Manufacturers are busy rolling out solutions to meet Tier 4 Final off-road requirements for diesel engine exhaust emissions, and the technologies look very similar to those already in use in the on-road market.
The biggest change from current Tier 4 Interim engines will be the addition of selective catalytic reduction (SCR) technology. SCR technology has been successfully used in Europe for several years, and was introduced to the on-highway diesel engine market to meet the last round of emissions regulations in 2010.
SCR requires the addition of diesel exhaust fluid (DEF), which is injected into a catalyst to reduce exhaust emissions. While this means yet another fluid to handle, there are many benefits, especially when paired with the previous exhaust gas recirculation (EGR) technology.
“Combining cooled EGR with SCR provides a very effective solution to balance NOx reduction between the engine combustion and exhaust aftertreatment,” says Kevan Browne, Cummins. “This enables the optimum point of fuel efficiency and performance to be maintained in the ‘sweet spot’ for longer than an SCR-only approach can achieve. The engine EGR rate is rebalanced for the Tier 4 Final engines.”
A Careful Balance
SCR systems were added to address the need to further reduce NOx emissions.
“It’s not possible to meet Tier 4 Final without SCR,” states Hakan Sterner, technical director for Scania Engines. “It’s the best way to reduce fuel consumption, be less sensitive to sulfur and keep a good transient response. With high EGR rates — which are required for Tier 4 Interim without SCR — it’s not possible to meet the emissions levels without a diesel particulate filter (DPF) [on engines above 174 hp].”
With SCR, DEF is consumed by injection into the decomposition pipe in the exhaust stream; dosing rates can vary by the engine supplier. “The fuel cost is so much higher than the cost for DEF, so it’s almost always a save to reduce the fuel consumption by increasing the DEF consumption,” says Sterner. “But there is a limit where increased NOx will give no reduced or very limited reduction of fuel consumption. There are also other limitations which reduce the possibility to just increase the injection of DEF — for example, the evaporation of DEF and efficiency in the catalytic converter.”
It is difficult to pinpoint the optimal DEF dosing rate for the SCR. “This is a complicated formula that varies between engine platforms, the range of exhaust temperatures and the efficiency of the spray pattern,” says Browne. “The design goal is to achieve the best balance of fuel economy with DEF dosing rates, DEF tank sizing and required refilling intervals.”
There has been a lot of work done to integrate the EGR and SCR technologies for optimum performance. “Until facing the NOx emissions levels required by Tier 4 Final, John Deere achieved the mandated NOx levels with its cooled EGR technology,” says Doug Laudick, manager of product planning, John Deere Power Systems. “With the additional 80% NOx reduction required for Tier 4 Final, we will combine our proven cooled EGR with an optimized SCR technology within the Integrated Emissions Control system.”
Many of the initial concerns with SCR have been worked out as the technology was adopted for on-highway applications. “SCR is an appropriate technology building block for Tier 4 Final, now that it is more mature for off-highway applications,” says Laudick. “The DEF supply chain infrastructure is better developed, and John Deere has optimized the performance of our SCR system.”
The company tracks total fluid economy as diesel fuel consumption plus DEF consumption, and has realized an improvement over Tier 4 Interim engines. “The total fluid economy with Tier 4 Final engines is expected to meet or improve upon that of our proven Tier 4 Interim engines with cooled EGR and exhaust filters operating on diesel fuel only,” says Laudick.