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.
So how are some of the OEMs coming along in introducing the Tier 4 interim technology into their equipment?
JBT AeroTech has been planning for a Tier 4 interim engine since May 2008, working with an engine manufacturer for a 260-horsepower engine for its Air First version of Tempest deicer. The engine uses a cooled EGR/DPF technology.
According to Ed Sachs, deicer engineering manager at JBT AeroTech, the engine is currently being outfitted with specific modifications, with delivery to the company’s facility occurring in July or August. The engine will be installed with validation and testing in the September timeframe. The company plans to demo the equipment around October.
Another GSE manufacturer, TLD, has been working with the Tier 4 interim technology for more than a year. The company has been working on Tier 4 interim implementation in both the U.S. and Europe, according to Peter Owitz, chief operating officer at TLD ACE.
Speaking specifically about the company’s GPUs, air conditioners and jet starters, Peter Owitz says the company has worked on designing equipment to fit both SCR and EGR/DPF technology for more than a year. The company is starting to test units with Tier 4 interim engines, he says.
Hobart Ground Power has also been working on the Tier 4 interim design for its products within the applicable horsepower range for 2011. Beginning the process in 2007, the company now has one engine with EGR/DPF aftertreatment currently in testing, according to Nat Phillips, engineering manager at Hobart.
Challenges for OEMs
The manufacturers have had to design the equipment to accommodate the larger engine and aftertreatment packages in the cabins. “For the engine manufacturers that are selecting DPF technology, total heat rejection will increase by between 30 and 50 percent,” Owitz says. “This will cause radiators to increase in size and, in some cases, unit volume to grow.”
He continues, “And for the engine suppliers that are choosing to use SCR technology, the unit will require additional space for another tank and the infrastructure to move the DEF around the unit.”
JBT AeroTech pointed to the availability of ultra-low-sulfur diesel fuel as another challenge for OEMs. “If you’re selling to a global community such as we do, we have to be very careful who our end-user is and be certain of the type of fuels that they’re using,” Sachs says. “The tier 4 engines utilize an ultra-low-sulfur diesel fuel. There are a lot of countries around the world that do not have that fuel as an offering. So it makes it a challenge for us as a manufacturer to make sure that we really understand who our customer is and where that unit is going to be placed.”
Impact on the Ramp
The manufacturers spoken with say they don’t expect much difference in ramp operations with the engines besides occasional maintenance practices.
“If the operator is using a piece of equipment that has an engine with an SCR in it, the only difference is that he will have to supply DEF into that unit,” Owitz of TLD says. “If it is a piece of equipment with a DPF, the operator will have no interaction with the engine aftertreatment due to the automatic regeneration cycle. If it gets interrupted for some reason or it is not allowed to run, there might be some operator action required.”
On the issue of low-duty cycle regeneration for cooled EGR/DPF on the ramp and whether or not the operation could facilitate enough heat for automatic (passive) regeneration, JBT AeroTech says the design of its model would promote it. “In our current products, it is the sole engine for the entire deicer so it’s running over a more varied engine RPM and we’re actually holding it at the higher loads much more,” Sachs says.
TLD also says the design of its products would adequately facilitate passive regeneration on the ramp.
And in its testing so far, Hobart has recorded sufficient passive regeneration. “The heat rejection of the engines has significantly increased and it appears that the ‘Passive Regeneration Mode’ is the primary operating mode for the DPF system,” Phillips says.
Heading into testing, JBT AeroTech and TLD say they don’t expect an improvement in fuel economy for either technology.
Hobart, on the other hand, has witnessed promising results in testing for fuel economy with the EGR/DPF technology, according to Phillips.
As for the cost increase over Tier 3, manufacturers say it can vary.
Tier 4 Final
With Tier 4 final approaching for 175 to 750 horsepower engines in 2014, several engine manufacturers have said they will combine the SCR and EGR/DPF technologies to meet the standard.
And the OEMs are paying attention, saying they have taken the Tier 4 final design into account to accommodate those engines when the time comes.