Fuel systems for CNG vehicles

Fuel systems for CNG vehicles

Best of 2014: Getting natural gas from tanks to engines can be a challenge.

December 30, 2014

Interest in vehicles powered by natural gas (NG) continues to grow, with increasing numbers of automakers and conversion companies willing to alter engines and add fuel tanks to new vehicles.

Getting fuel from those tanks to the engines, however, can often be an after-thought in the engineering process, something AFV Natural Gas Fuel Systems hopes to change.

The small Ohio startup is a spinoff of SSP, a more than 80-year-old company that specializes in stainless steel fittings. In 2009, SSP saw the growth rates of compressed natural gas (CNG) and liquefied natural gas (LNG) vehicles and realized that there would be a growing market for companies able to make specialized fuel lines.

“We try to get to the OEMs and the converters as early in the process as possible,” says Kevin Dickey, general manager of AFV. “The earlier we get involved, the more we can help them design a cost-effective system.” 

Tight spaces

The biggest challenge in designing natural gas fuel systems is the limited geometry that engineers have for fuel paths. Nearly all the cars and trucks getting conversions were designed to run on gasoline where fuel lines are routed from gas tanks generally mounted under the middle of the car to engines in the front.

NG-converted vehicles, on the other hand, often have tanks in odd places. Honda’s CNG Civic compact car, for example, puts the tanks in the trunk. Many commercial CNG vehicles have tanks attached to chassis rails, so the fuel has to take longer, more circuitous routes to get from tank to engine.

Dickey says customers have submitted designs with the fuel lines crossing over rear axles – dangerously close to drive shafts – and far too close to vehicle doors.

“The fuel line isn’t that important to them at first,” Dickey says. “They spend a lot of time working on the engine and the fuel tanks. When they get to the fuel lines, a lot of them say, ‘We just need to get it from the tank to the engine. That’s easy.’ That’s when they find out how hard this part of the job is.”

He adds that AFV can take CAD files, drawings, dimensional information, or sample tubes from fuel-system designers and either engineer or reverse-engineer those, searching for potential hazards and manufacturing problems. His team works closely with designers to address those problems before starting production with rigid stainless steel systems. 

Complex tube bends

AFV uses stainless steel tubes for its fuel lines. Diameters range from 0.25” for cars to 0.75” for light trucks, all the way up to 1.25” for commercial trucks. Dickey says the construction industry is putting out requests for 1.5” tube for excavators and other off-highway equipment, so AFV plans to tool up for those diameters as well.

After cutting the stainless steel tube to length and deburring the cut ends, technicians send the pieces to AFV’s tube-bending machine.

A multi-axis CNC machine, the computerized bender is key, Dickey says, because of the complicated lines engineers often design. The shapes are usually too complicated for manual processes, and producing parts with accurate, repeatable fit is critical to avoiding leaks, increasing system life, and reducing installation costs.

In addition to using CAD files for identifying placement hazards, AFV runs bent tube design through digital simulations to identify possible manufacturing problems. Many times, AFV has had to go back to clients because the bends they designed are simply not achievable with either computer-controlled or manual processes. Bends are sometimes sharper than the machine can produce. Zig-zag direction reversals might work on the designer’s computer, but a tube bender won’t have a place to grip the tube to make those bends.

“You’ve got a lot of really smart people out there designing fuel systems that have some creative tube bends,” states Dickey. “But they’re not familiar with the equipment, so they may have some designs that won’t work in the machine or might be expensive to produce.”

Working with vehicle designers, Dickey says AFV looks for ways to improve designs. For example, engineers have cut down fuel-line lengths by 18% from initial designs submitted by Mobility Ventures LLC, makers of the wheelchair accessible, van-like MV-1 vehicle.

“People get really receptive when you tell them you can lower weight and material use,” Dickey says. “It’s to their advantage to make these things easily manufacturable, so we don’t usually get much resistance when we talk about changing designs.” 

Testing for quality

Because rigid fuel-line routes are so specific, Dickey says AFV must use predictable, repeatable processes for every piece. The CNC-driven tube bender is a big part of that. Careful inspection to ensure quality is equally important.

AFV employees use multiple quality checks. Before work starts, each batch of stainless steel tube received gets chemically tested to ensure that its properties are up to the high-pressure specs that AFV guarantees its customers. Sister company SSP deals in large stainless steel volumes and handles testing.

After tube bending, a visual inspection machine measures the X, Y, and Z dimensions of the fuel line. Software compares the actual shapes to the 3D CAD dimensions, flagging any deviations. Dickey says the inspection system is a key advantage for AFV because it allows technicians to quickly measure 100% of its products.

“The other way of doing inspections is to use hard fixtures or roamer arms to check for dimensional accuracy, but these methods can be expensive, inflexible, slower, and not provide automatic dimensional documentation,” Dickey says. Shops tend to use them to inspect representative samples of parts, not every item. “Testing every unit is an important quality point for us. It gives us the ability to offer real assurances to customers that every piece meets their requirements.”

After visual inspection, AFV attaches fuel lines to test rigs to pressurize them, checking for leaks. Technicians can quickly test batches of tubes, ensuring that each one can handle the high pressures. 

Flexible lines

Rigid tube makes up the majority of AFV’s systems, but the company does use some flexible hosing for connectors. AFV recently began manufacturing steel-wrapped flexible hose assemblies in house, licensing polytetrafluoroethylene (PTFE) materials and designs from Titeflex.

Some companies have tried to use flexible lines for entire fuel systems, but Dickey says that can be dangerous. The harsh vibrations, typical to car and truck operations, can shift flexible lines, putting extra stress on connectors. A certain amount of give can alleviate vibration concerns, but too much can quickly lead to wear and damage. In addition, unlike gasoline and diesel, CNG is stored and delivered to the engine under intense pressure, so fuel lines must be significantly more robust.

“Our goal is to have the fuel system last the lifetime of the vehicle, if not longer. So we need to engineer things for durability,” Dickey explains. He adds that carefully designed, rigid systems can be more costly and difficult to engineer, but buyers get a return from increased safety and reliability. 

Market challenges

AFV and other NG support companies discuss long-term ownership costs frequently because the systems tend to be very expensive initially. At General Motors, for example, a GMC Savana cargo van with four CNG tanks costs $12,090 more than a gasoline-powered vehicle. Along with the need for a stronger filling station infrastructure, Dickey calls the high costs of the systems the leading barrier to more widespread use of the fuel.

“Volume cures a lot of these ills,” Dickey says. “As long as this is a niche product, you’re dealing to low-volume manufacturing, so it’s really tough to get the costs out.”

The biggest expense tends to be the NG storage tanks, Dickey adds. Some producers are using steel tanks, wrapped in carbon fiber, to ensure strength and durability. Fortunately, sales volumes for those tanks are rising as many industries look to natural gas as a fuel source. In addition to commercial truck and passenger car companies, tank orders are coming from generator producers, industrial equipment makers, and other industries, Dickey says.

“Those tank prices are going to come down rapidly,” he adds, saying he believes whole-vehicle conversion prices will drop very quickly in response. “We’ve got a technology here that lowers emissions, produces plenty of power, and saves people money on fuel. As soon as prices start to change, you’re going to see a lot of interest in natural gas.”


AVF Natural Gas Fuel Systems