Sales of natural-gas-powered trucks have leveled off since the drop in oil and diesel prices, but fleets already using the fuel continue to buy them, especially compressed natural gas. What are some of the basics involved in the latest CNG tanks and fuel systems?

It starts with range, because the number of miles covered in a run determines the required size and number of tanks. Weight and bulk are also important, as are certain parts that can add efficiency and reliability.

The storage capacity of CNG tank systems has improved, making the fuel a better option for longer hauls than in the past. About a decade ago, when storage was more limited, it was believe liquefied natural gas (LNG) would be the choice for long haul; because it is super cold and contracts, it can be stored in tanks in goodly amounts. Less CNG can be crammed into on-board vessels, but that was fine for local operations where trucks return home daily for refueling.

Since then, suppliers have improved CNG tank capacity and found ways to stack them more efficiently behind cabs and sleepers, reducing the need for saddle tanks. These advancements make CNG usable in reasonably long over-the-road operations.

“The trend definitely is more toward compressed natural gas,” says Robert Carrick, vocational sales manager for natural gas at Freightliner. “We estimate that more than 95% of the natural gas vehicles manufactured and delivered in 2015 were CNG.”

Freightliner’s partner is Agility Fuel Systems, and that equipment is installed at Freightliner’s plant in North Carolina and at an Agility facility nearby. That facility also works on Volvo trucks produced in Virginia, but many have tanks installed at a Fontaine Modification Center near the plant, says Frank Bio, Volvo’s director of alternative-fuels sales development.  

“With a couple key customers that have moved from LNG to CNG, we have seen a dramatic drop in LNG prep kits based on fuel availability, ease of maintenance, and overall value proposition,” says Kurt Swihart, marketing director at Kenworth. A mod center at the Chillicothe, Ohio, plant installs natural gas tanks and associated equipment per customers’ wishes, and some large dealerships are now qualified — something true of most builders.

In fact, Rush Enterprises, the nation’s largest dealer, last year bought a company called Momentum Fuel Technologies and now offers its own natural-gas fueling/tank system. It offers both side-mount and back-of-cab systems.

The company says its tanks are designed to maximize fill rates, with oversized plumbing with fewer bends and fittings plus a high-flow ball valve. In addition, fuel flows straight from the manifold to all tanks simultaneously. A pressure regulator allows fuel to be used at a lower psi for increased range.

Weight is also addressed, with lightweight aluminum and fiberglass composite materials and 3M nanoparticle-enhanced matric resin technology.

Momentum says its system has features that increase safety. For instance, quarter-turn ball valve handles on each tank make it obvious when the valve is closed and gas flow is stopped. In addition, high-pressure fuel lines are yellow and PRD supply lines are red to indicate a live channel that is always under pressure.

Agility showed its latest CNG installation at the recent Technology & Maintenance Council annual meeting in Nashville, Tenn. It featured four tanks with a total capacity of 160 diesel-gallons-equivalent (DGE), enough gas for about 600 miles, says Paul Mader, the company’s sales manager. Previous installations were limited to 400 to 500 miles, too short for the typical trip a driver can legally cover in a day.

The new system includes an aluminum-framed and -skinned cabinet that houses the tanks and associated lines, valves, gauges and regulator. The cabinet sits close to the cab — up to 6 inches closer than previous designs — to limit a truck’s length and free up frame space. The structure sits on rubber mounts affixed to the frame rails. Tanks are lightweight Type 4’s, with plastic cores wrapped in carbon fiber; they’re neck-mounted in rubber to resist spinning that can damage lines. Total system weight is 2,150 pounds.

Like all CNG systems, the tanks’ fill pressure is 3,600 psi, which can go several hundred psi higher when gas is hot and drops as it cools and is drawn out. Gas is regulated down to 120 psi for delivery through low-pressure lines to the engine.

Operation of the regulator is one item that gas-truck buyers should look at, says Kyle Takavitz, commercial director for Worthington Industries’ fuel systems business. Gas cools as it emerges from an orifice, which works to a customer’s benefit if the regulator is properly designed. “Ensure that you’re promoting gas to be as cold and dense as possible when it enters that engine,” he says. “Look for suppliers that can validate that performance.”

Worthington makes steel products including gas cylinders, but in the last two years it acquired a pair of gas-related companies that work with other materials and technologies, dHybrid Fuel Systems and Trilogy Engineered Solutions. Worthington also is among the partners in development of adsorbed gas storage, a technology that promises more gas to be stored in a given size tank, lower pumping and operating pressures, and therefore less overall cost.

A system also has high- and low-pressure filters that should be efficient, and need periodic cleaning, Takavitz says. How much maintenance depends on gas quality (the content of feed stocks and how well it was filtered at or near the well head), where it was transported (some pipelines are cleaner than others), time of year (which relates to system pressure), and how it’s being stored at a fueling station and on the truck.

Installation time is also cited as something to consider when choosing a system, with Worthington advertising that it can install high-pressure lines and tanks faster than its competitors, and Momentum saying it developed a “Fast-to-Fit” system using integrated brackets for quick, consistent mounting on the assembly line and aftermarket.