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[30] MITSUBISHI MOTORS CORPORATION Annual Report 2001
In 1987, MMC introduced one of the first vehicles to be
fitted with a high-performance 4WD system.
engine. During normal operation, the
engine is designed to run at a fixed,
low speed. This in turn reduces noise
and vibration, both internally and
externally.
The role of the diesel engine in the
HEV powertrain design utilized in this
vehicle is simply to generate electric
power for the motor. Since it is not con-
nected directly to the drive axle, the
engine can therefore be run highly effi-
ciently at a constant low speed. This
helps to minimize its exhaust emis-
sions. In addition, when the vehicle is
stopped or is moving off from a start,
the engine is not used and the vehicle
runs solely on electric power. This allows
the exhaust emissions of the engine to
be precisely matched to fuel economy,
making the vehicle compliant with
stringent new Japanese Short-term
Emission Regulations governing NOx
and particulate matter.
High-performance 4WD vehicle
development
The general aim of 4WD systems is to
share the load across all four tires
equally. This role is important as an
active safety technology since extract-
ing the best performance from each
tire helps to improve driveability,
while also allowing anybody to drive
the vehicle safely. In 1987, MMC intro-
duced one of the first vehicles to be
fitted with a high-performance 4WD
system, which featured a center differ-
ential connected to a VCU (viscous
coupling unit).
Since then, MMC has developed a
number of other high-performance
4WD systems. In one variety, an elec-
tronically controlled center differential
allows the vehicle to vary from full
4WD to a partial front-wheel drive
system with a front-rear power split of
30:70. Other systems feature AYC
(active yaw control), in which the cor-
nering abilities of the vehicle are
greatly enhanced through direct elec-
tronic control of the yaw moment at all
times via a combination of rear-wheel
drive and monitoring of the braking dif-
ferential between the two rear wheels.
In motor sports, particularly in ral-
lying, 4WD vehicles became the norm
since they were the best systems for
transmitting power from high-
performance powertrains to the road
surface. Yet most systems featured
only a VCU with a mechanical LSD
(limited slip differential), a combina-
tion that frequently failed to deal with
the constantly changing peculiarities
of rallying terrain. This fact left much
room for improvement to boost the
competitiveness of rallying vehicles.
Since rolling out its first 4WD system,
MMC has developed a number of other
high-performance 4WD systems.