Servicing CNG engines in the shop

Natural gas is a cleaner, cheaper fuel that can help fleets slash emissions and lower operating costs. And now, with last year’s introduction of Cummins’ 15L natural gas engine with power ratings up to 500 hp, the X15N, this domestically produced fuel is becoming a viable option for a wider range of fleets.

Natural gas has already gained widespread acceptance among refuse truck and transit bus fleets. There has also been solid adoption among vocational fleets that rely on 9L engines. “Now, for that second tier of Class 8 trucks that require at least 10L, we’re seeing the most interest from regional haul fleets,” said David King, product manager for natural and renewable gas engines at Cummins.

Reliable fueling is a big reason why. There are tens of thousands of public fueling stations across the country where tractor-trailers can fill up with diesel. On the other hand, according to data from the Alternative Fuels Data Center of the U.S. Department of Energy, there are only around 700 CNG (compressed natural gas) public fueling stations in the U.S. There are even fewer (50) LNG (liquid natural gas) stations, which is a big reason why CNG has taken off as the natural gas of choice in the transportation industry. The “return to base” aspect of a fleet lends itself to on-site fueling, or at least more predictable local fueling.

“Fleets with optimal logistics are able to capitalize on CNG-powered vehicles today,” said Victor Cummings, VP of service operations for Rush Truck Centers, a large network of commercial vehicle dealerships with more than 150 locations in 23 states and Ontario, Canada.

Regardless of whether you’re talking CNG or LNG, the natural gas engine on a natural gas vehicle (NGV) is the same. Quite frankly, that engine isn’t all that different from a diesel engine. There are some important distinctions though, especially as you work backwards toward the fuel tanks (cylinders). And because of those distinctions, some significant changes need to be made in the shop in order to service NGVs.

How are natural gas engines different?

To answer that question, King said he first likes to look at what is similar to a diesel engine.

“Both have an engine block, oil pan, lube system, and cooling system,” King pointed out, adding that in many cases, part numbers are identical. “But when you get to the cylinder head, the design is specific to the fuel type. There are unique cooling passages on a natural gas engine to keep the head and spark plugs cool. Speaking of which, sparkplugs and ignition coils replace the diesel fuel injectors since natural gas is a spark-ignited fuel.”

A natural gas engine also has a similar ECM, although the software is unique. There is also a spark and fuel-injection control module. “That’s because fuel is no longer injected into the combustion chamber,” King pointed out. “Fuel is now injected into the intake manifold.”

Natural gas engines do not have diesel aftertreatment system components such as a DPF, hydrocarbon doser, SCR, or DEF.

“You still have an EGR with a CNG vehicle,” said Javier Gonzalez, service director for Rush Truck Centers’ Peterbilt dealerships. “You also have something known as a three-way catalyst that’s designed to lower carbon monoxide, VOCs, and NOx emissions coming out of the engine. It’s essentially three bricks made up of precious metals that are located in the exhaust pipe, kind of like a sophisticated catalytic converter you’d find on a gasoline vehicle.”

What do these differences mean with respect to engine maintenance?

First of all, technicians need to maintain spark plugs on a natural gas engine. Cummins has taken steps to make it more convenient by aligning that interval with an oil drain. “We’ve worked to extend the oil drain interval on our newest natural gas engine, the X15N, by 50% over the 12L version,” King said. “We’re getting closer to diesel-like drain intervals now.”

Cummins’ natural gas engines also require oils that meet specific requirements regarding low ash content and the ability to handle higher moisture. King said Cummins worked closely with Valvoline to develop CES 20092, an engine oil specification for spark-ignition natural gas engines that can also be used in diesel and gasoline engines. Valvoline’s Premium Blue engine oil meets that specification, as does Chevron’s Delo 600 ADF engine oil.

In the grand scheme of things, the engine itself is a minor piece of the shift required for fleets to begin using natural gas.

“The biggest thing is just ensuring that the frame layout and fuel system are tailored to the need and application of the vehicle,” said Kyle Kimball, director of marketing at Kenworth Truck Company. “You never want to take up frame space with excess fuel that may not be necessary for a specific duty cycle.”

A CNG-powered commercial vehicle could have anywhere from two to six CNG cylinders on board, mounted either behind the cab, on the side, or on the roof. Those cylinders, along with the fuel system in general, represent the biggest difference between natural gas and diesel—which includes how fleets must go about servicing NGVs.

Standards and inspections

There are inherent safety risks associated with CNG, largely stemming from the unusually high pressure at which the fuel is stored (3,600 psi) onboard the vehicle.

“On a CNG vehicle, you’re seeing high pressure throughout the fuel system, from the fuel-pressure regulator all the way back to the cylinders,” Gonzalez said. “There are a lot of components in between that need to be regularly inspected. That’s much different than a diesel vehicle where the only high pressure you’ll see is from the high-pressure fuel pump, the fuel rail, and down to the injectors.”

Annual inspections are a key element of running a CNG fleet. Inspections are required to ensure that all fuel system components are mechanically sound.

“There are multiple standards and regulations that a CNG servicing facility must abide by,” Gonzalez pointed out. “One is the Federal Motor Vehicle Safety Standards (FMVSS) from the National Highway Traffic Safety Administration (NHTSA). Any vehicle over 10,000 lbs. must go through a 12-month inspection that includes the fuel cylinders (tanks) themselves, along with the fuel lines and other components that make up the CNG fuel system.”

Gonzalez said his technicians are looking for things like fuel cylinder imperfections. Type 4 cylinders are becoming the most common, which consist of a composite fiber resin and have a 20-year life expectancy.

“We use a depth gauge to look for abrasions in the resin,” Gonzalez said. “There is a specification set by ANSI and the NFPA (National Fire Protection Association) that includes inspection requirements associated with nicks, gouges, and abrasions. You can’t just rely on eyesight. And if a label is ever missing from a cylinder and you can’t verify the manufacturer, it must be taken off the vehicle and decommissioned.”

The same type of thorough inspection must be performed anytime a CNG vehicle is involved in any type of accident, even the mildest of fender benders. It all comes down to CNG vehicle safety, which is why a service facility must be certified to work on NGVs.

Setting up the shop

Fire risk is a key hazard that must be mitigated in order for a shop to become NGV-certified.

“If you plan to service vehicles indoors, they must be serviced in conditioned bays,” Gonzalez explained. “As dictated by the NFPA, these bays must be equipped with methane detection systems, shatterproof lighting, and proper ventilation.”

Due to CNG being in a gaseous state, a shop cannot afford to have a spark, so technicians cannot use cordless battery-electric hand tools or electric lifts. “Our certified CNG technicians are using manual wrenches and air-actuated tools,” Gonzalez related.

Speaking of “manual labor,” CNG engines can sometimes be more difficult to diagnose. Gonzalez said drivers may sometimes complain about misfires or low power, but sometimes there is no fault code. Understanding the air/fuel mixture and overall flow of the fuel system is important. Sometimes the root cause of a power issue is something really simple, like a malfunctioning fuel gauge that suggests there is CNG left in the cylinder when there really isn’t.

“A compression tester is a valuable tool when drivers have complaints like this,” Gonzalez said.

Another good specialty tool to have in the bay is a handheld methane detector. This helps a technician pinpoint leaks around the engine compartment or cylinder system.

As touched on earlier, on-site fueling can help local and regional fleets gain better control over their fuel supply. There’s a lot that goes into it, however.

Representatives from ANGI Energy Systems, a supplier of CNG refueling equipment and systems, said the first step is assessing site suitability. There needs to be adequate space, power supply, and access to a natural gas pipeline. There is also a lot of permitting and safety compliance measures that need to be taken. It’s crucial to budget carefully and take advantage of any potential grants or regulatory incentives that could help defray some of the upfront costs.

When switching to natural gas, there will be some upfront costs involved. Trucks cost more, shops typically require some upgrades, and technicians need special training to become certified. But there are also significant operational costs that can be avoided by running on CNG, making it worthwhile for a variety of light-, medium- and heavy-duty fleets.