DIAGNOSTICS AND MAINTENANCE OF MODERN EXHAUST GAS RECIRCULATION SYSTEMS

In a previous article we discussed the construction and principle of operation of exhaust gas recirculation (EGR) systems. Vehicles with complex double EGR systems were used for the discussion. After reviewing the construction and principle of operation of recirculation systems, we may now focus on the diagnostics and analysis of their operation. This article presents the EGR systems diagnostic procedure. A description and interpretation of the actual parameters will be effected using a VW EA288 engine, which was also presented in the previous article. In order to carry out basic EGR system diagnostics, it is necessary to focus on the appropriate parameters, which may include:

Air mass – required value (EGR map)
Air mass – measured value (using an air-mass meter)

Pressure in the intake manifold (boost pressure)

Boost pressure – required value

EGR HP valve opening position (high-pressure valve)

EGR LP valve opening position (low-pressure valve)

Exhaust gas flap position (for low-pressure EGR)

Amount of injected fuel

Lambda coefficient (broadband lambda probe)

NOx concentration (if a NOx probe is present)

Exhaust gas stream required for high-pressure recirculation (not always available)

Exhaust gas stream required for low-pressure recirculation (not always available)

Actual exhaust gas stream for high-pressure recirculation (not always available)

Actual exhaust gas stream for low-pressure recirculation (not always available)

Cylinder filling level (important in the case of low-pressure recirculation)

Figure 1. EGR system control diagram [source Konrad Reif. : Diesel Engine Management Systems and Components, Bosch Professional Automotive Information, Springer Fachmedien Wiesbaden 2014]

Figure 1 shows a conventional EGR system operation. Based on the desired injected fuel quantity and the engine rotational speed, the control system reads the required air value from the EGR map, equal to 250 mg/stroke. Air mass measured with an air-mass meter (air-mass meter 490 mg/stroke) is subtracted from the required mass value. The resulting value of 240 mg/stroke constitutes the input value for the EGR controller, which receives information to reduce air mass by 240 mg/stroke. By opening the EGR valve and closing the air throttle, the air mass controller strives to obtain an input value of zero. Once the task is complete, the air mass, measured by the air-mass meter, will equal the desired air mass marked by the EGR valve map.

Table1. Change of measured values showing EGR system operation

Details of the adjustment mechanism are shown in the table above. The table additionally shows that the EGR valve opening percentage is dependent both on the measured air mass parameter and the amount of injected fuel. As both the engine load and the injected fuel dose increase, the demand for fresh air also increases. In order to maintain the correct proportions of fuel and air, the engine controller needs to reduce the amount of recirculated exhaust gas in line with the EGR control map. Otherwise, the combustion process is disrupted, the engine will start to emit black smoke and will then stall. This principle was sufficient to manage this system in traditional recirculation systems. In today’s double recirculation systems, the control method takes into account the lambda coefficient and additional newly developed parameters.

In order to better visualise the process of diagnosing an EGR system in a more technically advanced engine, we will interpret and describe the actual parameter values measured while diagnosing an engine in a car that meets the Euro6 standard. This car is equipped with two-stage exhaust gas recirculation. As mentioned above, the subject of the analysis was a VW EA288 2.0 TDI 150hp engine. The popular VCDS device was used for the diagnostics. This device can provide a reading of engine parameters at device level and is used at certified VW service stations.

1. Idling

Figure 2. Reading of idling engine parameters during engine warm-up phase

Figure 2 presents engine parameters while idling. The engine is warming up, which is why both EGR valves are operating and opening values are approximate. 28–30%. Opening both EGR valves reduces the aspirated air mass to 235 mg/stroke: with the EGR valves closed, the air mass is approximately 490–510 mg/stroke. The effect of exhaust gas recirculation can be observed on the indication provided by the lambda probe (the lambda coefficient amounts to approximately 2.9–3, whereas without recirculation it would remain at 6.5–7) and on the nitrogen oxide sensor (the 71 ppm value is relatively very low, as with no recirculation during idling it would remain at 250–400 ppm)

IDE00021 – engine rotational speed (does not require a comment)

IDE00025 – refrigerant temperature (engine temperature)

IDE00100 – engine torque (this parameter provides information on the current engine load) IDE00348 – intake air temperature (this value is measured by the air-mass meter, directly upstream from the turbocharger)

IDE00472 – average injection amount (this parameter provides information on the actual amount of fuel in the stroke per one cylinder where the value is expressed in milligrammes per stroke)

IDE04003 – charge air temperature sensor (B1S1 stands for air temperature sensor upstream from the intercooler: B1S2, on the other hand, stands for the air temperature sensor downstream from the intercooler)

IDE04015 – exhaust gas recirculation (this parameter presents the opening range of exhaust gas recirculation valves where: EGR A stands for the high-pressure EGR valve, EGR B stands for the low-pressure valve and the values 29/29/0 are equal to % required value/% actual value/deviation value)

IDE05356 – calculated NOx concentration downstream from the turbocharger (this parameter provides information on nitrogen oxide concentration in the engine exhaust gas. The information is provided by the nitrogen oxide sensor reading: concentration is measured in ppm units (parts per million)

IDE07538 – intake manifold pressure sensor, actual calculated value (this parameter specifies the absolute value of the boost pressure, which is measured using the pressure sensor at the intercooler inlet)

IDE07778 – exhaust gas flap 1 regulating unit, row 1, feedback on position – actual value (this parameter provides information on the exhaust gas throttle position in the exhaust system for low-pressure recirculation: increased exhaust gas output is caused by a closed exhaust gas flap. The value of 90% should be read as the throttle being 90% closed, 0% as it being completely open)

IDE07801 – air-mass meter row 1, air mass 1, unprocessed value (this parameter provides information on the current air mass, measured by the air-mass meter located downstream from the air filter, where the amount is expressed in the same unit as the amount of fuel: milligrammes per stroke)

IDE10817 – lambda coefficient at the diesel particulate filter inlet (this parameter presents the lambda coefficient resulting from the amount of fuel and air: the reading is provided by the broadband probe, located downstream from the turbocharger)

IDE03383 – exhaust gas stream from the exhaust gas recirculation system (although this is not clear from the description, this parameter displays the current exhaust gas amount, expressed in kilogrammes per hour, flowing into the air intake system through high-pressure EGR valve 1: this is how the amount of exhaust gas is calculated)

IDE04468 – air mass from air meter 1 (the value provides information on the current air mass measured by the air-mass meter, expressed in kilogrammes per hour)

IDE07377 – exhaust gas recirculation valve 1, row 1, required mass stream value (this parameter provides information on the desired exhaust gas mass for high-pressure recirculation)

IDE07382 – exhaust gas recirculation valve 2, row 1, required mass stream value (this parameter provides information on the desired exhaust gas mass for low-pressure recirculation)

IDE07757 – exhaust gas recirculation valve 1, row 1, position feedback – actual value (the parameter provides information on the percentage position of high-pressure EGR valve 1: the value is read from the position sensor and converted into a value expressed as a percentage)

IDE07764 – exhaust gas recirculation valve 2, row 1, position feedback – actual value (the parameter provides information on the percentage position of low-pressure EGR valve 2: the value is read from the position sensor and converted into a value expressed as a percentage)

IDE09886 – exhaust gas recirculation valve 2, row 1 calculated mass stream (this is the low-pressure EGR valve 2 exhaust gas mass that is calculated based on the differential pressure sensor signal and the percentage opening of EGR valve 2)

Figure 3. Reading of idling engine parameters during the engine warm-up phase (different measured values)

Figure 3 presents an idling engine with a different parameters set (selection is limited by the diagnostic tester to 12 parameters).

As can be seen from Figure 3, parameters for both the exhaust gas and air mass are expressed in kg/h. The dependence between kg/h and mg/stroke is explained in the addendum at the end of the article.

The engine controller opens 2 EGR valves to the same extent: value is 28–30%. Air mass, measured by the air-mass meter, is shown as 25.2 kg/h. If exhaust gas from the two EGR valves is added to this value, we get the “cylinder filling” mass, which in this case is 25.2 + 10.8 + 15.1 = 51.1 kg/h.

How does the engine controller determine those parameters?

The calculation process starts with determining the cylinder filling level. For this purpose, the following parameters are used: intake manifold pressure, cylinder cubic capacity, cylinder filling coefficient, boost pressure and intake air temperature. The air mass measured by the air-mass meter (IDE04468) needs to be subtracted from the cylinder filling in order to obtain the total exhaust gas amount. In order to divide the exhaust gas amount between two EGR valves, the engine controller has the appropriate low-pressure EGR valve characteristics map built in. This characteristic uses the valve opening percentage (IDE07764) and the differential pressure between both the exhaust gas inlet and exhaust outlet (diagram in Figure 4).

An additional differential pressure sensor, presented in Figure 4, is used for measuring the pressure. The flow rate calculation mechanism is widely used in the automotive industry: based on the opening degree of any valve and the pressure differences caused by the operation of that valve, it is possible to determine its flow with specially designed flow characteristics.

Czujnik różaFigure 4. Differential pressure sensor for low-pressure exhaust gas recirculation [source Konrad Reif. : Diesel Engine Management Systems and Components, Bosch Professional Automotive Information, Springer Fachmedien Wiesbaden 2014] nicy ciśnień dla niskociśnieniowej recyrkulacji spalin [ źródło Konrad Reif. : Diesel Engine Management Systems and Components, Bosch Professional Automotive Information, Springer Fachmedien Wiesbaden 2014]

Figure 4. Differential pressure sensor for low-pressure exhaust gas recirculation [source Konrad Reif. : Diesel Engine Management Systems and Components, Bosch Professional Automotive Information, Springer Fachmedien Wiesbaden 2014]

The EGR flow characteristics output map provides the calculated mass of exhaust gas expressed in kg/h (parameter IDE09886). With the total amount of exhaust gas from both EGR valves, the controller subtracts the exhaust mass from EGR2 and obtains the exhaust mass for the EGR1 valve.

2. Engine operation with increased rotational speed

The situation looks slightly different when the engine rotational speed is increased to around 2,500 rpm (Figure 5).

Figure 5. Operating parameters for the exhaust gas recirculation system at an increased engine rotational speed

Once 1,500 rpm is exceeded, the engine controller closes the high-pressure EGR1 valve (0.00%) and starts low-pressure recirculation only (78.80%), which quantitatively provides 106.6 kg/h of fresh air for 78 kg/h of recirculated exhaust gas. The total cylinder filling is about 185 kg/h. With a small amount of fuel (5.89 mg/stroke), it is possible to maintain a lambda coefficient of 4.2, obtaining a low NOx emission of 126 ppm. As can be seen based on the parameters presented in Figure 5, exhaust gas recirculation is used to a large extent to keep the emission of nitrogen oxides low. The car being examined is also equipped with an SCR system, allowing even lower nitrogen oxide emissions, but this topic is discussed in another article in this series.

Figure 6. Operating parameters of the exhaust gas recirculation system during a road test performance with partial engine load

The engine operating parameters measured during steady driving at partial engine load (about 45% of the maximum torque) are presented in Figure 6. The engine is at normal operating temperature (90°C) and it generates 161 Nm of torque at a speed of 2,330 rpm. The amount of fuel delivered is 23.21 mg/stroke, with 540 mg/stroke of air where the lambda coefficient is 1.7.

According to theoretical calculations, the coefficient should reach about 1.6.

Example 1: lambda coefficient calculations: [Equation]

What could the reason for this difference be?

Recirculated exhaust gas is not included in the above example. The low-pressure EGR valve is 56% open, meaning that the residual amount of oxygen, originating from exhaust gas, gets inside the intake system upstream from the turbocharger. This is a tiny detail but should still be kept in mind. Furthermore, the example presents the high efficiency of the intake system, as the temperature of the intake air downstream from the air filter is 8°C, but after mixing with exhaust gas and being compressed by the turbocharger to 1.734 bar, the temperature value upstream from the intercooler is 112°C, after which it is only 30°C downstream from the intercooler. Such high efficiency can be obtained by cooling the air with a water jacket exchanger, located in the intake manifold, where an independent electric pump forces coolant circulation. The charge air cooling circuit, also known as the low-temperature circuit, is a separate closed system connected to the engine cooling system only by means of a shared expansion tank.

3. Full engine load

Figure 7. Operating parameters of the exhaust gas recirculation system during a road test performance at maximum engine load

The above-mentioned values can be compared to the operating point of the engine under maximum load (Figure 7). The parameters were recorded while the car was accelerating with the accelerator pedal pushed to the limit, when the maximum possible torque is generated by the engine. In order to obtain full load, the engine controller increased the boost pressure to approx. 2.5 bar and turned off the exhaust gas recirculation system completely. As might be noticed from the parameters, the amount of air increased to over 1,100 mg/stroke, allowing the fuel dose to increase to around 60 mg/stroke. The dose value is limited by the “smoke limiter” function, which is responsible for reducing the fuel dose based on the measured air mass in such a manner that the lambda coefficient does not drop below the assumed value of 1.3. As can be seen in Figure 7, the lambda probe parameter remained at the level of 1.33, which confirms that both injection and intake systems are efficient, maintaining the right proportions of fuel and air for the diesel engine at maximum load.

What makes this important is the fact that the above parameters can be used in order to perform engine diagnostics simultaneously. If the engine does not generate full power, e.g. because of damaged/obstructed injector nozzles, it is possible to compare the measured lambda coefficient with the calculated value at the same operating point. If the calculated value is higher than the measured value, the injectors will run out of fuel. However if the value is lower, the injectors will receive too much fuel. It is vital to remember that the fuel amount presented by the engine controller is a theoretical value, derived from the fuel pressure and the time required to activate the injector. The parameters that are measured directly are the air mass and the lambda coefficient.

4. Summary

This article presents the operation of advanced exhaust gas recirculation systems and clarifies many parameters that influence it in a direct or indirect way. To ensure better understanding of complex processes, the mechanisms behind various calculations made by the engine computer are also explained. Expert knowledge contained in this article concerning the interpretation of actual parameters can be used for diagnosing other engine control systems, including the injection system, intake and supercharging system, exhaust gas treatment system, sensors and actuating components. These topics serve as an initial knowledge base for subsequent articles devoted to diesel engine control systems, including exhaust gas treatment systems such as DPF, SCR, NOX Trap and others.

5. Addendum – explanation of air mass units

The mass parameter in milligrammes per stroke does not depend on engine rpm. The air mass measured in kg/h indicates a quantitative flow rate per time unit and depends on the engine rpm. Converting these parameters is not easy: for instance, kg/h must be converted into kilogrammes to milligrammes (multiply by 1,000,000) and hours to minutes (divide previous result by 60) and divided by engine rotational speed (engine speed is expressed in revolutions per minute, therefore in the previous step it was divided by 60). Lastly, the result obtained should be divided by the number of strokes per 1 revolution of the crankshaft (for a 4-cylinder engine this value is 2 and for a 6-cylinder engine it is 3).

6. Example 2

To reverse the conversion, milligrammes need to be converted to kilogrammes, i.e. in the first step, divide by 1,000,000 then multiply the result by the engine speed (e.g., 840 1/min); then multiply the obtained result by 60 (minutes will convert into hours) and multiply by the number of power strokes per crankshaft revolution (in our case it is 2). The result is expressed in kilogrammes per hour.

7. Example 3:

500 [mg/stroke] /1,000,000 = 0.0005 0.0005*60*840 [1/min]*2= 50.4 kg/h