Volkswagen Golf / Golf GTI / Golf Variant. Manual - part 85

the EPC warning light is turned off if no malfunction in the elec‐

tronic throttle control system is detected. In the event of a mal‐

function while the engine is running, the ECM will activate the

EPC warning light via the Instrument Cluster and at the same

time, a Diagnostic Trouble Code (DTC) is stored in the ECM

memory.

2.5

Engine Control Module (ECM)

The Engine Control Module (ECM) is a generic term for any em‐

bedded system that controls one or more of the electrical systems

or subsystems in a vehicle. It controls a series of actuators on an

internal combustion engine to ensure that driver commands (e.g.

to accelerate) are translated into appropriate engine perform‐

ance. It reads values from a multitude of sensors, interprets the

data, and adjusts the engine actuators accordingly. The ECM also

interacts with the transmission control module (TCM), ABS/trac‐

tion/stability control module and other vehicle function related

control systems.
ECM controlled systems and functions (performance and emis‐

sion related) will be introduced in the following chapters. These

include the OBD system, controller area network (CAN), throttle

control module, fuel supply, ignition, variable valve timing, ex‐

haust-gas recirculation, secondary air injection, exhaust system,

and EVAP system.

2.6

Malfunction Indicator Lamp (MIL)

When the ignition is switched on, the Engine Control Module

(ECM) performs checks on static system integrity (e.g. circuit in‐

tegrity, communications, etc). The Malfunction Indicator Lamp

(MIL) is switched on during this process via the Instrument Clus‐

ter. After engine starts, the ECM examines engine operation for

potential malfunction(s) or failure(s) that can lead to increased

emission values. If no malfunction is detected, the ECM switches

off the MIL via the Instrument Cluster.
In the event of a malfunction during the operation of the engine,

the ECM will activate the MIL via the instrument cluster and at the

same time, a Diagnostic Trouble Code (DTC) is stored in the ECM

memory. In OBD systems, the MIL can have up to three stages:

steady, flashing and Stop Vehicle. A steady MIL indicates a minor

fault (e.g. a failing oxygen sensor) whereas a flashing MIL indi‐

cates a more severe malfunction that could result in damage of

engine or exhaust system components (e.g. the catalytic con‐

verter) if left uncorrected for an extended period. This would also

indicate a severe fault. The three stages are 1. ON, then OFF; 2.

ON steady; 3. flashing constantly. The 3rd stage indicates dam‐

age may occur and driver must stop.

2.7

Controller Area Network (CAN)

Overview
The Controller Area Network (CAN) bus is a message-based pro‐

tocol that allows control units and devices to communicate with

each other using a shared network. With this system, control units

of the various electronic systems are no longer interconnected by

multiple separate cables. This does away with a large number of

electrical connections and results in a reduced likelihood of failure

of the device network.
Broadcast Communication
Each of the devices on the network has a CAN circuit and is

therefore is considered “intelligent”. All devices on the network

see all transmitted messages. Each device can determine if a

message is relevant or if it should be filtered out. This structure

allows modifications to CAN networks with minimal impact. Addi‐

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tional non-transmitting nodes can be added without modification

to the network.
Priority
Every message has an assigned priority. If two nodes try to send

messages simultaneously, the one with the higher priority gets

transmitted and the one with the lower priority gets postponed.

This arbitration does not affect other messages and results in

non-interrupted transmission of the highest priority message

2.8

Fuel Supply

Overview
The fuel supply system delivers fuel to an internal combustion

engine. With carburetors being replaced by fuel injections sys‐

tems in the late 1980s and 1990s, the most common types of fuel

supply system currently in use are throttle body injection (single-

point injection), multiport injection (MPI) and direct injection (DI).
Fuel injectors atomize fuel because high pressure is forcing the

fuel through a small nozzle in the injector into the intake air stream

or the combustion chamber. This process is often controlled by

the ECM and is dependent on data received from other sources

(e.g. mass air flow sensor, throttle position sensor, etc.) to deter‐

mine the precise amount of fuel needed for any given operating

condition. The primary advantages of fuel injection over carbu‐

retor are improved fuel economy, increased power output and

reduced emissions. The following sections will discuss each fuel

injection concept in detail.
Throttle Body Injection
Throttle body injection uses a single electrically controlled injector

at the throttle body. The fuel is drawn by an electric fuel pump out

of the fuel tank and flows through a paper filter into the fuel injec‐

tor. Since injection happens at the same location as the carbu‐

retor, very little engine redesign (intake manifold, fuel line routing,

etc.) is necessary. The cost saving of throttle body injection com‐

pared to other fuel injection methods encouraged vast adoption

in the late 1980s and early 1990s.
Throttle body injection system also inherits many disadvantages

of the carburetor. One of them being the inability to precisely con‐

trol the amount of fuel supplied into each cylinder, and is unable

to precisely control combustion and emissions. It also restricts the

design of intake manifold as any sharp bends in the intake path

will cause atomized fuel to accumulate on the outer wall of the

intake path. Supplying moderate engine heat to the intake mani‐

fold is also necessary to ensure that the fuel stay vaporized. This

results in a relatively high intake air temperature and compromi‐

ses performance.
Multiport Injection (MPI)
Multiport injection (MPI) consists of an injector for each cylinder

just upstream of the intake valve. The fuel pump delivers the fuel

into a high-pressure line where it flows to the fuel rail and injectors.

When activated by the ECM, each injector sprays fuel at the in‐

take port of its corresponding cylinder – this allows individual

cylinders to receive the right amount of fuel in a more precisely

timed manner. Sequential fuel injection mode can be applied to

activate each injector individually to improve engine response.

Lowered fuel consumption and emissions are also achieved.
Sequential multiport injection is still the most common fuel injec‐

tion system found on most economy cars thanks to its high

efficiency, control simplicity and low manufacturing cost (com‐

pared to direct injection). However, to further improve driveability

(performance) while reducing emissions and fuel consumption,

direct injection becomes a superior alternative.

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Direct Injection
Injectors in directly injected (DI) engines are mounted on the cyl‐

inder head and fuel is injected directly into the engine’s combus‐

tion chamber. In order to overcome the pressure in the

combustion chamber during compression and power stroke, in‐

jectors often operate at a primary pressure as high as 3000 psi.

At such extreme pressure level, no single fuel pump can supply

the required pressure directly from the fuel tank to the injectors.

Instead, a low-pressure and a high-pressure system are em‐

ployed. The low-pressure system principally utilizes the same fuel

systems and components for multiport injected engines. The

high-pressure system consists of a high-pressure fuel pump driv‐

en directly by the camshaft, a fuel rail (high-pressure accumula‐

tor), a high-pressure sensor and, depending on the system, a

pressure-control valve or a pressure limiter. The injectors are op‐

erated by the ECM to send a precise amount of fuel from the high-

pressure rail directly into the combustion chamber.
The distinctive difference between direct injection and other in‐

jection methods is that direct injection offers the flexibility regard‐

ing when in the combustion cycle the fuel is added and how. MPI

systems can only add fuel during induction; A DI system can add

fuel whenever it needs to. For example, fuel can be added during

induction to create a homogeneous charge then added again after

ignition to enhance power delivery under full load conditions.
VW/Audi Fuel Stratified Injection (FSI)
The goal of a stratified-charge operation is to form an ignitable

mixture near the spark plug at the instant of ignition. This means

that, instead of supplying the corresponding stoichiometric fuel

quantity to the amount of air in the combustion chamber, the fuel

interacts only with a portion of the air before it is conveyed to the

spark plug. The rest of the fresh air surrounds the stratified charge

allowing an ultra-lean condition with air-fuel ratio exceeding 50:1

in some instances. As less fuel is used to “burn” more air, stratified

injection helps to further reduce fuel consumption when the en‐

gine is operating in low-load conditions (e.g. highway cruising).

This is created by designing the combustion chamber so that a

“swirling” effect of the air-fuel charge is caused.

2.9

Ignition and Timing

Ignition
A spark ignition (SI) engine requires a spark to initiate combustion

in the combustion chamber. Voltage is supplied to the spark plug

where the electricity will arc across a gap at a voltage as high as

100 kilovolts. The ECM determines the precise moment to fire

each spark plug using ignition logic which is pre-programmed into

the ECM as a function of engine speed and load. An optimally

calibrated ignition system ensures consistent and reliable ignition

under all conditions. Knock or misfire as a result of incorrect ig‐

nition can lead to destruction of engine components or damage

of the catalytic converter.
Timing
Shifts in the moment of ignition (ignition timing) can result in in‐

creased emissions, decreased performance and fuel economy.

Whereas more spark advance improves power and fuel economy,

it also raises HC and NOx emissions. Excessive spark advance

can cause engine knock which is potentially destructive to en‐

gines. If the ECM detects knock from a signal sent by a knock

sensor, it will delay (retard) the timing of the spark. Excessive

spark retard lowers power output and produces high exhaust

temperatures, which can also harm the engine. Carefully de‐

signed ignition logic provides optimum timing that best balances

performance, fuel economy and emissions.

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2.10

Variable Valve Timing

Engines equipped with variable valve timing provide the option of

adjusting the phase of the camshaft with respect to the crank‐

shaft. This allows the ECM to control the time at which the valves

open or close, and therefore better assists engine “breathing” at

various engine speeds. When engine speed increases, the dura‐

tion of intake and exhaust stroke shortens so that less fresh air

can be drawn into the combustion chamber and less exhaust gas

can escape. In such a scenario, the ECM opens the intake valve

before the exhaust gas has completely left the combustion cham‐

ber, and their considerable velocity assists in drawing in the fresh

charge – this is referred to as “valve overlap”.
In addition to valve timing, some engines also employ variable

valve lift that switches to a more aggressive camshaft-lobe profile

as engine speed increases. A more aggressive camshaft-lobe

profile actuates valves more rapidly and lifts valves to a greater

magnitude in comparison to a normal camshaft-lobe profile. This

improves intake and exhaust flow rate, allowing engines to raise

maximum operating speed and power output.

2.11

Exhaust-Gas Recirculation (EGR) Sys‐

tem

Exhaust-Gas Recirculation (EGR) can be utilized to control the

cylinder charge and therefore the combustion process. The ex‐

haust gas that is recirculated to the intake manifold increases the

proportion of inert gas in the fresh gas filling; this results in a re‐

duction in the peak combustion temperature and, in turn, a drop

in temperature-dependent NOx emission.
Exhaust-gas recirculation is made possible by a connection be‐

tween the exhaust pipe and the intake manifold. Due to the

pressure differential, the intake manifold can draw in exhaust gas

via this connection. Together with the exhaust-gas recirculation

valve, the ECM adjusts the opening cross-section and therefore

controls the partial flow tapped from the main exhaust flow. A

malfunction in exhaust-gas recirculation system can result in per‐

formance loss and increased emissions. In such a scenario, the

Malfunction Indicator Lamp (MIL) lights up and a Diagnostic Trou‐

ble Code (DTC) is stored in the ECM memory.

2.12

Secondary Air Injection

Additionally injecting air into the exhaust pipe triggers an exo‐

thermic (release of heat) reaction. This leads to the combustion

of HC and CO components that prevail mainly during the warm

up phase. This oxidation process releases additional heat. Con‐

sequently, the exhaust gas becomes hotter, causing the catalytic

converter to heat up at a faster rate. For spark-ignition engines,

secondary-air injection is an effective means of reducing HC and

CO emissions after starting the engine and to rapidly heat up the

catalytic converter. This ensures that the conversion of NOx emis‐

sions commences earlier.
An electronically controlled valve operates the secondary-air

valve (a one-way check valve). The ECM actuates the pump and

the control valve, ensuring that secondary air can be injected at

a defined point in time. The secondary air must also be injected

as close to the outlet valve as possible in order to exploit the high

temperatures to utilize the exothermic (release of heat) reaction

effectively.

2.13

Exhaust Systems

Overview
There are three important functions of the exhaust system: to re‐

duce the pollutants in exhaust gas, muffle engine combustion

noise and to discharge exhaust gas at a convenient location on

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