Given all the current activity in developing new engine oils, this is a good time to look at the differences in oil appetites between heavy-duty trucks and light-duty vehicles. Many of you in the business know that there are significant issues around both types, and that two new categories, PC-11 and GF-6, will be introducing some new wrinkles in the engine oil story.
All engine oils have four essential purposes: to lubricate, seal, cool and clean. They accomplish their mission through a carefully crafted combination of base oils, viscosity index (V.I.) improvers and performance additives. The selection and proportions of these ingredients vary depending on the application in which the oil is going to be used.
Whats behind the changes were seeing these days? The motivations are essentially the same ones that have been on the scene since the 1970s, fuel economy and emissions. What is interesting now is that the issue of engine oil longevity has been stirred into the mix.
When you look at the most basic of oil properties, viscosity, you see one of the major differences between heavy-duty and light-duty oils. Since the early 1980s, the HD market has been owned by SAE 15W-40 multigrade oils. Thats because SAE 15W- 40 provides optimum viscosity and wear protection across a range of heavy-duty operating temperatures and yet has a low enough viscosity to make it easy to start the engine in most cold-weather conditions. Admittedly, there are times and places (Minnesota in January) where a lighter viscosity grade can be beneficial, and thats where SAE 10W- 30 and even SAE 5W-40 oils find a place.
By contrast, North Americas light-duty engine oil market has seen a continuous reduction in viscosity, from the 1970s when SAE 10W-40 ruled, then to SAE 10W-30, and up to today when SAE 5W-30 and 5W-20 together make up 80 percent of passenger car motor oil sales, according to Infineum USA. In the next decade, youll see SAE 0W-XX multigrades make inroads, too. The primary driver for this change has been the need to capture as much fuel savings as possible.
Ive noted before that U.S. light-duty fuel economy averaged about 15 miles per gallon in the 1970s versus todays federally mandated 27.5 mpg. The main improvements in fuel economy have come from an amazing amount of engineering design changes in the engine. Sophisticated hardware enhancements as well as on-board computers are just a couple of the innovations that have brought us to this point. Adding their increment, light-duty engine oil formulations have been able to reduce fuel consumption by somewhere around 5 percent to 7 percent, in the move from SAE 10W-40 to modern SAE 0W-20. Honda has gone lower still, to a newly minted SAE 0W-16 viscosity grade.
Fuel also has played a part in the evolution of engine oils. On the heavy-duty side, sulfur in diesel fuel caused real problems such as corrosion in the engine and the formation of deposits, not to mention the really stinky exhaust. On-road diesel was brought under control starting in the 1980s. U.S. restrictions drove sulfur content first from 0.5 percent down to 0.05 percent, and then to 0.0015 percent (15 ppm) in 2010. This change alone had a significant effect on diesel engine oil composition, because the total sulfur from base oils and additives needed to be sharply curtailed, too.
Gasoline also changed over time with the removal of lead antiknock additives as well as the addition of cleaners and chemicals to minimize tailpipe emissions. Ethanol has been added to gasoline for a number of years, to reduce carbon monoxide emissions and as a replacement for the octane booster MTBE.
In addition, the Energy Independence and Security Act of 2007 was passed to stretch the amount of fuel available through the use of ethanol. Currently, almost all U.S. gasoline contains 10 percent ethanol plus small amounts of deposit-control agents. Ethanol raises some interesting formulation issues for engine oils and the current debate over hiking ethanol levels to 15 percent is creating new worries.
Beyond shifts in viscosities and fuels, there are also additive differences between the heavy- and light-duty sides. If you break down the performance features of an engine oil, you find that it accomplishes its purposes of helping the oil either by protecting the engine surfaces; enhancing the base oils properties; or by protecting the base oil against the harsh environment of the engine.
Most of us first think about protecting the engine, and that is done through the use of antiwear agents and deposit control agents. The classic antiwear agent is zinc dithiophosphate (ZDTP). Heavy-duty engines historically have demanded higher quantities of ZDTP in their formulations than in gasoline engine oils, due to the heavier loads found in diesel engines. In addition, the more thermally stable forms of ZDTP are preferred.
Gasoline-fueled engines also use ZDTP but at far lower levels. The primary reason for this is that ZDTP contains phosphorus – a key antiwear constituent, but one which must be used sparingly because it can ruin the catalytic converter in the vehicles exhaust system.
Controlling deposits is critical to engine performance and is managed by detergent and dispersant components in the oils additive package. Because of the presence of acid-forming sulfur in the fuel, diesel oils have historically used higher levels of detergents. Calling these compounds detergents is a bit of a misnomer since they dont actually do a lot of cleaning up. Rather, they protect the metal surfaces and provide acid neutralization. Gasoline engines also need some detergency for the same reasons, but at lower levels.
Detergents also contain metallic components, and these contribute to the sulfated ash content of the engine oil. Both diesel and gasoline-fueled engines can tolerate some level of ash content in the oil but not too much. Gasoline engines have the additional concern that ash deposits might get past the combustion chamber and contribute to exhaust catalyst fouling. For diesel engines, the metallic components provide TBN to neutralize acidic blowby.
Dispersants are an important part of any additive package. In fact, they are most often the largest single component in the finished oil, after the base oil itself. They function by trapping sludge and varnish materials which form in the oil as a result of blowby from the combustion process. They trap these bad guys (often referred to as precursors) and keep them suspended in the oil before they have a chance to adhere to the surfaces inside the engine and harden into full-fledged deposits.
Both diesel and gasoline engine oils contain dispersants but the type employed may vary depending on the desired thermal stability of the material. Diesel engines operate at higher sump temperatures than gasoline engines, for example, and the oil finds its way into the ring belt area where thermal stability is critical. Rust protection also may come from the detergents in the additive package. Detergents prevent water from getting to the ferrous surfaces so rust cannot form. In some cases a surfactant will be added at very small dosages to improve the rust prevention properties.
Viscosity index improvers and pour-point depressants are included to enhance the properties of the oil. Both affect the range of temperatures in which the oil can function. Raising the oils V.I. allows it to operate at a higher maximum temperature, while pour-point depressants lower the minimum operating temperature to keep the oil flowing when the thermometer falls.
V.I. improvers start as long-chain polymers but they get chopped into shorter lengths as they pass through the engine, a process called shearing or shear-down. The primary difference between diesel and gasoline engine oil V.I. improvers involves the amount of permanent mechanical shear-down of viscosity which occurs. Diesel engine builders are strongly concerned that too great a drop in oil viscosity after shear could result in premature engine failures, while gasoline engine designers want the oil to shear to as low as possible for its grade, in order to maximize fuel economy benefits.
Foaming in both types of oil is controlled by antifoam agents which are usually either silicone based materials or specialized methacrylates. A corollary problem is air entrainment, and it is also controlled by these same materials. However, the composition of the overall additive system has a major effect on these properties. Both engine types are susceptible to damage caused by foaming or air entrainment, and heavy-duty engine manufacturers are so concerned with air entrainment that theyre trying to develop a new test to include in the PC-11 engine oil category.
Fuel economy has been integral to the gasoline engine oil additive package requirements for nearly 40 years, taking the form of friction modifiers plus viscosity reduction. The friction modifiers in use are essentially surfactants which lower the friction between metal parts that are in relative motion. The largest amount of friction in an engine occurs in the ring belt area, where up to 60 percent of the total engine friction is generated. Of course engine design is key, and things such as lower tension rings have helped.
Diesel engine fuel economy is in its infancy by comparison, and currently driven only by viscosity. Looking back at the introduction of SAE 15W-40, one of the benefits was a fuel economy gain versus the monogrades that dominated before. The new PC- 11 category will go one step further and define SAE 10W-30 as a fuel economy oil worthy of separate notice (currently called PC-11B). The amount of fuel economy gain will not be identified with a test, just the lighter viscosity grade.
So there you have it; engine oil evolution depends on the type of application – heavy-duty versus passenger car and light truck – the fuel quality, and the oil chemistry. Whether there are any breakthroughs in additive technology remains to be seen. However, engine design will continue to drive oil composition in terms of base oil type and viscosity demands. My guess is that lower viscosity and more advanced base oils are in our future.