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Volvo 850 turbo engine operation theory

Volvo 850 service & repair manual

THEORY/OPERATION – TURBO

1995 Volvo 850

1995 ENGINE PERFORMANCE Volvo – Theory & Operation
Volvo 850 Turbo, T-5, T-5R & 850R

INTRODUCTION

This article covers basic description and operation of engine performance-related systems and components. Read this article before diagnosing vehicles or systems with which you are not completely familiar.

AIR INDUCTION SYSTEM

TURBOCHARGERS

Turbo models use a water-cooled turbocharger, mounted directly to exhaust manifold, with a wastegate assembly attached to rear of turbine housing. Turbocharger consists of a turbine/compressor assembly, oil supply system and wastegate. Other components include impellers, impeller shaft, bearings and impeller housings.
The safety valve of system is a pressure-actuated wastegate that prevents excessive intake boost pressure. The wastegate is controlled by the turbo control valve. This 3-port solenoid valve monitors boost pressure and prevents turbo lag. The control valve is activated by turbo control unit, which receives signals from throttle position sensor, fuel injection ECU and turbo pressure sensor.
If boost pressure exceeds safe limits, engine damage may result. The wastegate opens when exhaust pressure exceeds a predetermined limit and allows exhaust gases to by-pass compressor. Turbocharger operation requires a large quantity of clean oil to prevent bearing failure. Engine oil pressure provides constant lubrication to system.
At idle and light throttle, turbo engine operates like a standard engine. When more power is required, exhaust gases from exhaust manifold enter turbocharger’s turbine housing and flow through turbine blades. Exhaust flow and turbine speed increase as throttle opens and RPM increases. Impeller turns with turbine and forces air into compressor housing and intake manifold. As impeller and turbine speed increases, boost pressure also increases.

COMPUTERIZED ENGINE CONTROLS

850 turbo models use a Motronic 4.3 injection and electronic ignition system, equipped with self-diagnostic capabilities.
Systems use an Electronic Control Unit (ECU) that receives input from engine monitoring sensors. These sensors include camshaft sensor, coolant temperature sensor, oxygen sensor, knock sensor, mass airflow meter/sensor, timing pick-up and throttle switch. ECU uses these input signals to control air/fuel mixture for emission control, fuel economy and good driveability. Ignition and fuel controls are provided by a single ECU.

CONTROL UNITS

Electronic Control Unit (ECU) provide precise control of fuel, ignition and turbo operation. System has self-diagnostic capabilities.
850 turbo uses a single ECU for ignition and fuel controls, located in right front of engine compartment.
NOTE: Components are grouped into 2 categories. The first category is INPUT DEVICES, which are components that control or produce voltage signals monitored by control unit. The
second category is OUTPUT SIGNALS, which are components controlled by control unit.

INPUT DEVICES

Available input signals include: Acceleration Sensor
Acceleration sensor is mounted to firewall at right rear of
engine compartment. Sensor consists of a piezoelectric vibration pick- up that detects vertical acceleration of vehicle, for example driving on a bumpy road. ECU uses this sensor signal to determine if irregularities in crankshaft rotation are due to cylinder misfiring or due to driving on an uneven road surface.
A/C Switch
Signals fuel injection ECU of A/C operation. This allows fuel injection ECU to control idle speed with idle valve.
Air Temperature Sensor
Information gathered from air temperature sensor is combined with information from pressure sensor to calculate intake air mass.
Engine Coolant Temperature Sensor
Engine Coolant Temperature (ECT) sensor is a negative temperature coefficient type, meaning its resistance lessens as temperature increases. Four cylinder sensor unit has 2 resistors. One resistor is connected to fuel injection ECU and the other resistor is connected to the ignition ECU.
Knock Sensor
Knock sensor detects knocking and sends signal to Motronic or ignition ECU. ECU is able to gradually retard ignition timing to each individual cylinder. If knocking does not stop, a signal is sent to Motronic ECU to enrich air/fuel mixture.
Mass Airflow Meter
This meter measures intake air mass. Measure sensor is a heated wire which is maintained at 250 F (120 C) warmer than intake air. Motronic ECU is able to calculate mass of intake air by measuring amount of current required to maintain wire temperature. When engine
is turned off, any contaminants on wire is burned off by heating wire to greater than 1800 F (1000 C).
Mass Airflow Sensor
This sensor uses a hot film, rather than a heated wire to measure intake air mass. Since working temperature is high at 338 F (170 C), and flow and temperature-sensitive resistances are mounted on side of hot film, a burn-off function is not required.
Heated Oxygen Sensor (HO2S)
Also known as a Lambda probe, this heated oxygen sensor generates an electrical signal proportional to air/fuel mixture. Motronic ECU uses this information to adjust amount of injected fuel. The turbo uses 2 oxygen sensors.
Throttle Position (TP) Sensor
The TP sensor signals Motronic or ignition and fuel injection
ECUs when throttle is fully closed or fully open.

OUTPUT SIGNALS

ECU processes information from input sensors and sends appropriate voltage control signals to control devices.
NOTE: For theory and operation of each output component, refer to system indicated after component.
CHECK ENGINE Light
See CHECK ENGINE LIGHT under SELF-DIAGNOSTIC SYSTEM.
EGR Solenoid Valve
See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pump
See FUEL DELIVERY under FUEL SYSTEM.
Idle Valve
See IDLE SPEED under FUEL SYSTEM.
Ignition Control Unit
See IGNITION SYSTEM.
Power Transistor & Ignition Coil
See ELECTRONIC IGNITION under IGNITION SYSTEM.

VOLVO 850 TURBO FUEL SYSTEM

FUEL DELIVERY

Fuel Pump
The Volvo 850 is equipped with an in-tank fuel pump. Fuel pump is equipped with check valves to hold fuel pressure in system when ignition is off. Fuel from main pump is sent through an in-line fuel filter. Fuel is then sent to fuel pressure regulator where pressure is maintained at a constant pressure in relationship to manifold
pressure. Excess fuel returns to fuel tank via a return line.
Fuel Pressure Regulator
Pressure regulator is a sealed unit which is divided by a diaphragm into 2 chambers (fuel and spring chambers). Fuel chamber receives fuel through inlet side from injector fuel rail. Spring chamber is connected to intake manifold vacuum. At idle, intake manifold vacuum is high. Diaphragm is pulled back by intake manifold vacuum. Any excessive fuel is returned to fuel tank. As throttle is depressed, intake manifold vacuum decreases. Regulator spring overcomes manifold vacuum, increasing fuel pressure.

FUEL CONTROL

ECU calculates base injection pulse width by processing signals from various engine sensors. Information from crankshaft position sensor (RPM) is used to trigger timing of fuel injection. During normal driving conditions, injection duration is regulated in reference to mass air meter/sensor, engine speed, oxygen content of exhaust gases and coolant temperature. Under full throttle conditions,
a richer fuel mixture is provided for increased power and to reduce combustion heat in engine and catalytic converter.
Fuel Injectors
Each injector incorporates a solenoid, plunger and needle valve which opens and closes an orifice. Control unit supplies current through auxiliary relay for a predetermined period, opening all injectors simultaneously to inject atomized fuel. Injection takes
place twice per revolution while starter motor is running and once per revolution under normal driving conditions. Fuel is injected into intake manifold close to each intake valve.

IDLE SPEED

Engine idle speed is controlled by ECU depending upon engine operating conditions. ECU senses engine operating conditions and determines best idle speed. Idle speed is controlled by varying air passage inside idle valve.
Idle Valve
Idle valve uses a solenoid or motor to control by-pass air. Signal from ECU determines idle speed by controlling amount of by-pass air.

VOLVO 850 IGNITION SYSTEM

ELECTRONIC IGNITION

ECU computes correct timing of each ignition pulse in response to signals from RPM sensor, Camshaft Position (CMP) sensor, Mass Airflow (MAF) sensor, Engine Coolant Temperature (ECT) sensor, Throttle Position (TP) sensor, Knock Sensors (KS) and Transmission Control Module (TCM).

IGNITION TIMING CONTROL

Ignition Timing Advance Control
Ignition timing is totally controlled by an ECU. Ignition timing is based on preprogrammed information and modified by inputs from engine sensors.
Knock Sensor
Two knock sensors are used. Knock sensors are fitted to cylinder block to sense detonation inside cylinders. When detonation
is detected, ECU retards ignition timing in each cylinder individually until knocking stops. If knocking continues, ignition control unit signals turbo control unit to reduce boost pressure in stages.

EMISSION SYSTEMS

FUEL EVAPORATIVE SYSTEM (EVAP)

Evaporative emissions system is designed to prevent fuel vapor from entering atmosphere. Fuel system is completely sealed and vented only through a carbon canister. System consists of pressure/vacuum relief fuel filler cap, rollover valve, charcoal canister, purge valve and various connecting hoses.
Fuel pressure/vacuum relief filler cap allows excessive tank pressure to vent. It also allows air into fuel tank if vacuum should become excessive due to a malfunction in fuel evaporation system. Fuel tank vapor is vented by a line through rollover valve to charcoal
canister. Rollover valve is located in vent line close to fuel tank. Valve is designed to prevent fuel spillage if vehicle rolls over. Valve is open until vehicle is at a 45-degree angle or more from horizontal position.
Canister Purge Valve
Charcoal canister is filled with activated charcoal. Fuel vapor from tank is absorbed by charcoal when engine is not running. When engine is running faster than idle, canister purge valve opens and fuel vapor is drawn into engine and burned.

EXHAUST GAS RECIRCULATION (EGR)

EGR System
EGR system operates by returning some exhaust gases to engine to be mixed with air/fuel mixture. This exhaust gas, which is
basically inert at this stage, lowers combustion temperature. Reducing combustion temperature reduces amount of oxides of nitrogen (NOx) released into atmosphere.
EGR Solenoid Valve
When engine coolant temperature is less than 115 F (45 C), solenoid receives no current and EGR system is inactive. With engine at operating temperature, solenoid receives current from relay and opens vacuum line to EGR valve. EGR valve is opened completely by negative pressure. Even the slightest throttle opening opens idle switch. Time relay cuts current to solenoid, disconnecting EGR system for about 5 seconds and avoiding HC and particle build-up during acceleration from idle.
EGR Electronic Vacuum Regulator Valve (EVRV)
EGR valve is supplied with a vacuum control signal from connection in lower section of the EVRV. Vacuum in intake manifold is supplied to upper connection. EVRV stabilizes signal from intake manifold and converts control module signal into a modified vacuum signal for controlling EGR valve.

VOLVO 850 SELF-DIAGNOSTIC SYSTEM

Vehicle is equipped with self-diagnostic systems. A CHECK ENGINE light glows to signal driver of a system malfunction. Fault codes are retrieved through the diagnostic unit, located in right front of engine compartment. The diagnostic unit is equipped with an LED indicator, activation button and function select cable.

CHECK ENGINE LIGHT

NOTE: CHECK ENGINE light is also known as Malfunction Indicator
Light (MIL).
CHECK ENGINE light is located on instrument panel. Light will illuminate when ignition switch is turned to ON position (bulb check) or when emission-related systems are malfunctioning during normal engine operation.

 

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