Although there is a lot of focus these days on alternative power systems such as batteries and fuel cells, internal combustion engines will continue to play an important role in heavy-duty applications. As such, it will be necessary for manufacturers to keep evolving their designs to optimize performance and reduce emissions.
“Almost every region of the world has air pollution regulations in place or is developing them,” says Gregory Pelton, senior manager – Sales and Application Engineering Construction, Industrial and Agriculture at MTU America Inc. “These ever more stringent emissions regulations require the deployment of new or optimized technology.”
MTU, like most engine manufacturers, has developed a platform of dual certified engines that meet EU Stage V and U.S. Tier 4 Final emissions regulations while also offering a solution for lesser regulated areas. Stage V is the most stringent regulation to date for the off-highway equipment industry. Developing engines that meet it as well as other regional regulations eases installation and design costs for OEMs by giving them the flexibility of using the same engine for various emissions regulations.
John Deere's 13.6L engine features a next-generation ECU utilizing advanced model-based controls which enhance reliability and transient control.John Deere Power Systems
Nick Block, director, Worldwide Marketing & Sales, John Deere Power Systems (JDPS), says leveraging the company’s experience to refine the technologies it has developed over the years aids engine optimization. Areas on which JDPS is focusing its efforts include improvements in package size, reducing overall engine weight and integrating new generations of electronic control systems, which offer greater capabilities.
“In the future, we anticipate there will be a completely new generation of engine electronic control systems that will be more capable,” Block says. “One trend we know will continue to evolve in the coming years is less reliance on physical sensors as engine control units continue to become more advanced and enable greater engine control and diagnostics performance.”
JDPS used a next-generation engine control unit (ECU) in its 13.6L engine. The new ECU utilizes advanced model-based controls that enhance reliability and transient control.
Technology Improves Performance
Efficiency is a key area in which many manufacturers focus their design efforts. For JDPS, this includes looking at ways to improve fuel efficiency. To do so, it is addressing combustion and air handling efficiency, as well as managing friction losses by leveraging advanced modeling techniques.
For its 13.6L engine, JDPS used a clean-sheet approach. Designing the engine from the ground up better enabled the company to address many of the areas it wanted to improve. “One of the objectives of that project was to manage in-cylinder efficiencies,” Block notes. “We designed this engine for increased firing pressures to accommodate fast fuel burn while minimizing heat losses, leading to increased performance.”
"We recognize that if we’re going to make our transportation more sustainable, we need to make engines more efficient," says Laurence Fromm, executive vice president of business development, Achates Power.
An illustration of Achates Power's opposed-piston engine design.Achates Power Inc.
Fromm explains that in a conventional engine, there is a piston which goes back and forth in a cylinder. But in an opposed-piston engine, every cylinder has two pistons that come together. The piston crowns are what forms the combustion chamber. “There's a lot of benefits to that architecture,” he says.
As there is no longer a cylinder head, efficiency can be improved because heat losses typically associated with cylinder heads are eliminated. In conventional engines, cylinder heads get very hot and aggressive cooling is needed, and all the heat is wasted energy. “We can take more of the energy in the fuel and turn it into useful work,” says Fromm. Studies have shown an opposed-piston engine can be 13% to 15% more efficient due to the reduced heat loss and other benefits.
Achates Power is developing an advanced combat engine for the U.S. Army which doubles the power pack density of other engines.Achates Power Inc.
Fromm says the engine it is developing with Cummins Inc. doubles the power pack density of other engines. “We have a smaller engine, a smaller cooling system, and because we’re more efficient, we have a smaller fuel system,” he explains. “That allows us to deliver twice as much power per unit volume.”
This is important for a combat vehicle because it can move and accelerate faster and more easily climb hills, all of which makes it easier to engage and evade enemies.
By not having a cylinder head, heat losses are minimal; heat rejection is about 30% less than a conventional engine. This means the cooling system size can be reduced about 30%, as well, which helps reduce space claim for the engine system. The engine itself is also smaller while providing more power density due to it being a two-stroke engine.
“We get a power stroke in every cylinder on every revolution of the engine, which is inherently more power dense than a conventional engine," says Fromm.
Expanding Fuel Options
Libertine FPE is currently developing opposed free-piston engine technology, which replaces the crankshafts with software control. Doing so would allow every combustion event to run optimally using real-time compression ratio control. This will enable engines to be even cleaner and more efficient, says Sam Cockerill, CEO of Libertine FPE.
It will also enable a fossil-free future. In particular, it will allow the use of renewable fuels like ethanol and methanol. Making fuels that act like petrol and diesel is expensive and energy inefficient. “Making simple molecules like ethanol and methanol, it's easy to do, it's cost effective, it's more energy efficient and it has lower greenhouse gas impact,” Cockerill asserts.
Shown is a concept for Libertine's end product, which brings together the electric components as well as control hardware and software into a platform engine developers can use to command compression ratios cycle by cycle.Libertine FPE
He notes this is a challenge engineers have been trying to overcome for several years with various designs, but generating electrical power while maintaining piston motion control has proved troublesome. Advances in power electronics, embedded controls and magnetic materials in recent years has provided new opportunities.
Enabling greater control over compression ratios provides more flexibility in the types of fuels engine manufacturers can use.Libertine FPE
This means it could be possible to bring alternatively fueled engines to market faster, helping manufacturers and end use customers meet emissions reduction targets sooner. “If you replace the crankshaft with linear electrical machines, it's a fuel flexible engine,” says Cockerill. While there are a few aspects that still need to be considered such as material compatibility in the fuel lines and injector choice, in general, having control over the compression ratio enables fuel flexibility.
This in turn means that an engine can be designed to run on future fossil-free fuels such as ethanol and methanol while being backward compatible with fuels that are in use today. Cockerill says this flexibility solves the "chicken and egg" problem that new engines designed exclusively for new fuels face.
There is a lot of interest in Libertine's technology for heavy-duty on- and off-highway engine applications. This is because battery-electric solutions alone will not be sufficient to meet emissions reduction goals due to the size, weight and power density needed, as well as lack of charging infrastructure in many cases. As such, internal combustion engines and the ability to use alternative fuels will be necessary in these applications.
“Renewable fuels and internal combustion engines, those work today,” Cockerill says. “I think they will be a big part of the future.”
Many acknowledge that electrifying heavy-duty applications will be difficult, necessitating a mix of options to decarbonize the industry.
Meeting Current and Future Emissions Regulations
Reducing emissions will continue to be an area of focus for manufacturers as they and their customers look to minimize their environmental impact in the coming years. Governments around the world are also implementing stringent carbon reduction goals that will necessitate OEMs and engine manufacturers find ways to reduce emissions further.
The latest emissions regulations have mainly focused on the heavy-duty on-highway market, such as the Phase 2 Greenhouse Gas (GHG) standard for medium- and heavy-duty vehicles through model year 2027 in the U.S. However, many in the industry anticipate similar regulations will make it into the off-highway market at some point in the future.
“The on-highway emissions regulations have typically paved the way for subsequent off-road standards,” says Pelton. “The Phase 2 GHG standards are a great example of what most in the industry expect to see in off-road equipment, as well. It will become whole vehicle optimization with the diesel engine itself playing only a part in the total solution. This will require even stronger partnerships with our OEM customers as we will have to work together even more closely to produce products demanded by both the users and whatever regulations are in place.”
Shown is an Achates Power's 10.6L heavy-duty diesel engine in a Peterbilt 579 truck, which will be driven in fleet service in California later in 2021.Achates Power Inc.
To reduce NOx, it is necessary to quickly get the selective catalyst reduction (SCR) system as hot as possible to ensure only harmless nitrogen and oxygen come out of the vehicle’s tailpipe. An opposed-piston engine is able to achieve this as it has very high enthalpy (thermodynamic quantity equivalent to the total heat content of a system) to heat up the catalyst while achieving low engine out NOx.
“The reason we can do this is because in the opposed-piston engine, we have intake ports on one side of the engine and exhaust ports on the other side. It's the piston motion that opens and closes the ports. And that port timing allows us to scavenge the cylinder in one direction. So, the exhaust ports open first, the exhaust goes out, the intake ports open, we put new fresh air in, the ports close and we compress and ignite,” explains Fromm.
“We’re able to dynamically change how much exhaust gas we leave in the cylinder,” he continues. “When we want to generate a lot of enthalpy, we go into what we call a catalyst light off mode where we reduce the flow through the engine.”
Maximizing the amount of exhaust gas that stays in the cylinder enables it to get hotter as it sits in the cylinder; the hotter it is, the better able it is to quickly light the catalyst. Because this exhaust gas has already been combusted, it has the effect of internal exhaust gas recirculation (EGR), which is commonly used for reducing NOx. Whereas a conventional engine will have to pump exhaust gas in and out, the opposed-piston engine is able to reduce this extra work through its dynamic control of the gas and leave it in the cylinder.
“We have engines on our dynamometer now that we've measured the ability to meet the 2027 California requirement for a 90% reduction using conventional aftertreatment systems available today,” Fromm points out.
“Conventional engines have been improved a lot over the last 30 or 40 years,” he continues. “But every time we try to make them less harmful in terms of CO2 or criteria emissions, it’s getting harder because we’ve already wrung so much out of it. We’ve reached the point of diminishing returns; it’s getting more expensive to make it better.”
With the opposed-piston engine, however, he says there is still room to continue making improvements. “The opposed-piston engine has built-in advantages that allow us to go beyond what’s going to be required in 2027 to reduce both CO2 and NOx even more.”
The Integrated Emissions Control System incorporates the components right into the system rather than utilizing a separate mixing pipe, enabling it to be more streamlined and compact.John Deere Power Systems
To meet these and any other new emissions regulations, it will be important to focus engineering efforts on combustion technology. “We need to continue to consider engine design elements such as power cylinder, cylinder heads, air systems and fuel injection parameters," Block comments. "It’s not one single thing, but rather a variety of refinements for system optimization.”
Managing air flow through the engine will be important, as well, especially if the on-highway NOx reduction standards go into effect for off-highway applications. Improving the efficiency of aftertreatment systems will also be important.
JDPS' Integrated Emissions Control System (IECS) is an example of the improvements that can be made to an aftertreatment system. According to Block, it incorporates the components right into the system rather than utilizing a separate mixing pipe. Improvements in the catalyst and substrate technologies allow for smaller sizes while also reducing the amount of metal previously used. “These combined improvements allow for an improved, streamlined aftertreatment design that is more compact, flexible and lower in cost," he adds.
Virtual analysis will play a key role in continued development efforts. Block says it allows JDPS to define and optimize engine subsystems to achieve performance enhancements before creating any hardware or building the combustion system. “We’re continuing to explore new technologies that enable advanced combustion concepts for continued improvement of the diesel engine,” he concludes.
