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MODERN EXHAUST GAS TREATMENT SYSTEMS IN THE CONTEXT OF CHANGES IN ENGINE OIL PROPERTIES. DIESEL PARTICULATE FILTER AND IMPACT ON ENGINE OIL DEGRADATION

1. Introduction – Basics

It is obvious to anyone who is familiar with the principles of the operation of internal combustion engines that a malfunction of the engine and the sensors and actuators that support it can lead to the failure of the exhaust aftertreatment systems. Examples of such failures and engine management system malfunctions are numerous, the most common being mechanical wear (including oil consumption), injection system problems (e.g. faulty or incorrectly coded or out of tolerance injectors), faulty turbocharger (incorrect wastegate actuator setting, VNG variable geometry), malfunction of the flow meter and/or boost pressure sensor, faulty electrical connections, etc. 


Let's consider whether the reverse of this statement is also correct? Can faults in exhaust aftertreatment systems lead to engine failure? In the event of failure of exhaust aftertreatment systems, can engine oil lead, in extreme conditions, to engine failure? 


Since the answer is not obvious, we propose an answer to this question with the following: 


In order to familiarize yourself with the basics, we encourage you to take a training course on lubrication and lubricants available on Castrol's free e-learning platform: "Learning Campus": 


https://thelearningcampus.bp.com/uxp/login  


If the basics of tribology and lubrication of mechanisms are known, then we can move on to the next part of this article, and here we will only remind you of the necessary concepts. 


Castrol training courses for business partners include a substantive part devoted to car engine oils. There we learn that reducing friction and wear are the basic functions of a lubricant, but it must also perform many other functions (e.g. protecting surfaces against wear and corrosion, cooling the engine, preventing the accumulation of contaminants and keeping them suspended in oil, keeping clean surfaces, sealing the combustion chamber etc.). 


Therefore, it is worth remembering that engine oil is required to perform a multitude of functions to ensure that the engine is able to operate efficiently for long periods of time without damaging itself and without causing harmful effects to the environment. 
The various functions of engine oil are related to its individual components from which it is made.  


For example, keeping the engine components clean (especially pistons and piston rings) and keeping contaminants suspended in oil (mainly oxides, varnishes and solid particles) are functions supported by additives called detergents and dispersants. 


Also, the use of polymers known as viscosity modifiers maintain the oil's desired flow characteristics appropriate to the application, thus ensuring effective lubrication of engine components over the entire range of external and internal temperatures in which the engine must operate. 


These polymers slow down the decrease in oil viscosity at high temperatures and, at low temperatures they allow to lower the pour point. This behavior justifies the name Viscosity Index Improvers (VII). 

2. Evolution of exhaust aftertreatment systems and their impact on engine oil and its components

Over time, the technology used in engine engineering has evolved significantly. Today, we want engines to be more efficient, more powerful, smaller and less polluting at the same time. We will now briefly discuss the new challenges engine oil faces with the advent of advanced exhaust aftertreatment systems. 


In simple terms, it is no longer enough for the oil to fulfill only those functions listed at the beginning of the article (related only to the proper operation of the engine), but it must fulfill new functions related to the proper operation of exhaust gas treatment systems. 


Consistently, today we describe engine oil using features such as the content of Sulphated Ash, Phosphorus and Sulfur (SAPS) 


Certain additives, such as phosphorus and sulfur compounds, have proven to be very useful in increasing the performance of classical oils (anti-wear properties, resistance to oxidation, contribution to keeping the engine clean) but equally, a high concentration of them can poison and damage the catalyst and lead to the accumulation of SAPS in the engine particulate filter.   


The sulphated ash is a result from the combustion of engine oil (largely due to the use of anti-wear additives and detergents in the oil component).  Since sulphated ash deposits are not combustible, they cannot be removed in the regeneration process of the diesel particulate filter. Therefore, these ashes accumulate in the DPF filter and reduce its efficiency until it is completely blocked.  


For this reason, we can already draw the first practical conclusion:  Never add any additives to the oil, regardless of how their manufacturers praise them!!! 
If these products are made using metallic or ash-forming compounds after combustion, they can actually have a positive effect on some of the engine oil's basic functions (like higher engine cleanliness), but at the same time they can irreversibly damage the exhaust aftertreatment system. 
 

3. What happens to engine oil during use

During the use of an oil in the engine (the oil drain interval), the lubricant will be contaminated with a multitude of substances (such as fuel, particles from the wear of metal parts, oxides, acids, water, etc.) 


These contaminants cause physical and chemical changes in the lubricant, which can lead to viscosity index loss or thermal degradation. If the changes are significant enough, this can lead to viscous or thermal breakdown of the oil, which in turn leads to engine performance problems, including the risk of catastrophic engine failure. 


"Viscosity breakdown" is a phenomenon generated mainly by dilution of engine oil with fuel or the occurrence of places in the lubrication system that strongly shear the oil film and at the same time viscosity modifier polymers (at which the main bonds between molecules break) reducing their effectiveness in maintaining a high viscosity index. 


The effect of the loss of viscosity is that the oil film formed between the two surfaces moving relative to each other will be thinner and will not be able to withstand as much stress that an oil without loss of viscosity can easily handle; this can lead to increased engine wear, especially in the crankshaft plain bearings and piston rings.

Fig 1. Possible effects of viscosity loss in the crankshaft bearings
Fig 1. Possible effects of viscosity loss in the crankshaft bearings 

Thermal degradation is basically a change in engine oil properties due to excessive heat and/or prolonged exposure to heat. This significantly reduces the ability of the oil to maintain a protective oil film and leads to its degradation. This process is also enhanced by the presence of impurities in the oil.

 

This phenomenon (thermal degradation) and its effects will be discussed in the next article.


Particle filter and viscosity loss (Viscosity breakdown)

 

Since the introduction of the Euro 4 standards, the treatment of exhaust gases has become more complex; a particulate filter (PF) was added to the already present catalytic converter. It is generally abbreviated as PF (particulate filter), but the more common names are DPF (diesel particulate filter) and GPF (gasoline particulate filter). The following text mainly discusses the DPF due to its popularity, but the processes discussed are the same for both diesel and gasoline engines.

 

DPF/GPF has the initial role of retaining particulate matter in the exhaust gases, thus avoiding their release into the atmosphere and the associated pollution. During engine operation, the gases practically pass through the ceramic wall of the filter, while the soot is stored in the filter.

ceramic tubes
When soot accumulates more and more in the filter, it is necessary to "clean the filter". This process is usually called regeneration or afterburning. The start of the regeneration process is initiated by the engine control unit (ECU). This occurs when the signal from the DPF pressure sensors indicates that the pressure difference before and after the filter has exceeded a certain threshold, which suggests that the filter is full. The filter can also be regenerated based on a certain number of starts and hours of operation. 
This regeneration is actually a controlled combustion of accumulated (carbon-rich) soot particles. 
How is combustion initiated? The temperature rise of the particulate filter is precisely controlled (using at least two temperature sensors placed upstream and downstream of the filter). Basically, to raise the temperature, the system uses what it has at hand - that is fuel.  
There are systems that have an additional injector that dispenses a controlled amount of fuel directly into the exhaust system before the diesel particulate filter; burning this fuel increases the temperature and regenerates the filter. 
Fig 3. Example of an exhaust system with additional injector for particle filter regeneration
Fig 3.Example of an exhaust system with additional injector for particle filter regeneration
  1. Injector (for DPF regeneration) 
  2. Primary catalyst
  3. Pressure sensor at the output of the primary catalyst
  4. DPF inlet temperature sensor
  5. Secondary catalytic converter (in the same housing as the DPF)
  6. Pressure sensor upstream of the particulate filter
  7. Particulate filter
  8. DPF outlet temperature sensor
  9. DPF outlet pressure sensor
This system has an injector (usually supplied with fuel from an in-tank electric pump or a transfer pump, thus at a pressure at least two orders of magnitude lower than that supplied to the injectors serving the engine) located in the exhaust system upstream of the diesel particulate filter. 
Fig 4a. Example injector used to regenerate DPF - Renault
Fig 4a. Example injector used to regenerate DPF - Renault
Fig 4b. Example injector used to regenerate DPF - Toyota
Fig 4b. Example injector used to regenerate DPF - Toyota
Another common approach (which has the advantage of being cheaper to implement) is to use injectors already present in the engine. Their main role is still "supplying" the engine with fuel (in diesel systems directly to the combustion chamber), but they have the option of performing a second function consisting of introducing an additional amount of fuel into the engine, which does not burn in the combustion chamber and goes to the filter for its regeneration. 
Fig 5.   Section in the exhaust system (example- Mercedes). You can see that what is called, as a spare part, the "diesel particulate filter / DPF" is a structure consisting of a ceramic body - catalytic converter (about 1/3 of the volume) and the diesel particulate filter itself (about 2/3 of the volume).
Fig 5. Section in the exhaust system (example- Mercedes). You can see that what is called, as a spare part, the "diesel particulate filter / DPF" is a structure consisting of a ceramic body - catalytic converter (about 1/3 of the volume) and the diesel particulate filter itself (about 2/3 of the volume).
A visual representation that describes this process is the current and voltage signals by which the injectors of the common rail system are controlled. Here is an example:
Figure 6. The control signal (in current and voltage) of a piezoelectric injector, captured using a Bosch FSA oscilloscope; the capture was made in a Mercedes-Benz E 320 CDI car, engine OM 642.920, during the filter regeneration process (process initiated using the Bosch KTS diagnostic tester).
Figure 6. The control signal (in current and voltage) of a piezoelectric injector, captured using a Bosch FSA oscilloscope; the capture was made in a Mercedes-Benz E 320 CDI car, engine OM 642.920, during the filter regeneration process (process initiated using the Bosch KTS diagnostic tester). 

In Figure 6, we can see that during the engine's idling cycle, the injector opens (and closes) four times, allowing a well-controlled amount of fuel to enter the combustion chamber at high pressure. 


The first three signals (two pre-injection and main injection) are the "portion of diesel" that a given cylinder receives to ensure proper fuel combustion and perform the engine's intended work. The last highlighted signal, called post-injection, illustrates the part of the diesel that does not contribute to the engine's torque, but is only used to perform burn-in of the diesel particulate filter. This last injection, which occurs with a delay, does not occur until the ECU has started the diesel particulate filter regeneration process. 


The fuel that is sent to the combustion chamber during post-injection will essentially leave the combustion chamber at the moment of opening the exhaust valve, thus reaching the exhaust manifold and then the catalytic converter and diesel particulate filter, ensuring cleaning (burning off the soot) of the latter. 


Unfortunately, some of the injected fuel ends up in the piston rings, flows down the cylinder liner and finally mixes with the engine oil, causing engine oil to be diluted with fuel. This is inevitable due to the construction and operation of the system and at the same time it is the biggest disadvantage of this system!!! 
Prolonged use of the vehicle in city traffic, the use of fuel containing too much biofuel, or operation with faults in the exhaust gas recirculation system or in the sensors and actuators that manage engine operation are just some of the operating conditions that can lead to situations where filter regeneration may not be possible be carried out correctly. As a result, the ECU will initiate DPF regeneration very frequently and dilution of the oil with fuel will increase rapidly. 

A breakdown in engine oil viscosity can therefore occur even shortly after an oil change if the engine management and exhaust gas treatment systems are not working properly. The most dangerous problems include the operation of a car with a DPF filter, in which large amounts of sulphate ash, sulfur and phosphorus have accumulated. This can cause the post-combustion procedure to be triggered very frequently, which in turn will cause the oil to rapidly dilute with the fuel. If the DPF filter is not replaced in this case, there will be a rapid deterioration in the performance of the engine oil with all its associated consequences. 

What is worth remembering here?

 
Exhaust aftertreatment systems, in particular DPF, must be kept in perfect technical condition; failure of this system may lead to rapid dilution of engine oil with fuel and the occurrence of viscosity loss, and in extreme conditions to serious engine failure. 


Before changing the oil, it is recommended to diagnose the system (reading the error codes if any have been registered) and assessing the level of filling of the diesel particulate filter. 


A scenario is possible where the diesel particulate filter is 95% filled with soot particles and ashes, so the ECU will trigger regeneration of the particulate filter just after an oil change. If the engine oil is changed now, very soon after the service, the regeneration process will occur and the engine oil will be partially diluted with fuel. 


For systems using secondary injection, starting the filter regeneration procedure by a diagnostic tester before replacing the oil will ensure the longest time until the first spontaneous regeneration process occurs and thus the risk of oil dilution with fuel will be lower! 


To ensure that the engine oil you use has the optimum properties and performance (including a specific additive package to protect your engine and the aftertreatment system it will work with), choose the right oil for your car by visiting the Castrol website

product finder
This way you ensure that you choose the right oil, specifically designed to protect your engine - both directly, by ensuring optimal lubrication, and indirectly, by protecting the aftertreatment system.