Systems for reducing exhaust emissions from diesel engines

Exhaust gas recirculation (EGR) is an integral part of diesel engine's control system, as its high efficiency continues to have a very positive effect on reducing nitrogen oxides (NOx) concentration. It consists in introducing exhaust gases into the engine intake system in order to reduce the amount of oxygen and the combustion temperature. As inert gases the exhaust gas contains large amount of water vapor, which helps to decrease temperature of warmed up engine, whereas when the engine is cold the exhaust gas recirculation effectively increases its temperature in the first minutes of operation. 

 

There are two basic types of recirculation systems:

• high-pressure recirculation system, consists in taking exhaust gases from the exhaust manifold and introduce them into the intake manifold,
• low-pressure recirculation system, consists in taking exhaust gases from behind the particulate filter and introducing them before the turbocharger.

2.1. High-pressure recirculation system

High-pressure recirculation is a classic system that has evolved a long way in automotive. Today's recirculation systems are elaborate; in addition to the EGR valve itself, there is often an exhaust gas cooler and a switching system with which it can be turned off. An air damper is also an important component, with the help of which the engine controller can force a greater involvement of exhaust gases in the recirculation process. Virtually every modern diesel engine car has an exhaust gas recirculation valve, as it has a very high efficiency in reducing nitrogen oxides, already at the stage of engine operation.

The hot exhaust gas from the exhaust manifold is routed via valve 7 to radiator 5 or additional valve 6. If the valve 6 is closed the exhaust gas is routed via the cooler; when the valve 6 is open, the exhaust gas bypasses the cooler. In the engine temperature range up to about 30°C it is advantageous to bypass the exhaust gas cooler in order to heat up the combustion chamber faster. The management algorithm can vary, depending on the system because many times the exhaust gas cooler is used to heat the coolant directed to the passenger compartment heater. At high operating temperatures and high engine loads operation of the cooler is necessary because it lowers the temperature of the circulated exhaust gas thus reducing its volume. This process is very beneficial for low nitrogen oxides emissions and at the same time has a positive effect on the combustion process, the engine runs much quieter and its operation is "softer". Introduced exhaust gases into the intake manifold cause reduction of the volume of sucked air through the engine intake system. The engine controller registers the volume change in the air mass using the flow meter (item 10) and makes adjustments to the EGR valve operation. If the exhaust volume is not sufficient, the engine controller closes the air damper forcing additional exhaust flow. When the engine is under load, exhaust gas recirculation must be turned off because it would interfere with the turbocharger plus the engine needs then the maximum amount of air. When the engine is under load the nitrogen oxides emissions rise sharply.

 

2.2. Low-pressure recirculation system

In Euro 6-compliant vehicles apart from the high-pressure recirculation, a low-pressure recirculation is also used i.e., recirculated exhaust gas is taken from the exhaust system and fed into the engine intake system before the turbocharger (Fig. 2).

The exhaust gas is introduced in front of the turbocharger's compression wheel, which requires it to be very clean so an additional filter is required for this type of EGR. This role is taken over by the already used particulate filter (component 6). This system also uses an exhaust gas cooler (9), but without the possibility of disconnecting (bypassing) it, it is permanently mounted. The EGR valve is in the form of an exhaust gas throttle (8). Due to the slight pressure difference between the exhaust pipe and the compressor inlet a so-called exhaust gas piling up flap is used to force the exhaust gas to flow properly. When the engine is running under load (turbocharger operation) the exhaust gas flap is switched off. The control of the exhaust gas flow is realized by the mass air flow meter. In mixed-mode systems (LP EGR + HP EGR) there is an additional "differential pressure sensor for low-pressure recirculation" that is mounted between the particulate filter outlet and the compressor inlet.

Depending on the fault code DTC, it may be a flow malfunction, mechanical blockage, incorrect signal from the position sensor, output voltage too low or too high and many others.  The following is a sample list of fault codes DTC associated with the exhaust gas recirculation system.

3.1. Interpretation of fault codes DTC P0400 - P0402

Fault codes (DTCs) from P0400 to P0402 refer to abnormal flow. Diagnosis of these errors is mainly based on the operation of the air mass flow meter. The engine controller compares the measured air mass against a reference/template mass derived from the EGR valve control map (Figure 4). The EGR valve map has 2 inputs and one output - the inputs are: engine speed and fuel dosage in the mg/cycle, while the output is the required air mass also given in mg/cycle. The engine controller opens the EGR valve to equalize those masses, and if there is a fault that contributes to the flow disruption (carbon build-up, clogged radiator, faulty EGR throttle) then the ECU records the corresponding fault code (DTC).

3.2. Interpretation of fault codes DTC P0403 - P0404

Fault codes (DTC’s) P0403 and P0404 are related to the mechanics of the EGR valve actuator. After driving a polarity-varying PWM electrical signal (DC electric motor moving the valve mechanism), the DC-Driver module inside the ECU analyzes the motor's current consumption and determines its position using a built-in potentiometric position sensor. If the current draw of the electric motor driving the EGR valve mechanism is too low or too high, the ECU detects P0403 error. This can be caused not only by the defect in the EGR valve itself, but also by a malfunction in the electrical system. P0404 fault occurs when the required voltage values from the valve position sensor are not reached during the end-stops test. The test of the end positions of the actuator is carried out very often, for example after turning off the engine and ignition switch.

 

3.3. Additional diagnostic problems of the EGR system

 

If the diagnosis performed showed a defect in the exhaust gas recirculation valve that qualifies it for replacement, it should be remembered that the engine controller diagnoses the efficiency of the entire EGR system and it consists of additional components that should also be verified. In the diagnostic process of the EGR system it is imperative to pay attention to such components as among others: flexible hoses, exhaust gas ducts, exhaust gas cooler, air damper and air mass flow meter. On cars with mixed recirculation this list expands to include additional components i.e.: the exhaust gas flap, charge air cooler, particulate filter with exhaust gas pressure sensor, differential pressure sensor for low-pressure recirculation, and the low-pressure EGR valve itself. All of these components can generate a number of error codes, so care must be taken to ensure that they are working properly.

 

After replacing the EGR valve it is necessary to perform an adaptation with a diagnostic tester so that the engine controller "learns" anew the operating ranges of the installed component. Failure to adapt the system can again cause rapid damage to the EGR valve. 

How to check if the exhaust gas recirculation system is working properly?

 

Once we know the structure and principle of operation of the recirculation system then we can focus on analyzing its operation. To do this it’s need a diagnostic tester with the ability to read many, preferably all possible measured values so-called real time parameters (actual values). The analysis can be done at a standstill and while driving. In the diagnostic device you should look for such parameters as for example:

 

• air mass required value 
• air mass measured value (from the flow meter)
• Intake manifold pressure (boost pressure)
• Boost pressure required value
• HP EGR valve opening position (high pressure)
• LP EGR valve opening position (low pressure)
• Exhaust gas flap position (for low-pressure EGR)
• Fuel injection dose- Lambda ratio (wideband lambda probe)
• NOx concentration (if there is an NOx probe)
• Required exhaust gas stream for high-pressure recirculation (occurs in specific types)
• Required exhaust gas stream for low-pressure recirculation (occurs in specific types)
• Actual exhaust gas stream for high-pressure recirculation (occurs in specific types)
• Actual exhaust gas stream for low-pressure recirculation (occurs in specific types)
• Cylinder charge (relevant for low-pressure recirculation)

In addition to these elements there are many others that are indirectly affected by exhaust gas recirculation. 

 

How to analyze the correctness of the detailed parameters divided into HP EGR, LP EGR and mixed systems will be presented in subsequent articles in a series on exhaust gas aftertreatment systems.

 

References:

^[1] Konrad Reif. : Diesel Engine Management Systems and Components, Bosch Professional Automotive Information, Springer Fachmedien Wiesbaden 2014.