Teamwork transforms new tech into cleanest, most fuel efficient X15

Improving fuel economy in a heavy-duty diesel engine while lowering emissions is no small feat.

For decades engineers have been aiming to achieve the combination knowing customer motivations to run cleaner for more miles per gallon. 

That’s what makes Cummins 2027 X15 such a stimulating topic among engineers, fleet owners and maintenance managers who know the interests all too well. 

With the base engine fuel economy gain of up to 4%, Cummins has gotten past longtime engineering hurdles with its latest X15 and has left industry leaders impressed, eager to learn more. 

Mike Roeth, longtime engineer and executive director at the North American Council for Freight Efficiency (NACFE), is no stranger to pursuing more efficient and cleaner burning Class 8 trucks. Prior to helping found NACFE in 2010, he amassed 25 years of experience working in the trucking industry.

Cummins’ success in lowering emissions while boosting fuel economy had Roeth recalling an emissions project on a heavy-duty truck from the early 2000s.

"When we did that first major NOx reduction with EGR, we did lose a significant amount of fuel economy in that effort," Roeth said. "We lost 4 or 5% from the engines before it. The industry has been taught that if you lower NOx, you might lose fuel economy."

Andrea Lukas, Director of Market Product Management for Cummins’ engine business, credits the company’s engineers for working together to utilize new ways of breaking through old barriers. 

"You're used to that tradeoff where if you have to meet new emissions requirements, usually it is at the detriment of fuel efficiency," Lukas said. "Kudos to the engineering teams here who have really kept in mind the value of fuel economy, what that means to the fleets, how that impacts their businesses and trying to be very conscientious in that tradeoff."

Engine Technology Forum executive director Allen Schaeffer pointed out how new, data-driven features in engines like the Cummins 2027 X15 are playing a growing role in performance improvement. 

"Continuous improvement in internal combustion engines is alive and well," Schaeffer said. "What we're seeing is a remarkable range of new engine models integrating advanced technology features like start/stop and engine brakes that further enhance the power and performance of these proven engines. All this is happening while meeting emissions standards that are even nearer to zero than today."

Maximizing efficiency through powertrain gains

Improved fuel efficiency with the 2027 X15 starts with the engine which can deliver up to a 4% fuel economy improvement over the 2024 engine. 

Efficiency further improves with the fully integrated powertrain featuring Eaton Cummins Endurant XE™ and Cummins-Meritor Super-Fast axles which can provide up to a 3% gain in fuel efficiency for a total of 7%."To start with, as we are thinking about the engine, fuel efficiency comes from BSFC [Base Specific Fuel Consumption] improvement, which includes choosing the right turbocharger, piston thermal management, etc.," explained Prithivi Raj Babu, Cummins Senior Director of Powertrain, Bus and Technology Integration. "However, there is always the tradeoff and beyond that you cannot go and then keep improving the base engine fuel efficiency. That is where the whole powertrain comes in." 

Dual SCR cannisters paired with two 5kW heaters more effectively reduce emissions in aftertreatment. A belt-driven 48-volt alternator under the hood powers the heaters without requiring batteries.    

The new Endurant XE™ features new small step gear ratios optimized for fast axle ratios to minimize fuel consumption and improve performance. A deeper first gear ratio maintains low-speed performance for fast axle ratios and the overall system will be specified for direct drive cruise while having an overdrive when needed.

The Cummins-Meritor axle and braking systems deliver additional value with a new RPL35/RPL20 driveshaft combination with reduced weight and controlled by active torque management. The Permalube RPL Series was the first-of-its kind in the industry to offer permanently lubricated and sealed for life drivelines to reduce maintenance and cost. 

A new 14X HE SuperFast Ratio axle accommodates lower numeric differentials (2.05 and 1.95) for fuel economy benefit without impacting drivability. Calibration further optimizes fuel economy benefit. 

"It is a direct drive which means that in steep grades, it's not going to be as efficient. It is going to be good on flat roads and through small rolling hills," Babu said. 

Additional fuel economy powertrain benefits are realized through customizable features like Smart Torque 2, SmartCoast (Cruise + Pedal), Torque Droops, Predictive Cruise Control, Predictive Road Speed Governor, Dynamic Power, Hill Roll Out, Predictive Gear Shifting and Predictive SmartCoast Extension. The majority of the listed are predictive features that rely on data streams including GPS to analyze road conditions ahead of time, such as grades, and then adjust acceleration, gear setting and braking to maximize fuel economy. 

Customers can make preferred adjustments according to their needs. 

“We have the ability to sit with that end customer, understand their duty cycle and provide feature trim patterns specific for them,” Babu said.   

Better fuel economy begins with the engine team

The 4% fuel economy improvement comes thanks to several advancements to the 2027 X15 at the base level. 

Lukas explained that a newly designed piston bowl provides a higher compression ratio for more efficient combustion. Changes to the piston skirt area and the height of the piston pin reduce friction. 

“As we move through the crank rotation that also leads to improved fuel economy,” Lukas said. 

The new 48-volt aftertreatment system developed by Cummins Emission Solutions not only decreases emissions it also improves fuel economy thanks to that larger, twin SCR canister design. 

"The catalysts have increased but the size of the aftertreatment also allows for improved flow which reduces pack pressure and optimizes catalyst utilization," Lukas said. "More efficient airflow and reduction of back pressure leads to fuel economy improvements. Horsepower goes to making the wheels go round instead of expelling that exhaust." 

Cummins Turbo Technologies developed a new turbo for the 2027 X15. Changing to a roller element bearing allowed for tighter clearances and greater efficiency. Engineering also made improvements to aerodynamics for the compressor and turbine wheels. A special coating further reduced clearances and an improved actuator also enhanced performance. 

Better fuel delivery starts with new injectors and a more compact, opposed plunger fuel pump. 

"Each plunger has its own roller. There is no sliding which minimizes the movement and improves efficiency," Lukas said. "We've also added active inlet metering to each of the plungers to ensure precision fuel control."

Cummins various businesses all pulled together to not only reduce emissions for the 2027 X15 but also to improve fuel economy. 

“This is where having those complementary businesses and being able to work directly with other components and other suppliers under one roof so to speak is really beneficial,” Lukas said.