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Thinner Days Ahead?


For nearly two decades, emissions mandates have been the leading concern for builders of diesel engines for trucks and buses, observes Bengt Otterholm of Volvo Group in Gothenburg, Sweden. Few regions of the world were unaffected, as regulators targeted unhealthy diesel particulate matter and oxides of sulfur and nitrogen. From Seoul to Sao Paolo, Toronto to Taiwan, much of the world is now at or close to the Euro V emission standard for new equipment, and many countries are preparing to roll up to Euro VI over the next four years.

Truck manufacturers did their part with an array of technologies to reduce emissions. They pressed for improvements in diesel fuel and lubricant quality, and installed computerized engine controls, exhaust gas recirculation, diesel particulate filters and selective catalytic reduction on their equipment.

At this point, such technologies are all-pervasive in the developed world, and markets in Asia, Central and South America are catching up. Over 80 percent of Volvos new production is fitted with after-treatment components, Otterholm told the recent ICIS Pan American Base Oils and Lubricants Conference. The varying emissions mandates, like Japans PNLT, Euro VI, and the U.S. EPAs 2010 emissions standards, are converging now, and by 2015 new on-highway vehicles will generate virtually zero particulate matter (PM) and Nox.

To hit the Euro VI mark, Volvo is exploring two engine concepts. The first couples EGR with a turbocharger. Untreated exhaust from this set-up would contain just 2 or 3 grams of NOx per kilowatt hour (g/kWh), and on-board diesel particulate filters and selective catalytic reduction will cut that by another 80 to 87 percent.

The second concept engine forgoes the EGR, but includes the particulate trap and a more hard-working SCR with up to 93 percent efficiency. This would be more cost-competitive, but the engine itself produces 4 to 6 grams of NOx, noted Otterholm, who is lubricants coordinator for Volvo Truck Technology.

In both cases, the post-SCR exhaust would have no more than 0.4 grams of NOx – fully compliant with Euro VI. And either way, he acknowledged, Volvos big challenge will be to satisfy the requirement that these systems fulfill their jobs for up to 700,000 kilometers.

Next Up: Fuel Economy

With NOx emissions approaching their vanishing point, whats next? Since regulations have driven us to almost zero emissions on our new vehicles, fuel economy is our next big issue, said Otterholm. And this is being mainly driven by customers. Large fleets and individual owner-operators alike want to reduce their operating costs, conserve resources and reduce CO2 emissions. And thats where lubricants come in, particularly through viscosity effects and friction modification.

Engine friction – from pistons and liners rubbing, the valvetrain, and splash losses – eats about 4 percent of the power in each gallon of fuel, Volvo estimates. Energy losses from accessories like the oil pump and driveline friction together waste another 6 percent. Heat losses take a whopping 13 percent, and waste-heat recovery tops the engine designers list for improvement.

Still, theres a lot of potential gain from the trucks lubricants. In fact, each 1 percent gain in overall engine efficiency can translate to a 2 to 3 percent improvement in fuel economy. And even 1 percent better fuel economy is a lot of money to a fleet, Otterholm stressed.

After studying the effects of viscosity on fuel economy, Volvo determined that SAE xW-30 multigrade engine oils may offer quick gains versus 15W-40, which is by far the most popular grade for diesel engine oils. Of the 30-weight choices, Volvo thinks the most promising choices would be a standard 10W-30 (with high-temperature high-shear viscosity of 3.5 mPas), or perhaps a 10W-30 with a modified HTHS of 3.0.

In tests run under simulated long-haul conditions, with no cold starts and only a fully warmed up engine, Volvo saw fuel economy gains of around 0.7 percent with a standard SAE 10W-30 versus 15W-40. When using a 3.0 HTHS oil, the 10W-30 delivered 1.2 percent gains. Some of this work has been confirmed by field data, Otterholm said. However, it saw little additional improvement in trials with 5W-30 or 0W-30 – at least not enough to justify such a big step.

Three-pronged Risks

Multigrade 30s are not risk-free for heavy-duty engines, Otterholm emphasized. He cited three areas of particular concern:

Durability. An engine lubricated with a 3.0 HTHS oil will have lower hydrodynamic losses when running flat on the highway – thats good – but under loaded or in stop-start conditions it will spend more time in the mixed and boundary lubrication regimes, where the most valvetrain, piston and liner wear occurs (bad!). This is where friction modifiers may help. With friction modification, you can help to postpone the time when friction starts to climb.

Backward compatibility. Older engines will not be serviceable with SAE 10W-30s that have reduced HTHS viscosity – and its a rare fleet thats willing to buy and segregate two classes of 10W-30s for its powerplants. Intermingling thus seems inevitable, which bodes ill for engine life. If we cannot secure backward compatibility with low-viscosity oils, we wont hit the market. If only a small percentage of vehicles in a fleet use it, the workshop wont adopt it until the fleet percentage goes higher.

Oil pressure issues. Oil pressure needs to be match ed to the oils viscosity, to ensure piston-cooling jets, rocker arms and other pressure-controlled engine parts respond. And if lower-viscosity oil were used, pressure alarms would need to be reset for every engine type, too. The oil will flow and get where its needed, but some ports are operated by oil pressure, and all of them would have to be recalibrated to the lower pressure, Otterholm explained. Also, the ports would still have to work with the higher-viscosity oils, to handle the risk of fleets switching to a thicker oil.

Enough Time

This technical upheaval is not unique to todays vehicles, said Gary M. Parsons of Chevron Oronite, who also spoke at the Pan American conference. I remember having these same conversations when they started putting catalytic converters on cars – and now we have 500-horsepower cars which all have the technology. It shows that if you give the oil and additive companies and OEMs enough time, we are able to meet the challenge.

Like Otterholm, Parsons sees the priority for lubricants shifting from emissions to fuel economy. With PM and NOx at near-zero, theres little room for further reduction, he pointed out. But regulators dont go out of business. Where will they go now? Theyll go regulate CO2 emissions. And theyll do that by pushing down fuel consumption.

Another driver, he added, is the rising cost of fuel. Todays consumers, not just regulators, are demanding better fuel economy.

Additives will play a key role as technology enablers, to meet the new fuel economy standards, said Parsons, who is global OEM and industry liaison manager for the additive company in Richmond, Calif. Heat is already one of the biggest hurdles, he noted. Thanks to advanced thermal management and EGR, todays engines operate about 10 degrees C higher than their predecessors. So thats a much more severe environment for the oil.

To handle the heat and deliver better fuel economy will require dispersants to handle soot, antioxidants to improve thermal stability, and friction modifiers to control friction. Base oils are also likely to change, as lower volatility is needed to stem the evaporative losses which lead to oil thickening.

Finally, lower-viscosity oils are sure to be on the agenda. Using a lower-viscosity engine oil gets you a relatively small fuel economy benefit, maybe 1 percent or so, Parsons said, but a fleet manager finds this very appealing – to capture fuel economy with just a change of oil. Its the low-hanging fruit of fuel economy, and very easy to do.

The tricky part, he added, is to pick that low-hanging fruit while assuring extended drain intervals and engine durability, so that the fleets total cost of ownership does not suffer. Heavy-duty OEMs are adamant on engine durability, since thats a major competitive selling point for their trucks. Paccar, for example, expects 90 percent of its MX engines to hit one million miles before their first overhaul, Parsons said.

The gains in fuel economy cant be at the expense of engine durability, he reiterated. So reducing viscosity will also require other formulation decisions.

Proving Performance

Theres also the urgent question of what test to use to measure the fuel economy improvement. The most widely recognized heavy-duty fuel economy method is SAE Standard J1321, a costly field test, but theres also Volvos D12D test, which uses a 12.1-liter six-cylinder diesel on a dynamometer. It follows the European Stationary Cycle of specified speeds and loads, and compares a candidate oils fuel economy performance to that of a reference oil, usually an SAE 15W-40.

Chevron Oronite tested a variety of oils – two SAE 5W-30s, a 10W-30, a 5W-40, and a 10W-40 and 15W-40 – in the D12D, and compared them to a 15W-30 baseline. The outcomes pointed to two basic facts: First, they confirmed that lowering the oils high-temperature viscosity indeed has the biggest impact on fuel economy. And second, the oils standard detergent-inhibitor additives had very little impact at all.

The D12D test operates mostly in the hydrodynamic lubrication regime, versus boundary lubrication or mixed lubrication, Parsons explained. And the tests showed that d.i. additive chemistry has a negligible direct impact on fuel economy at average on-highway conditions.

Friction modifier additives, however, clearly affected fuel economy during those parts of the test cycle that involved boundary lubrication. Pursuing this thread, Oronite performed another set of tests. This time, four 5W-30 oils, each with a different friction modifier chemistry, were compared to the non-friction-modified 5W-30. Sure enough, fuel economy got a notable boost over the baseline – except in the case of one chemistry where it actually worsened.

This last result highlights the difficulty of selecting the right friction modifier, and matching it to the real-life road conditions that a vehicle may encounter. However, if properly selected, Parsons concluded, friction modifiers can impact fuel economy, and the benefits appear to be retained to some degree.

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