CNG 101

What is CNG?

It’s the same gas that you use to cook and heat your homes, just compressed into cylinders. Natural gas is a mixture of hydrocarbons, predominantly methane (CH4). As delivered through the pipeline system, it may also contain hydrocarbons such as ethane and propane and other gases such as nitrogen, helium, carbon dioxide, hydrogen sulfide, and water vapor.  Natural gas has a high octane rating and excellent properties for spark-ignited internal combustion engines.  It is non-toxic, non-corrosive, and non-carcinogenic.  It presents no threat to soil, surface water, or groundwater.

Most natural gas is extracted from gas and oil wells. Much smaller amounts are derived from supplemental sources such as synthetic gas, landfill gas and other biogas resources.

Natural gas accounts for approximately one quarter of the energy used in the United States. Of this, about one third goes to residential and commercial uses, one third to industrial uses, and one third to electric power production. Only about one tenth of one percent is currently used for transportation fuel.

Benefits of Natural Gas as a Fuel

Natural gas vehicles produce lower emissions than vehicles running on gasoline or diesel. In addition, NGVs do not emit the particulate matter that creates smog-like haze and that causes harm to human respiratory and cardio vascular systems.

Another benefit of natural gas as a transportation fuel is that it is a domestic resource that can help the United States reduce its dependence on foreign oil. The cost of a gasoline-gallon equivalent of CNG compares favorably to that of gasoline, so driving a natural gas vehicle may save you money.

Natural gas reduces carbon monoxide, non-methane hydrocarbon and NOx emissions. Natural gas does however increase total hydrocarbon emissions.

Fuel Definitions and Characteristics

In the utility industry, natural gas is sold in standard cubic feet, or SCF. In the automotive industry, gaseous CNG is sold in gasoline gallon-equivalents, or GGE. The DOE states that “The GGE of CNG is 123 CF …(based on 929 Btu/CF of CNG and 114,264 Btu/Gallon of gasoline).” By mass, a GGE of NG has been defined as 5.66 pounds.

Pipeline NG is typically 95-96% methane(>88% is required for motorfuel); LNG (see below) is about 99% methane. NG’s simple chemical makeup (CH4) is easily broken down during combustion, thus it’s very clean burning for domestic or vehicle use. While NG is piped into the home at <1 psi, the same gas is either compressed to 3000-3600 psi for on-board storage, or chilled to minus 260 degrees Fahrenheit to become a clear cryogenic liquid (LNG) at 1/600th its gaseous volume. While LNG saves space, it requires doublewalled insulated tanks to keep it cryogenic; if not, it boils off. LNG is considered impractical for passenger cars because of tank size, but an attractive alternative for truck and bus use.

Compared to gasoline, NG contains considerably less thermal energy. At 29,000 Btu per gallon; a bi-fueled vehicle running on CNG has somewhat limited range. A dedicated NGV with cranked up compression ratios, however, can take advantage of NG’s 120–130 (R+M)/2 octane rating for a considerable improvement. Natural gas is not visible, so suppliers add Mercaptan (with its familiar rotten-egg scent) to NG so it can be detected; building codes may require methane detectors if NGVs are to be stored indoors.

Infrastructure

Stationary and mobile vehicle “fast-fill” compressor stations can be set up wherever NG is available. These tend to be expensive, but overnight “timed-filling” offers a less expensive option. Only a small compressor is required for timed-filling; “at-home” filling is not uncommon. Because natural gas temperature and pressure are inter-related, fast-filling raises tank temperature and pressure, thus affecting fill capacity. By contrast, timed-filling keeps tank temperatures down so more NG can be compressed and stored for greater vehicle range. For refueling of line-haul trucks and buses, both CNG and LNG fill-sites are being established at strategic locations on certain interstate highways forming a system of “Clean Corridors.”

The Nuts and Bolts of CNG Vehicles

The outstanding difference between a dedicated gasoline auto and an NGV is the pressure at which CNG is stored on board—typically 3000 to 3600 psi. The robust cylindrical tanks required to contain such pressures take up space, add weight, and affect handling. The two or more tanks needed for gasoline-comparable range may be located between the frame rails and/or behind the rear axle. Light-duty CNG vehicles often have tanks behind the back seat or in the trunk/pickup bed. Some low-ride buses have CNG tanks on the roof. CNG tanks may be constructed of steel or aluminum and/or composite materials. Tanks are manifolded together and safety valves, regulators and other unique components ensure safety and reliable operation of the NG fuel system. On bi-fuel vehicles, the NG components parallel the normal gasoline delivery system. If the vehicle is aftermarket converted to natural gas, NFPA-52 regulations determine how, and often where, CNG components are to be installed.

A Word on Tank Safety

Some people have anxieties over servicing (or driving) an NGV, usually over possible tank rupture and fire. These fears are understandable, but extensive tests show NGV fuel systems to be safer than gasoline systems.

Very few tank-related incidents have occurred. Just like SCUBA tanks, however, CNG tanks must be handled properly and be periodically inspected, tested and replaced. Tanks are robust and are rated well above working pressures, are equipped with pressure relief devices (PRDs), and are vented to the outside.

 

The NG Fuel System

On-board CNG tanks are filled from a fast-fill, or a time-fill compressor station. A standardized quick-connect fill-valve (with breakaway provision) is located at the normal gasoline cap location (or possibly in the grill or under the hood), and feeds CNG to the on-board tanks. Each tank has its own pressure relief device and shutoff valve. From the tanks, CNG is routed through hefty stainless steel lines to an externally accessible quarter-turn manual shutoff valve, then to the engine compartment.

All NGVs use pressure-reducing regulators to drop and stabilize fuel pressure to workable levels (typically ~100 psi). A primary or multistage regulator, located along the frame rail and/or under the hood, is usually warmed with engine “coolant” to prevent freeze-up. On naturally aspirated NGVs, a secondary regulator drops pressure to near atmospheric.

A vacuum or ignition controlled valve serves as a fuel “lockoff” to shutoff the fuel supply to the engine. Fuel is delivered to the intake system via either a fuel mixer, a “throttle body” injector assembly, or port injectors. Naturally aspirated bi-fuel vehicles often feed NG into the intake system via a variable venturi “mixer,” which sits above the gasoline system’s throttle body or carburetor. Sometimes gaseous fuel is released to the intake air stream using a fixed venturi “spray bar” or a “fuel ring,” which resembles a stovetop burner.

Electronically controlled systems may use a bank of EFI gaseous injectors (similar to gasoline port-injectors) for metering the fuel to the intake. Some engine families use port NG injection systems. Because NG is gaseous, it displaces incoming air. Thus, bi-fuel vehicles tend to lose power (~10%) on NG because of lost volumetric efficiency. Only dedicated NGVs can make up for this loss with raised compression ratios. Dedicated NGVs may also use a unique catalytic converter, but no EGR or knock sensor.

This is how it works:

     1. CNG enters the vehicle through the natural gas valve or receptacle (1) into high pressure cylinders (2).
     2. When natural gas is required by the engine, it leaves the cylinders and passes through the master manual shut-off valve (3).
     3. The gas enters the engine through the high pressure fuel line (4).
     4. Gas enters the regulator (5), which reduces pressure from up to 3,000 psi (pounds per square inch) to approximate atmospheric pressure.
     5. The natural gas solenoid valve (6) allows natural gas to pass from the regulator into the gas mixer or fuel injectors (7).
     6. Natural gas mixed with air flows down through the carburetor or fuel injection system and enters the engine’s combustion chambers.

How is a vehicle converted to natural gas?

Existing gasoline-powered vehicles have CNG components added at our conversion center and Natural Gas then becomes the primary fuel used. Storage cylinders are placed underneath vehicles like vans or pickup trucks, or in the trunk of vehicles like cabs and other cars. Stainless steel lines carry the fuel to a regulator (which reduces the pressure) in the engine compartment, then through a fuel-air mixer, and into the intake manifold to be introduced for combustion. There is a fuel selection switch (usually dash mounted) that allows the driver to manually select CNG or gasoline. A recent development has some conversion systems using fuel injection to supply the fuel directly into the combustion chamber.

Combustion and Emissions

Ignition demands are higher for NG: it’s harder to ignite, and the timing curve must be adapted (advanced) for the slower and longer burntime. NG’s higher stoichiometric ratio (17.2:1) requires closed-loop bifuel adaptation via added electronics for HO2S, knock sensor, and EGR functions. OBD-II systems must remain intact, and engines must be certified under EPA regulations. Natural gas vehicle (NGV) pre-cat HC and CO emissions are inherently low. Dedicated NGV tailpipes easily pass a “white glove” inspection; in fact, Honda calls their dedicated Civic GX the “cleanest on earth.”

From the exterior, most CNG and LNG vehicles look similar to gasoline or diesel vehicles, minus the pollution and the noise. All can be identified by a diamond-shaped label on the rear of the vehicle.

Required Cylinder Inspections

 

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