Oil analysis in the Tier 4 era
With EPA emissions compliant engines everything’s the same — just more intense.
| January 01, 2013 |
Engines run hotter. Power density increases. EGR rates rise. Combustion produces more acid. Soot thickens the oil like flour in a roux. And the emissions that used to vent out the exhaust stack are swallowed up by the oil or trapped and scalded inside downstream exhaust filters.
As if that weren’t enough, now everybody wants 500-hour oil changes instead of the conventional 250-hour intervals.
Welcome to Tier 4
It’s a rough neighborhood, but fortunately lube oil standards have been toughened up to meet each of these challenges. The key to keeping your engines from getting mugged in this environment is to keep a close eye on the health of your oil using oil analysis.
“The key to keeping your engines from getting mugged in this environment is to keep a close eye on the health of your oil using oil analysis.”
If you have Tier 4 Interim or Tier 4 Final engines you’re still testing for the same indicators of oil health and life – viscosity, TBN, soot, glycol contamination, the oxidation number and dirt/silica. But in this harsher environment the impact of any or all of these elements may be increased.
There are a number of things that Tier 4 engines do to increase viscosity. These include:
• High temperature oxidation. Heat forms polymers in the lube oil that are thicker than the oil itself. They eventually become insoluble and turn into sludge, which further oxidizes and becomes deposits that hamper engine functions.
“Diesel engine oil temperatures are usually around 100 to 120 degrees Celsius,” says Dr. Ken Chao at John Deere Power Systems. “But every time you increase oil temperatures by 10 degrees Celsius, its oxidation rate doubles.”
EGR, the routing of exhaust gas back into the combustion chamber to reduce NOx emissions, accounts for most of this heat. But manufacturers have also ramped up the power density in many of these engines, producing more ponies per cubic inch of displacement, which also increases heat load.
- Evaporation. Lube oil is a mixture of small and large molecules. When it heats up the smallest molecules tend to evaporate out of a liquid state, thus thickening the oil.
- Soot. When microscopic particles of carbon in the exhaust are routed back into the combustion chamber as part of the EGR cycle, the particles are picked up by the oil and thicken it. “You’re taking 50 percent of the emissions and putting them back into the engine with EGR,” says Ken Hill, vice president of sales and marketing for WearCheck USA.
- Contaminants. Normal wear metals and the dirt and dust that get past the air filtration system also increase oil viscosity.
Conversely, there are a number of things going on in the new engines that can cause your oil to decrease in viscosity. These include:
- Shear down. Multigrade oils contain additives called viscosity modifiers, specialized molecules that increase the viscosity when the oil gets hot and decrease it when it gets cold. Eventually high shear rates inside sliding or rolling junctions “shear” or tear apart these molecules reducing the oil’s ability to thicken in high heat situations. So, for example, your 5W-30 weight oil starts performing more like a 5W-20.
- Fuel dilution. A certain amount of unburnt fuel will migrate into the lube oil, diluting its additive concentration, making it less effective and causing it to thin.
Curiously when you have fresh oil, the shear down of the viscosity polymers happens within the first 50 hours or so, Chao says. Viscosity decreases during that initial period of time and then gradually increases as oxidation takes over and other contaminants thicken the oil. Maintaining the lower end of the viscosity is especially critical, Chao says, since most engine wear occurs during cold starts.
EGR engines throw increasing amounts of air back into the engine. Air is 70 percent nitrogen and in the high heat, low oxygen environment of an EGR loop, some of this nitrogen is converted to nitric acid, a highly corrosive, inorganic acid. Sulfur compounds in diesel fuel and lube oil can also form sulfuric acid, although this is less of a problem today with ultra low sulfur diesel (ULSD) fuel and low-ash oils.
Today’s oils are formulated with alkaline detergent additives that neutralize most of this acid. What the industry calls “reserve alkalinity” declines over time as more of the acids form. The measurement for alkalinity is called the Total Base Number and when it decreases to a certain point you have to change the oil or risk corrosion of the internal parts of the engine.
In addition to increasing viscosity, soot has other undesirable effects on today’s engines. Soot is abrasive, and in high levels can cause excess engine wear. “If the size of solid particulates is equal to the thickness of the oil film between two rubbing surfaces, you will get wear,” Chao says.
“You used to have a regular engine cooler and an oil cooler, but now you have an EGR cooler too,” Hill says. “You have more coolant going through the system and more plumbing in the system. We’re seeing an increase in coolant contamination because of that.”
As a result, there is a greater opportunity for glycol from the cooling system to migrate into the lube oil from leaks in the system. Glycol itself is relatively unstable and disappears rapidly in the oil, but it leaves behind traces of sodium and potassium, which are measureable and indicate you have a coolant leak. If your oil sample shows high levels of sodium and/or potassium, a pressure test on your cooling system can confirm the existence of a leak.
Coolant leaks, even small ones, should not be ignored. The Technology Maintenance Council guidelines call for zero glycol contamination in a system, says Hill. If you ignore the leak you’ll end up with premature wear on the rings, liners and valves.
Many Tier 3 and Tier 4 engines boost turbocharging to get better power density from the same engine sizes. As long as your intake air filters are working properly this isn’t an issue. But any breach in the filter system will accelerate dust and dirt contamination in direct proportion to the increased amount of turbocharging.
Extended oil service intervals
“The oil companies have done a good job of creating oil formulas that disperse the soot in EGR engines and manage the excess acids,” Hill says. The newest oil standard, the American Petroleum Institute CJ-4, was developed specifically with EGR engines in mind.
What concerns Hill is that many manufacturers have gone from recommending 250- hour to 500-hour oil change intervals during this same time period. The engines and the oils are designed to handle this, Hill says. But what happens if something goes wrong between 250 and 500 hours, and nobody knows it? Working a normal eight- hour day, a machine might not see an oil change for more than three months.
A big fleet may balk at spending $10,000 a year to do oil samples on all their machines, Hill says. But if that oil sampling prevents just one engine from catastrophic failure, then it has paid for itself for years to come.
“If 85 percent of the samples come back normal, they say, ‘why do it?’” Hill says. “But that other 15 percent is going to hurt you.”
If you do want to extend drain intervals to 500 hours, Hill recommends you do an oil sample at 250 hours before deciding to continue up to the 500-hour mark.
SCR relieves the heat
Many of the off-road engine manufacturers will adapt to the 2014 Tier 4 Final emission regulations by using SCR (selective catalytic reduction) to reduce emissions of NOx. SCR engines don’t need as much EGR and turning down the EGR will help reduce heat and soot and all the challenges they bring.
Several manufacturers have already released SCR-based engines for their off-road equipment, including Case, and several more have announced their intention to go the SCR route.
SCR systems spray a mist of urea and water into the exhaust, which when run through a catalyst reduces the NOx to simple water and nitrogen. These engines require a separate tank for the urea solution and the hardware and sensors to provide the dosing. But the on-highway world has found favor with SCR, and, says Chao, these systems deliver better fuel economy.
About Our Experts
Ken Hill has more than 25 years in laboratory operations and technical services for the oil analysis industry. His responsibilities include marketing and development of programs for OEMs and end users. Hill is an 18-year member of the Association of Equipment Management Professionals (AEMP) and has earned that organization’s Certified Equipment Support Professional (CESP) designation.
Dr. Ken Chao has a doctorate in chemical engineering with specialization in tribology (friction wear and lubrication). He has global technical responsibility for the Deere brand of engine oil formulations used in more than 2,000 different applications. Prior to joining John Deere Power Systems, Chao worked at the University of Dayton Research Institute developing high temperature lubricants for the U.S. Air Force.