Automotive Engineering - June 2023 - 29

ELECTRIFICATION FEATURE
CATEGORY
Continuous
key-on loads
Transient loads
LOAD
ECUs, radio, exterior & interior lights,
fog lamps, brake, trailer light, sensors
and actuators
OBD testing
Vehicle safety
operation
Vehicle
performance
Continuous
key-off loads
Heated steering, electric brake boost,
torque, air conditioning, heater,
vents, cooling fans vectoring, positive
temperature coefficient (PTC) heater,
vacuum pump, coolant pump, oil
pump, driver assistance system
Electrical actuators related to
evaporative emission control system
(EVAP), exhaust gas recirculation
(EGR), heating, ventilation and air
conditioning (HVAC), exhaust gas heat
recovery (EGHR), gasoline particulate
filter (GPF)
Steering, anti-collision braking
system, vehicle alarm, stability
control system
Air handling (Turbocharger), thermal
management (coolant and oil pumps)
ECUs, clock, radio, vehicle alarm,
lights, OBD
Table 1: Six categories of electrical loads.
Control strategy for three vital
functions
The proposed strategy has three functions: low SoC
detection, auxiliary load prioritization, and auxiliary
battery charging.
Fig. 2 illustrates a high-level power and communication
diagram of the proposed strategy. The vehicle is
assumed to be under a charging event. The ABMS detects
the SoC if the auxiliary battery falls under a SoC
threshold. After that, the BCM passes that information
from the ABMS to the PCM. The PCM then turns off
the loads based on their priorities. The PCM also commands
the APU to start transferring energy from the
HV battery into the auxiliary battery.
Fig. 3 presents the flowchart of the proposed strategy.
It is important to point out that the park duration in the
dashed block is only applicable to the key-off use cases.
The SoC low threshold is used to determine whether the
SoC of the auxiliary battery will fall under a SoC threshold.
If that is the case, the next step is to determine if any of
the loads can be turned off based on the load priorities.
Following that procedure, the APU gets turned on to
recharge the auxiliary battery. Depending on the SoC,
some of the loads are turned back on also based on
their priorities. When the SoC high threshold is reached,
the APU stops recharging the auxiliary battery.
AUTOMOTIVE ENGINEERING
Fig. 1: The auxiliary system architecture in the hybrid-electric vehicle.
The low SoC detection for vehicle key-on use case begins once the
vehicle starts. The PCM continues receiving CAN signals such as auxiliary
battery, SoC and voltage from the ABSM. As the auxiliary loads
draw power from the auxiliary battery, the SoC decreases. Once the
SoC drops below the SoC low threshold (e.g., 10%), the low SoC condition
is determined. Otherwise, the PCM continues monitoring these
CAN signals.
Six load categories
Six categories of auxiliary loads have been defined in Table 1. The
first five categories are applicable for this key-on use case. The priorities
for these five categories from high to low are: 1. Continuous keyon
loads; 2. vehicle safety operation; 3. OBD testing; 4. transient
loads, and 5. vehicle performance.
The continuous key-on loads have the highest priority because
they provide the fundamental functions for any of the vehicle functions
at the higher levels. Following that, the loads that support vehicle
safety operation are ranked second. The loads for OBD testing
are ranked third and are required by regulations. Transient loads such
as those from HVAC are considered more important than the loads
for vehicle performance because they provide basic driving experience.
The loads to support vehicle performance are ranked the last as
they are optional for driving the vehicle.
Ideally, all five categories of loads should be satisfied. However,
when making a trade-off between energy consumption reduction
and maximum performance, some of the loads could be temporarily
and/or conditionally turned off.
APU charging strategy for key-on use case
To help explain the optimal charging strategy proposed in this paper,
Fig. 4 illustrates the relationship between the APU efficiency and the
June 2023 29
BOTH IMAGES: KUMAR/FORD
Automotive Engineering - June 2023

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