Cummins provided extensive details of the design and engineering involved in developing the new HELM version of its X15 diesel engine. The company says its new engine will offer up to a 7% improvement in fuel economy compared to the current EPA 2024-certified X15 while also meeting all 2027 emissions targets. SAE Media was invited to tour the company’s headquarters in Columbus, Indiana, where journalists were given a comprehensive update on the hardware powering the latest X15. 

The current crop 
Cummins’ present stable of EPA24-compliant X15s carried over largely the same specs as the EPA21 variants. The 2024 X15 comes in three flavors depending on the application. 

The Efficiency Series leads off the range with an output of 400-500 hp (298-373 kW) and a torque range of 1,450-1,850 lb-ft (1,965-2,508 Nm). The Performance Series is the middle child with a power rating of 525-565 hp (391-421 kW) and 1,650-2,050 lb-ft (2,237-2,779 Nm) of torque. Topping the range is the Productivity Series, which ranges from 430-605 hp (320-451 kW) and 1,650-2,050 lb-ft (2,237-2,779 Nm). 

All three variants feature a variable geometry turbocharger (VGT) that reportedly offers higher boost pressures at lower engine speeds. Cummins states that the 7th generation VGT provides 5% higher turbocharger efficiency through new wheel options and a multi-clearance shroud. The current-generation VGT also features a patented sliding nozzle ring. 

Evolution of engineering 
Cummins representatives gave a comprehensive presentation on many of the design details of the new EPA27 X15 including updates to the cooling system, fuel system, piston design and serviceability changes. “The X15 first went into production in 1998 and now has been revised a total of nine times to match the different emissions regulations,” said Tom Marsh, program leader at Cummins. 

“If you look all the way back to when we initially developed the X15 and imagine the technology level in terms of computing power, it was very much hand calculations and, in some cases, still drawing boards. We really didn’t have the capability we have now in terms of computational fluid dynamics and all of our finite-element process tools,” Marsh explained. 

“What you’ll see is that we’ve carried over a lot of the fundamental architecture that we had in previous generation X15s,” Marsh said. “We’ve taken all of the existing technology and developed it with the new global HELM platform that allows us to engineer smaller, lighter and more efficient engines.” 

The block is one area where Cummins leveraged the latest design tools and capabilities. “If you compare the old block to the latest one, it’s about three inches shorter than the existing engine,” Marsh said. “We’ve also done a lot of work on the structural analysis and how we make the block stiff but light.” 

Marsh continued, “If you look at the design, we have all sorts of patented technology to keep liners straight in the bores and develop the walls and structure of the block to integrate the gear housing, which makes the whole system structurally stiffer. What that’s allowed us to do is reduce the block weight by roughly 50 pounds. However, the overall system weight is the same due to an increase in weight for the aftertreatment system.” An aftertreatment heater was one of the major updates for the EPA27 X15 engines (see sidebar). 

Other significant design changes for the HELM X15 include a new DOHC cylinder head that enables valvetrain technology such as variable valve timing, the addition of crankcase ventilation to meet regulatory requirements, a thermal recirculation valve in the fuel system to aid in cold starts, and a belt-driven 48-volt alternator. Maximum output for the HELM X15 is stated at 605 hp and 2,050 lb-ft. 

Cool under pressure 
Cummins states that much of the gain in efficiency for the EPA27 X15 came from a thorough revision of the cooling system. “From a structural design perspective, we completely redesigned the way we routed the coolant through the engine,” Marsh said. “One of the big downsides of the original X15 is that it was designed without an EGR system. So, when we added EGR, there were compromises made in the cooling system because the block wasn’t originally designed for it.” 

“As a result, we carried more pressure drop in the cooling system than would be ideal,” he explained. “The new X15 has been designed with all these future technologies in mind such as cooled alternators. We really worked on making sure the system was optimized in terms of pumping coolant in a way that minimized pressure drops because that’s just lost energy. The system also features dual thermostats with different opening temperatures. This mitigates pressure drops as each thermostat opens for their individual temperatures.” 

Marsh stated that the new X15’s cooling system has only 65 kPa of pressure drop based on Cummins’ in-house tests. “If you look at the new tools that enabled this head design, you can see how much progress has been made to enable us to design for heat loss, actual flow interface and cavitation,” he said. “When you look at a totally optimized system, we’ve been able to drop the pressure loss to a third of what it was in the old engine.” 

Marsh continued, “When you have areas of stagnant flow, you tend to get cavitation and you’re not taking full advantage of that flow to remove heat. So now the flow is focused around the top portion of the cylinder where we can really take a lot of the combustion heat out of the process.” 

Marsh also explained some of the serviceability aspects of the new system. “One cool aspect about the modular water pump and loop pump is that the housing, bearings, seals and impeller are completely serviceable. You can just pull that entire assembly out of the engine and service those individual components.” 

A fraction of friction  
According to Marsh, the other secret to the efficiency gains of the new X15 came from major revisions to the piston geometry. “We’ve developed pistons over time through new technologies and welding techniques,” he said. “The piston is actually a multipiece design that is a complete welded assembly.” 

Cummins engineers made the bowl smaller to improve combustion efficiency in addition to other revisions. “The real difference is in the height of the piston. We moved the centerline of the pin closer to the firing deck, which reduces friction in the power cylinder,” Marsh explained. “The closer we can get that pin towards the foreign deck the lower the friction and side loading is going to be. That’s why the piston is so much shorter with less skirt. It’s very much an evolution, but in a much more compact design.” 

The cylinder liners also underwent major changes for the EPA27 engine. “We changed and improved the flow velocity thanks to the coolant system design changes,” said Jared Holcomb, on-highway product manager, Cummins. “We moved to a press fit block to liner interface which adds more support to the liner and should improve our bore distortion, which then equates to oil consumption seal and combustion seal improvements.” 

Marsh explained the importance of reducing friction losses in an engine of this size. “As you look at the design of the engine, what you’re trying to optimize is how much side load you’ve got for a given stroke,” he said. “You’re trying to reduce the friction on the walls. We’ve been able to lighten the whole piston up, shorten the compression height by moving the pin up, and that means you can get away with a different height of the connecting rod as well.” 

The harshest duty cycle that the X15 will experience is in Australia with the road trains, which has impacted the design of the engine for North America, Marsh said. “As we’ve gone through development of the existing X15, things like the press fit liner have been developed already. Because we’ve been manufacturing that for a specific market, we’ve already developed the capabilities to integrate this technology from a manufacturing perspective,” he explained. “There’s a lot of things like that where we’ve learned somewhere else and then pulled the technology into the product for North America.” 

Flipping filters 
One of the most noticeable changes for the new X15 is the change from canister style to cartridge filters for both fuel and engine oil. “With steel canister filters, when you need to change oil, you take it off and throw the whole thing in the trash,” Marsh explained. “Also, when you drop the filter off the engine it’s full of hot oil and very heavy. It’s not a great solution for the environment or for the person servicing it. The new design is a reusable plastic housing with a media that you change as a cartridge by pulling it out of the top.” 

Engineers added a drain to aid the serviceability of the filter. “You just put a hose on there, spin the housing cover loose, the oil drains out and then you just drop the filter off the engine. It’s much simpler to service when it’s lighter, and you’re only throwing away the media,” Marsh said. “We’re doing the exact same thing on the fuel filter side as well. We chose not to do a top load design because as you service them, any debris that’s on the top can get caught where the filter goes. We wanted to be able to drop the whole thing off and clean it before reassembly.” 

Forward thinking 
With the latest X15, Cummins has exhausted a lot of resources and engineering hours to produce a powertrain that still meets customer expectations while also satisfying the looming North American regulations that go into effect in 2027. 

“A lot of what we’ve been trying to do is improve overall CO2 performance and make the engine lighter, while at the same time maintain the robustness of the product while adopting the global platform,” Marsh said.  

Holcomb added, “The importance of the latest X15 is that we’re meeting those new NOx regulations but still offering improved fuel economy and similar DEF consumption.” 

Big change: Heat treatment 
One of the most significant changes for the 2027 HELM X15 diesel engine compared to the current EPA24 unit is on the aftertreatment side of the package. The 2024 X15 engines utilize a single module aftertreatment system. The EPA 2027-compliant X15s will move to a new twin module aftertreatment package that reportedly achieves drastically lower emissions levels while using the same basic technology. 

Another significant update for the new X15 aftertreatment system is the addition of an integrated heating element. Cummins states that this approach allows for engine efficiency and performance to be optimized without compromising thermal management. Independent heating of the aftertreatment system enables a reduction in NOx without necessitating a major increase in catalyst volume to meet the new regulations. 

The electric heating system brings the catalyst to operating temperatures more quickly at startup, which improves low-temperature performance and minimizes nitrous oxide (N2O) emissions. N2O is more strictly regulated by the EPA in its Phase 3 greenhouse gas regulations and is penalized >250x more than CO2.  

The heating element also enables on-demand thermal management. Cummins states that this system is scalable to also meet future regulator changes and is completely maintenance-free.