CNG fueling is straightforward, but . . .

In my previous blog, Behind the wheel of a CNG-powered big rig, I discoursed on my experience with driving a compressed natural gas (CNG)-powered Freightliner Cascadia 113 tractor pulling a loaded van trailer. I got to do this at a recent Daimler Trucks North America (DTNA) event in Napa Valley, CA.

DTNA is the leading heavy duty truck manufacturer in North America. It produces and markets commercial vehicles under the Freightliner, Western Star and Thomas Built Buses nameplates.

The subject of this blog is fueling a CNG-powered tractor. The particular Cascadia 113 I drove had back-of-cab CNG storage tanks and a CNG fuel tank on each side.

Like a diesel-powered vehicle, there is a gauge on the dashboard instrument panel that indicates how much CNG is left in the onboard storage tanks.

Different techniques

Fueling with natural gas is relatively easy, but different from gasoline or diesel. You latch a special fitting at the end of the CNG fuel hose to the fueling nipple on the CNG fuel tank, then turn on the dispenser to transfer CNG into the vehicle.

With gasoline or diesel, which is a liquid, the fueling stops abruptly when the vehicles' tank is full and the gas or diesel has backed up into the filler, shutting off the flow from the pump.

With natural gas, there is no abrupt stop. Rather, the fueling just slows down as the tank fills. When the pump determines the vehicle has reached maximum pressure, the pump will stop and the readout on the pump will indicate "100 percent." Simply unlatch the fitting from the fueling nipple and return the hose back to the dispenser.

The CNG dispenser displays the pressure at which the tank is being filled and calculates and shows how many gasoline gallon equivalents (GGEs) are delivered into the vehicle.  

To determine how many DGEs (diesel gallon equivalents) you received, calculate by multiplying the GGEs by 1.13, Robert Carrick, Freightliner Trucks' manager of natural gas sales, explained to me. That is the DGE quantity.

This is required as energy content is different in a gallon of gasoline than it is in a gallon of diesel, he pointed out.

Do not be alarmed by the hissing sound when you disconnect the hose. This occurs because CNG for vehicle fueling is pressurized at 3,600 psi.

Fill choices

Natural gas is delivered by a pipeline to a fueling station. There, the gas is compressed, dried and stored at 4,500 psi. It is dispensed into a vehicle at 3,600 psi.

The faster you fuel with CNG, the more heat you gain with gas, said Carrick. As the fuel warms up it becomes less dense and, therefore, contains less energy by volume when the fuel system reaches the rated pressure.

For this reason, you are usually able to get more CNG into a tank with a time-fill versus a fast-fill application, he said. With time-fill, the fuel is slowly dispensed over time so that the heat of compression is lower, allowing a fuller fill than with fast-fill.

This can be alleviated for fast fill stations if the station is a modern station with temperature and pressure compensation, noted Carrick.

Safety issues

No special safety equipment is required for CNG fueling, I learned from discussions with Carrick; Greg Treinen, Freightliner's segment manager, product marketing - vocational and natural gas; and Brian Daniels, the company's product manager - vocational and alternative fuels. As for training, a simple three-minute video is all that is required to familiarize the driver with fueling technique.  

Compressed natural gas is non-toxic (meaning there are no poisonous ingredients in it that can be absorbed into the blood once inhaled), has few associated health risks and is non-corrosive.

Plus, there is no potential for ground or water contamination in the event of a fuel release.

Compressed natural gas is lighter than air, they pointed out. If there ever was a leak in a CNG fuel system, the gas would simply disperse into the atmosphere, significantly lowering the risk of ignition.

Carrick, Daniels and Treinen said natural gas has an ignition temperature of 1,100 degrees Fahrenheit, meaning it takes a higher temperature to ignite natural gas compared to gasoline or diesel.

My next blog will address some of the maintenance issues with CNG-powered vehicles.