CNG Fuel System Basics – Transit Bus


⇒Storage Pressure 3600psi

⇒Engine Management Pressure 45-100psi.

⇒Cylinder Life Span – 15/20/25 years

⇒Ignition Temperature 1200°F

⇒5.660 pounds = 1 Gasoline Gallon Equivalent (GGE) (≈125 scf)

⇒6.360 pounds = 1 Diesel Gallon Equivalent (DGE) (≈ 140 scf)

⇒Octane Rating 117




Natural gas is odorless and colorless, the local gas company adds chemicals (odorants) to give it a pungent smell, similar to rotten eggs. Natural gas is lighter than air, leaking natural gas will generally rise and disperse safely; in an enclosed shop area it can pose a danger. The nominal pressure in a CNG system is 3600psi at 70°F, pressure can rise to 4400psi on warmer days.

To be safe always:

⇒Follow manufacturer recommended safety practices

⇒Keep any ignition source away from CNG system

⇒Eliminate any source of static electricity

⇒Conduct work on ventilated area

⇒Never tightened or loosen any component while under pressure

⇒Be familiar with the system you are working on!




Transit bus engines powered by natural gas work like gasoline powered spark-ignited engines. The CNG fuel system on the bus transfers high-pressure natural gas from the storage cylinders to the engine. The high pressure gas is reduced to a lower pressure compatible with the engine fuel management system. Once the fuel is in the engine the air-fuel mixture is ignited by the spark plugs just like a gasoline engine.

About 12000 transit buses in the united states are fueled by CNG; thousands of refuse trucks and delivery fleets are also powered by CNG.

Natural gas as a vehicle fuel is safer than gasoline or diesel, CNG is lighter than air, if a CNG leak should occur, the gas will disperse rapidly upwards into the atmosphere and dissipate. Diesel, gasoline are heavier than air, and will pool on the ground when a leak occurs, creating a fire hazard.

The risk of fire with CNG is very low when handled properly, the ignition temperature is about 1,200°F, compared with about 600° F for gasoline. It also has very narrow flammability limits, it will only burn in concentrations of 5-15%, below 5% is too lean and above 15% is too rich to burn. The high ignition temperature and limited flammability range make accidental combustion of CNG unlikely.


System Components:

cng, regulators, gauges, defuel switch, fuel lines,
Generic System Components

 Generic Layout:

cng layout
Generic CNG Fuel System Layout

System Operation:

When fueling – The CNG enters the bus through the fill valve/fill manifold; flows through the one-way check valve and into the storage cylinders.

During engine operation – the gas leaves the cylinders through the fuel lines and flows through the Manual Shut-Off Valve and high pressure fuel filter. The gas then travels through the high pressure regulator where the storage pressure (3600psi) drops to a lower operational pressure of about 125psi. The fuel now flows to an electronic fuel shut off valve and into the low pressure filter and then into the low pressure regulator. At the low pressure regulator the fuel drops to engine management pressure which is between 45-100psi, depending on type of engine. After the low pressure regulator fuel flows into the engine through a modulated solenoid (single point injection) to premix with the air as in enters the intake. On other engines the fuel is injected into the intake ports through set of injectors (multi-point injection).


Fill Manifold:

At the fill manifold several components make come together depending on the vehicle. in most cases, the fill manifold will house the fill valve, the pressure gauges, the defuel valve, and the manual shut-off valve.

defuel valve, proxinity swtich, pressure gauge
Generic Fill Manifold


Check Valve:

The check valve is designed as a safety measure to prevent fuel flow in reverse. It allows free fuel flow to the CNG cylinder and blocks flow in reverse.


CNG Fuel Cylinders:

The natural gas is stored in high pressure storage cylinders made of steel, aluminum or composites. The nominal storage pressure is 3600psi.

fuel cylinders, fuel tanks, cng tanks,
CNG Cylinders


The cylinders are categorized into 4 categories:

1.  Type 1 is an all steel cylinder made of metal formed in the shape of a cylinder. these types of tanks are the heaviest tanks on the market, but also the most affordable or cost-effective. Type I tanks are typically painted with a protective coating on the outside of the tank.

2. Type 2 cylinders are made with a thinner, steel liner.  The metal liner is wrapped with a composite wrapping around the body of the cylinder; this is called a “hooped wrapped” cylinder. The wrapping is made of glass fiber or carbon fibers or a combination of both.  The metal cylinder takes about 50% of the pressure and the wrapping the rest. Type 2 cylinders are lighter than Type 1 cylinders due to the reduction in metal and the use of lighter-weight composite materials. 

3. Type 3 is made with an all aluminum cylinder liner wrapped entirely in a composite or fiberglass wrap. This is what is called a “fully wrapped” cylinder. The composite material wrapped around the metal liner is typically made of glass fiber or carbon fibers.

4. Type 4 is an all composite cylinder, it does not use any metal it is their structural design.  These cylinders are made with a “gas-tightthick rubber membrane.  The plastic liner is then reinforced with a composite material using the “full wrapped” method, where the entire cylinder is wrapped.   The composite material wrapped around the metal liner is typically made of glass fiber or carbon fibers.

All cylinders are manufactured to meet an industry standard. The main difference between cylinders is cost and weight, type 1 cylinders are the least expensive and also the heaviest.


Cylinder Shut-Off Valves:

All cylinders are equipped with an electronic or manual shutoff valve. Some cylinders have an electronic solenoid inside the tank; others a manual valve on the manifold attached to the tank, yet others have both a manual valve and solenoid valve on the tank manifold.

electric shut-off, manual shut-off, cng shut-off, cylinder valve
Shut-Off Valves


Pressure Relief Device:

Each cylinder must have at least one pressure relief device. The PRD is designed to release cylinder pressure in case of a fire. The PRD is made of a fusible material seal that melts when the temperature on the device has reached 219°F.

The release of the pressure will keep the cylinder from bursting from the increase in pressure due to heat.

Keep in mind that the fuel line between the PRD and the fuel cylinder is always under pressure even if the cylinder valve is closed. To relieve pressure from the PRD or PRD line the cylinder must be defueled.

The PRD is a single use component and must be replaced after activation or when leaking. There are no serviceable parts on a PRD.


Fuel Lines:

Fuel lines are made of stainless steel for longevity and rust prevention. The lines are manufactured to be pressurized to 1.5 times service pressure and not to burst below 2.5 times service pressure.

sample figures:

3600psi – service pressure/5400psi – line max pressure/9000psi – burst pressure


Manual Shut-Off Valve:

The manual shut-off valve is designed to shut off fuel flow to the components past the one way check valve. There are some vehicles that will hold pressure on the high pressure side of the system even when the valve is in the OFF position. Ensure you know the layout of the system you are working on and always follow safety procedures!

shutoff valve, 1/4 turn valve
Manual Shut-Off Valve 1/4 turn


High Pressure Filter:

Positioned on the high-pressure side of the vehicle system between the storage cylinder and the high pressure regulator. The high pressure filter purifies the fuel by removing moisture and contaminants. The filter is a coalescing type and has an efficiency rating of 1 micron.

cng filter, coaslescing filter
High Pressure Filter


High Pressure Regulator:

The high pressure (HP) regulator drops the fuel pressure from cylinder pressure of 3600psi to approximately 99 -150psi.

The regulator is warmed through engine coolant recirculation to prevent it from freezing up during the pressure reduction process. The regulator has a pressure relief valve set to open at 270psi + 60psi, any pressure relieved due to a regulator malfunction will be vented via an externally mounted vent line.

cng regulator, hp regulator
High Pressure Regulator


Low Pressure Regulator:

The low pressure regulator drops the fuel pressure to below 45-100psi. The regulator may be externally mounted or integrated into the engine intake manifold. Depending on the fuel system this regulator may also be warmed with engine coolant to prevent it from freezing up during the pressure reduction process.

Low Pressure regualtor, cng regulator, impco regulator
Low Pressure Regulator – External Mounted


Low Pressure Filter:

The low pressure coalescing filter(s) is usually located near the engine. The low pressure filter removes residual contaminants and oil from the fuel before it enters the engine. Consult the engine manufacturer for maintenance and replacement guidelines.


Shut-Off Solenoid:

The shut-off solenoid shuts fuel flow to the engine under key-off condition.


Fuel Metering Single-Point:

Fuel flow to the engine is metered through a control valve with a PWM signal. The valve is normally closed NC.

cummins FVC, fuel control
Fuel Control Valve


Fuel Metering Multi-Point:

Another method of delivering CNG fuel to the engine is through a multi-point injection system; the fuel is delivered closer to the intake port of each cylinder.

Direct Injection:

Some later model engines are experimenting with direct injection into the combustion chamber similar to a diesel engine.


System Maintenance:

Filter Change:

See article on “CNG Fuel Filter R&R”.

CNG Fuel Filter R&R

Cylinder Maintenance:

END-OF-LIFE – CNG fuel cylinders have a useful life of 15, 20, or 25 years, as designated by the manufacturer. All cylinders must be replaced when they reached their expiration date, cylinders will have a label that states “DO NOT USE AFTER (EXPIRATION DATE).” On some buses the cylinder expiration date may also be found in the fueling connector area.

CYLINDER INSPECTION – CNG cylinders are exposed to environmental, chemical, and road hazards that can damage and threaten the integrity of the cylinder. Cylinders should be inspected by a qualified inspector every three years (36 months) or every 36,000 miles, whichever comes first in accordance with government regulations. Cylinders must also be inspected after any fire, accident or other incident that could cause damage to the cylinder.

CYLINDER REPLACEMENT – CNG cylinders should be replaced by a qualified personnel and a repair facility with the right equipment to safely vent the CNG from the tank and purge the tank with nitrogen to eliminate any pressure or fire potential associated with residual CNG in the cylinder. Once the cylinder is safely purged of any natural gas, the expired CNG cylinder must be rendered unusable usually by drilling two ½ or larger holes in the body of the cylinder; then discard following local regulations.


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