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The MTU Hybrid PowerPack for railcars combines a Stage V diesel engine with an electric machine used as a motor or generator, and an MTU battery system that stores energy recovered during braking. (MTU)

All options on the table

Powertrain developers are leaving no stone unturned in their quest to trim emissions in commercial vehicles and off-highway equipment.

The push to trim engine emissions is prompting industry-wide efforts to use substitutes for diesel fuel. Alternative fuels and electrified powertrains are making significant inroads, though design teams for diesel power aren’t conceding the future. A number of paths for fueling the future were recently examined by a range of panelists at SAE COMVEC Technology Connection in Indianapolis. Alternative fuels including gasoline and natural gas will provide short-term solutions while electrified powertrains make headway in more applications.

Natural gas is gaining adherents who like both its cleanliness and low cost. The same factors are driving research in gasoline as an alternative fuel for diesel engines. Electrification is becoming more attractive in some commercial-vehicle environments, and its role is expected to increase as both emissions and noise regulations put restrictions on fossil fuels.

Even so, diesel fuel proponents aren’t backing down from the competition. Instead, they’re racing to commercialize technological advances that trim emissions. Most panelists in multiple technical sessions predicted that diesel’s dominant role in many off-highway applications and large on-highway trucks will continue for at least a few decades.

“In off-highway, internal combustion will remain the dominant force well beyond 2050, with fields like material handling and closed environments being notable exceptions,” said Shelley Knust, executive director of off-highway engineering at Cummins.

Panelists noted that diesel has made significant strides in NOx and particulate reductions over the past several years. Design teams involved in fuel and powertrain development are continuing that trend.

“Diesel still remains one of the most energy-efficient fuels known to man,” said Scott Woodruff, director of global mining and oil & gas businesses at MTU America. “The cleanliness of fuels is evolving as the move to low-sulfur fuels increases. As low-sulfur fuels become more prevalent around the world, engineers can develop cleaner engines.”

Low-sulfur fuel is far from the only technique for trimming diesel emissions. Aftertreatment technologies are improving, and engines are evolving to reduce fuel consumption. In on-highway medium- and heavy-duty vehicles, techniques like stop-start and cylinder deactivation are starting to move towards the mainstream.

“Cylinder deactivation is working out really well for us,” said Charles Roberts, a director at Southwest Research Institute (SwRI). “Fuel economy is improved, and engines like to run in that mode.”

Alternative fuels
COMVEC panelists described a number of alternative fuels that are vying to chip away at diesel’s dominant role in commercial vehicles. Reducing emissions without making major changes to engines is a central goal of many programs.

A panelist from Aramco detailed research into using gasoline in diesel engines. Results have shown that using lower-priced gasoline doesn’t cause extra wear to most engine components, but exhaust gases are much cleaner.

“When you inject gasoline, you get vanishingly low emissions with reductions in NOx,” said Michael Traver, chief engineer for commercial transport research at the Aramco Research Center near Detroit. “We used multi-shot injections on a diesel engine. We ran tests for 800 hours and found that the lubricity effect was okay. We saw some cavitation in the injector, but we feel we can redesign the injector and minimize the problem.”

Natural gas is a more mainstream approach, having already seen significant acceptance. It offers both lower pricing and cleaner performance compared to diesel, but the engine’s purchase price is still an issue.

“For a 6.7-L product, the benefit of natural gas is that it’s the lowest NOx engine with 50-90% lower emissions compared to diesel,” said Yemane Gessesse, director of engineering at Cummins Westport Inc. “The initial pricing of LNG [liquefied natural gas] versions tends to be more expensive than the diesel counterparts. However, long-term total cost of ownership favors natural gas engines because of the price differential between natural gas and diesel.”

Charging towards electrification
While developers explore different ways to improve ICE systems, they’re also focusing on ways to add electric power to vehicles. Nearly every speaker during a multi-session Fueling the Future track addressed the benefits and challenges of joining the electrification bandwagon.

Electrification based on batteries, and possibly on hydrogen fuel cells, is a part of every company’s powertrain technology programs. Though long-term goals call for eliminating the internal-combustion engine (ICE), hybrids are being examined for many near-term commercial-vehicle programs. Much of the foundational work has been completed but integrating everything efficiently remains a major concern.

“Optimal benefit comes from a fully integrated approach,” said Michael Weinert, VP of engineering and manufacturing at John Deere Power Systems. “There are quite a few challenges in off-highway for the electrification. It requires tight integration of the diesel engine, transmission, inverters and batteries. There are many different types of applications, so you need a scalable approach so you can reuse technologies.”

Panelists noted that some of the challenges are similar for both electrified and ICE powertrains. The diversity of commercial-vehicle applications makes the market more difficult than passenger cars. There’s no one-size-fits-all strategy, so engineers must develop scalable architectures that can be adapted for different vehicle sizes and operating environments.

“The systems approach will be dependent on the application,” Knust said. “A high power consumption, continuously operating powertrain will look very different than a powertrain on something that’s intermittent and some capable of regeneration. The infrastructure will also be a key factor. A closed environment like an airport or a port where there’s available power and regeneration is viable will be very different than agriculture or other areas.”

Though most of the regulators behind the push to electric power view it as emissions free, engineers note that clean diesel can be more environmentally friendly than electric power. Coal-powered generators can end up creating more air pollution than diesel engines. In some regions like China, electrical generators do not match the cleanliness of diesels.

“When you focus on electrification, you have to look at well to wheels,” said Carl Hergart, director of powertrain and advanced engineering at PACCAR Inc. “In China, when you analyze electronic production, it turns out the cleanest solution is diesel. You also have to look at the time needed to refuel. It takes about 10 minutes to fill a diesel tank; an electric vehicle takes at least an hour per charge.”

Focus on fuel cells
Electrification is not the only alternative power source that’s viewed as a panacea. Hydrogen fuel cells are also being studied as a clean air solution. They provide many benefits over rechargeable battery packs used in today’s electrified powertrains.

“Hydrogen can provide a simplified architecture,” said Dennis Backofen, team manager for fuel cell model-based development at IAV GmbH. “It also saves weight. For a long-haul vehicle, a fuel cell’s weight is 1,200 kg for the entire hydrogen powertrain (900-kg tank, 200-kg fuel cell system and 100-kg battery), compared to a battery pack that weighs 3,700 kg.”

Fuel cells aren’t going to show up in commercial-vehicle production runs in the near future, but many COMVEC speakers said they are watching the technology closely. Hydrogen is already being tested in rugged environments where their clean operation is matched by efficiency gains compared to diesels.

“Ships are using fuel cells. They can integrate fuel cells on decks to reduce the amount of wiring on the ship,” said Iain Ridley, director of business development at AVL Powertrain Engineering. “Companies can put fuel cells around the ship instead of setting one diesel engine deep in the bottom of the ship. That reduces the amount of wiring significantly.”

The technology is advancing fairly quickly, speakers said, but adoption in powertrains will be driven by the availability of hydrogen. In applications like trains, where refueling stations are easier to implement than those used by trucks and off-highway equipment, real production usage may come earlier for commercial vehicles.

“If fuel cells continue to become more reliable, engineers can think about replacing engines with a fuel cell,” said MTU’s Woodruff. “But we still need to deal with hydrogen production. Still, it’s not too far away. Companies are using fuel cells in rail trials today.”

Panelists also noted that government support may play a role. The technology may see acceptance in Asia before it appears in the U.S. “The Japanese are looking at fuel cells as a near-term solution, not a long-term solution,” said SwRI’s Roberts. “They have the will to do it. In the U.S., fuel cells won’t emerge as quickly.”

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