Automotive Engineering - August 2021 - 15

COVER STORY
Making the case for
IN-WHEEL
MOTORS
IWMs offer many benefits in packaging, vehicle dynamics,
safety and potentially lower cost for urban shuttle and
delivery vehicles, among others.
M
uch of an IC engine-powered vehicle's ride,
handling, sound and overall character derives
from the engine. Some believe that electric
vehicles (EVs), propelled by electric motors
with no intake or exhaust sound and less gearing and
NVH, limit the opportunity for vehicle differentiation. They
argue that the powertrain will become a commodity and
that competitive advantage will need to be achieved
through other areas, such as styling and infotainment.
I contend that the exception to this view is the in-wheel
motor (IWM), a technology that enables quantum improvements
in propulsion efficiency, ride dynamics, active
safety, and vehicle design. IWMs enable " turn-on-a-dime "
operation, a relevant feature for dense urban environments
and safe vehicle entry/egress from the sidewalk. Moreover,
the IWM has the potential to extend the revolution - started
by the now-ubiquitous EV " skateboard " architecture - in
how vehicles are developed, manufactured and serviced.
IWMs are increasingly being developed and tested by
OEMs and suppliers as part of the corner module. A
growing number of advocates in the mobility-engineering
community believe they are an inevitable solution for future
EVs. Therefore, it is useful to understand what has
prevented IWM commercialization so far, beginning with
the commonly cited concern: unsprung mass.
Challenges and barriers
IWM can be integrated into the vehicle in various ways
as shown in the accompanying images. They illustrate
AUTOMOTIVE ENGINEERING
IWMs can improve EV
efficiency, dynamics, safety, and
manufacturability - when unsprung
mass is addressed in their design.
by Chris Borroni-Bird
how IWMs can increase the unsprung mass - the components between
the suspension system and the road surface, which includes wheels,
tires, brakes and parts of the suspension system itself.
From a vehicle-dynamics perspective, it is useful to consider a vehicle's
mass as a combination of sprung and unsprung mass. A vehicle's
primary resonance mode (~1 Hz) is mainly caused by the vehicle's
sprung mass bouncing on the suspension system. Unsprung
mass, however, adversely affects ride comfort because it creates a
more pronounced secondary resonance mode (~7-10 Hz, due to its
inertial lag bouncing on the tire). It also makes handling more difficult
because the suspension must work harder to keep the tires in
contact with the ground.
IWMs at each corner might perhaps double the vehicle's unsprung
mass. But the consequences can be mitigated by tuning the suspension
shock absorbers and reinforcing the shock-mount structure (to
manage increased loads into the body structure). Moreover, ride deterioration
is less perceptible for certain types of vehicles, like trucks and
SUVs, that have a higher sprung:unsprung mass ratio (which drives the
magnitude of this secondary resonance). Passenger expectations for
ride comfort also may be less demanding for these vehicles.
In 2010, Lotus Engineering modified a 2007 Ford Focus by adding
30 kg to each wheel to simulate the effect of adding Protean's
IWM unsprung mass. Data comparing ride and handling characteristics
before and after the modification showed that increased unsprung
mass can be addressed with typical ride-and-handling optimization
techniques.
Other concerns with IWM include durability, thermal management
and electrical safety. Although there is no mass-produced automotive
IWM yet, development activity is expanding. Substantial mileage is
being accumulated in the lab and on test vehicles. Lessons learned
July/August 2021 15
PROTEAN ELECTRIC
Automotive Engineering - August 2021

Table of Contents for the Digital Edition of Automotive Engineering - August 2021
