The Clean Air Nonroad Diesel Rule finalized in 2004 introduced Tier 4 emissions standards for diesel engines. Engines with 175 to 750 horsepower will enter into the Tier 4 interim phase in 2011, with the final to be concluded in 2014.
Tier 4 Interim Design
The Tier 4 interim ushers in the need for aftertreatment technology for many engine models.
To meet the requirement for 175 horsepower to 750 horsepower engines, engine manufacturers have selected between two types of Tier 4 interim design paths.
One type of Tier 4 interim design uses cooled exhaust gas recirculation (EGR). The engine re-circulates cooled exhaust gas from the exhaust manifold back into the intake manifold, which results in lower combustion temperatures and reduced NOx.
The system utilizes a diesel particulate filter (DPF) to capture particulates that occur due to lower combustion temperatures. A diesel oxidation catalyst in the DPF causes a chemical reaction with the exhaust gas, creating increased temperatures to clean the particulate filter.
The EGR/DPF system generates sufficient heat to eliminate the particulate automatically (passive regeneration) the majority of the time. When passive regeneration does not keep up with the soot buildup in the particulate filter, the system automatically switches to active regeneration to increase the heat levels to clean the filter. Engines operating at light duty cycles may have more active regenerations to keep the particulate filter clean.
A second design uses selective catalytic reduction (SCR) exhaust aftertreatment. The method uses aqueous ammonia that is injected into the exhaust stream in the form of diesel exhaust fluid (DEF). NOx is then converted by a catalyst into diatomic nitrogen, CO2 and water vapor.
EGR/DPF vs. SCR
The two types of designs present benefits and challenges.
Manufacturers utilizing EGR/DPF, such as Cummins Inc., claim one of several benefits of the design is the technology has been proven in on-road vehicles since 2007.
The design does require periodic maintenance of the DPF. Cummins has set the maintenance schedule at about every 5,000 hours of operation. The engine also requires an ultra-low-sulfur diesel, which is not readily available in all parts of the world.
Training may also be required to acquaint the operator with the aftertreatment operation. “The training will give the operator an understanding of what the dashboard lamps are telling him,” says Robert Tonkin, industrial sales manager at Cummins Inc. “There will be a dash lamp that comes on to let the user know [active regeneration] is going on. It doesn’t affect the operation; the operator can still use the equipment like they always do.
“There may also be an inhibit switch on the dash that allows the operator to turn off the active regeneration for operations reasons, or if the dash lamp indicates a stationary regeneration is required, the switch can be used to initiate the stationary regeneration. It is important that the inhibit switch is not left in the defeat mode as it can cause excessive soot buildup in the particulate filter,” he says.
Engine manufacturers utilizing SCR technology, such as MTU Detroit Diesel Inc., claim the benefits include a smaller overall engine footprint in the cabin. “With SCR, the engine footprint is essentially the same,” says Dee Wise, application engineer at MTU Detroit Diesel. “No additional equipment, really, to speak of anyway on the engine itself. We have an additional fluid tank that needs to be mounted somewhere and a pump that goes with that. There is also a catalyst which must be used. It is roughly the same size as the average equivalent DPF.”
An SCR engine does require the use of DEF. MTU Detroit Diesel puts DEF usage at a 4-5% proportion to the amount of diesel fuel consumed.