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The Emission Mission

TAMPA, Fla. - It would be an understatement to suggest that truck exhaust is cleaner than ever. Tighter controls on the combustion process are limiting the gases that contribute to smog, while exhaust...


TAMPA, Fla. – It would be an understatement to suggest that truck exhaust is cleaner than ever. Tighter controls on the combustion process are limiting the gases that contribute to smog, while exhaust systems now capture most of the tiny flakes of Particulate Matter that would otherwise burrow into your lungs.

The allowable levels of NOx (oxides of nitrogen), which were cut in half during the latest round of emission standards, come courtesy of the lower combustion temperatures made possible by Exhaust Gas Recirculation systems and Caterpillar’s ACERT technology. New Diesel Particulate Filters (DPFs) introduced this model year can capture up to 90% of the Particulate Matter.

But despite these advances – and costs in the form of added equipment, higher engine heat rejections, and lower fuel economy – the trucking industry won’t have much time to catch its breath.

Manufacturers are now in the midst of designing strategies to address the next round of US Environmental Protection Agency standards, which will be unveiled in 2010.

This time, regulators will require another 85% reduction in NOx, along with the onboard diagnostic systems to ensure this happens.

“We all may not be going in the same direction, but we’re all going toward the same goal,” said Caterpillar’s Brendt McClusky, during presentations to maintenance executives attending the annual meeting of the Technology and Maintenance Council of the American Trucking Associations.

What this means is that post-2010 trucks will come fitted with yet another generation of equipment…although there’s a question of what this equipment will look like.

Approaches being investigated so far include:

* Selective Catalytic Reduction (SCR) – The technology that’s already being used in Europe introduces a mixture of urea into the exhaust stream, where the heat of the exhaust converts the mixture into ammonia and water, enabling a chemical process that turns NOx into a harmless mixture of water vapour and nitrogen.

* Homogeneous Charge Ignition (HCI) – This hybrid of a gasoline engine’s spark-driven ignition and a diesel engine’s compression ignition involves mixing fuel before it’s injected into the combustion chamber. The result is a lower local flame temperature that produces less NOx. The process will require some precisely controlled valve timing, said Chuck Blake, Detroit Diesel’s senior technical sales support manager. “Sometimes you need to re-induct the exhaust and hurt your scavaging (the effect that pulls gases out of the exhaust).” However, the mechanical action of a crankshaft eccentric can be used to change the compression ratio.

* Hydrogen-Enabled Combustion (HEC) – “This is not, ‘get some hydrogen and throw it in the tank,'” Blake stresses of the technology. Instead, HEC would use diesel fuel to generate hydrogen, which would be ignited by an electric spark in a “plasma reformer.” The highly charged ionized gas (the plasma) would then be used to regenerate a trap designed to capture NOx.

* The Lean NOx Trap (LNT) – Eaton has announced that it is working on a solution that combines a fuel dosing unit, a fuel reformer catalyst, a Lean NOx Trap (LNT) catalyst, and an SCR catalyst. Essentially, the lower NOx is achieved in two steps – with the LNT and the SCR. Unlike systems that draw the ammonia from urea, the Eaton system would use the LNT to convert NOx into nitrogen and produce ammonia. That ammonia would then be stored in the SCR catalyst, converting any NOx that slips past the LNT.

There would be several advantages to embracing the solution that involves urea, said Volvo Trucks North America’s Mark Louzon.

“The base engine you’ve come to know from the ’04 or ’07 world, that engine should pretty much remain the same.” Engine manufacturers would even be able to generate more NOx in the name of efficiency or fuel economy, and simply address the unwanted gas with the Selective Catalytic Reduction technology, he said.

The question of what a urea distribution network would look like is still a matter of discus- sion among groups including the Alliance of Automobile Manufacturers, the Engine Manufacturers Association, and Truck Manufacturers Association. But the fluid itself could be delivered through bulk storage tanks, large bulk dispensers, mid-sized “totes” that are hauled to a fleet yard, or 10-litre jugs. It’s believed that a 2010 engine would require about one part urea to 100 parts of diesel fuel. And if the European experience offers any indication, the fluid will also cost about 50 cents per litre.

“One per cent usage is fairly minimal,” added Blake. “It’s equivalent to windshield washer fluid.”

Regulators also consider urea – an organic, water-soluble and nitrogen-based solution — to be an environmentally friendly fluid.

Still, there are maintenance-related issues that would need to be addressed. The urea is delivered by sensitive valves that require filters, and urea tanks need to be re-filled on a regular basis. The size of the urea tank on each truck may be dictated by a vehicle’s duty cycle, while weight-sensitive operations may need to consider smaller tanks. And when exposed to extreme cold, the urea could also turn into slush, possibly requiring a heater to ensure that the solution continues to flow through the valves.

Then there’s the question of what would happen if the urea tank runs dry. Initially there would probably be a warning light, chimes and buzzers to alert a driver, said Louzon. The US Environmental Protection Agency has also proposed controls that would keep the driver from re-starting a urea-deprived truck once it’s shut down, or perhaps de-rating the engine’s power.

Cummins, meanwhile, is in the midst of testing NOx-absorbing catalysts in pick-up trucks. This year, you can actually buy a Dodge Ram that will meet the 2010 emission standards.

“We take the exhaust from the engine, and we run it through the Diesel Oxidation Catalyst (DOC) to create heat…converting NOx to nitrogen dioxide,” explains Bill Stahl, Cummins’ director of OEM service. The exhaust flows from the DOC, to a NOx Adsorber Catalyst (NAC), the Diesel Particulate Filter, and then through the muffler. When you feed the NAC with hydrogen, NOx and carbon monoxide, you end up with carbon dioxide, nitrogen and water vapor.

“The work being done today validates the capability of this type of technology for 2010, and new technology equals environmental progress, so we are truly cleaning the air,” he added.

This approach presents its own engineering challenges. The three individual regeneration cycles occur at different temperatures and durations, with no two regenerations operating at the same time. Then there’s the question of whether oil drain intervals will need to change to accommodate the technology, and whether fuel mileage will be sacrificed in the process.

But just because NOx and Particulate Matter are on a steady decline to oblivion, don’t expect the US EPA to stop there.

The nasty components of NOx are 90% nitrogen oxide, and 10% nitrogen dioxide, unburned hydrocarbon (diesel fuel), and the Particulate Matter in the form of carbon, unburned hydrocarbon and ash. The levels of carbon dioxide are unregulated for now, but with the growing focus on the so-called Greenhouse Gases, it will undoubtedly gain more attention in the years to come.

You can worry about that when we approach 2013.


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