The EGR system (EGR, AGR) in a car serves to reduce the level
of nitrogen oxide emissions. The operation of the system is based on
replacing part of the oxygen mixture (air) with an oxygen-free mixture (exhaust
gases), thereby reducing the likelihood of nitrogen oxidation in the car’s
cylinders. Contrary to the persistent stereotype, no “afterburning” of the
exhaust gases occurs, only a decrease in the amount of oxygen in the mixture
supplied to the cylinders.
Since the exhaust gases have a high temperature, cooling is
required before they are supplied to the cylinders, otherwise the intake
mixture will be overheated. To cool the exhaust gases, liquid heat
exchangers are used, in which the engine cooling circuit fluid removes excess
temperature from the exhaust gases passing through the coolers. Coolers
can be with a bypass valve (passes gases past the coolers when the car is not
warmed up) and without these valves, with thermostats in the cooling circuit
(to turn off the flow of liquid when the car is not warmed up) and without
them. Both the bypass valve and the thermostat are designed to speed up
the warm-up of the car and start the USR system into operation earlier.
To regulate the amount of mixed exhaust gases, a valve is
used, usually an electromagnetic one. The engine control unit controls the
valve and at the same time monitors its position through a position sensor,
correcting its operation.
Operating conditions of the USR system
To reduce the negative impact of the USR system on the
dynamic characteristics of the car, the speed of engine warm-up, and the
quality of fuel combustion, the conditions and framework for the operation of
the USR system have been established:
The engine has an optimal operating temperature. At low
or high temperatures, the USR system turns off or reduces the degree of its
participation in mixture formation.
The temperature of the intake air must be within certain
limits. At low or high temperatures, the USR system turns off or reduces
the degree of its participation in mixture formation.
The USR system is switched off above 3000 rpm.
The USR system turns off or reduces its influence under high
loads.
Additional functions performed by the USR valve
The EGR system valve is often assigned two more functions
that are not related to reducing nitrogen oxide emissions:
The function of relieving excess pressure in the intake tract
during a sharp decrease in engine speed from high load mode. At this point
the valve opens for a short time while there is a pressure difference from
inlet to outlet.
Burning function of the particulate filter (DPF), if the car
is equipped with it. When burning the DPF filter, the throttle valve is
closed and the EGR valve is opened slightly to enrich the mixture and allow
unburned fuel vapors to reach the particulate filter.
Typical malfunctions of the USR system
As a rule, malfunctions relate to the USR valve and its
position sensor, due to which the engine control unit cannot provide accurate
control of the flow of exhaust gases to the intake. Starting from EURO-4,
malfunctions of the USR system require immediate elimination, since this
usually limits engine power.
Typically, malfunctions manifest themselves as follows:
The USR valve is working, the valve position sensor is
faulty. The system cannot determine the correct position, but engine
performance is not affected. With EURO-3, the car continues to operate
normally, with EURO-4, power limitation is activated
The USR valve is faulty, stuck in the closed
position. The engine does not deteriorate its characteristics, with EURO-3
you can drive, with EURO-4 the power limit is activated.
The EGR valve is faulty, stuck in the open or slightly open
position. The engine runs worse, there may be a loss of power, and excess
smoke from the exhaust pipe. With EURO-3, the power limitation mode is not
activated; with EURO-4, power limitation is activated.
The EGR system cooler is leaking. Often there are no
signs other than a drop in coolant level without visible leaks. Generally
not dangerous, but theoretically the leak could be large and cause water hammer
in the engine.
Disabling the USR system
In addition to replacing faulty USR system components, you
can partially or completely disable the operation of the USR valve.
Positive aspects of disabling the USR system:
When the valve is completely turned off, you can forget about
problems with the USR system, this is important for long trips and cars where
this system does not work reliably (for example, LR Discovery 3).
Improved response to the gas pedal, reduced
delay when starting off and acceleration. This is due to the fact that
there is always a maximum of oxygen in the cylinders. When the USR
operates, the cylinders are filled with a mixture of air and exhaust gases, and
for full performance, several engine revolutions are required to “weather” this
mixture.
Disabling the USR system reduces smoke,
especially the “spitting” of smoke during acceleration.
Negative aspects of disabling the USR system:
a decrease in the environmental friendliness of
the car in terms of nitrogen oxide emissions (but an improvement in smoke!).
disabling the USR system requires the removal
of the DPF filter, if the car has one.
Here we'll answer some basic
frequently asked questions about removing an EGR system to clear up some
misconceptions
QUESTION 1 : Does disabling USR lead to
increased cylinder temperature and premature engine wear ?.
ANSWER 1 : More precisely, this is a misunderstanding
of the processes in the engine with the USR system. As written above, at high
loads, when the maximum temperature occurs in the cylinders, the USR valve must
be closed in any case. We can only talk about a slight increase in the
“minimum” temperature in the cylinders at minimum engine loads.
QUESTION 2 : Does disabling the USR leads
to worse engine warm-up ?.
ANSWER 2 : The practice of many years of
using a car with a working USR and a disabled one shows that there is no
difference in the speed of warming up the car. As was written above, the USR
system practically does not work on a car that is not fully warmed up.
QUESTION 3 : Does disabling the USR leads
to disruption of mixture formation and worsens engine operating parameters ?.
ANSWER 3 : Engine management systems are
adaptive enough to correctly handle the situation with the USR system disabled.
In addition, as described above, there are many conditions for the operation of
this system, and in fact it does not always work even in normal mode.