A broader view of efficiency drives the Cummins X15

Maximizing fluid economy in a heavy-duty diesel powertrain begins long before a new truck delivers its first load.

Advances in engine hardware and software have made Cummins’ X15 more efficient by making the most of not just the use of diesel fuel but other costly fluids—oil and diesel exhaust fluid (DEF). When paired with sound fleet management practices and careful vehicle specification, those efficiencies can help fleets and owner-operators reduce total cost of ownership.

While fuel economy often dominates discussions around efficiency, Cummins engineers say a broader view is needed—one that accounts for all operating fluids and how they interact across a range of duty cycles.

Managing DEF consumption

Beyond fuel economy and extended oil change intervals, Cummins advises fleets to take a closer look at DEF consumption, an operating cost that can significantly affect total cost of ownership.

DEF is required in 2010 and newer diesel trucks equipped with selective catalytic reduction systems to reduce emissions. When injected into the aftertreatment system, DEF converts nitrogen oxides into nitrogen and water vapor to meet state and federal air quality standards.

Balance is the key. Relying too heavily on DEF to reduce emissions can offset fuel economy gains, said Kyle Richardson, X15 product manager.

Cummins’ X15 was engineered to meet emissions regulations without increasing DEF to the point of compromising its total fluid economy. Excessive DEF use, Richardson said, can drive up operating costs over time.

DEF consumption varies by duty cycle. In general, higher fuel use results in higher DEF use and long idle times combined with demanding routes can accelerate DEF consumption. Cummins reports that DEF use in the X15 typically ranges from 3% to 5% of diesel fuel consumption.

For a Class 8 truck using 15,000 gallons of diesel annually, a 5% DEF rate equates to 750 gallons. At $4.50 per gallon, that adds about $3,375 to annual operating costs. At a 7% rate, DEF consumption rises to roughly 1,050 gallons, or $4,725. Across a 10-truck fleet, that difference amounts to more than $13,000 per year.

In a business where one percent gains in fuel economy earn headlines, DEF consumption is often overlooked, Richardson said.

“One of the challenges we have with fleets is that they don't measure their DEF usage,” Richardson said. “They buy in bulk, fill it up and go. They’re tracking diesel costs, but not always the DEF cost that goes with it.”

Roughly half of heavy-duty trucks in the United States still operate on pre-2010 powertrains that do not require DEF, which Richardson said contributes to a learning curve for fleets transitioning to newer equipment.

“We’d like to empower the industry to look at DEF cost alongside diesel cost. That’s where you start to see how engineering decisions affect total cost of ownership,” Richardson said. 

DEF prices can fluctuate based on urea and natural gas markets, both of which are used in its production. Prices spiked in 2021 following increases in raw material costs. Demand for DEF in the U.S. is expected to grow substantially over the next decade as emissions regulations continue to shape the market.

Extending oil life in the X15

In addition to benefitting from continued engineering improvements in the X15’s aftertreatment system, fleets and owner-operators can also improve TCO through the engine’s extended oil drain intervals. 

For several years, customers passed along engine oil samples to Cummins OilGuard program. In-depth analysis of the data collected helped engineers gain valuable insights into the engine’s performance as well as oil life. 

Those insights were incorporated into the 2024 X15 through a variable maintenance monitor embedded in the engine control module, said Kristopher Ptasznik, Cummins powertrain TCO and consultancy leader for North American customer support.

“You can extend your oil drains based on the duty cycle of any given individual truck in your fleet,” Ptasznik said. “Instead of having a fixed duty cycle you can do variable to try and extend it out. You don't need to do sampling for that though physical sampling is still available.” 

More flexible oil change intervals can also help fleets better align maintenance with operational demands, reducing downtime.

“Downtime for maintenance is often overlooked as an issue but it's real,” Ptasznik said.  “To change the oil, you’re often driving the truck out of route, paying for fuel, paying the driver and adding wear and tear before the work even begins.” 

Waiting on a technician or an available bay can also require more valuable time. That is where oil life monitoring can really shine. 

“If you can eliminate one or two maintenance events over the life of the truck just by extending an oil change and warranty is backing that extension because of the algorithm that's in the ECM, then it's a snowball effect of every little bit of savings adding up to a net positive total cost of ownership,” Ptasznik said.

Richardson added that onboard diagnostics now provide greater visibility into remaining life for maintenance items such as oil, fuel and DEF filters, allowing fleets to address multiple service needs during a single visit.

“You’re aligning your downtime,” he said. “You’re addressing issues earlier and avoiding additional service stops.”

Spec’ing for efficiency

Fuel economy can be an exciting topic. The X15 Efficiency Series engine turned plenty of heads after providing up to 5% better fuel economy following the integration with the Eaton Cummins Joint Venture and Endurant transmission. 

A 5% fuel economy gain is big news in the trucking industry. Making full and regular use of Cummins’ efficiency advancements is even bigger news. That is where careful truck spec’ing comes into play. 

“If a truck is spec’d wrong, you’re not going to get the fuel economy you want,” Ptasznik said. “And changing major components later can be expensive, so it’s important to get it right upfront.”

Routes, loads, terrain and driver preferences all influence fuel economy, making a one-size-fits-all approach ineffective.

“Two customers might both say they’re linehaul, but their operations can be very different,” Ptasznik said. “Once you understand where and how the truck will run, you can make better decisions that support efficiency and total cost of ownership.”

Taken together, Cummins engineers say fluid economy—fuel, oil and DEF—offers a more complete picture of operating efficiency. Managing all three, they say, is key to reducing costs and keeping trucks productive over the long haul.