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 turbos 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 VSVs 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 VSVs 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 wastegate is closed, all engine exhaust must pass through the turbo(s) to exit. When the wastegate is open, some exhaust can escape before the turbo(s) and will exit directly into the downpipe, cats, etc.
There is a control line connected directly from the turbochargers compressor discharge to the wastegate actuator, so whatever boost pressure is being made by the turbo(s) will be sent to the wastegate actuator. As boost pressure rises, so will the pressure in this line, which causes the wastegate to open.
As exhaust energy escapes by the opening of the WG, the turbos will slow down, and boost pressure will fall, which will cause the control line pressure to fall, and the wastegate to begin closing. By this inverse proportional action, the wastegate can control boost mechanically.
Mounted in the control line between the turbo and the WG, is the wastegate 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 wastegate closed longer. The higher the duty cycle of the wastegate VSV, the more delayed the opening of the wastegate and the higher the boost.
If the wastegate VSV should fail, it will close and the wastegate 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 pre-spool.
This pre-spool 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 energizes 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 pre-spool and the initial opening of the EGCV.
Here is the sequence of events from single to true twin turbo operation.
- At idle the WG, EBV, EGCV and IACV valves are all closed.
- From 1500 to 3500 RPM, and low loads, the above valves remain closed, and the system operates only on the #1 turbocharger. (Single mode)
- Around 3500 RPM, the ECU will duty cycle the EBV open, and allow the #2 turbocharger to prespool. If/when #2 boost pressure is high enough, the mechanical reed valve will open, allowing #2 boost to join #1 boost in the common system. (Transition mode).
- As RPM and load increases, 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
