In recent years, engine manufacturers have been required to reduce the levels of nitrogen oxides (NOx) in diesel engine exhaust to meet Tier 3 emission standards required by the Environmental Protection Agency (EPA). One of the reasons for this mandate by the EPA is because NOx has been associated with respiratory disease and cancer. This requirement is accomplished by changes in engine designs that include retarded timing, raised piston rings, selective catalytic reduction and the use of exhaust gas recirculating (EGR).
New engine designs use EGR to control NOx emissions by recirculating exhaust gases back into the combustion chamber to be burned a second time, thereby reducing emissions associated with health risks. The amount of exhaust gas introduced into the combustion chamber will displace oxygen, creating cooler combustion. In doing this, many of the exhaust contaminants end up in the engine lubricating oil.
Diesel engine oils are now exposed to a higher level of contamination that can degrade the oil and damage engine parts. There is concern that exhaust gas recirculation can have a detrimental effect on engine durability and its effects on the oil. Oils exposed to the EGR environment show an increase in soot content, acid number (AN) and viscosity, while the engine and oil are both exposed to corrosive/acidic gases and particle buildup.
Cooled EGR occurs when the engine coolant absorbs exhaust gas heat before entering the combustion chamber. Because the engine coolant takes up the heat from exhaust gases, the engine cooling system runs hotter, therefore the oil gets hotter. Oil oxidation rate doubles with every 18 degrees Fahrenheit. Oil sump temperatures could be running up to 40 degrees hotter.
Engine and oil manufacturers are striving to accommodate the harsh environment brought on by the use of EGR systems. Engine oil manufacturers have reformulated oils to combat the deleterious effects of the EGR environment so they will be able to provide the required protection that current engine designs need. This has led to the latest CI-4/CJ-4 API engine oil ratings, which are currently under the PC-9 designation.
Production of sulfuric acid due to current sulfur levels in diesel fuel and nitric acid from NOx compounds that are recirculated back into the engine through the EGR will require lubricating oils with a higher base number (BN) and detergency to counter-act the damaging effects of these acidic contaminants. The PC-9 oils will therefore have a higher BN and detergency level than in the past. Diesel engines using EGR systems to lubricate oils will also require higher a level of dispersancy because of increased soot loading in the oil. Without increased dispersancy, the higher levels of soot and particulate matter will not stay in suspension in the form of smaller material, increasing wear to liner, ring and valve train.
The new American Petroleum Institute (API) engine oils are expected to be licensed sometime this year. Engine manufacturers are developing tests that will assess the effect of EGR technology, and are developing materials capable of withstanding increased abrasive and corrosive wear. One such test is the Cummins M11 Exhaust Gas Recirculating Test. The test was developed to evaluate engine wear, deposit formation and oil performance of heavy-duty diesel engines with EGR systems during operating conditions. Mack and Caterpillar have also developed tests for evaluating oil and engine performance in an EGR environment.
Oil analysis has become more important than ever. These changes in diesel engine design, which include EGR systems, are pushing the performance requirements of diesel engine lubricating oil. Testing is becoming critical in monitoring the oils ability to function properly as well as evaluating the health of the engine. Previous soot limits of 1.5 percent were normal in most heavy-duty diesel engines. Soot limits of three percent are now generally accepted, and higher levels are expected in the future.
In addition to the stress that higher temperatures put on the engine oil, mixture with exhaust gases can act as a catalyst for oxidation and nitration (a form of oxidation) in the oil. A routine oil analysis may start showing an unacceptable increase in viscosity when all other parameters and time on the oil appear normal. An improperly operating EGR system can severely aggravate this problem. Waste gate components in an EGR system can be particularly susceptible to surface scuffing damage. If an exhaust gas recirculating system is not operating properly, the lubricating oil can rapidly deteriorate. Sometimes going so far as to turning the oil into an oxidized, acidic sludge.
Used diesel engine oil testing parameters may focus more on infrared analysis (FTIR) for oxidation, nitration and sulfation than in the past. Previously, diesel engines without an EGR system did not have excessive oxidation and nitration problems unless there were significant mechanical problems or poor maintenance. The catalyzing effects of the contaminants introduced into the engine and its lubricant make the oil much more prone to oxidation, nitration and sulphation. This means the application of FTIR analysis will become a more relevant and useful tool for measuring used diesel engine oil life and serviceability.
With the new engine technology involving EGR systems, oil change intervals may suffer even with the advent of the PC-9 formulation. Condition-based oil change intervals based on standard laboratory analysis may be cut back initially because of increases in oxidation contaminants and soot. Oil change intervals will be proven by the lubricants ability to handle the added stress by maintaining an acceptable level of alkalinity reserve (base number), proper viscosity limits through dispersancy and antioxidants and wear control.
Once again, oil analysis will be a significant determining factor establishing condition-based oil change intervals with the new developments in engine design and oil formulation. Without acquiring new test data under standard oil analysis conditions, it is hard to predict what limits we are likely to see in the future. Some wear metals relating to the liner, ring and valvetrain, such as chromium, may increase. Soot levels are definitely expected to increase well beyond the nominally accepted level of three percent. Other test parameters such as BN, AN, viscosity, oxidation and nitration will keep the same limits; it is simply a question as to how much faster these limits will be reached.
One tool that could be incorporated into an engine lubricating oil system in order to help attain desired extended oil drain intervals is the use of bypass filtration. Standard engine oil filtration is built directly into the oil circulating system. These systems are full-flow because they have to participate in the circulating systems ability to lubricate the engine.
Because of the flow rate and filter design, factory full-flow filtration systems may not filter particulates smaller than 15 microns. It is up to the oil to control particulate contaminants and soot. Bypass filtration does not have to directly participate in the lubrication of the engine; it just needs to clean the oil. Therefore, oil is filtered at a much lower flow-rate through a denser medium. Particulate contaminants are then removed at a greater quantity in a smaller size range. The ability to remove a higher level of contaminants via bypass filtration could enhance oil life.
Some engine manufacturers are currently designing a new generation of EGR systems that will help reduce some of the performance and maintenance drawbacks. All this is when heavy-duty truck owners are trying to extend oil drain intervals to limits that were unheard of ten years ago. Currently, the goal of the new API classification is to keep oil drain intervals at their present extended level. With current diesel engine design and emission requirements, oil analysis is going to be an important piece of the puzzle in uncovering what these changes will mean regarding the service life of diesel engine oils to truck fleet owners and maintenance operations in the future.
Source:David Doyle, CTC Analytical Services, "EGR Systems and Lubricating Oil in Diesel Engines". Practicing Oil Analysis Magazine. July 2002