Diesel — Cleaning up, but is it enough?
Diesel’s fuel economy offers one of its strongest advantages. And the reason it’s most commonly used by trucks, buses, agricultural and other nonroad equipment, locomotives and ships. Like gasoline, diesel is also widely available. It also shares the disadvantages of gasoline, in particular its harmful emissions. In order to assess the role diesel plays in fueling transportation, we must assess its key characteristics.

Diesel Usage
Besides long-haul trucking where diesel is used 100%, vehicles that most commonly rely on diesel fuel are those that make short hauls with frequent stops and starts: waste haulers, school buses, transit buses and delivery vehicles. Diesel emissions are greatest during starting and stopping, which means these vehicles are producing tremendous amounts of emissions. These are also the vehicles that go through residential areas, exposing neighborhoods and their residences to the harmful emissions they produce.

Long Engine Life
Diesel engines have an extremely long lifetime. While in some respects this is seen as an advantage, it is also problematic. The standards that regulate diesel are reaping slow rewards, because they only apply to new original equipment manufacturer (OEM) engines and vehicles. Diesel engines are commonly rebuilt, without having to meet new standards. The rebuilt engines can last another 30 years.

“Clean” Diesel
Ultra-low-sulfur diesel, aka clean diesel, has some challenges that offset its promise. The emissions performance of vehicles powered by clean diesel have not been verified on the road. Clean diesel isn’t widely available in the US. And low-sulfur diesel buses require expensive pollution reduction devices that’s long-term performance is unknown.

A recent study found that natural gas school buses contained higher levels of air pollutants and toxic air contaminants. However, this study compared a “Green Diesel” engine fueled by ultra-low sulfur diesel and using a catalyzed particulate trip designed to meet 2007 emissions standards, with an in-use NG engine with no after-treatment and designed to meet 1998 standards. The results of such a comparison can only be considered when buses equipped with the same level of technology  are compared. The California Air Resources Board (CARB) retested diesel and natural gas buses with the equivalent technology, and found natural gas to be best on almost all emissions.

While diesel will get cleaner under 2007 standards, there are fuel alternatives that are already clean today. Natural gas is one of them.

Falling Short of Emissions Standards
Despite efforts to “clean up” diesel, diesel technologies available today cannot meet stricter new emissions standards. Over the next few decades, heavy-duty diesel vehicles will have to undergo dramatic changes to comply with ever stricter standards.