Summary
The use of Low and Ultra-Low Sulphur Diesel fuel has been proven safe and effective in real world field use in passenger vehicles and heavy duty trucks. There are some known issues with fuel system lubricity, especially in older engines, but proper maintenance protocol will minimize any impact on fleet operations. Transit fleet locomotives are centrally maintained making compliance with scheduled service specs much easier.
Analysis
Sulphur in diesel fuel manifests itself as sulphate particulate emissions in diesel exhaust and contributes to Sulphuric Acid formation and fallout as Acid Rain. Particulates are a known health hazard and the target of ever-tightening EPA ambient air quality standards. By 2015-2017 diesel locomotives will use Diesel Particulate Filters [DPFs] to eliminate 90% - 95% of all particulate mass emissions. In the interim the use of Ultra-Low Sulphur diesel fuel [ULSD] is a positive step and an enabler for future application of DPFs.
Diesel fuel as refined can have upwards of 3000 ppm sulphur depending on the grade of crude oil feedstock. Low Sulphur diesel has <500 ppm and ULSD <15 ppm. Hydrogenation is typically used to remove sulphur at the refinery after distillation.
Sulphur is not a lubricant but the process used to reduce the Sulphur also reduces the fuel's inherent self-lubricating properties. Lubricity is a measure of the fuel's ability to lubricate and protect the various parts of the engine's fuel injection system from wear. Diesel engines use very high pressure fuel injection systems and fuel lubricity is important to protect the precision internal injector components and surfaces. When California first implemented low sulphur fuels many in-use older trucks experienced fuel leaks due to incompatible fuel system seals and O-rings, etc. Newer engines are designed to use ULSD without such effects.
To ensure safe operation with ULSD ASTM International (formerly the American Society for Testing and Materials) adopted the lubricity specification defined in ASTM D975 for all diesel fuels which went into effect January 1, 2005.
The same process that removes the sulphur also reduces aromatic content and density of the fuel, thereby decreasing the BTU energy content per gallon by ~1%. This may result in slightly reduced peak power and fuel economy.
When high exhaust gas recirculation [EGR] rates are used soot can accumulate in the crankcase lube oil and must be suspended to prevent scouring. If a DPF is used the oil additive package must use a low-ash composition to reduce Phosphorus, etc. accumulation on the DPF. New oil spec API CJ-4 addresses these issues without more frequent drain intervals on newer DPF equipped engines.
