The Year of Urea

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Fluid concerns in 2007 may be focused on the introduction of Ultra Low Sulfur Diesel, but the biggest fluid-related challenge of all may come in 2010.

Several manufacturers have suggested that they may need to inject urea into the exhaust stream to meet the next round of exhaust standards, in a process known as Selective Catalyst Reduction (SCR).

The idea of using urea seems like a simple chemistry experiment. A chemical trap known as a “NOx adsorber” captures NOx, urea is then injected into the exhaust stream, and the resulting hydrocarbon-rich environment releases oxygen and converts the NOx into a harmless mixture of nitrogen and water.

The approach has been used on diesel generators for years, but trucks require the SCR equipment in a smaller package, capable of withstanding the rough ride of today’s roads.

The challenges relating to introducing ULSD pale in comparison to those involving urea. The first question among them is how the fluid would be distributed. If it comes in the form of small, easily handled jugs, it may require frequent top-ups. Environmental Protection Agency officials admit that they’re skittish about what would happen if the jug runs dry. If too much urea is injected, some ammonia will escape. If it’s too low, the NOx isn’t reduced. And just how much will it cost? While the fluid is being used in Europe, those fleets have the advantage of a price that’s set much lower than the cost of diesel.

International sales manager Daniel Webster suggests the required amount of urea could be significant, with fuel supplies including between 5% and 10% of the mixture of fuel.

Urea presents potential maintenance challenges as well, not the least of which is its ability to corrode brass, copper and mild steel. (Although, it is compatible with stainless steel, plastic and some aluminum.) It also freezes at 11 Fahrenheit, decomposes above 280 Fahrenheit, and can be a mild irritant.

Even manufacturers are split on the issue.

“Several engine manufacturers have indicated that SCR is the only viable path for meeting the 2010 EPA standard – however, our research indicates SCR might not be the best choice for on-highway applications,” Caterpillar noted in a recent press release. The company points out that the SCR systems could be vulnerable to damage, and that there have been challenges to limiting the amount of the fluid being injected during the combustion process.

Then there are the operation-related challenges: “Do you de-rate the engines when they run out of urea?” Webster asks as an example. “Do we have onboard diagnostic systems that would raise a flag?”

For now, the tests continue – and there are positive signs that manufacturers will find ways to meet the 2010 standards.

Detroit Diesel, for example, says it’s on track to meeting a target of 50% brake thermal efficiency, which measures the share of the fuel’s energy that is converted into power during the combustion process.

“In current production heavy-duty engines we usually see a brake thermal efficiency of about 41% or 42%, so for us to demonstrate greater than 45% efficiency as part of our advanced engineering effort is a significant achievement for Detroit Diesel,” said president and chief executive officer Carsten Reinhardt.

Cummins, meanwhile, recently announced that it’s working with the California South Coast Air Quality Management District to build and demonstrate a nine-litre ISL engine that meets 2010 standards with the combination of a Diesel Particulate Filter and NOx adsorber.

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