Two lubricant properties have been in the industry news quite a bit lately, prompting a reader to e-mail me that they seem to be related in some way: As viscosity goes up, volatility goes down, he wrote. Yes, I replied, that is true, primarily because of the refining processes used to produce base oils.
Those of you who have been reading my columns for the last several years know I have an ongoing concern about base oils, the largest component in all engine oils. The inescapable relationship between viscosity and volatility plays into my belief that we are heading in the direction where base oil viscosity will reach the point of diminishing returns, and rather soon at that.
Lets look at the issues first. Viscosity of engine oils is going down. No one really can dispute that. Currently, the number one selling grade of passenger car engine oil in the United States is SAE 5W-30. However, a number of engine manufacturers have already started recommending SAE 0W-20 for their latest models and more will be following.
The obvious advantage of lower viscosity engine oils is improved fuel economy. All you have to do is look at the Sequence VID engine test results required for oils meeting API SN Resource Conserving and ILSAC GF-5 specifications. The sum of fuel economy improvements over the span of the oils life must be at least 1.9 percent with SAE XW-30, but SAE XW-20 oils are required to show a minimum 2.5 percent improvement. So a vehicle manufacturer can capture 30 percent more fuel economy from the lighter weight!
Recently, the SAE Engine Oil Viscosity Classification task force, which is the keeper of SAE J300, updated that +100-year-old standard to include three new viscosity grades: SAE 8, SAE 12 and SAE 16 (see page 10). What this means is that were headed even lower in viscosity. The three new grades are defined by their high shear rate viscosity, the property most related to fuel economy benefits.
The following table shows the high shear rate viscosity for the new grades, compared with SAE 20 and 30:
I vividly remember the spirited debate back in the 1980s over what the high shear rate viscosity limit should be for SAE XW-30, as it was becoming the dominant grade at the time. People were wearing lapel buttons which said things like 2.9 is Fine or 2.7 is Heaven. (Weve gone a long way from there!)
Certainly improvements in engine design have had a major impact on what viscosity can successfully lubricate modern engines. Weve gone from engines which could deliver about 0.5 horsepower per cubic inch (16.4 cc) to engines which now deliver 1.5 hp/cu.in. Along with that, we have on-board computers that control most aspects of engine operation.
Heres an example of how engine hardware is working harder, from Owen Wyrley-Birch of Lubrizol. Speaking in December to the ICIS Pan American Base Oils & Lubricants Conference, he outlined how far todays engines have advanced in the past 25 years. The popular Ford Explorer, for example, ships now with an engine that is 50 percent smaller than back in 1991, yet delivers 65 percent more power – 240 hp now, versus 145 hp then. The vehicles curb weight has grown by 12 percent to 2.25 tons, and its interior volume is 65 percent larger, yet it now zooms from 0 to 60 mph in just 6 seconds.
All that, plus the SUV today averages 28 miles per gallon (24 mpg with the 6-cylinder option), for 43 percent more fuel efficiency than its ancestor. You can count up similar improvements at nearly every original equipment manufacturer. As Wyrley-Birch put it, Modern engines are smaller, more powerful, lower emissions and just fun!
While we celebrate this gain, and engine oils contribution, we also have to acknowledge viscositys difficult side. The other property were talking about here – volatility – has a major impact on engine operation and engine oil life.
First, volatility is measured as the amount of oil lost by evaporation due to temperature. The test method most often cited is ASTM D5800, known as Noack volatility. As I wrote on this subject a couple of months ago, Noack measures the amount of oil lost after one hour from a weighed sample that is heated to 250 degrees C. The Noack method involves the application of certain correction factors, and there is continuing discussion about the tests precision, particularly its reproducibility. However, it is the current industry standard.
Concerns with volatility are three-fold.
First, it can result in a less than optimum amount of engine oil in the crankcase, which in turn means more stress on whats left. Todays oil life systems and engine monitors that alert you when its time to change the oil are based solely on an algorithm of crankshaft revolutions and operating temperatures. They do not measure such things as remaining oil volume. Ive heard from quick-lube operators that customers often come in for an oil change with less-than-full crankcases. In addition, they report, whats left is pretty thoroughly worn out.
Second, some of the oil lost to evaporation is actually recirculated into the intake manifold, where these volatiles can impact combustion and promote deposits in the combustion chamber. Worse, the newest small-displacement engine designs, equipped with direct fuel injection and turbochargers, are exquisitely sensitive to oil in the combustion chamber, which can result in severe pre-ignition detonations and catastrophic engine failures. The upcoming GF-6 passenger car engine oil upgrade will include a new engine test designed to measure the impact of oil formulations on this condition. Volatility of the oil is a concern here.
Third, the loss of oil from the system affects the viscosity of the oil that remains, just like cooking down a syrup on your stovetop. The thickened engine oil works against fuel economy simply because its heavier and creates drag. If an SAE XW-20 is supposed to give 2.5 percent fuel economy improvement overall, but loses a portion of its volume and becomes an SAE XW-30, there is an obvious loss of friction-reducing characteristics. While the loss may be pretty minimal, another increase in fuel economy performance will be required for the 2017 model year vehicles. And of course, it will get worse in 2025 when we see another major step change in mandated fuel economy.
OK, so getting back to the mailbag, my reader said he tried to graph the relationship between viscosity and volatility but couldnt make it look quite right. Thats no surprise; depending on the base oil API Group, there are other factors at work here. As we move to lower viscosity engine oils, we also move to lower viscosity base oils, and that poses a problem for refiners.
Base oil refining is pretty much a matter of separating cuts by boiling range. The viscosity you achieve is a result of the average molecular weight of the boiling range cut. (Other actions carried out on the cut, such as dewaxing, also have some effect on viscosity but essentially the viscosity is set by the molecular weight.)
The volatility of each cut is set by the boiling range as well. So if a cut is pretty broad, its volatility will be relatively higher than if the cut is narrow. But the flip side is that a broad cut gives a higher yield while a narrow cut reduces yield. There it is then, the refiners dilemma: Do I make it broad and have a lot to sell? Or do I make it narrow and have a lot of light ends and heavier ends to deal with?
For a long time, volatility wasnt an issue so wider cuts were the choice. However, as base oil volatility became more important, the cuts began to narrow. With the mid-1990s advent of API Group II base oil refining processes such as hydrotreating, the overall viscosity of the base oil stream dropped since all of the feed was being cracked and reduced in molecular weight. Group III made it even lower.
So here we are with the need to reduce volatility at the same time were reducing engine oil viscosity. That means narrower and narrower cuts and even lower viscosity grades anyway. Some automakers (Daimler for one) are talking about specifying engine oil Noack volatility limits of 10 percent maximum – as opposed to the 15 percent max seen now for all SAE grades, including SAE XW-16, under the current GF-5 spec and the GF-6 upgrade.
You can imagine what an SAE XW-8 viscosity grade engine oil will look like. The volatility will be no higher than 15 percent and possibly lower, with a base oil having a viscosity of about 3 centiStoke at 100 C. Thats going to take a whole lot of narrowing of cuts. Theres also going to be a lot of demands placed on the additive packages to manage friction and protect the engine and emissions system.
By the way, Ive focused my comments on passenger car engine oils because they are currently the ones with the lowest viscosity targets, but heavy-duty engine oils are moving in the same direction. See page 22, for example, to read about Ryder Systems switch from SAE 15W-40 to 10W-30 in their entire trucking fleet. Also, the next heavy-duty upgrade, PC-11, is making room for an SAE 10W-30 with reduced high shear rate viscosity, to gain some fuel economy benefits. However, the volatility limit on this grade will remain at 15 percent maximum.
Europes diesel engine manufacturers have already begun to push down volatility while at the same time lowering viscosity. For example, the Volvo VDS-4 engine oil spec has a 13 percent maximum Noack volatility on both SAE 10W-30 and 15W-40. Going further, Iveco has a heavy-duty SAE 0W-20 with Noack below 12. The push for fuel economy is universal and with it the need for low viscosity, low volatility base oils.
For me, the bottom line on this subject is that we need to move forward quickly with improvements in refining to gain the required low-vis, low volatility base oils needed for the next stage of engine oil technology. Well be supplying engine oils for the foreseeable future and they have to be right.
There, Ive said my piece and the soapbox-on-wheels can go back to its place until the next time.
Industry consultant Steve Swedberg has over 40 years experience in lubricants, most notably with Pennzoil and Chevron Oronite. He is a longtime member of the American Chemical Society and SAE International, where he was chairman of Technical Committee 1 on automotive engine oils. He can be reached at steveswedberg@cox.net.