In theory it would be possible to use the waste heat in exhaust gas to reduce the fuel consumption of commercial vehicles by a few percent. To date, the idea has not made any breakthrough because the payback times for the additional technology were still too long. However, using the waste heat from the engine in addition to the energy in the exhaust gas makes application of the Organic Rankine Cycle (ORC) a viable proposition. IAV has developed the “Advanced Thermal Management” system which combines evaporative cooling with the ORC. It could go into volume production as early as the next generation of engines.
Only about a third of the energy in fuel is used for propulsion, with the remainder currently being lost in the form of waste heat. Part of it could be converted back into mechanical or electric energy and put to good use. Besides Stirling engines and thermoelectric generators, the ORC is shown to be particularly suitable. In comparison with other heat recovery technologies, it has the best cost-benefit ratio.
In most applications, consideration has so far only been given to the waste heat in exhaust gas as the source of energy for the ORC. “This is capable of reducing a vehicle’s fuel consumption by 3 to 3.5 percent”, reports Oliver Dingel, Senior Technical Consultant for Energy Management in the Commercial Vehicle Powertrain division at IAV. “Forwarders usually demand that investments in new technology pay for themselves within two years. This is currently not possible if you only use the waste heat in exhaust gas.”
Engine heat reduces fuel consumption by 4.5 percent
A further major source of heat, however, is the engine which at the moment is cooled with a liquid water-glycol mixture at a temperature of around 100 degrees Celsius. IAV’s “Advanced Thermal Management” (ATM) system uses both heat sources as energy supply for the ORC. A water-ethanol blend (Rankine fluid) first flows through the heat exchanger in the exhaust system and then through the engine. This way, the ORC can recover far more energy. “Fuel consumption now falls by at least 4.5 percent, reaching the break-even point set for heat recovery in the truck”, Dingel says. “The energy can be transferred to the engine or drive shaft by mechanical means or fed into the vehicle electrical system by means of a generator. At the moment, however, there are still not enough power consumers in trucks for this.”
Using ATM, however, demands some rethinking in terms of engine cooling. It is no longer based on heating a liquid medium but uses the targeted, forced evaporation of the fluid in the engine (designed boiling). Depending on load point, this takes place at far higher pressures (up to 40 bar) and temperatures than conventional cooling. At first sight, this appears to impair the transfer of heat to the cooling medium because with conventional cooling, the temperature difference between the hot components and the medium is very high. “Although it is substantially less with ATM, evaporation provides a much better heat transfer”, Dingel explains. “Through phase transition, the fluid can take up a far higher amount of energy, which leads to a drastic reduction in the mass flow demand. As a result, ATM also manages with a very small coolant pump.”
Condenser increases cooling capacity
Downstream of the engine, the evaporated medium flows into the expander where it expands and performs mechanical work for recovering heat. Liquefaction then takes place in a condenser which replaces the radiator used today. This reveals a further benefit of ATM: the condenser has better efficiency. For the same cooling capacity, it requires a surface area some 40 percent less than a conventional radiator. If the cooling area is left unchanged, far more heat can be discharged into the environment than with a conventional radiator. This provides the capability of running the ORC at high loads too, for example while hillclimbing, without significantly reducing the energy recovered as a result of higher fan power consumption.
Using ATM involves modifications to the cylinder head and to the engine block. “In future, cooling ducts must be configured in a completely different way”, Dingel says. “But this is not expected to increase costs.” IAV has already tested the new cooling system on a passenger car cylinder head and carried out simulations to verify its efficiency in the commercial vehicle sector. Following successful development on behalf of a client, ATM could go into volume production as early as the next generation of truck engines.