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June 09, 2020   |   Diesel Technology Forum

Policy Insider

Four Keys to the Future for Diesel: HYBRIDS

Expanded use of hybrid technology is just one way that diesel engines will play a continued key role to save fuel, and lower greenhouse gas and other emissions across all applications.

Four key strategies will define the future for diesel technology: getting emissions closer to zero, improving energy efficiency, increasing use of low carbon renewable biofuels and hybridization.

This segment explores the role of hybrid technologies.  

Disruptive trends are altering the future of mobility, work and societies at large. Connected vehicles, automated driving, shared ride services, and an anticipated shift to fully electric power for personal vehicles are all taking shape at some level. 

Where does that leave today’s mainstream technologies – gas and diesel? 

From governmental estimates to international consulting firms, authorities agree that both gasoline and diesel technology will continue to be important for many decades into the future for continued progress on societies greatest challenges – achieving cleaner air and reducing greenhouse gas emissions. Especially in the near term, gasoline and diesel technologies will remain vital as other technologies are yet to reach commercial availability at scale and with still unknown impacts from the global pandemic, continued improvements in gas and diesel will be essential.   

That’s why improving efficiency while reducing emissions from internal combustion engines (ICE) continues to be a primary goal for engine, vehicle and equipment makers. Hybridization is a technology that captures wasted energy, storing and applying it to useful work and may be a greater player in the future. In close integration with the internal combustion engine, electric motors, controllers and energy storage systems work with conventional drivelines and transmissions to propel the vehicle or machine with overall less energy consumed.    

Are hybrid systems a consideration for all vehicles or machines? No. The key considerations for whether or not hybridization makes sense is how a vehicle or machine is used, whether its duty cycle or work patterns are good candidates for capturing wasted energy and applying it to the machine’s useful work, thereby reducing fuel consumption. For example, long haul trucks driving mostly steady-state highway speeds are less-effective applications, while pickup and delivery vehicles with stop and go routes like transit buses or refuse haulers make far more sense. Fuel prices, overall vehicle prices, market conditions and trucking trends also weigh heavily into the offerings of hybrid systems.

There are essentially three types of hybrid systems, parallel, series and mild. Parallel systems are ICE and electric motors working together to deliver power directly to the vehicle drivetrain and are the most common variant. A series hybrid system uses the ICE to generate electricity to power electric motors. These so-called “mild hybrid” technologies are increasingly common and used to provide energy to enable engine start/stop features for vehicles, but cannot propel the vehicle. Full hybrid systems can propel a vehicle or machine for some period of time on stored energy alone before switching to the ICE.

For commercial vehicles, hybrid systems have been most popular and incorporated into public transit buses, where they have been available for well over a decade. Combined with a downsized diesel engine, the stop and go driving cycle of a transit bus is ideal for hybrid technology, while also saving on brake maintenance, a major expense for transit agencies. Public transit agencies in Washington, DC, San Francisco, New York and other major metropolitan areas all have hybrid electric buses in their fleet.

Diesel Hybrid Bus

For trucks, medium size large box trucks hauling heavier loads, but typically shorter driving range than a longer haul tractor trailer, are the application of the most interest. However, consideration of hybrid technology is waning in these segments as greater interest in all electric and fuel cell technologies emerge. Hybridization may still be a strategy for complying with increasingly stringent greenhouse gas emissions for commercial vehicles in some applications. 

Cummins recently introduced its electric hybrid PowerDrive utility truck, a Kenworth T370 that is a versatile hybrid system, offering both parallel and series capabilities, and features the ability to switch in real time between two hybrid and two pure electric modes, optimizing the powertrain for the best fuel economics in any driving situation.

Beyond commercial trucks, off-road equipment makers are working on innovative ways to develop reliable, efficient, cost-effective alternatives to traditional pure diesel-powered drivetrain systems and have product on the market today.

One of the first was Caterpillar’s D7E, a hybrid drive dozer that debuted in 2010. This track-type tractor utilized an electric drive system, whereby the diesel engine operates in a fuel-efficient steady state mode generating electricity that in turn drives the machine propulsion and hydraulic systems, delivering up to 30 percent more fuel efficiency, with additional gains in productivity and material moved per hour.

CAT Hybrid Wheel Loader

Wheel loaders are often used to move and load bulky loose materials like stone and gravel, mulch and other materials. These units have repetitive motion in the cycle, and a range of manufacturers have various hybrid options available, including Caterpillar, CNH, Deere and Volvo.

John Deere was among the first to introduce electric drive technology in off-highway equipment, as it produced the 644K hybrid loader in 2013 and the 944K hybrid loader in 2015. Hybrid-electrics drive smoothly and efficiently recapture energy to slow the loader when the operator lets off the accelerator. This lessens the load on the engine and reduces fuel consumption by as much as 25 percent, along with quieter operation.

Achieving 50 percent reduction in fuel consumption and 35 percent fewer greenhouse gas emissions is a key reason why hybrid technology is making inroads into construction equipment. Volvo CE’s concept LX1 wheel loader, now in test service in California with Waste Management, has exceeded expectations including demonstrating that it is able to do the work of a wheel loader the next size up. 

CNH Industrial – its FPT and Steyr tractor divisions - have developed a concept farm tractor without a conventional transmission where the tractor’s diesel engine serves entirely as a generator operating in the optimum steady state efficiency mode, powering electric motors to drive each of the wheels and power take off points.  Fuel savings for the concept is reported at 10 percent.

Beyond the construction site, hybrid systems are also featured in the marine and rail sectors. 

Marine vessels like workboats, tugs and pushers, require tremendous horsepower to maneuver ocean freighters and tow heavy barges. Caterpillar and boat-builder Sanmar teamed up for a new hydraulic hybrid propulsion system that compared to traditional tugs will dramatically reduce fuel consumption and carbon emissions as well as through life maintenance costs.  Given the propensity for tugboats to work in waters near populated coastlines and in big city ports, the ability of this technology to mitigate climate change in a meaningful and measurable way is both promising and exciting.

Yanmar has a hybrid application in a new propulsion system that outrivals conventional propulsion engines in efficiency and functionality, using the diesel engine to power shipboard generators that supply electrical power to motors that drive propulsion systems and separately power electrical loads of the boat. 

Cummins offers marine diesel electric solutions in a wide range of marine vessel applications, as does MTU and Rolls-Royce Power Systems with its fully integrated marine hybrid diesel electric systems launching in 2020.   

Beyond large engines in marine applications, hybrid drive systems also now extend to rail applications. MTU hybrid power packs allow for diesel or diesel electric train operation enabling flexibility particularly in Europe where some portions of passenger rail are electrified, but many still rely on diesel power. These settings, where a combination of diesel and electric power are available to train operators, position both hybrid and conventional diesel-only settings to be utilized for maximum efficient train operation. 

Hybrid Diesel Trains

As is evidenced here, the diesel engine is perfectly suited for hybridization in a growing number of applications, both on the road, in the water, on the rails, in the fields and on the jobsites of the future. The expanded use of hybrid technology is just one way that diesel engines will play a continued key role to save fuel, and lower greenhouse gas and other emissions across all applications.  


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Allen Schaeffer
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