18 E-AXLES
ABOVE: ZF’s mSTARS
(modular semi-trailing arm
rear suspension) axle system
VehicleDynamicsInternational.com • November/December 2019
Just add wheels
A key characteristic of ZF’s mSTARS system is its modularity. Depending on the
customer’s request, a relatively simple basic axle can be extended to an electric
or a standard powertrain module and/or an AKC rear-axle steering system. Only
minor modifications to the car body are required for installing the system into
vehicles in lieu of the hitherto existing, conventional axles. This means that the
number of platform variants can be reduced and the possibilities widely extended
in a modular way.
The concept also reduces development costs for car manufacturers because ZF
takes over the individual fine-tuning of the internal interfaces in the axle toolkit. The
mSTARS axle conforms to the complex conventional multilink suspension standards
of compact premium and sports vehicles. By means of the AKC active rear-axle the
steering system can be combined with all modular axle configurations – as can the
active damping systems – enhancing agility, comfort and stability. ADAS can also
regulate better, more safely and more comfortably because the rear wheels are
actively involved in the steering.
The automotive industry appears to have struck gold
with the e-axle concept. To some, this means in excess of
300kW of permanent assistance on the front of a milliondollar
hypercar; to others it may be a ‘hang-off’ system
providing four-wheel-drive traction for a family crossover;
or perhaps an affordable route into electrifi cation for a
low-cost city car. There’s an e-axle, it would appear, for
every occasion.
Research carried out by IHS Markit last year predicted
a compound annual growth rate of 46% for rear e-axles
and 35% for front e-axles until 2023. That would see the
annual production of front e-axles grow from around
750,000 units to more than 4.5 million per year over
the same time period.
The reasons for adopting an e-axle architecture are
compelling. Integrating as much as possible of the electric
drive hardware into one unit – potentially including
the power electronics and associated ancillaries – can
reduce the total packaging volume of the drivetrain,
as well as cutting down on weight and cost. There are
also performance benefi ts, with the potential to reduce
electrical losses by minimising the use of components such
as high-voltage cables, connectors and cooling units. Plus,
each individual part can be optimised specifi cally for use
in that package.
That’s not to say the use of an e-axle is a foregone
conclusion, as Catalin Fliscu, hybrid control team leader
at AVL, explains, “Packaging tends to be a big issue if you
want to hybridise an existing platform. If you can reduce
the size of the fuel tank or perhaps raise the boot fl oor
slightly you can maybe drop an e-axle into the back of a
front-wheel-drive platform. That also gives the benefi t of
four-wheel drive or electric-only rear-wheel drive, which
is seen as a sporty attribute. You can’t always do that,
though. Sometimes it’s easier to fi nd the packaging space
for a P2 installation in the engine bay.”
Several OEMs have successfully integrated e-axles into
legacy platforms, but for the next generation of vehicles
the decision to equip them with this technology is
likely to be taken at the start. Potentially, this enables
manufacturers to pursue a modular architecture, covering
/VehicleDynamicsInternational.com