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Is it time for a diesel in your family car? Two things occurred recently which got me thinking about future automobile engines and the lubricants they will use. The first was an article in the Arizona Republic (front page, Feb. 19) about diesel engines. The headline read Your next car may be a diesel. The second was watching Al Gores Academy Award-winning rant on the environment, An Inconvenient Truth.

The News

The newspaper article pointed out some of the improvements that have occurred in diesel technology which make it a viable candidate for U.S. passenger car service. Beyond the fact that diesel engines are inherently more efficient than gasoline engines, some recent engineering and fuel changes have added to that advantage.

First, the addition of computer controls has improved combustion and emissions controls. Fuel injection systems have been improved and turbo-charging has been added. As a friend of mine who has driven a turbo-diesel Volkswagen says, It really pins you to your seat. Thats something that American drivers value.

Second, the introduction of cleaner diesel fuel in the United States also improves the climate for the diesel engine. Ultra low sulfur diesel (15 ppm) is now required for over-the-road diesel use. Although emissions controls for diesels are still challenging, this does make cleaner exhaust from diesels a little easier to obtain.

If you compare price and performance data for gasoline- and diesel-fueled 2006 Volkswagen Jettas, youll find the following to be true.

Given the concerns being raised about energy conservation, this looks like a pretty good deal.

When President Bush delivered his 2007 State of the Union message to Congress, one of his stated goals was to reduce gas consumption by 20 percent by the year 2017. The increased use of diesel-powered vehicles in the marketplace would appear to go a long way towards achieving that goal.

First, U.S. automakers will have to climb on board. With the exception of the Jeep Liberty mid-size SUV, Detroits Big Three today only offer diesels on full-size pickups, at hefty price premiums. Today U.S. buyers who really want to sit behind the wheel of a diesel passenger car must shop imports such as VW, BMW and Mercedes.

An Inconvenient Truth

For those of you who havent seen it, An Inconvenient Truth posits that the world is in imminent danger of total environmental disaster due to increases in carbon dioxide (greenhouse gas) in the atmosphere. Atmospheric levels of CO2 have increased due to industrialization, increasing the amount of infrared radiation which is retained and raising atmospheric temperatures (global warming). Global warming will result in all sorts of negative consequences such as melting icecaps, upsets in ocean currents, etc. While Al Gore is convinced of this, there are a number of reputable scientists who are not so sure.

What Al did say, which is of interest, is that the United States has the lowest Corporate Average Fuel Economy requirement (CAFE) of several major areas of the world. According to the Pew Center on Global Climate Change, the United States ranks lowest of five countries, the European Union and even California in terms of fuel economy limits. (See graph on page 12.)

His point is that reductions in energy consumption will reduce greenhouse gases and limit the effects of global warming. Whether or not you agree with the global warming scenario, reducing fuel consumption and greenhouse gas emissions cannot be a bad thing – if it can be done in a practical manner.

What It All Means

Putting the two items together, it would seem that a case could be made for increasing the number of diesel-powered automobiles in the United States. It is not a major stretch to change from one kind of internal combustion engine to another and avoid the difficulties which are inherent in more exotic technologies such as hybrids and alternate fuels. The entire infrastructure is in place to deliver diesel fuel as opposed to gasoline, and service stations already have tankage which could handle the change. In fact, many stations already have diesel tanks and pumps.

The question then is: What does this mean for engine oils?

The trend in automotive engine oil (gasoline) formulation has been to lower viscosity and reduce friction to increase fuel economy. This trend has been going on for at least 20 years. In addition, improved oxidation stability has been introduced, again to improve fuel economy. Emissions have not been neglected, as phosphorus levels have been reduced to protect catalyst systems. As always, the trade-offs have been increased volatility due to lower viscosity oils, and concerns about wear with reduced phosphorus.

A typical ILSAC GF-4 product, the current engine oil specification, can be characterized as follows:

Viscosity grade

SAE 5W-30

Phosphorus, wt%

0.08 Max; 0.06 Min

Volatility, %

15 Max at 700 degrees F

Compositionally, this formulation requires the use of special low-viscosity, low-volatility base stocks. It also contains an additive package which is heavy on antioxidants and friction modifiers as well as secondary anti-wear agents.

Compared to gasoline engine oil formulations, a diesel oil formulation is normally a higher-viscosity, more heavily formulated product. Wear, deposit control and oxidation resistance are prime concerns. Because of the improved efficiency of diesel engines, extreme measures to improve fuel economy dont seem as critical. Many thought that the new requirements for over-the-road diesel emissions would raise the need for control of metallic components, sulfur and phosphorus. It now appears that these concerns were not as serious as first believed.

Typically, an API CI-4 PLUS product (the one most widely used now) would be characterized as follows:

Viscosity Grade

SAE 15W-40

Phosphorus, wt%


Ash Content, wt%

1.0 to 1.2

Sulfur content, wt%

0.4 to 0.6

While base stock cuts dont have to be as low in viscosity, there is an advantage to more highly processed materials. Heavy-duty engine tests seem to respond better to higher saturate level stocks.

Designing engine oil for the new passenger car diesel will require some new testing protocols. The European ACEA standards might provide a good starting point. Unfortunately, many European engine manufacturers are requiring not only ACEA claims but their own proprietary specifications. That makes a standard more difficult to achieve. Questions about ACEA test methodology have been raised in ASTM, particularly with regard to test precision and standardization. Either the ACEA standards or API C categories for diesel oils could be used on an interim basis until ASTM, API and the other interested parties can complete their own test development and specification-setting process.

Time to Market

If we are to consider the use of diesel-fueled engines in North American passenger cars, and the primary benefit is fuel economy, what happens to the marketplace? For a number of years now, the automobile population of the United States has been turning over at the rate of about 7 percent to 8 percent per year, meaning it takes about 14 years to completely turn over all automobiles. Obviously, there are some vehicles that dont last that long and some that last longer. So if the median age of automobiles on the road is seven years, and if the auto industry completely converts to diesel engines for passenger car production for model year 2008, were looking at 2015 before the marketplace will tip towards diesels. That should provide enough time for the fuel distribution system and the engine oil suppliers to completely develop new products and delivery systems to meet demand.

Will it happen? Is the U.S. marketplace up to this challenge? Only time will tell, but improved fuel economy, stringent emissions controls, and improved engine designs have been with us for many years. The oil and auto industries have always responded. Im betting they can do it again.

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