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Several months ago, I wrote on the subject of viscosity (How low can we go? September and October 2007). Those articles described the long-term trend to lower-viscosity engine oils, based primarily on the need for additional fuel economy. I also noted that there seemed to be a minimum viscosity limit below which we could not go. What is the logic of this? And is it possible were at the end game for viscosity?

Lower viscosity means two things to the vehicle manufacturer or engine designer: Improved fuel economy, and improved low-temperature cranking and pumping. Both of these have been big problems in the past. Low-temperature pumpability and cranking limits are now in place without much need for improvement at this time – but fuel economy gains are always needed.

Since the passage of U.S. fuel economy legislation in the 1970s, viscosity of engine oils has been an important part of the auto industry strategy. Remember that lowering engine oil viscosity has a benefit in reducing friction and ultimately reducing fuel consumption. In those days, SAE 10W-40 was the dominant multigrade, and mono-grades were a big part of the market – but not for long.

By 1980 viscosities were dropping and monogrades were going out of style. This trend accelerated, and now we are seeing SAE 5W-20 and SAE 5W-30 as the dominant passenger car grades. SAE 0W-20 is also being promoted, and with it comes (at least in my mind) concern about engine oils viscosity and its ability to protect the engine.

For those who havent seen it before, the graph on page 8 is what is called the Stribeck Curve, relating viscosity to friction in a rolling bearing assembly. There is a similar curve relating sliding friction between two surfaces, such as the rapid speed changes and reversals in a piston engine.

The component functions on the graphs lower axis, indicate the absolute Viscosity, the relative velocity between the surfaces (Speed) and the load applied between the surfaces (Pressure).

Just a casual look at the curve shows that as viscosity goes down, the coefficient of friction goes down, too, until a critical point is reached. Then, further viscosity reductions increase friction, especially when the speed is low as well. At a certain point the engine goes into elastohy-drodynamic, or mixed, lubrication mode. Not much further down from there takes the engine into boundary lubrication mode and into significant wear.

What is the Issue?

Fundamentally, the issue is whether there is sufficient viscosity at operating temperatures and pressures to provide adequate protection against wear. In the 1980s, the discussion went back and forth in SAE technical committees about what limits to set on high-temperature/high-shear viscosity. Slogans like 2.9 is fine and 2.7 is heaven were thrown around. It wasnt until 1992 that SAE Standard J300 (Engine Oil Viscosity Classification) finally included HTHS viscosity limits. These limits reflected what industry gurus felt were needed to protect the engine from wear.

In an engine, most of the friction losses due to surface interfaces are in sliding friction. Somewhere around 70 percent of frictional losses are in the ring/liner interface. This is seen when used liners are examined for wear. Within the ring travel area, there is some level of wear, with more on each end where the ring travel stops and then reverses. Bore polishing is an extreme case of wear in this area. Sliding followers on the cam are another area of wear that has drawn a great deal of attention over the years. To date, all of these areas have been successfully protected by the lubricant.

So, how do the various viscosity grades compare when it comes to higher Continued from page 6 temperature and fuel economy benefits? The chart above compares viscosity, HTHS and relative fuel economy benefits for a number of viscosity grades, ranging from SAE 30 to SAE 0W-20.

While OEMs (especially automotive) are comfortable with these limits, it does raise the question as to where the breaking point is regarding fuel economy vs. wear protection. Engine oils have contained zinc dialkyldithiophosphate (ZDDP) antiwear agents since the 1940s. The actual antiwear component of ZDDP is phosphorus, which poisons exhaust catalysts when engine oil gets into the combustion process (which it does).

In the early 1970s, ZDDP levels contributed upwards of 0.14 wt. percent of phosphorus in the finished engine oil. That came down to 0.10 percent maximum for ILSAC GF-3 engine oils, introduced in 2000. Todays GF-4 oils are at 0.08 percent maximum, with the prospect of this limit continuing for GF-5. This reduction in ZDDP addresses the OEMs need for continuous improvement in tailpipe emissions.

This obviously begs the question as to whether other antiwear agents which are benign to catalysts might be used in the engine oil additive package. The answer, at least so far, is there arent any others.

For the heavy-duty OEMs, the same issues are in play, although with some variations. The standard viscosity grade for heavy-duty engine oils today is SAE 15W-40, with over 85 percent of the U.S. market. It offers fuel economy benefits versus a monograde SAE 30 (about 1 to 2 percent better). Its HTHS viscosity limit is 3.7 centiPoise, which provides good protection against wear in diesel engines.

Remember, diesels are more robust in design, and with good reason. The brake horsepower and torque of a diesel engine is significant and it operates at lower RPM than most gasoline engines. In these powerplants, using higher-viscosity oils protects against wear in both rotating and sliding conditions. ZDDP in diesel engine oils is now at levels where adequate protection is obtained. However, heavy-duty OEMs have similar emissions issues as passenger car makers, and they needed some reduction in phosphorus from levels used in API CI-4 PLUS oils, where 0.14 wt. percent phosphorus was the norm. In response, todays new API CJ-4 oils have limited phosphorus to 0.12 percent maximum.

Heavy-duty OEMs would like to see fuel economy gains in their engines as well. Ultra low sulfur diesel fuel (ULSD) is now required in over-the-road operations and is more expensive than premium gasoline! One answer would seem to be to go to SAE 10W-30, to gain some fuel economy benefits versus our SAE 15W-40 work-horse grade. However, the concern is that HTHS viscosity would drop to the point where more wear would result. Given the expense of a new engine or a rebuild of an existing engine – as well as lost revenues while the truck is being repaired – most fleet owners would not want to take that risk.

What is the Motivation?

On the passenger car side, one dilemma is the fact that fuel economy claims for engine oils are not truly realized by the driving public. Doing some simple math, a 1.7 percent improvement in fuel economy using SAE 5W-30 engine oil in a vehicle which gets about 22 mpg (such as my new Nissan Quest minivan, with 3.5L dual overhead cam V-6 engine which generates 235 HP) should mean a gain of about 0.4 mpg, versus an SAE 20W-30. However, the driving patterns for my car wash out any of that effect, so I dont see the gain. My engine is protected by the oil so Im happy, but paying the extra amount for fuel economy performance – its not free – is not a good deal.

So youre asking yourself, why do the OEMs want better fuel economy with each round of engine oil improvement? The answer is really quite simple: In order to certify their engines for government Corporate Average Fuel Economy (CAFE) requirements, the OEMs must use engine oils that are generally available and competitively priced. The procedures they use to measure engine fuel economy across their fleets are precise and do benefit from the minor fuel economy gains delivered by using lower-viscosity, friction-modified engine oils..

The current CAFE requirement for any OEMs fleet is an average 27.5 mpg. Thats scheduled to go to 35 mpg by 2015, so the heat is still on to gain fuel economy. (Admittedly, my cars fuel economy alone doesnt meet the CAFE standard required of Nissan. However, the rest of Nissans fleet of cars offsets my gas hog so that Nissan can be assured of hitting the golden 27.5 mark.)

What is the Alternative?

First, lets recognize the great achievements of the OEMs since the introduction of CAFE. In 1975, when CAFE legislation first was passed, automobiles were powered by up to 400-plus-cubic-inch (6-plus-liter) carbureted engines. Fuel economy was about 14 mpg on average and tailpipe emissions were pretty atrocious. By 2000, engines were smaller – typically 2 to 4 liters – fuel injected, and stringently emission-controlled. All of these gains were due to the development of computer controls for fuel metering and exhaust mixing, plus catalysts. If you dont take your hats off to the OEMs for this, youre not paying attention!

Even so, I think it is imperative to stop the lower viscosity madness. Dropping to SAE 0W-20 gains the OEMs just 0.5 percent vs. SAE 5W-20 when figuring their CAFE number, or maybe 1.0 percent vs. SAE 5W-30. With all of the technological advances the OEMs have been able to achieve with hardware and software, I think these fractions are lost in the fog. Certainly, the individual consumer cant see it. However, lowering HTHS viscosity by even 0.3 cPs opens the door to increased wear and durability issues that we consumers dont want to face and the OEMs shouldnt want to deal with as well.

I suggest that we consumers need to look at the vehicles were driving; maybe 235 horsepower is more than we need or want. (In my case and others, theres often no choice of powertrains.) We need to think in terms of appropriate performance and more economy. We need to think about the decisions being made for us by our representatives in Congress, which have led us into this situation.

There are many ways to achieve greater energy independence, which is at the heart of this fuel economy question. Surely, if cooler heads prevail we can find an answer that gives us all what we want: Freedom to go where we want, when we want, without concerns about fuel costs or vehicle performance and durability.

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