Alarge part of the energy in fuel is still being lost unused in exhaust gas as it leaves the exhaust pipe. This makes recuperating waste heat a core technology for improving the eﬃciency of the powertrain in passenger cars, commercial vehicles and mobile work machines. IAV is looking closely at the Organic Rankine Cycle (ORC) which provides the best ratio of cost, weight and beneﬁt. On the test bench, prototypes have demonstrated that the technology is robust and meets expectations.
There are various driving forces that make recuperating waste heat an attractive option for passenger cars and commercial vehicles. On the passenger car side, the legislator is demanding ever lower CO2 fleet limits from the OEMs, in the commercial vehicle segment, on the other hand, the focus is on saving fuel. “It
is here that recovering waste heat pays off the fastest”, says Oliver Dingel, Senior Technical Consultant for Energy Management at IAV. “Introducing such systems in the commercial vehicle market is more a matter of the diesel price than it is of future statutory limit values. In the case of passenger cars, however, which only do about a tenth of the mileage of a commercial vehicle and use a fifth of the fuel, recuperating waste heat can play a key part in reducing CO2 emission to below 95 g/km.”
Potential for reducing consumption depends on vehicle and application
For the car, and depending on cycle, it can cut fuel consumption by up to 8 percent – less in the NEDC, more in the WLTC and RDE cycles relevant to the future. With commercial vehicles, the potential saving is in the region of 3 to 5 percent. Owing to higher consumption levels and mileages, the additional investments pay off far more quickly. “Together with the colleagues from Consulting4Drive, we have evaluated various scenarios in different markets”, Dingel says.
IAV champions the ORC for recovering energy from exhaust gases. This steam process makes it possible to convert gained energy either into mechanical work and feed it directly into the powertrain or to generate electric energy through a generator. “In particular, feeding back electric energy makes sense when a high level of power is demanded by electrified auxiliary drives”, Dingel says. “Even in the WLTC, ORC can deliver four to five kilowatts of power which can be used for charging a traction battery on the fly.” This also makes it a perfect addition to a hybrid drive in the car. Whereas this shows its effect in particular while driving in urban areas, ORC really shines on long interurban trips. In the commercial vehicle, electrifying auxiliary units by introducing the 48-volt electric system based on ORC technology can open additional perspectives.
Proven on the test bench
IAV has examined a Curtis turbine and a singlecylinder reciprocating expander as an energy converter for the ORC. Both exist as prototypes and have already proven themselves in extensive trial runs on the test bench. In combination with a commercial vehicle engine, the reciprocating expander provided twelve kilowatts of mechanic power and an efficiency level of 13.5 percent. Its particular benefit for linking it mechanically to the combustion engine: even at varying engine speeds and media pressures, efficiency remains at a high level. The Curtis turbine was linked to a generator and reached a maximum electric output or ten kilowatts and an efficiency level of up to 11.5 percent. Working at high speeds of up to 70,000 revolutions per minute, the turbine generator unit is very compact, making it ideal for feeding in the electricity recovered from exhaust gas energy.
In all investigations, the steam for the two expanders was generated in IAV’s prototype heat exchangers. Their underlying principle provides flexibility in adapting it to different installation conditions and, weighing very little, combines low pressure losses with high levels of efficiency. In technical terms, all components have meanwhile been perfected to such an extent that they have passed the advance engineering stage at IAV and found their place in the commercial vehicles section.
Ready to be taken to manufacturing maturity level
IAV could start developing a solution for OEMs or component suppliers straight away. “We have already built several generations of ORC prototypes”, Dingel says. “This gives us a wealth of non-OEM-specific knowledge and expertise.” Following extensive investigations and in-house engineering, the tool and testing chain is now complete. IAV can design heat exchangers and expanders and produce prototypes, has a facility for tests with ethanol and simulates integrating ORC systems into vehicles.
From simulation, design, prototyping, operating and control strategy to trials and investigations on the test bench or in the vehicle: IAV has everything it takes to integrate ORC systems. Interdisciplinary cooperation among IAV’s many different specialist sections plays a key part – because integrating an ORC system into the powertrain not only affects the electric system but also the cooling system, exhaust gas aftertreatment and engine control. We have expertise in all interfaces between the subsystems”, Dingel explains. “This means we can integrate the ORC system into the vehicle so skillfully and seamlessly that it delivers optimum performance across all operating ranges.”