Powertrain & Energy - July 25, 2012 - 6

Tech Report
Waste heat recovery by Behr improves fuel
economy for long-haul trucks
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.
cording 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 prototypical waste heat recovery (WHR) system,
developed by Behr, is based on the Rankine cycle. It
showed an efficiency improvement of 5% on
average, which, according to the supplier, is the
direct equivalent to a potential fuel-efficiency
increase in the same magnitude.
“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, ac-
6 July 25, 2012
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%.
SAE Powertrain & Energy
Powertrain & Energy - July 25, 2012

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