Even though new oil formulations are typically developed to support changes in engines and the addition of new components, since the mid-1980s, emission regulations have been a major driving force behind diesel engine lubrication formulation.
Recently, as a result of The Clean Air Act (CAA), the U.S. Environmental Protection Agency (EPA) now regulates diesel engine exhaust emissions. The CAA regulations have focused on two emission components: nitrogen oxides (NOx) and particulate matter (PM). NOx levels increase when flame temperatures increase in the combustion chamber and are small gaseous molecules emitted in the exhaust gas. PM emissions by contrast are made up of tiny suspended particles, such as soot, hydrocarbons and sulfates, all of which are by-products of the combustion process. Both have been proven to be detrimental to human health and the environment, which is why tighter controls have been set.
While these new regulations are good for the environment, they present very real challenges to engine manufacturers, lubricant formulators and fuel suppliers as they work together to develop products that can deliver high performance while meeting strict emissions mandates. Likewise, diesel engine designs, such as EGR (exhaust gas recirculation) and timing, have placed a harder load on today’s engine oils, which in turn have led to an increased need for both accurate and customized oil analysis programs. In fact, oil analysis will become an increasingly important part of the new lower emission diesel engine maintenance programs.
U.S. Heavy-duty Emissions Requirements
In the United States, emission standards continue to tighten (Figure 1).
The 1998 EPA regulations have reduced NOx emissions 20 percent below the levels set in 1994, and 90 percent below the pre-1988 period. Diesel engine manufacturers are modifying engine designs to comply with EPA regulations. All 1999 model year and rebuilt electronically controlled engines for on-highway service employ retarded diesel fuel injection timing in both urban transient and steady-state operating cycles. Retarded timing helps meet the emissions challenge by lowering NOx emissions; however, it increases soot levels in the crankcase, placing higher demands on the engine oil. Oils that do not control soot well can cause valve train wear, filter plugging, bearing failure, sludge formation, fuel economy loss and reduced engine life.
The New API Performance Level CI-4
To meet the NOx emission requirements, most heavy-duty diesel engines will be equipped with cooled exhaust gas recirculation (EGR) after-treatment systems. EGR replaces some of the intake air in the combustion chamber with exhaust gas to lower the peak flame temperature, reducing the NOx level. These systems allow more advanced fuel injection timing for better fuel economy, but they also introduce corrosive acids (e.g., sulfuric acids) from sulfur in the fuel and nitric acid into the engine (Table 1). Likewise, the change in flame characteristics along with the recirculation of exhaust gases cause increased soot build-up in the crankcase.
As a result, lubricants formulated to protect engines equipped with EGR systems should be able to control higher operating temperatures and soot loads and neutralize more acids to prevent corrosive wear in the power cylinder components - cylinders, rings and bearings. Oils claiming API CI-4 must demonstrate the following characteristics:
To meet these requirements, lubricant manufacturers have reformulated their products, increasing the level of basic additives (as determined by base number), as well as improving the base oil quality to better control the effects of EGR by-products.
EGR’s main environmental effect is a 50 percent reduction in NOx (from 4 grams per brake horsepower hour (g/bhp-hr) to 2 g/bhp-hr). The system allows more time for advanced fuel injection and will recover some fuel efficiency. Some users are afraid that EGR engines will have no fuel efficiency benefit and some predict these engines to be less fuel efficient.
Engines equipped with EGR systems have more parts and are more complex than other diesel engines. EGR engines cost more and their maintenance costs are higher. It is possible that EGR engines will require shorter drain intervals and consequently will disrupt existing maintenance schedules. In addition, technicians must be retrained and will need updated tools. Also, the higher operating temperature will have more impact on the components, potentially causing them to fail prematurely.
EGR also affects the engine lubricant. Exhaust gases must be cooled from 1,200°F to 250°F (650°C to 120°C) by the engine’s coolant system; therefore, the engine coolant system will run hotter and overall engine temperature will be higher. Crankcase lubricant temperature can be up to 40°F (22°C) higher (remember, oil oxidation rates double with every 18°F/10°C). This lubricant temperature increase will affect oxidation stability, risking a rise in viscosity and leading to varnish and sludge formation. Additionally, combustion by-products (containing sulfur, nitrous compounds, carbon and other contaminants) are recirculated back to the engine, escalating the formation of sulfuric and nitric acids that increase oxidation rate. Additionally, blow-by combustions consume antioxidants and detergents, causing corrosive wear in bearings, rings and liners. The increase in soot concentrations must be controlled by dispersant additives that, in case of depletion, can cause viscosity increase, antiwear additive depletion, filter plugging, reduction of lubricant flow, valve train wear, bearings failure, sludge and loss of fuel efficiency. Lubricant formulators have met these challenges by using more additives and enhancing base stock quality, which has increased lubricant cost (Table 2).
Resetting Parameters for In-service Lubricant Analysis
Because the lubricant’s ability to protect the engine, control contamination and maintain the engine’s condition are critical for fleet reliability, oil analysis is more important than ever.
While standard oil analysis tests are needed to determine the lubricant’s physical and chemical properties and to provide a baseline oil condition, the following parameters have changed for most new API CI-4 lubricants:
Lubricant Health and Degradation
Lubricant degradation is a chemical (irreversible) deterioration of the lubricant. It is caused by the base oil combining with oxygen, sulfur and nitrogen to form harmful compounds. It can also be caused by additive depletion due to reactions with contaminants such as heat, air metal particles, soot, fuel and glycol.
The catalyzing effects of the contaminants introduced into the engine by the EGR process make the oil much more prone to degradation. Used oil analysis testing will likely be more dependent on FTIR analysis than in the past for the measurement of oxidation, nitration and sulfation products (Table 3).
Viscosity is one of the most important properties of lubricating oil, and an indicator of the lubricant’s film strength. It is important to monitor the conditions that can affect it. Also, the oil’s viscosity is an indicator of contamination from soot, glycol, fuel and oxidation. In EGR engines, viscosity will be affected (increased) by higher levels of soot, oxidation from higher temperatures, and nitration by NOx (Table 4).
Oxidation occurs when the base oil is attacked by oxygen. Heat, pressure and catalytic materials accelerate the oxidation process. By-products of oxidation form lacquer deposits, corrode metal components, and increase the oil’s viscosity, impairing its ability to lubricate. FTIR is an effective direct means of measuring the oil’s oxidation level in a diesel engine. Oxidation will be affected in EGR engines by higher operating temperatures, nitration and acids.
Nitration is caused by oil degradation in a reduced oxygen environment and results in nitrogenous by-products. These compounds contain acidic precursors that may combine with water to form nitrous acids in the lubricant. These acids attack the oil, reducing additive effectiveness and increasing the rate of oil degradation, which creates varnish, lacquer, sludge and engine deposits. Infrared analysis is used to directly measure nitration products in the engine lubricant. Nitration can be a problem for EGR engines due to NOx compounds forming deposits reacting with the lubricant.
Sulfation is the formation of compounds containing sulfur from the base oil’s reaction with oxygen, heat and sulfur (from base oil or diesel fuel). Sulfurous compounds form deposits, lacquer, varnish and sludge. They deplete additives and can react with water to form sulfuric acids that corrode the metals and degrade the lubricant. Sulfation is also measured by FTIR. In EGR engines, the lubricant will be more prone to sulfation due to the effect of the reintroduction of sulfurous compounds in the lubricant so special attention should be paid to sulfation levels in EGR engines.
Acid number (AN) is the quantity of acid or acid-like constituents in the lubricant. An increase in the lubricant’s AN from its original value (baseline) is a cause for concern and should be investigated. An increase in AN usually indicates lubricant degradation due to oxidation, nitration or the presence of acidic products from oil degradation, contamination or combustion. In EGR engines, oxidation and acid formation can potentially cause the acid number to increase, though it is not commonly used for testing in-service diesel engine oil.
Base number (BN) measures the amount of alkaline detergent additives in the lubricant that are capable of neutralizing the acidic products of combustion. BN decreases during service as the alkaline additives protect the engine, neutralizing acids and controlling high-temperature deposits. The higher production of sulfuric acid and nitric acid in the EGR engines will require BN to be closely monitored to measure the additives levels that control the damaging effects of these acidic by-products. BN is typically used to establish an optimal drain interval and will become important to setting new oil change intervals for EGR-equipped engines. It is likely that BN values in EGR engines will decrease faster than non-EGR engines because there are more acidic products to neutralize.
Soot is formed during the combustion process and enters the crankcase with combustion gas blow-by. Soot is 98 percent carbon by weight, and has an original size of 0.01 to 0.05 micron, but tends to agglomerate to form larger particles in the crankcase. Soot levels generally increase with mileage and fuel consumption. Excess soot increases the oil’s viscosity, leading to higher temperatures, higher pumping costs, power loss and the risk of lubricant starvation, especially at start-up. An oil’s ability to disperse soot is critical to preventing soot-polishing wear caused by the effects of soot on the oil’s antiwear additives. If wear occurs in the valve train, fuel economy will suffer as injection timing and valve timing will move from their optimum settings.
Soot loads in the lubricant can be expected to increase dramatically in EGR engines, causing increased temperature and viscosity, dispersancy failure, deposits and wear. Actual extended oil drains need to be carefully monitored due to increased soot.
Coolant contamination (glycol) of engine oil can lead to a catastrophic failure if left undetected. Usually, about one-half of coolant solution is composed of ethylene or propylene glycol and the other half is water. Glycol in the lubricant promotes varnish and deposits formation. Additionally, coolant contamination can cause acid formation, bearing corrosion, additive precipitation forming oil balls (reaction of lubricant additives with glycol and oil), loss of dispersancy, filter plugging, oxidation and viscosity change. An inhibitor package is typically added to the coolant to control corrosion and cavitation, the leading causes of coolant leaks in engines.
EGR requires the gases to be cooled by the engine’s cooling system; therefore, the cooling system runs hotter, requiring the system to be monitored closely to prevent additive depletion and possible damage to the cooling system or cavitation that can cause engine leaks in wet-liner engines.
Fuel dilution refers to raw or cooked fuel that has contaminated the crankcase oil, generally indicative of mechanical malfunction, leakage or abnormal operating conditions. Fuel dilution is obviously dangerous because there is significant reduction in film strength due to a reduction in viscosity. However, other conditions such as soot load, base oil volatility and glycol contamination can also affect viscous thinning. It is possible that a crankcase oil could be thinning from fuel dilution and/or viscosity index (VI) improver sheardown and thickening from volatilization and/or rising soot load - all at the same time, without change in viscosity. A combination of viscometry, flash point and soot measurement should be used to detect fuel dilution.
Oil Change Intervals and Limits
The expectation of extended oil change intervals (40,000 miles to 50,000 miles or 300 hours to 500 hours) in engines with EGR systems will be challenged. Condition-based oil change intervals based on laboratory oil analysis may be shorter because of increases in oxidation contaminants and soot. Oil change intervals need to be based on the lubricant’s ability to maintain an acceptable level of alkalinity reserve (BN), oxidation stability, proper viscosity limits through dispersancy, antioxidants and wear control.
In general, BN, AN, viscosity, oxidation, nitration and sulfation will keep the same limits as for pre-EGR engines/lubricants. It is just a matter of how quickly these limits are reached. However, other warning limits will change for EGR engines (Table 5). Soot levels, for example, are expected to increase from the actual level of 3 percent to around 5 or 6 percent. Soot concentration is important because it provides a general indication of combustion efficiency and identifies abnormal blow-by. Also, while soot load may be high and well-dispersed, in one case contaminants like water and glycol can disrupt dispersancy, leading to rapid agglomeration and deposition of soot onto machine surfaces. The analyst must take this into account when responding to water or glycol alarms in systems carrying a high soot load.
The importance of oil analysis in EGR engine maintenance programs should not be ignored. While EGR is a good thing for the environment, it presents challenges to those responsible for engine maintenance, including lubrication. However, with a carefully planned oil analysis program, transition to a new era of diesel engine design and operation should be possible without compromising equipment reliability and durability.