PART A - Engine, Engine Management and Turbocharger

3. The Turbocharger

3.1 General Description

The turbocharger (make Garrett, type T3) with integral wastegate and intercooler, used for the Sierra Cosworth engine, is similar to the turbocharger that is fitted to the Escort RS Turbo engine. In addi*tion, the Sierra Cosworth turbocharger is watercooled. The principle of operation of both turbo*chargers are similar and described in the Escort RS Turbo Workshop Manual.

The turbocharger is designed to give boost pressure at relatively low engine speeds, thus giving a useful increase of torque under normal driving conditions.
The operation boost pressure is 0.7 bar: Maximum boost pressure is limited to 0.9 bar above atmospheric.

Turbocharger assembly
1 = Turbine housing with integral waste gate
2 = Watercooled intermediate housing
3 = Wastegate actuator
4 = Connection - boost pressure control Valve to Wastegate actuator
5 = Connection - Compressor to boost pressure control valve
6 = Compressor housing
7 = Exhaust manifold assy.
8 = Boost pressure gauge
9 = Intercooler
10 = Air cleaner
11 = Relief connection (vent)
12 = Boost pressure control valve
13 = Air Charge Temperature (ACT) Sensor
14 = ECU

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PART A - Engine, Engine Management and Turbocharger

3. The Turbocharger

The lubricating oil for the turbocharger is taken from the main oil pressure system of the engine (the oil feed pipe is connected to the oil pressure switch). The return oil flows back to the sump through a pipe of relatively large cross-section.

The watercooled intermediate housing of the turbocharger prevents over*heating of the turbine shaft bearings and the turbocharger housing. After switch-off the coolant will prevent excessive temperature build-up of
the unit.

The coolant feed pipe for the turbo*charger intermediate housing is connected to the thermostat housing . The return coolant flows back to the expansion tank.

The integral wastegate regulates the flow of exhaust gases. Due to the engine's demand for boost the exhaust gases are either allowed to the exhaust turbine or through the bypass duct into the vehicle exhaust system circumventing the exhaust turbine.

The wastegate actuator, which is connected to the wastegate by a rod, is controlled by the ECU via a boost pressure control valve.

Pipe connections

1 = Oil feed pipe
2 = Oil return pipe
3 = Coolant feed pipe
4 = Coolant return pipe

1 = Wastegate actuator
2 = Actuator rod
3 = Wastegate
4 = Turbine housing
5 = Turbine impeller

An intercooler (air to air radiator) is fitted to cool the charge air.

The turbocharger is maintenance-free.

3. The Turbocharger

3.2 Boost Pressure Control

The Sierra RS Cosworth is equipped with electronic/mechanical boost pressure control.

The ECU is programmed to control boost via a solenoid operated control valve on receipt of sensor signals .

The boost pressure control valve is directly connected to the high pressure side of the turbocharger by a hose.

Depending upon whether the valve is open or closed it will either "vent off" a portion of the boost to the inlet side of the compressor (aircleaner) or direct it to the wastegate actuator

1 = Exhaust turbine
2 = Compressor impeller
3 = Wastegate actuator
4 = Boost pressure control valve
5 = Air charge temperature (ACT) sensor
W = Wastegate connection
C = Compressor connection
R = Relief connection (vent)
M = ECU

With engine in operation, the boost pressure control valve is open, allowing air charge to "vent off", depending on receipt of signals from the ECU.

Boost pressure control valve in situ
1 = Hose - Wastegate Actuator
2 = Hose - Turbocharger high pressure side
3 = Hose to aircleaner ("vent off")

Boost pressure control valve open, allowing air charge to "vent off', The wastegate is closed.

The boost pressure control valve will be closed gradually when the engine speed is above 6000 RPM or when the air charge temperature exceeds 70° C.

When these signals from the RPM / TDC* sensor and the Air Charge Temperature (ACT) sensor are received by the ECU, it closes the solenoid control valve gradually applying full turbo pressure to the wastegate actuator which then fully opens the wastegate. A high amount of the exhaust gases are allowed through the bypass duct into the vehicle exhaust system circum*venting the exhaust turbine. The boost pressure is reduced to approximately 0.3 bar.
The reduction in boost pressure will reduce the temperature to acceptable working level.

The By-pass Valve (Recirculating Dump Valve)

A special by-pass valve (located next to the intercooler) is directly connected to the air chamber. the intercooler and the inlet trunking by hoses. The bypass valve prevents a pressure build up of air charge in front of the throttle plate when the throttle plate is suddenly closed. With the throttle plate in closed position, the vacuum in the air chamber opens the by-pass valve and an amount of charged air will be by-passed from the intercooler back to the inlet trunking.

Limitation on Engine Revolutions

Boost pressure control valve closed allowing air charge to open the wastegate via the actuator

By-pass Valve
1 = Valve
2 = Connection to air chamber
3 = Connection to intercooler
4 = Connection to inlet trunking

In case the boost pressure exceeds 0.9 bar, the fuel injection at the injector valves will be interrupted.
At 6500 to 6800 engine RPM, the boost pressure is reduced to approximately 0.3 bar by the boost pressure control valve.
At 6800 to 7000 engine RPM, the ECU is programmed in such a way to interrupt every second injection (the engine will remain in performance but without increase of engine RPM).
At speeds above 7000 RPM, injection will cut off totally, controlled by the ECU.