Cummins’ new X15 diesel engine is ready to take on EPA’s crazy-stringent 2027 emissions regulations. The company says the engine not only meets the emissions requirements, but it’s also lighter than the current engine, delivers up to 7% better fuel economy, and retains its beefy 600-hp/2,050 lb.-ft. rating.
The engine itself has been more than six years in the making. Cummins says it’s a clean-sheet design built on the legacy of the 2017 X15 – the first year Cummins did away with the “IS” in the ISX15 designation. That engine was a quantum leap forward from previous versions.
In August, Cummins gave truck journalists a closer look at the 2027 X15 during a media briefing at company’s corporate headquarters in Columbus, Ind.
“We are investing heavily in our new series of products because the existing platforms were not capable of meeting the future regulations,” said Jim Nebergall, Cummins’ executive director of market strategy. “We’re investing now for today, tomorrow and the next 30 years.”
The X-series engines first appeared in 1998, replacing N-series engines, such as the N14. That platform would not have met then-pending 2004 and 2007 emissions regs.
Putting the drastic emissions cuts into context, in 1998 NOx (nitrogen oxides) limits were set at 4 grams per brake-horsepower/hour and the PM (particulate matter) limit was 0.1 gram.
The 2027 standards cut NOx to 0.035 grams (35 milligrams) and PM has been chopped to 0.005 grams (5 mg).
In other words, by 2027, NOx emission will have been cut by more than 90% over the life of the X-series engines.
Along with the cuts to criteria pollutants (PM and NOx) come mandated reductions of various greenhouse gases, including methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2).
It’s usually a trade-off between NOx emissions and greenhouse gas. With each step down in NOx, fuel economy went in the same direction. Cummins claims it’s different this time.
Greenhouse gas correlates to fuel efficiency. Every gallon of diesel fuel burned produces about 22.4 pounds of CO2. The only way to reduce GHG is to burn less fuel.
“These state-of-the-art engines are very efficient, with very low internal friction and very efficient combustion,” Nebergall explained. “We’re able to deliver both improved fuel economy and lower NOx. That’s unheard of historically.”
Updated engine platform
Cummins has made some significant upgrades to the base 2027 engine, mostly targeted at complying with the upcoming regs. Some are intended to reduce the maintenance burden and improve reliability.
A new piston design features a reshaped bowl and redesigned cylinder for better combustion and a higher compression ratio. The skirts are significantly shorter, which reduces friction within the cylinder.
The fuel pump has been redesigned to eliminate some of the problems that plagued previous versions. It’s capable of 2,400 bar injection pressure (34,800 psi) for better control over the formation of soot (PM) and improved efficiency. It incorporates a thermal recirculation valve to aid in cold starts, and is lubricated by fuel rather than engine oil.
A DOHC cylinder head design enables advanced valvetrain capability such as variable valve timing. Cummins is keeping the variable geometry turbocharger design, but improvements offer better air management capability.
Cummins has also incorporated various weight-saving alterations, including a composite rocker cover and oil pan as well as some changes to the block itself.
Overall, Cummins says the engine is lighter than the current 2024 version, but weight neutral when including the aftertreatment system, which is said to be significantly heavier than current versions.
Physically new to the engine is a larger CM3550 electronic control module. It supports more advanced cybersecurity measures, which are a new regulatory requirement – in place largely to prevent hacking. There’s also a new 48-volt belt driven alternator on the engine dedicated to powering a 5-kw heating system associated with the aftertreatment system.
Integrated powertrain
Cummins’ acquisition of Meritor in 2022 opened access to axles, drivelines, and brake products the company didn’t have previously. Coupled with Eaton’s Endurant transmission, Cummins now brings a full powertrain to market. Engineers wasted no time deepening the drivetrain integration.
According to Nebergall, the optimum driveline spec’ includes an Eaton Endurant HD transmission with a .77 overdrive in 12th gear, coupled to Cummins’ (formerly Meritor’s) 14X HE drive axles with a 2.05:1 drive ratio.
“With this new powertrain, we’re seeing a 3-4% increase in fuel efficiency from the engine alone,” he said. “We see an additional 1-3% improvement from the downsped drivetrain, for an overall improvement of about 7%.”
Stable aftertreatment architecture
EPA’s 2027 NOx reduction mandate is a game-changer for engine makers. The step down from 200 mg (current standard) of NOx to 35 mg in 2027 represents an 80% reduction. The challenge with NOx reduction is that it has historically meant a decrease in fuel economy. Cummins says it will meet the NOx reduction requirements while improving fuel mileage.
That means the aftertreatment system for 2027 will be working a bit harder – and smarter — than previous versions.
Cummins will continue using exhaust gas recirculation (EGR). It’s a proven method of getting NOx to the lowest level right out of the engine — before it goes to the aftertreatment system.
It’s an engineering balancing act. Increasing the amount of EGR lightens the load on the aftertreatment system but usually comes with fuel economy and performance penalties.
Dialing back the percentage of EGR forces an increase in DEF consumption, but it gives back fuel economy.
“We are going to deliver very impressive fuel economy improvements while also having similar maintenance to today and similar DEF consumption,” Nebergall insists. “Even though we are pushing NOx down, we are not dramatically altering DEF consumption.”
The basic structure of the entire aftertreatment system is similar to what we have today, but it’s been optimized for performance and space savings. Exhaust first flows through the diesel oxidation catalyst (DOC) and diesel particulate filter (DPF). The catalyst volume is the same, but it’s shorter and fatter.
Exhaust then flows to the SCR catalyst, which now has two modules arranged side-by-side. The shorter, parallel path allows the catalyst to maintain temperatures across the catalyst better than a longer thinner module.
The system still has a DEF delivery system, and it will continue to require fuel-fired regens from time to time to rid the DPF of soot.
The two modules are physically larger and heavier than current systems, but Nebergall said overall, “they are weight neutral when factoring in the weight reductions on the engine.”
5-kW heaters
New for 2027 are two heating coils designed to provide more consistent thermal management of the aftertreatment system, regardless of the engine’s operating state. NOx conversion is more efficient when the catalyst is hot but keeping the system hot when the engine is idling or in a low-load state is challenging.
“These heaters give a lever to create heat that’s independent of what the engine’s doing,” Nebergall explained. “If the engine is idling and the aftertreatment is cooling off, you can control the heater to heat the system up. Or if it’s a duty cycle where the engine isn’t working very hard, such as when the truck is bobtailing, we can still keep the aftertreatment system warm.”
The 2027 emissions regs not only call for greater levels of NOx reduction, the testing and certification procedures are much more rigorous. Previously, engine makers had a bit of wiggle room in “light-load” duty cycles and at start-up, but the new rules call for compliance in any duty cycle, in any operating condition.
Cylinder deactivation has been considered as a strategy to add heat to the catalyst for better NOx conversion performance, but Cummins has chosen to use this arguably less complex array of heaters.
“We have designed the engine to enable cylinder deactivation so we can add it if we need it for the greenhouse gas regulations coming in 2030,” Nebergall said. “We don’t think it’s necessary for 2027.”
The two heaters – one at the inlet of the DPF, the other at the SCR inlet – are powered by a belt-driven 48-volt alternator mounted on the front of the engine. The system provides on-demand heat when it’s required, but Nebergall said in most duty cycles, like linehaul at highway speed, it won’t be used.
It’s a simple resistive-wire heater requiring no maintenance. It’s easily replaceable, as is the generator.
Emissions system warranty
EPA’s 2027 reg also included a mandatory emissions systems warranty that has to cover the function of the system over the so-called expected life of the engine. It really applies to the first owner of the vehicle, in many cases.
The warranty period increases from five years/100,000 miles (160,000 km) to 10 years/450,000 miles (720,000 km). This warranty isn’t optional; it’s mandated in regulation. And the cost will be significant.
Nebergall pointed out during his presentation that many heavy-duty customers already buy the 500,000-mile (800,000 km) extended warranty packages, which in most cases gets them through to the end of their trade cycle. If the trade cycle is shorter, it can add residual value to the truck at resale.
For vocational customers with longer trade cycles but lower mileages, the 10-year term will apply.
“The cost of this warranty is baked into the base price, and the customer no longer has the choice to buy it or not,” Nebergall noted. “The system cost is going up for the entire industry.”
Final thoughts
Throughout his presentation at this unveiling of the X15 diesel, Nebergall stressed reliability was a key factor in the redesign of the engine. The strategies described here seem to lean in that direction. They are clean and straightforward and employ mostly technology that has been in service for a decade or more.
First impressions suggest this approach will yield a relatively trouble-free experience for customers, but we’ve learned in the past that even the best laid plans can go south.
Customers will have enough to be concerned about with the steep price increases coming with the 2027 engines — universally, by the way. Not just for Cummins. Let’s hope Nebergall and his team have done their jobs.
