May 2008: Feature Story

After-Treatment Technologies

In order to reduce NOx and PM by products in higher horsepower off-highway diesel power plants, engine makers are turning to a number of after-treatment systems — technologies like diesel oxidation catalysts (DOC), NOx adsorbers (NAC), selective catalytic reduction (SCR) and cooled exhaust gas recirculation (EGR). Some manufacturers keep hinting on what technology they will be using, but more are still being a little cryptic.

“We expect to see the introduction of systems such as diesel oxidation catalysts, depending on horsepower band. We will certainly see the introduction of enhanced technologies, both internal and external to the engine, to reduce NOx formation and PM,” explains Tim Cresswell, product marketing manager at Perkins Engines (a designer and manufacturer of high-performance diesel engines for 75 years). “It is the advent of such after-treatment systems that will be the most evident and physically challenging in terms of engine packaging. Diesel particulate filters and the accompanying filter regeneration equipment will add to the overall installation volume. A typical particulate filter assembly, though hugely efficient, potentially carries a significant weight and size penalty, causing a rethink for OEMs when it comes to equipment design.”

It seems each manufacturer is turning to a number of these technologies to most efficiently reduce emissions and please customers, but there is a prominent VHS/Beta battle of two main technologies — SCR and EGR. Selective catalytic reduction or SCR is a technology that uses a urea-based solution to reduce NOx. The urea solution is mixed with engine exhaust gases in the catalytic converter turning the NOx to oxygen, nitrogen and water. SCR is already used in European truck and car engines, and some U.S. manufacturers have announced that they will begin using the technology in on-highway engines beginning in 2010 (Detroit Diesel and Freightliner for example).

But most North American companies seem to be turning to cooled exhaust gas recirculation (EGR) strategies, which lowers an engine’s peak combustion temperature to lower NOx output. Both sides have made compelling arguments. The EGR camp says that SCR requires additional components and is overly complex; also, urea may not be widely available in the Tier timeframe and SCR is unproven in North America.

While not many companies have announced they will use SCR technology in North America as of yet, Tognum subsidiary MTU Detroit Diesel unveiled at CONEXPO-CON/AGG its Tier 4 Interim compliant Series 900 engines outfitted with SCR technology. Plans call for both the Series 900 (120 to 322 bhp) and Series 500 (349 to 644 bhp) engines to use SCR to meet the Tier 4 Interim standards that take effect in 2011.

Companies like MTU Detroit Diesel counter that it is a proven technology not only in European vehicles, but also in a wide number of other applications, including coal-burning energy plants in the United States. They argue that urea availability issues are not so difficult and that SCR is the only way to meet emissions standards without sacrificing fuel economy. In fact, proponents of SCR insist they will deliver a significant fuel economy improvement over today’s engines. Yet, big North American companies like John Deere and Cummins have noted they will be using EGR in their engine systems — at least for Interim Tier 4.

“An SCR system injects liquid urea into the exhaust stream to reduce NOx. While this is effective, it also requires that the vehicle or machine be fitted with a separate tank, a sophisticated urea injection system and an EPA-required, tamper-proof diagnostic system,” explains Laudick. “Because urea freezes, heating systems for the tank and delivery lines are required. In addition, urea is not conveniently available in most parts of the world at the present time, which creates access and storage concerns. SCR may be an appropriate technology for Final Tier 4 regulations, when the technology is more developed for off-highway applications.”

But companies like MTU Detroit Diesel believe that the adoption of SCR technology could be the best overall solution.
According to Bryan Mangum, marketing and communications manager at MTU Detroit Diesel: “Urea is a common substance that is easily made, non-toxic and requires no special handling. We believe infrastructure will be quickly built out, and we will also be able to fully support our customers through our own extensive North American distribution network. With regards to gelling temperature, urea’s is about the same as diesel’s, so keeping urea flowing is no more of a challenge than keeping fuel flowing.

“That fact is that none of these new technologies are without their challenges,” continues Mangum. “MTU has experience with all of them and our philosophy is to match the best technology to the needs of each specific engine. In the case of our Series 500 and Series 900 engines, SCR was the optimal solution. It is a cost-effective means of preserving the engines’ inherent fuel efficiency and will require no major changes to engine design. SCR also does not increase heat rejection, which can be a major problem with other emissions control technologies.”