Automotive Engineering - June 2023 - 31

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Fig. 4: Relationship between the APU efficiency and the percent of APU
load over its max load.
Prioritizing auxiliary loads for key-off
Similarly, the categories of loads are ranked for the key-off
use case. Instead of having five rankings, this use case only
has three: continuous key-off loads; OBD testing, and transient
loads.
The continuous key-off loads that are much less the continuous
key-on loads replace the continuous key-on loads.
The loads for OBD testing are ranked as the second. The OBD
tests are obligatory due to regulatory requirements. The transient
loads such as HVAC are ranked last. They are necessary
for achieving and possibly exceeding the life target of the
vehicle. However, they are not as important as the other two
categories of loads.
The APU charging strategy for vehicle key-off use case is
the same as the charging strategy for the key-on use case.
The difference comes in with the amplitude of the loads.
The loads during key-off are significantly less than the loads
during key-on. The charging strategy still adjusts the operating
point of the APU to ensure it only works at its maximum
efficiency.
With more offering of drive-assist functionality by vehicle
manufacturers, there is increasing pressure on auxiliary battery
to improve its efficiency and implement more advanced
energy management strategy to maintain SoC during
vehicle operations. As such, we propose this strategy to
detect low SoC condition, prioritize auxiliary loads as required
and effectively manage charging of the auxiliary battery
by APU at high operating efficiency level during the
key-on and key-off use cases.
Vivek Kumar is a vehicle controls engineer at Ford
Motor Co. His specialized research focuses on
diesel, gas, hybrid / BEV controls and algorithms.
He is also associate editor for the SAE International
Journal of Engines. He can be reached at
vkumar54@ford.com.
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Automotive Engineering - June 2023

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