|
How Supra Sequential Twin Turbo Work
Article by John Cribb
The main principle of this system is to be able
to route engine exhaust through one or both turbo(s) in the most
efficient manner possible, in order to achieve useful boost at
both low and high loads.
At low engine RPM and load, all engine exhaust
is routed through the #1 turbine in order to have boost at low
rpms; this is called single mode. As engine load and RPM
increase, the system will bring the #2 turbo online in a
controlled manner, so there is no shock to the turbo or the
engine, nor (in theory) any spikes or flat spots in the engine
torque curve. This is known as the transition mode. Finally when
both turbo’s are online and boosting in parallel, this is called
true twin turbo mode.
There are 4 sets of VSV's (Vacuum Solenoid
Valve), actuators, and control valves for the STTS. Each VSV
allows stored air from a small accumulator (pressure tank) to
pass to, or bleed from the actuators. Each of the 4 control
valves has an air operated actuator, and an ECU controlled VSV.
Two of the control valves, the EGCV and IACV, have only two
positions, either open or shut as their VSV’s are controlled by
On/Off signals from the ECU, while the other two, the EBV and WG,
can be controlled to any position between 0 and 100%, as their
VSV’s are operated by PWM duty cycle signals from the ECU.
Following is a definition and description for
each control valve:
Wastegate (WG):
This normally closed valve is located in a “Tee” of the engine
exhaust, before the turbocharger turbines. When the WG is
closed, all engine exhaust must pass through the turbo(s) to
exit. When the WG is open, some exhaust can “escape” before the
turbo(s) and will exit directly into the downpipe, cat’s, etc.
There is a control line connected directly from the
turbochargers’ compressor discharge to the WG actuator, so
whatever boost pressure is being made by the turbo(s) will be
sent to the WG actuator. As boost pressure rises, so will the
pressure in this line, which causes the WG to open. As exhaust
energy “escapes” by the opening of the WG, the turbo’s will slow
down, and boost pressure will fall, which will cause the control
line pressure to fall, and the WG to begin closing. By this
inverse proportional action, the WG can control boost
mechanically.
Mounted in the control line between the turbo’s and the WG, is
the WG VSV. When this VSV is duty cycled by the ECU, it acts as
a “bleeder” and reduces the pressure in the control line, which
keeps the WG closed longer. The higher the duty cycle of the WG
VSV, the more delayed the opening of the WG and the higher the
boost. If the WG VSV should fail, it will close and the WG will
receive full manifold pressure, which will open it much sooner,
reducing boost pressure. The configuration of this VSV in
“bleeder” mode is also known as a “fail-safe” configuration.
Exhaust gas Bypass Valve (EBV):
This normally closed valve is located downstream of both the #1
and #2 turbines, but before the EGCV. At about 3500 rpm, the ECU
“duty cycles” the VSV for this valve causing it to open
gradually. Normally, the exhaust from the #2 turbine is blocked
by the closed EGCV, so when the EBV opens, there is now a path
for a small amount of exhaust gas to flow through the #2 turbine
and exit, this allows the #2 turbine to “prespool”. This
“prespool” smoothes the transition from 1 to 2 turbos, and
cushions the shock of the EGCV opening. The EBV valve is
sometimes confused for a wastegate, but it is located after
the turbine wheels instead of in front of them, so it is not
a 2nd wastegate.
Exhaust Gas Control Valve (EGCV):
This normally closed valve is located in the #2 turbine exhaust
discharge piping, and it serves to block the exhaust flow
through the #2 turbine. When this valve is closed, all exhaust
flow must pass through the #1 turbine. At about 4000 rpm, and
after the EBV has opened, the ECU energises the EGCV VSV to open
the EGCV. This unblocks the discharge from the #2 turbine so
exhaust gas can now flow unrestricted through the #2 turbine and
out the exhaust system. This brings the #2 turbo up to full
operating speed.
Intake Air Control Valve (IACV):
This normally closed valve is located in the intake system, just
after the #2 compressor discharge. When this valve is in the
closed position, boost is blocked from the #2 compressor, but
more importantly, no backflow from #1 is possible which might
cause #2 to spin backwards. Just after the EGCV is opened, the
ECU energises the IACV VSV to open the IACV. This allows the
full boost pressure from #2 compressor to join with boost coming
from #1 compressor and the system is now operating in true twin
turbo mode.
There is also a mechanical 1 way reed valve within the same
housing of the IACV, and in parallel with it, which allows boost
from #2 to enter the common manifold if its pressure is equal
to, or greater than the #1 boost during prespool and the initial
opening of the EGCV.
Here is the sequence of events from single to true twin turbo
operation.
1. At
idle the WG, EBV, EGCV and IACV valves are all closed.
2.
From 1500 to 3500 RPM, and low loads, the above valves remain
closed, and the system operates only on the #1 turbocharger.
(Single mode)
3.
Around
3500 RPM, the ECU will duty cycle the EBV open, and allow the #2
turbocharger to prespool. If/when #2’s boost pressure is high
enough, the mechanical reed valve will open, allowing #2 boost
to join #1 boost in the common system. (Transition mode).
4. As
RPM’s and load increase, the EGCV will open, allowing more flow
of exhaust gas from the #2 turbine. At about the same time, the
IACV will also open, allowing the #2 compressor to flow into the
intake system. Once the EGCV and IACV are both open, the two
turbochargers are operating in True Twin mode, with (in theory
again) equal exhaust and intake flows.
Boost pressure of the overall system is always controlled by the
duty cycled VSV for the WG
|
