Automotive Engineering - March 2023 - 26

BALANCING THE 'DOMINOS'
in EV thermal
management
Excessive internal valve leakage is an
inefficiency that worsens upstream
system inefficiencies. Effective valve
design is the solution.
by Steve Hokky
E
lectric vehicle thermal management requires OEMs and suppliers
to control many variables that potentially create excessive
heat. Designing thermal-management systems (TMS) to effectively
control these variables is critical to overall EV performance.
Internal leakage and other coolant-valve performance criteria
are among the variables affecting EV performance, most notably driving
range. Valve design is fundamental to a robust, thermally efficient
and leak-free system, according to engineers at Robertshaw Controls,
a specialist in plastic coolant-valve innovation and TMS design.
The delicate balance
Excessive heat generation around multiple EV systems represents an
inefficiency that prevents them from operating at optimal temperatures.
To manage this inefficiency, the TMS is tasked with simultaneously
maintaining optimal temperatures in and around multiple components
and systems. Lithium-ion battery temperature control receives
a lot of attention, but it is one of many components and systems
to consider. It is also critical to protect the " brains " of the vehicle
- the ECU or ECM - from extreme temperatures. Accordingly,
some ECUs and ECMs are equipped with their own chiller. Typically,
the vehicle gearbox also has a cooling loop. Finally, the cabin often
has a cooling loop with a coolant valve for cooling the air.
The TMS must simultaneously maintain optimal temperatures in
multiple systems - a delicate balance. If one system's operating temperature
increases significantly above optimal, the TMS compensates
by diverting colder fluid from other systems' cooling loops to the
" hot " system's loop. Although this process immediately lowers the
hot system's temperature, during a trip of sizable distance it can push
operating temperatures higher around other systems that have experienced
the cooling loop diversion. This thermal " domino effect "
compounds the original inefficiency of the upstream system's excessive
heat production and eventually impacts EV performance.
As an example, consider how the thermal domino effect impacts
the battery - and, ultimately, driving range. When the battery temperature
rises above the optimal 21° C (70° F), its cooling loop needs
colder fluid from another loop. Meanwhile, the pumps and compressor
that feed the battery's cooling loop with colder fluid need to
work harder, increasing the energy demand on the battery. If the TMS
does not respond with peak efficiency (measured by the time it takes
to lower the battery's temperature to optimal), the larger and more
abrupt the stored-energy loss from the battery's most recent charge.
Ultimately, TMS inefficiency reduces the vehicle's driving range
compared with its potential range. Over the long term, inefficient
TMS performance also diminishes battery life.
26 March 2023
Maintaining the optimal battery operating temperature
becomes more critical, not to mention challenging,
in extreme outside temperatures. A 2019 AAA study
revealed the potential impact of outside temperatures
on battery performance (https://www.sae.org/
news/2019/03/aaa-ev-range-thermal-effect-study). The
study indicated that an outside temperature of 35° C
(95° F) decreased driving range in five different EV
makes and models, on average, by 4%, and by 17% with
the HVAC system in use. Note that when temperatures
around EV components and systems are below optimal,
TMS efficiency is equally or more important.
The challenge in a cold operating environment is not
excessive heat production, but too little heat production.
In the AAA study, when the vehicles were operated
in outside temperatures of 7° C (20° F), the result
was an average 12% decline in driving range - and a
41% drop when HVAC systems were operated.
Internal valve leakage
How rapidly or gradually the thermal domino effect
occurs depends on overall TMS operating efficiency,
which hinges on variables such as internal leakage in
coolant valves. In contrast to external leakage (i.e.,
physical loss of coolant), internal leakage means diversion
of coolant from one loop to another. Excessive
internal valve leakage is an inefficiency that worsens
upstream inefficiencies.
Among its TMS design solutions, Robertshaw offers
Ranco electronically-controlled multi-port valves (typically
2, 3, 4 and 5 ports) that move coolant throughout
cooling loops with minimal energy consumption and
AUTOMOTIVE ENGINEERING
OEMs should prioritize
several coolant-valve
properties to make their
TMS designs flexible,
space-efficient and
operationally reliable.
https://www.sae.org/news/2019/03/aaa-ev-range-thermal-effect-study https://www.sae.org/news/2019/03/aaa-ev-range-thermal-effect-study
Automotive Engineering - March 2023

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