During a recent technical presentation at the Mahle headquarters, Behr revealed a prototypical waste heat recovery (WHR) system for long-haul trucks that has demonstrated a fuel-efficiency benefit of around 5% on the test rig during stationary and transient operation.
“Currently the biggest driver of commercial-vehicle drivetrain optimization work is emissions legislation. While that is not likely to change, the diesel price level together with oncoming low carbon regulations for trucks also require better fuel efficiency,” said Dr. Ing. Eberhard Pantow, Head of Advanced Engineering Engine Cooling, Truck at Behr.
As only around 40% of the energy contained in the fuel can actually be used to propel the truck, according to Behr, it is attractive to harvest at least some of the 30% of energy that is currently lost via the hot exhaust gases and exhaust gas recirculation (EGR).
“If a waste heat recovery system is fitted, the thermal efficiency of this WHR system is directly equivalent to a lower fuel consumption of the same magnitude,” Pantow said, explaining the rationale why the automotive supplier, based in Stuttgart, Germany, has built a WHR prototype, which is already designed to fit realistically into the packaging space long-haul trucks offer.
The WHR unit is based on the Rankine cycle (in practical terms, a variation of the steam engine process) and has been fully tested with water as working fluid. Parts of the process have also been tested with ethanol. During rig testing according to the European Stationary Cycle, the system demonstrated an efficiency of between 4.5 and 5.5%, depending on the individual part of the cycle. The tests were conducted on a full engine with typical long-haul truck dimensions at roughly 2 L displacement per cylinder. Also, the full exhaust gas aftertreatment systems were installed.
“It turned out that the motorway [highway] part of the cycle proved to be particularly beneficial at 5% and more of efficiency,” Pantow said. The total WHR system weight is currently around 100 kg (220 lb); “however, that is inclusive of high safety margins and before further optimization measures, which will be part of industrialization,” Pantow added.
Testing the system according to the European Transient Cycle showed that the highway benefits in particular are also confirmed during transient operation: during the urban section the WHR efficiency was 4.2%, during the rural section it was 3.5%, and during the highway section it was measured at 5.2%.
The Rankine closed-loop power delivery cycle consists of four major elements: A small pump feeds the liquid working fluid to two vaporizers, which serve to turn the working fluid to vapor. Both vaporizer units are positioned downstream of the aftertreatment components, one in the exhaust gas recirculation flow (EGR vaporizer), the other one in the main exhaust flow (tailpipe vaporizer). Within these two vaporizers the working liquid is preheated, then vaporized, and is subsequently superheated to produce a dry steam. This super critical steam flows into an expander.
“We chose a two-cylinder reciprocating engine with 650 cc displacement to turn the thermal energy in the exhaust flow into mechanical power,” Pantow explained. After passing through the expander, the working fluid enters a condenser where it is returned to the liquid state. In the Behr WHR, this condenser is cooled by the engine cooling system. From here the cycle begins again.
During development the supplier’s engineers were able to bring down the exhaust gas back pressure increase to a few mbar, according to Pantow. They also minimized the need for extra cooling, which would otherwise impact WHR efficiency. As it is, the most relevant part of the map—long-haul highway driving at medium engine load and constant rpm levels—requires the least cooling, which is good news as the system targets long-haul trucks.
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“Under full load conditions and during urban driving, the extra coolant and fan requirements can eat up 1% of the WHR efficiency,” Pantow said.
One of the important oncoming decisions will be the choice of the working fluid. While water has specific benefits inasmuch as it requires fewer changes in the engine cooling system, ethanol offers advantages because it does not freeze and is more benevolently controlled during load change situations. During the technical briefing, the Behr expert highlighted the benefits of ethanol but said that “this decision will ultimately depend on an industry-wide discussion.”
Asked about how the recovered energy is best fed back into the drivetrain, Pantow said that both the direct mechanical use and the electrical storage have advantages: “If we use the kinetic energy from the expander directly as mechanical driving force, the total system efficiency will be higher. However, there are driving situations where there is no need for the recovered energy. If the expander energy is converted to electrical energy, it can be stored and used freely, independent of the driving situation. The downside of this thermo-electric process is the amount of loss, which is caused by the conversion.”
During the oncoming further development work, the supplier is optimistic that there is even more potential to increase the WHR system’s efficiency: “For one, we have not yet optimized the system design details for individual working fluids. Secondly, we have had some heat losses through the many sensor accesses we integrated into the prototype.”