat from the exhaust line and used a two stage turbine to recover energy and to distribute it directly to the driveline via a mechanical transmission. Due to the less efficient internal combustion engine the waste heat potential was much higher than for today’s diesel engines, i.e. the reached fuel economy was very high with around 12%.
Honda has developed a water based Rankine cycle for a personal car with and petrol engine taking the waste heat from the exhaust. A piston expander produces mechanical power which is provided to the driveline.
The BMW turbo steamer project has not leaded to a car prototype yet. A low temperature organic Rankine cycle and a high temperature water steam Rankine cycle recover heat from the exhaust line and the engine coolant, the recovered energy is converted into mechanical energy by two piston machines.
The EU FP7 NoWaste programme, which is made up of well-known automotive manufacturers such as Volvo, Fiat and AVL, has documented the target performance for a recovery system: 1) improve the fuel economy of the truck by more than 12% 2) cost less than €4500 for the whole system and 3) weigh less than 150kg. The systems are also based on the organic rankine cycle (ORC) in which a refrigerant is boiled by the exhaust gas and this in turn is used to either generate electricity (e.g. for refrigerated trucks) or preheat the diesel to improve the fuel efficiency.
With OnS technology a 2x improvement in heat transfer would be able to reduce the size of the exchanger by half. The heat exchanger is the heaviest part of the system and any reduction will significantly help in developing an efficient system.