The Exhaust Gas Recirculation System (EGR) - Troubleshooting
The Exhaust Gas Recirculation System - Troubleshooting
Tighter statutory regulations have made it necessary to reduce exhaust
emissions even further. This applies to both diesel and petrol engines.
Emission of nitrogen oxides is reduced with the aid of so-called exhaust
gas recirculation. In the case of petrol engines, fuel consumption is also
reduced in part-load operation.
What influence does
exhaust gas recirculation
have on combustion?
At high combustion temperatures, nitrogen oxides are produced in the
engine's combustion chamber. Recirculating part of the exhaust gas to
the fresh intake air reduces the combustion temperature in the
combustion chamber. The production of nitrogen oxides is avoided on
account of the low combustion temperature.
The following table shows the exhaust gas recirculation rate for diesel and
petrol engines:
How does exhaust gas
recirculation take place?
A distinction is made between two kinds of exhaust gas recirculation:
“inner” and “outer” exhaust gas recirculation.
In the case of inner exhaust gas recirculation, the process of mixing
exhaust gas and fresh air/fuel mixture takes place within the combustion
chamber. In all 4-stroke engines this is done by the valve overlap of intake
and exhaust valve particular to the system. On account of the design, the
exhaust gas recirculation rate is very low and can only be influenced to a
limited extent. Only since the development of variable valve timing has it
been possible to actively influence the recirculation rate, depending on
load and rpm.
EGR system
Outer exhaust gas recirculation takes place via an additional pipe between
the exhaust manifold/pipe and the intake manifold and the EGR valve.
The first systems were controlled by a poppet valve, which is opened or
closed by a vacuum element (pneumatic drive). The suction line pressure
served as a control variable for the vacuum element. This meant that the
position of the poppet valve depended on the engine's operating state.
To achieve more influence over the exhaust gas recirculation rate,
pneumatic check valves, pressure limiting valves and delay valves were
installed. Some systems also take the exhaust gas backpressure into
account as control pressure for the vacuum element. In some operating
states exhaust gas recirculation is switched off completely. This is made
possible by installing electrical switchover valves in the control line.
Despite these possibilities of influence, the system was still always
dependent on the engine's load state and the suction pipe vacuum this
implied to control the vacuum element.
To meet the demands of modern engines and become independent from
suction pipe vacuum, electrical drives were developed for exhaust gas
recirculation valves. At the same time, sensors for recognizing valve
position were integrated.
Electrical EGR valve
These developments enable exact
control with short adjustment times.
These days, direct current motors
are also used as electrical drives,
alongside stepper motors, lifting
and rotary magnets. The actual
control valve has also been
modified over time. In addition to
needle and poppet valves of
different sizes and dimensions,
rotary and flap valves are also
used today.
Components of an exhaust
gas recirculation system
- Exhaust gas recirculation valve
- Pressure converter
- Thermal valves
These have a similar task as pressure converters, but work dependent on
temperature. Pressure converters and thermal valves can also be
combined.
Potential faults and their
causes
The EGR valve is certainly the greatest fault source on account of the high
loads. Oil mist and soot from the exhaust gas soot the valve and the
cross-section size of the valve opening is reduced over time until it is
completely blocked. This results in a continual reduction of the
recirculated exhaust gas quantity, which is reflected in exhaust gas
behaviour. The high thermal load favours this process even further. The
vacuum hose system is also often responsible for faults. Leaks lead to a
loss of the required vacuum for the EGR valve, and the valve no longer
opens. An EGR valve not working due to lack of vacuum can of course
also be caused by a defective pressure converter or a thermal valve not
working properly.
There are various possibilities of checking the exhaust gas
recirculation system. These depend on whether or not the system is
capable of self-diagnosis. Systems that are not self-diagnosis capable
can be checked with a multimeter, a manual vacuum pump and a digital
thermometer. But before these time-consuming tests are started, a visual
inspection of all system-relevant components must be carried out.
This means:
■ Are all vacuum lines airtight, connected correctly and laid without
being bent?
■ Are all electrical connections on the pressure converter and changeover
switch connected properly? Are the cables OK?
■ Are there leaks on the EGR valve or the connected pipes?
If no faults are found during the visual inspection, the system must be
checked using further tests and measurements.
Testing vacuum modulated
EGR valves on petrol
engines
The following procedure must be used when testing vacuum modulated
EGR valves:
Valves with one membrane
With the engine switched off, remove the vacuum line and connect the
manual vacuum pump. Generate a vacuum of approx. 300 mbar. If the
valve is OK, the pressure may not drop within 5 minutes. Repeat the test
with the engine running and warm. At a pressure difference of approx.
300 mbar, idling must deteriorate or the engine die. If the valve is fitted
with a temperature sensor, this can also be tested. To do this, remove the
temperature sensor and measure resistance. The approximate resistance
values for the individual temperatures are listed in the following table:
EGR valves on diesel
engines
Use a hot air gun or hot water to heat the system. Use the digital thermometer to check the temperature and compare the measured values with
the reference values.
Valves with two membranes
Valves with laterally offset vacuum connections are only opened by one
connection. These can be located above one another or offset laterally on
one level. Valves in which the vacuum connections are arranged above
one another work in two stages. Above the lower connection, the valve is
partly opened, above the upper connection the valve is completely
opened. Valves with laterally offset vacuum connections are only opened
by one connection. The connections are colour coded. The following
combinations are possible:
■ Black and brown
■ Red and brown
■ Red and blue
The vacuum supply line is connected to the red or black coded connection.
Leak tests are carried out under the same conditions as for valves with
one membrane, but must be carried out on both vacuum connections. To
check the vacuum supply to the valve, the manual vacuum pump can be
used as a manometer. It is connected to the EGR valve supply line. The
prevailing vacuum is indicated with the engine running. In the case of valves with connections arranged above one another, the manual vacuum
pump must be connected to the line of the lower connection, with laterally
offset connections to the line of the red or black connection.
Leak test on an EGR valve
EGR valves on diesel engines can be tested in the same way as
those on petrol engines
A vacuum of approx. 500 mbar must be created using the manual
vacuum pump with the engine switched off. This vacuum must be
maintained for 5 minutes and may not drop. A visual inspection can also
be made. To do this, create a vacuum again using the manual vacuum
pump via the vacuum connection. Observe the valve rod (connection
between membrane and valve) through the openings. They must move
evenly when the manual vacuum pump is actuated.
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