Automotive Engineering - April 2021 - 26

Optimizing hybrids

COVER STORY

FOR COST AND
EFFICIENCY
A

B

C

D

E1

48V
MHEV P0

HEV P2

E2

Figure 9: Relative powertrain costs for
various technology packages.

400V
HEV P2.5

PHEV P2.5

PHEV Serial / REX + Direct Drive
4-Cylinder NA

4-Cylinder TC

3-Cylinder TC

Wiring & HCU
On-board charger
DC/DC Converter
Battery

Transmission &
e-drive

100%

CO2 measures
EAS Eu7
ICE Baseline
B

C1

D

to provide low-end torque. Thus, the characteristics of
the charging unit can be tailored to operate in the
range of highest charger efficiency at rated power.

The ICE for high-voltage HEV and PHEV
Combining ICE powertrains with high voltage systems
provides simplification potential on the ICE by limiting its
operation range, reducing the ICE power rating even
more than a 48V system. For example, in combination
with an e-motor delivering up to 80kW at 400V, the ICE
power rating can be reduced to 160kW.
Since the e-motor delivers high torque at low speed,
a quite extreme reduction of low-end torque from the
ICE up to mid engine speeds allows simplification of
the boosting system to a wastegate TC, removal of the
VVL system, and potentially also VVT system.

The ideal ICE for a serial hybrid or
range extender
The largest step in ICE simplification can be achieved
with serial-hybrid or range-extender configurations.
As the ICE is not mechanically coupled to the drivetrain, a reduction of the operation range to preferred load
and speed ranges with high efficiency is feasible and transient as well as idle operation is irrelevant. For vehicle
operation outside the city mode, direct connection of the
ICE to the drive train may be advantageous, limited to
higher vehicle speeds in highway operation.
Maximum vehicle performance in high-voltage hybrid
applications, in parallel mode with ICE connected to the
drivetrain, is achieved with the combustion engine and
e-motor combined.
So far, even with moderate ICE power, the acceleration
and performance of hybrid versions is significantly beyond the conventional variants. For these applications,
the question is whether a naturally-aspirated Atkinson
engine, or a smaller turbocharged engine is the better
26 April 2021

E1

E2

option. The NA engine shows its best BSFC in a rather narrow area of the
map, whereas a TC-engine achieves a wider sweet spot.

Efficiency versus cost
The trade-off between add-on cost and CO2 savings on one side and
robustness regarding RDE-requirements on the other side is an essential
factor for the final selection of the optimum technology packages. In
Figure 9, the cost for different powertrain architectures and technology
packages leading to comparable system power levels are visualized.
Starting point for each configuration is an EU7 emission- compliant (i.e.,
full map in stoichiometric operation) base engine together with a conforming aftertreatment system (again for expected EU7 targets). These
two cost blocks together represent the 100% line in the diagram.
Moving from 48V to 400V, the main cost driver is the larger battery
required for plug-in hybrids to increase the electric-driving range.

Conclusions
A modular engine family architecture with common machining and
assembly concepts for the main components provides a basis to
integrate different technology modules for different applications.
By adapting charging and valvetrain systems, the full-load curve
shape and fuel consumption maps can be tailored to best match the
specific powertrain configuration, from pure ICE through mild- to
full-hybrid applications.
This modular technology " component box " is a cost-effective and
flexible way to cope with future fuel-consumption and emission limits, in particular when considering an uncertain distribution of variants in future vehicle platforms.
Balancing overall powertrain complexity with tailored technology
modules on the ICE dependent on the degree of electrification allows
significant cost reduction up to 40% for highly electrified variants
compared to the ICE-only baseline. It at least partially compensates
the cost penalty of the additional electric components.
This is a condensed version of SAE Technical Paper 2020-01-0839, " A modular
gasoline engine family for hybrid powertrains: Balancing cost and efficiency. "
It can be ordered or downloaded from SAE International at www.sae.org

AUTOMOTIVE ENGINEERING

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