Three-way catalyst FAQs: What fleets should know

For fleets considering the Cummins X15N natural gas engine, aftertreatment is part of the maintenance and operating cost equation. The X15N uses a three-way catalyst designed around a simple idea: control emissions without adding fluids, filters or routine aftertreatment service.

There is no diesel exhaust fluid (DEF), no diesel particulate filter (DPF) to clean and no regeneration process to manage. That can mean fewer maintenance tasks and fewer reasons for the aftertreatment system to take a truck out of service. Those points closely match the priorities Cummins subject matter experts identified for communicating the X15N story: simplicity, maintenance, uptime and total cost of ownership. 

What is a three-way catalyst?

A three-way catalyst is the emissions aftertreatment system used with the X15N. It helps control three pollutants in the exhaust: nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO).

The basic technology is familiar. It works on the same principle as the catalytic converter used in gasoline-powered passenger cars, but it is designed for heavy-duty operation. 

How does the three-way catalyst work?

As exhaust leaves the engine, it flows through the catalyst. Precious metals inside help drive chemical reactions that convert NOx, hydrocarbons and carbon monoxide into less harmful outputs such as nitrogen gas (N2), water and carbon dioxide.

The engine helps make that possible by carefully controlling the mixture of air and natural gas during combustion. Keeping that mixture where it needs to be creates the right exhaust conditions for the catalyst to work effectively. In other words, the engine and aftertreatment system do not work independently. They are designed to work together. 

Does the X15N three-way catalyst require DEF?

No. The X15N does not use DEF.

That means there is no DEF to buy, store or refill and no DEF filter to replace. Fleets also avoid the maintenance tasks associated with managing a DEF system. For operations looking to simplify their maintenance workload, that removes an entire set of service items from the equation. 

How does this system affect maintenance?

The biggest difference is how little attention the catalyst itself requires. There is no DPF to clean, no DEF filter to replace and no routine catalyst service to schedule. Cummins describes the three-way catalyst as having no maintenance or replacement period and being designed to last for the life of the vehicle.

For a maintenance team, fewer aftertreatment service points can make planning more straightforward.

How can the three-way catalyst help reduce operating costs?

Some savings come from things the fleet no longer has to purchase or service. X15N-powered vehicles do not consume DEF. Fewer aftertreatment maintenance requirements can also mean fewer associated service costs and less shop time.

The exact savings will vary with the fleet, application, vehicle use and maintenance practices.

Can the three-way catalyst help reduce downtime?

It can help reduce downtime related to aftertreatment maintenance. With fewer routine aftertreatment service requirements, there are fewer maintenance events that may require technician attention or take a truck out of operation.

For a fleet, even a routine shop visit has consequences. The truck is not working while it is being serviced. Reducing those service needs can help support vehicle availability

How is the three-way catalyst packaged on the vehicle?

The three-way catalyst is packaged as a muffler and can be mounted vertically or horizontally.

Those options give vehicle manufacturers flexibility when fitting the aftertreatment system into different chassis layouts. The relatively simple package also supports easier vehicle integration.

Is the three-way catalyst maintenance-free?

Yes. The X15N three-way catalyst is designed as a maintenance-free aftertreatment system, with no filters to clean or replace.

Maintenance-free does not mean the engine itself requires no maintenance. Regular X15N service still matters. That includes maintaining the ignition system and replacing spark plugs at the recommended intervals. Proper engine maintenance helps maintain good combustion and protect the catalyst over its service life. 

Does the three-way catalyst need regeneration?

No. The X15N three-way catalyst does not require DPF regeneration because it does not use a diesel particulate filter.

Instead of collecting soot in a filter and periodically burning it away, exhaust simply passes through the catalyst as the vehicle operates. There is no separate regeneration process for the fleet or driver to manage. 

How is this different from SCR/DPF aftertreatment?

The two systems take different approaches to emissions control.

Diesel aftertreatment systems that use selective catalytic reduction (SCR) rely on DEF, while a DPF captures particulate matter and requires periodic cleaning. The X15N three-way catalyst does not use DEF or a particulate filter.

That makes the X15N aftertreatment system fluid-free and filter-free. Cummins SMEs specifically recommended focusing on those factual differences rather than positioning the technology as a criticism of diesel. 

Why does this matter for return-to-base operations?

Return-to-base fleets often have predictable routes and planned periods when vehicles are available for maintenance. Keeping that schedule simple can help trucks spend more time on their routes and less time waiting for service.

With no DEF system to manage, no DPF cleaning and a catalyst intended to last the life of the vehicle, fleets have fewer aftertreatment-related maintenance events to build into that planning. 

Is the three-way catalyst only about emissions?

No. Controlling emissions is its primary job, but the way the three-way catalyst does that job can also affect fleet operations.

No DEF means one less fluid to purchase and manage. No DPF means no filter cleaning or regeneration. A maintenance-free catalyst means one less component with a routine service schedule.

Taken together, those differences can support uptime, reduce maintenance complexity and contribute to lower total cost of ownership. That broader economic value was a central theme in Cummins' SME discussion of the technology. 

What should fleets know before adopting X15N?

The three-way catalyst is an important part of the X15N value story, but it is not the only factor.

Fleets should consider how natural gas fits their routes, vehicle use, fueling infrastructure and business goals. Where the fuel and application make sense, the X15N adds another advantage: an aftertreatment system designed to do its job without asking fleets to manage DEF, filter cleaning or routine catalyst service.