VC-Turbo Engine
The world's first production variable compression ratio engine that simultaneously achieves high efficiency and high power while synchronizing with the driver's intentions
The VC-Turbo engine utilizes a multi-link system that continuously varies piston top dead center (TDC)? bottom dead center (BDC) positions, allowing free control of the compression ratio critical factor of power and efficiency, on demand. This makes it the world’s first production engine to achieve both overwhelming high power and surprising fuel efficiency - two performance characteristics that ordinarily oppose each other.
System operation
Making variable compression possible
A
gasoline
engine
compresses
an
air-fuel
mixture
that
enters
into
the
cylinder
before
igniting
and
combusting
it.
A
higher
compression
ratio
yields
greater
efficiency;
however,
there
is
a
limit
to
the
compression
ratio
because
abnormal
combustion
(knocking)
can
occur
due
to
the
rise
of
mixture
air
temperature.
At
cruising
speeds
when
the
intake
airflow
is
low,
the
limit
of
compression
ratio
is
high.
When
accelerating,
and
the
intake
airflow
is
high,
the
limit
is
low.
Additionally,
the
limit
becomes
lower
for
turbo
engine
that
intakes
compression
air.
It
is
ideal
to
change
compression
ratio
according
to
the
load
condition.
With
a
conventional
engine,
because
the
pistons
and
crankshaft
are
connected
directly
by
the
connecting
rod,
the
compression
ratio
is
fixed.
A
VC-Turbo
engine
uses
a
multi-link
system
in
place
of
a
traditional
connecting
rod
to
rotate
the
crankshaft,
and
an
actuator
motor
changes
the
multi-link
system
endpoint
in
order
to
vary
the
pistons’
reach
to
transform
the
compression
ratio.
This
makes
it
possible
to
vary
the
compression
ratio
continuously
as
needed
within
the
range
of
8:1
(for
high
load)
to
14:1
(for
low
load).
The
optimal
compression
ratio
can
be
continuously
set
to
match
the
operation
of
the
accelerator
pedal
by
the
driver.
How it works
- When a change in the compression ratio is needed, the actuator motor moves the actuator arm.
- The actuator arm rotates the control shaft.
- The rotation of the control shaft moves the lower-link(L-link), changing the multi-link system angle.
- The multi-link system adjusts the vertical position of the piston stroke within the cylinder, changing the compression ratio.
Optimization of the link layout
As the angular change of the upper-link (U-link) when the piston moves up and down is small, the upper-link (U-link) remains more vertical as it moves down smoothly. This reduces friction with the cylinder walls and contributes to improved fuel economy. The reciprocating motion of the piston between TDC and BDC becomes symmetrical, helping to reduce vibration.
System mechanism
The VC-Turbo engine uses a newly developed high-efficiency, wide-range turbocharger and electric waste gate. This allows precise control of the boost pressure with minimal turbo lag to generate powerful on-demand output. Under low load, the electric VTC (Valve Timing Control) continuously adjusts valve timing to reduce pumping loss by means of an Atkinson combustion cycle. It realizes high fuel efficiency in combination with higher compression ratio.
Research for VC-Turbo was started in 1998. Several methods were utilized to realize the VC-Turbo. Developing a variable compression system using a link mechanism, optimizing the link layout, using advanced analysis to produce precision part shapes, and creating a new high-precision heat treatment were just a few technological hurdles. As a result, Nissan became the world’s first auto manufacturer to start mass production of a variable compression engine.
- Reducing pumping loss by means of an Atkinson cycle In an ordinary gasoline engine, when cruising and at other times when power is not needed, the throttle valve is closed to reduce the intake airflow. However, this increases the airflow resistance (pumping loss), which is one obstacle to improving to fuel efficiency. With an Atkinson cycle, the open/close timing of the engine intake valves is actively controlled so that they open later during cruising, reducing the intake airflow in order to control the power. Compared with a conventional engine in which the intake airflow is controlled only by the throttle valve, this makes it possible to reduce pumping loss and improve fuel efficiency.