By Eric Brisbon

Those of us who are mechanically minded and enjoy older vehicles, usually get a kick out of viewing a driveline of a vehicle from yesteryear. The simple, straight forward approach of providing power to the wheels, along with ample work room, brings with it a certain sense of nostalgia. The relative ease of maintenance and service leaves one with a sense of longing. Times have changed.

Fossil fuel burning engines succumbed to environmental and political pressures to change. They needed to significantly improve the tailpipe emissions while continuing to enhance the overall life and economics of use. Diesels have the added requirement of having to smoke less, run quieter and smell better.

There are many items added to modern diesels to address emission requirements. Diesels have two principle measures that have been scrutinized and regulated for decades. Nitrous Oxide (NOx) and Particulate Matter (PM) have been continuously reduced to meet governmental laws and be socially acceptable. Unfortunately, when NOx is reduced, PM tends to increase and vice versa. As a result, a parallel approach was taken to reduce emission through the combustion with after treatment (such as Urea) catalysts for NOx. The exhaust stream is then treated with additional after treatment such as particulate filters and scrubbers for particulate control.

Catalysts have been used for years in both gasoline and diesel engines. However, there is one element that diesel fuel contains which can create chemical reactions that are potentially harmful. The sulfur contained in diesel fuel converts to SO2 (sulfur dioxide). High sulfur levels have adverse effects on catalysts efficiency, consequently, the lower the sulfur content of fuel the higher the reduction in emissions.

Particulate filters work well but require cleaning to keep them operational. Continuously Regenerating Particulate Filters (CRPF) and catalyzed diesel filters both suffer with high sulfur fuels as efficiency of their processes are impeded. Therefore, it’s an obvious conclusion that reducing the level of sulfur in diesel fuels is a natural progression to aid in meeting emission requirements.

One large side effect of low sulfur fuels is the potential problems it creates on the fuel systems handling it. Historically, sulfur provided the required lubrication to keep fuel injection equipment running for many miles with little to no appreciable wear. The Environmental Protection Agency (EPA) has dictated that since 2010, 100% of on-highway diesel fuel must be Ultra-Low-Sulfur Fuel (ULSD). The sulfur used for lubrication in diesel fuel can be compared to the lead in gasoline, creating a cause/effect problem when it was removed, hence the popularity of “lead substitutes”. This situation is not unlike problems we have had with valves and valve seats burning in the gasoline -fired spark ignition engines when the lead was removed, which demonstrates consequences from removing the sulfur as well.

There are various types of diesel fuel injection systems that have developed over the years as higher pressures and more control of the injection cycle was required. High pressure pumping requires very tight tolerances. It is not uncommon to see a diametral clearance of two microns. As fuel systems have grown in peak injection pressures, the duty factor of its lubricants has had to follow suit. On many of these, the diesel fuel provided the only lubrication to fuel injection equipment. Obviously, lubricity is crucial and modern fuel systems have had to work on many new and innovative tribological programs to address the reduction of sulfur and resulting loss of lubricity. Fuel additives, dyed diesel, various coating and even Biofuel have all grown in stature due to ULSD.

However, to really understand the issues, you must look at the fuel itself. Diesel fuel is rated by a scale called Cetane. Cetane is the inverse of the Octane rating used for gasoline and it provides an overall measure of the time lag to combustibility. In addition, there are several additional components such as pour point, cloud point, lubricity, and sulfur content. The interesting thing about Cetane is that a diesel engine runs best with the Cetane rating between 48-51. By law, the minimum Cetane rating in Europe is 51. In the US it is not uncommon to find Cetane rating of only 40!

One of the more common issue is the use of alcohol in the additive. Because of the suspended levels of paraffin in diesel fuel, when the temperature drops (pour point) the paraffin solidifies, creating “gelling or waxing”. Alcohol is used to stop this, especially in extreme situations. However, the alcohol changes everything about the fuel, and modern diesel systems cannot handle the result.

The other trend in todays’ diesel world is the use of vegetable oil (fry oil), home heating oil, or other blended fuel. This is a hot topic because it became popular to offset the higher cost of diesel. The problem is that while most 20-year-old powerplants will run for a time, newer engines with advanced fuel systems cannot absorb the added contamination of the alternatives. NASA Langley Research, who has built reactors to create an algae-based diesel fuel from sea water and seaweed, ultimately did not consider that newer common rail systems cannot tolerate the contaminates. The project failed.

Should you have an older diesel, or a re-manufactured replacement and you start having trouble with premature wear or loss of performance, remember that the fuel system may not have been designed for the fuel you are using. It may be time to go with updated fuel injection equipment from a reputable re-manufacturer or experiment with fuel supplement. Just be sure you DO NOT use an alcohol-based additives.