The efforts of engine manufacturers to design new powerplants that meet tightened EPA standards in 2007 have been complemented by the efforts of coach manufacturers to integrate those engine packages into their new models.

EPA regulations now require that new diesel engines run cleaner, with a more than 50% reduction in nitrogen oxide and 90% reduction in soot and ash. They must also run on ultra-low sulfur diesel fuel, which has no more than 15 parts per million (ppm) of sulfur versus the previous standard of 500 ppm. To accomplish this reduction in emissions, engine manufacturers had to make some key changes to their products’ exhaust gas recirculation systems, cooling capacity and crankcase design.

In turn, this forced coach builders to come up with strategies to integrate these new engines, as well as downstream components, such as diesel particulate filters (DPF) and dashboard DPF status gauges, into their product design. The end result is that coach manufacturers such as Prevost, MCI and Setra are offering models that have upgraded features to accommodate the engine packages and, in some cases, offer improvements over previous models. At the same time, engine manufacturers are touting possible improvements in the performance of their 2007 engines. Here are a few examples of the changes incorporated by these heavy-duty coach builders.

Slight reconfiguration needed
To accommodate the new 2007 engine, engineers at Prevost had to reconfigure the interior of the coaches in a way that maintained seating capacity. In the Prevost H-Series, this was accomplished by replacing the conventional door on the restroom with a sliding door. Prevost officials say the sliding door provides greater access to the lavatory.

But the engines are only part of the equation in terms of reducing emissions to meet 2007 standards. The other key component is the DPF, which helps to curtail the emission of particulate matter. The integration of the DPF into coach design, which replaces muffler componentry, was one of the key challenges facing manufacturers.

At Prevost, engineers decided to install the DPF vertically. According to company officials, this orientation has the benefit of reducing heat generated in the engine compartment and eliminating the need for special heat precautions for mechanics during engine maintenance. They said it also makes the engine more accessible and provides easier access to major components, such as fans and radiators.

With less accumulation of dirt and grime, the DPF and its sensors are better protected and the lifecycle of the device is extended, Prevost officials added. This is not an insignificant detail because the DPFs are expensive to replace.

Prevost engineers also developed a patent-pending diffuser to replace the exhaust pipe. It features a special flush design to prevent water infiltration and to cool the exhaust more effectively. At a distance of six inches from the diffuser, gas temperatures are 50% lower compared with a traditional exhaust pipe.

“It was a real challenge to integrate the new EPA-compliant engine into our 2007 coaches while maintaining serviceability, practicality, safety and performance,” said Claude Picard, Prevost’s vice president of marketing. “Our engineers have really exceeded our expectations with this new engine installation.”

Staying low has benefits
At MCI, engineers took a different approach, placing the DPF under the vehicle. They said this strategy has several key advantages, the welfare of technicians chief among them.

According to MCI officials, the DPF weighs nearly 150 pounds. Placing it low helps technicians reach the DPF without a ladder or man lift. The placement also ensures that passenger and baggage room isn’t affected. Plus, a skid plate helps protect the DPF against bouncing stones or other road debris. Secure installation helps guard against theft of the DPF, which uses semi-precious metals.

MCI engineers also took the opportunity to fine-tune a few other coach systems. “We’ve used this EPA-mandated change to improve our coaches in some significant ways,” says Bryan Couch, vice president of product planning. “For 2007, cooling systems will offer more capacity and the coaches will run more quietly.” To accommodate the DPF, onboard diagnostics have been refined to alert drivers to DPF status.

MCI also has improved the serviceability of the top-selling J4500 model and flagship E4500 by introducing a flip-down radiator and adding an engine compartment access panel. The D-series rear cap has been redesigned as well, giving an overall sleeker appearance to MCI’s workhorse model.

MCI offers a wide selection of powertrain options. Operators can choose from Caterpillar C-13, Detroit Diesel S-60 and Cummins ISM engines paired with Generation IV Allison or ZF ASTronic 12-speed transmissions.

Some changes are minimal
At Setra, the integration of the 2007 engines and DPF did not have much impact on the design of the S 417, according to Tom Chezem, vice president of sales. “We don’t have any physical changes,” he said. “The coach structure remained the same, and the DPF simply replaced our muffler and is stored in the same compartment.”

The DPF is mounted vertically in a compartment directly behind the driver’s side rear wheels, which provides secure but easy access behind a flip-up panel.

Setra is offering the 2007-compliant Detroit Diesel S-60 engine in its 2008 model, featuring an incremental increase in torque and horsepower. The engine system will feature a new high-flow water pump to ensure proper flow to the new EGR cooler and engine cooling system.

The S 417 will also offer the ZF independent tag axle as standard equipment in 2007 and the ZF ASTronic 12-speed transmission as optional equipment, along with the Generation IV Allison B500R transmission. Both transmissions come standard with an output retarder.

With the installation of the compliant engine package, changes will be evident to the operator of the S 417. The dashboard has been modified to add a set of status indicators and warning lamps for the engine and DPF systems.

These dashboard status indicators are important because the engine and DPF can require immediate attention if the filter gets clogged and pressure begins to build up in the engine. The indicators also tell the operator if passive or active regeneration of the DPF (a chemical or thermal event that burns soot collected in the filter) is not taking place, requiring manual regeneration or some other type of service.

Engine makers eye new emissions challenges

By Paul Hartley

With each round of diesel-emission reductions, the challenges for engine makers such as Caterpillar, Cummins, Detroit Diesel and International Truck and Engine Corp. multiply. The EPA’s most recent regulatory hurdle, which took effect Jan. 1, mandated the addition of several new components and more sophisticated electronic controls to monitor vehicular operating conditions and make adjustments as needed.

EPA ’07 was actually sort of a mixed blessing for engine companies. Although it required deep cuts in nitrogen oxides and particulate matter, it also eliminated the parallel standards that had differentiated urban buses from all other on-highway heavy diesel vehicles.

“The latest change reduced some of our development work,” says Roe East, general manager of bus business for Cummins. “It also decreased the paperwork necessary for filing certifications with the EPA and California’s Air Resources Board.”

Administrative tasks aside, the new rules kept engineering teams busy for the past five years, designing and testing systems that would lower emissions without hampering performance and reliability. That work resulted in the following changes to on-highway diesel engines:

Probably the single most expensive, and most discussed, piece of new hardware is the diesel particulate filter. Although DPFs have been used on some urban buses for a while, the ’07-compliant versions are said to be “active aftertreatment devices” because they’re capable of on-demand cleaning, or regeneration, when internal soot levels reach unacceptable limits. In such instances, a dash-mounted warning light signals the need for manual cleaning, which is initiated with a dash-mounted switch or button.

Manufacturers using cooled exhaust gas recirculation (EGR), first widely deployed in 2002, have “optimized” (i.e., increased) the amount returned to the intake manifold — this to further quell NOx formation during combustion. Cat’s ACERT engines now come with a variation of this technology, too, but they draw spent gases from the backside of the particulate filter instead of the discharge end of the turbo. Added EGR flow, of course, results in greater heat rejection and higher engine compartment temperatures. To counter this problem, ’07 engines are designed with more robust cooling systems. The improvements, ranging from larger radiators and fans to higher capacity water pumps and oil coolers, vary by model and manufacturer. Still, this is one area that will likely receive even more attention soon, as the full effects of summer driving become apparent. More than their predecessors, the new engines rely heavily on fully integrated electronics to precisely control combustion, facilitate communications between various systems and monitor the performance of components. This is reportedly just the beginning of what will become the EPA’s next focus, after the 2010 mandates have been met.