Profitable Uptime And Lower TCO for Natural Gas Aftertreatment

Profitable Uptime And Lower TCO for Natural Gas Aftertreatment

A simplified aftertreatment system on Cummins’ natural gas engines offers strong uptime and no routine maintenance in a lightweight package, and it has improved further.

Cummins’ three-way catalyst system on the X15N, ISX12N, L9N and 6.7N has long provided fleets with maintenance-free aftertreatment. Unlike diesel systems, the three-way catalyst does not require routine service, helping fleets avoid downtime associated with those maintenance intervals.

As manufacturers, industry leaders and legislators work to address diesel aftertreatment requirements, which often involve diesel exhaust fluid (DEF) sensors and diesel particulate filter maintenance, Cummins’ natural gas engines continue to improve in power, range and emissions.

Cummins’ latest natural gas engine, the X15N, offers diesel-like power with the company’s proven, simple three-way catalyst aftertreatment, which has been improved over earlier designs.

“There have been some improvements to our air-fuel ratio control,” said Scott Baize, Cummins Spark Ignition Regional Technical Platform Leader for North America. “The 15-liter has a wide-band oxygen sensor, which is more accurate outside of a strict stoichiometric air-fuel ratio point.”

The term stoichiometric in engine applications refers to an ideal ratio of air and fuel during combustion that burns the fuel thoroughly with the fewest emissions possible.

“On natural gas it varies depending on fuel quality, but it is roughly 16.5 to 1. For every pound of fuel, you need 16 and a half pounds of air to keep that catalyst happy,” Baize said.

With an ideal air-fuel ratio in place, Cummins’ natural gas combustion provides a thorough burn and ultimately reduces the work needed from aftertreatment to further eliminate emissions.

“There is a lot that happens on the control side of the engine. That air-fuel ratio control is very important. Getting the exact right amount of fuel to match the air is important to make sure the catalyst works properly and provides good emissions reduction. We’re reducing NOx by over 99% with that three-way catalyst.”

David King, Cummins product manager for natural gas engines, noted that the three-way catalyst is also lighter than diesel aftertreatment and easier to mount because of its smaller size.

Depending on the configuration in the X15N, aftertreatment can weigh up to 150 pounds less than diesel. Combined with a lighter engine design, the X15N can offer a roughly 500-pound weight reduction compared with the X15 diesel engine, King noted. Longevity is also built in.

“It’s meant to last for the life of the vehicle,” King said. “It doesn’t have a maintenance or a replacement period. It takes up less space, it is lighter and you just forget that it is there for the most part.”

Why simple can be better

Cummins’ three-way catalyst gets its name from the job it has performed for the past three decades: removing nitrous oxides (NOx), hydrocarbons and carbon monoxide (CO) in natural gas combustion. It is the same catalyst used in aftertreatment systems in gasoline-powered passenger cars, known as catalytic converters.

“You don’t do any maintenance on the catalyst in a passenger car and neither do we,” Baize said.

Keeping up with spark plug changes every 60,000 miles helps keep the catalyst operating in top shape. For the X15N, spark plug changes can be performed at 60,000-mile oil change intervals or up to 65,000 miles with Valvoline NextGen One Solution.

“It’s important to maintain your engine, especially the ignition system,” Baize said. “Spark plugs need to be replaced regularly, and when they are, the catalyst should last for the life of the vehicle. Poor spark plug maintenance can cause misfire, and if that continues over time, it can damage the catalyst. The spark plugs need regular attention, but the catalyst itself does not require maintenance.”

While the Cummins converter itself has remained largely unchanged, there have been some changes to the ratios of platinum, palladium and rhodium in the catalyst to further reduce emissions. Those precious metals convert NOx, CO and hydrocarbons into water and carbon dioxide.

The latest Cummins natural gas aftertreatment is compliant with the strict 2027 EPA and California Air Resources Board emissions requirements, and it does so without DEF, which averages around $4 a gallon, or regular DPF cleanings, which can cost more than $1,000.

“Two key factors make a three-way catalyst work,” Baize said. “One is precious metal content, and the other is the catalyst’s oxygen storage capability. That allows it to store oxygen when needed to help convert NOx, then release it to help convert CO and hydrocarbons. The goal is to maintain the right oxygen balance so all three emissions can be converted effectively.”

Diesel and natural gas each have advantages depending on the application, but natural gas can become the stronger fit when fuel prices and aftertreatment-related downtime are factored in. The average price of diesel is well above $5 a gallon, according to AAA, while natural gas at public stations is 60% below that price.

Savings at private stations are even better. The Alliance for Clean Transportation reports up to a 90% reduction in fuel costs for fleets that build and manage their own natural gas fueling system.

Fuel savings will grow, King added, as fleets adopt more natural gas vehicles (NGVs) and consume more clean-burning natural gas.

“It’s a great time to start your payoff quickly right now,” King said. “While we can’t forecast fuel prices, we can generally say natural gas will offer a better price than diesel going forward.”

In addition to higher fuel costs, DEF- and DPF-related issues that result in downtime can add up quickly.

“When we think of unscheduled downtime, we think about how a fleet has to lease a truck or pay a driver to sit around,” King said. “They’ve got a load that just might be late, which can result in a penalty fee. Maybe they’ve got a subcontract where somebody else picks up that trailer. Whatever the case, downtime can cost a lot more than you think.”