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‐
GTI 2014 ➤
Generic Scan Tool - Edition 04.2015
2. Description and Operation
13
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.
GTI 2014 ➤
Generic Scan Tool - Edition 04.2015
14
Rep. Gr. ST - Generic Scan Tool
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.
GTI 2014 ➤
Generic Scan Tool - Edition 04.2015
2. Description and Operation
15
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
GTI 2014 ➤
Generic Scan Tool - Edition 04.2015
16
Rep. Gr. ST - Generic Scan Tool
Нет комментариевНе стесняйтесь поделиться с нами вашим ценным мнением.
Текст