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ASE F1 Compressed Natural Gas Vehicles study materials
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ASE F1 Compressed Natural Gas Vehicles practice questions

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Question 1CNG Fuel System Fundamentals, Safety, and Regulations

Which document establishes the federal safety standards that CNG fuel system containers must meet for installation on a motor vehicle in the United States?

  1. SAE J1616
  2. FMVSS 303
  3. FMVSS 304 Correct answer.
  4. NFPA 52

Explanation

FMVSS 304, Compressed Natural Gas Fuel Container Integrity, sets the federal performance requirements for CNG containers used on motor vehicles. NFPA 52 covers vehicular fuel system installation practices but is not the federal container standard. SAE J1616 addresses fuel quality and refueling recommendations.

Question 2CNG Fuel System Fundamentals, Safety, and Regulations

A CNG cylinder installed on a vehicle must be qualified and marked to show compliance with which standard?

  1. ISO 9001
  2. ANSI/CSA NGV2 or FMVSS 304 Correct answer.
  3. ASTM D4814
  4. SAE J20

Explanation

Vehicle CNG cylinders must be qualified to a recognized container standard such as ANSI/CSA NGV2 and marked as compliant. FMVSS 304 requires that the container meet specified performance criteria. ASTM D4814 is a gasoline specification, SAE J20 covers coolant hose, and ISO 9001 is a quality management standard.

Question 3CNG Fuel System Fundamentals, Safety, and Regulations

What is the primary purpose of the manual shut-off valve located at the CNG cylinder?

  1. To measure the quantity of gas remaining in the cylinder
  2. To regulate fuel pressure delivered to the injectors
  3. To vent gas to atmosphere during normal refueling
  4. To isolate the cylinder from the rest of the fuel system for service or emergency Correct answer.

Explanation

The cylinder valve is the first isolation point in the system, allowing the cylinder to be sealed off for service or in an emergency. Pressure regulation is done by the regulator/reducer, refueling does not vent to atmosphere, and quantity is measured by the pressure gauge or fuel gauge.

Question 4CNG Fuel System Fundamentals, Safety, and Regulations

In a bi-fuel CNG vehicle, what does the fuel selector switch allow the driver to do?

  1. Choose between operating on CNG or on gasoline Correct answer.
  2. Adjust the CNG regulator output pressure
  3. Override the fuel gauge reading
  4. Manually open the cylinder shut-off valve

Explanation

A bi-fuel vehicle carries both CNG and gasoline, and the selector switch lets the driver choose which fuel the engine uses. Regulator pressure is set by the regulator, the cylinder valve is a separate manual device, and the gauge is not overridden by the selector.

Question 5CNG Fuel System Fundamentals, Safety, and Regulations

What is the function of the pressure regulator (reducer) in a CNG fuel system?

  1. It increases cylinder pressure during refueling
  2. It stores CNG for later use during acceleration
  3. It reduces high cylinder pressure to a low pressure suitable for the injectors Correct answer.
  4. It filters particulate matter from the gas

Explanation

The regulator/reducer steps the high cylinder pressure down to a low, controlled pressure for the fuel injection system. It does not store fuel, its main job is not filtration, and cylinder pressure rises during refueling due to the fill, not the regulator.

Question 6CNG Fuel System Fundamentals, Safety, and Regulations

What does the term 'service pressure' refer to on a CNG cylinder?

  1. The pressure at which the cylinder is normally filled and operated Correct answer.
  2. The minimum pressure needed to start the engine
  3. The pressure at which the burst disc ruptures
  4. The pressure inside the fuel line at the injectors

Explanation

Service pressure is the nominal pressure at which the cylinder is designed to operate and be filled. Burst disc rupture pressure is a separate safety rating, starting pressure is unrelated, and injector line pressure is much lower.

Question 7CNG Fuel System Fundamentals, Safety, and Regulations

Why is natural gas considered to have a wide flammability range that demands careful leak detection?

  1. It ignites only when compressed above 3,600 psi
  2. It cannot burn in air at all
  3. It burns only at a single exact mixture ratio
  4. It can ignite over a broad range of gas-to-air mixtures Correct answer.

Explanation

Methane has a relatively wide flammability range in air, so leaks can create ignitable mixtures over a broad concentration band. It does not burn at only one ratio, it does burn in air, and ignition does not require high pressure.

Question 8CNG Fuel System Fundamentals, Safety, and Regulations

What color is CNG typically identified by in vehicle fuel system labeling and cylinder markings?

  1. Red
  2. Green
  3. Yellow
  4. Blue Correct answer.

Explanation

CNG is commonly identified with blue labeling in vehicle fuel system markings and industry practice. Red, yellow, and green are used for other hazards or identifiers, not the standard CNG color association.

Question 9CNG Fuel System Fundamentals, Safety, and Regulations

A technician is inspecting a CNG cylinder and finds deep gouges and exposed fiber in the composite wrap. What is the correct action?

  1. Remove the cylinder from service and evaluate it per the manufacturer's criteria Correct answer.
  2. Reduce the fill pressure by half and continue operating
  3. Sand the damaged area smooth and return the vehicle to service
  4. Apply a protective coating over the damage and continue use

Explanation

Structural damage such as cut fibers or deep gouges can compromise cylinder integrity and requires removal from service and evaluation against manufacturer limits. Sanding, coating, or reducing fill pressure does not restore structural strength.

Question 10CNG Fuel System Fundamentals, Safety, and Regulations

Why must CNG fuel lines be routed and secured away from sharp edges and heat sources?

  1. To reduce the weight of the fuel system
  2. To improve fuel economy at high speed
  3. To prevent abrasion and heat damage that could cause a leak Correct answer.
  4. To keep the fuel lines electrically grounded

Explanation

High-pressure fuel lines can be damaged by chafing or heat, which could lead to a leak, so routing and clamping protect them. Grounding, weight reduction, and fuel economy are not the reasons for this routing requirement.

Question 11CNG Fuel System Fundamentals, Safety, and Regulations

What is the purpose of the pressure relief device (PRD) fitted to a CNG cylinder?

  1. To release gas slowly during normal driving to regulate pressure
  2. To allow the driver to manually vent the cylinder
  3. To supply gas to the engine at a constant pressure
  4. To vent cylinder contents in a fire to prevent catastrophic rupture Correct answer.

Explanation

A PRD is a safety device designed to relieve pressure, typically by venting gas, when exposed to fire or excessive temperature to prevent a violent rupture. It does not vent during normal driving, is not a driver control, and does not regulate engine supply pressure.

Question 12CNG Fuel System Fundamentals, Safety, and Regulations

A vehicle is being refueled and the technician hears a continuous hiss at the receptacle after the nozzle is removed. What is the most likely cause?

  1. Normal equalization of pressure
  2. A leaking or stuck check valve at the receptacle Correct answer.
  3. A failed pressure regulator
  4. Low cylinder pressure

Explanation

A continuous leak after nozzle removal suggests the receptacle check valve is not sealing, allowing gas to escape. Normal equalization is brief, and neither a failed regulator nor low cylinder pressure causes a hiss at the receptacle.

Question 13CNG Fuel System Fundamentals, Safety, and Regulations

Which practice is required before performing service on a CNG fuel system?

  1. Loosen the fuel line fittings to vent slowly
  2. Close the cylinder valve and bleed residual pressure from the lines Correct answer.
  3. Open the cylinder valve fully to equalize pressure
  4. Disconnect the battery and leave the system pressurized

Explanation

Safe service requires isolating the cylinder at the valve and bleeding trapped pressure from the downstream lines before opening the system. Opening the valve, leaving it pressurized, or loosening fittings under pressure is unsafe.

Question 14CNG Fuel System Fundamentals, Safety, and Regulations

What does the fuel pressure gauge on a CNG vehicle primarily indicate?

  1. The regulated injector supply pressure
  2. The refueling nozzle pressure
  3. The engine oil pressure
  4. The pressure remaining in the cylinder, which correlates with fuel quantity Correct answer.

Explanation

The cylinder pressure gauge reflects the remaining gas quantity, since pressure drops as gas is used. Injector supply pressure is much lower and separately regulated, oil pressure is unrelated, and nozzle pressure is a refueling station value.

Question 15CNG Fuel System Fundamentals, Safety, and Regulations

Why is a gas detection sensor often installed near the ceiling in a CNG service bay?

  1. Ceiling sensors are cheaper to install
  2. Natural gas is heavier than air and settles at floor level
  3. Natural gas is lighter than air and tends to accumulate at high points Correct answer.
  4. Ceiling placement avoids electrical interference

Explanation

Methane is lighter than air, so a leak rises and accumulates near the ceiling, making high placement effective for detection. Gasoline vapor is heavier than air, but natural gas is not, and cost or interference are not the reasons.

Question 16CNG Fuel System Fundamentals, Safety, and Regulations

What is the role of the high-pressure filter in a CNG fuel system?

  1. To reduce gas pressure to atmospheric
  2. To store gas for cold starts
  3. To trap particulate contamination before it reaches the regulator and injectors Correct answer.
  4. To remove moisture from the gas by condensation

Explanation

The high-pressure filter captures particles that could damage or clog the regulator and injectors. It is not a moisture condenser, does not reduce pressure, and does not store gas.

Question 17CNG Fuel System Fundamentals, Safety, and Regulations

When a CNG cylinder reaches the end of its approved service life, what must be done?

  1. It may continue in service if it still holds pressure
  2. It must be removed from service and disposed of per the manufacturer's instructions Correct answer.
  3. It may be re-rated by the technician for higher pressure
  4. It may be converted to store other gases

Explanation

Cylinders have a defined service life after which they must be retired and disposed of according to manufacturer and regulatory requirements. Holding pressure does not extend life, technicians cannot re-rate cylinders, and conversion to other gases is not permitted.

Question 18CNG Fuel System Fundamentals, Safety, and Regulations

Which of the following best describes the difference between CNG and LNG?

  1. CNG is stored as a liquid, while LNG is stored as a gas
  2. Both are stored as liquids at ambient temperature
  3. Both are stored as gases at atmospheric pressure
  4. CNG is stored as a compressed gas, while LNG is stored as a cryogenic liquid Correct answer.

Explanation

CNG is compressed gas stored at high pressure at ambient temperature, while LNG is refrigerated to a cryogenic liquid state. The other options reverse or misstate these storage states.

Question 19CNG Fuel System Fundamentals, Safety, and Regulations

A technician must confirm the working pressure rating before connecting a service hose to a CNG system. Why is this important?

  1. Higher-rated hoses always improve fuel economy
  2. Lower-rated hoses filter gas better
  3. Working pressure affects the color of the gas
  4. Hoses and fittings must be rated for the system's maximum pressure to avoid failure Correct answer.

Explanation

Service equipment must be rated for at least the maximum pressure it will see, or it can burst and injure the technician. Fuel economy, gas color, and filtration are unrelated to hose pressure rating.

Question 20CNG Fuel System Fundamentals, Safety, and Regulations

What is the purpose of the excess flow valve or check valve in the CNG fuel line?

  1. To cool the gas before it reaches the engine
  2. To measure fuel consumption
  3. To limit gas flow in the event of a line break Correct answer.
  4. To increase flow during acceleration

Explanation

An excess flow or check valve restricts flow if a line ruptures, limiting the release of gas. It does not boost flow, cool the gas, or measure consumption.

Question 21CNG Fuel System Fundamentals, Safety, and Regulations

During a cylinder inspection, a technician notes the requalification or inspection interval stamped on the label. Why must this interval be followed?

  1. It can be extended indefinitely by the owner
  2. It is only a suggestion with no safety value
  3. Regulations and standards require periodic inspection to verify continued cylinder integrity Correct answer.
  4. It applies only to gasoline tanks

Explanation

Periodic inspection or requalification is mandated to confirm the cylinder remains safe for continued service. It is not optional, applies to CNG cylinders, and cannot be extended at the owner's discretion.

Question 22CNG Fuel System Fundamentals, Safety, and Regulations

A vehicle with a CNG system is involved in a collision that damaged the cylinder area. What is the appropriate course of action?

  1. Reduce fill pressure and continue operating
  2. Return to service if no gas is currently leaking
  3. Remove the vehicle from service and have the cylinder and system inspected per the manufacturer's procedure Correct answer.
  4. Replace only the fuel gauge and continue

Explanation

Collision damage can compromise cylinder integrity even without a visible leak, so the system must be inspected per manufacturer procedures before returning to service. Absence of a current leak, gauge replacement, or reduced fill pressure does not establish safety.

Question 23CNG Fuel System Fundamentals, Safety, and Regulations

Why does a CNG cylinder's pressure drop significantly as fuel is consumed even though much gas remains?

  1. The cylinder volume shrinks as it empties
  2. The regulator removes gas from the cylinder permanently
  3. Pressure in a fixed volume falls as the amount of gas decreases Correct answer.
  4. The gas changes chemical composition as it burns

Explanation

In a fixed volume, pressure is proportional to the amount of gas present, so pressure falls as gas is used even though usable fuel remains. The gas composition does not change, the regulator does not remove gas, and the cylinder volume is essentially constant.

Question 24CNG Fuel System Fundamentals, Safety, and Regulations

A technician observes frost forming on the regulator during engine operation. What does this most likely indicate?

  1. A blocked fuel filter
  2. Expansion cooling of the gas as pressure drops across the regulator Correct answer.
  3. A refrigerant leak from the air conditioning system
  4. Excessive engine coolant temperature

Explanation

Gas expanding across the regulator absorbs heat, which can chill the regulator and form frost, especially under high flow or cold conditions. It is not an A/C refrigerant leak, coolant temperature issue, or filter blockage.

Question 25CNG Fuel System Fundamentals, Safety, and Regulations

Why must vent lines from pressure relief devices be routed to discharge safely away from occupants and ignition sources?

  1. To direct released gas to a location where it will not create a fire or injury hazard Correct answer.
  2. To reduce the weight of the venting system
  3. To allow the driver to hear the vent operate
  4. To increase the venting pressure

Explanation

If a PRD vents, the released gas must be directed where it will not endanger occupants or ignite, which is why vent line routing is specified. Routing does not increase pressure, is not for driver awareness, and is not a weight-reduction measure.

Question 26CNG Fuel System Fundamentals, Safety, and Regulations

Which federal regulation establishes the initial qualification and periodic requalification requirements for CNG fuel containers used on motor vehicles?

  1. ANSI/CSA NGV2
  2. NFPA 52
  3. 49 CFR 571.304 Correct answer.
  4. 49 CFR 180.205

Explanation

FMVSS 304 (49 CFR 571.304) sets the initial performance and marking requirements for compressed natural gas fuel containers. Periodic requalification and in-service inspection requirements are found in 49 CFR 180.205 and 49 CFR 173.34. NFPA 52 is a consensus standard for CNG vehicular fuel systems, and CSA NGV2 is a manufacturer standard.

Question 27CNG Fuel System Fundamentals, Safety, and Regulations

Which document must be consulted to determine the correct replacement interval for a CNG fuel system component such as a coalescing filter element?

  1. The manufacturer's service information for that specific system Correct answer.
  2. The ASME Boiler and Pressure Vessel Code
  3. The vehicle owner's manual only
  4. The state vehicle inspection handbook

Explanation

Component service intervals are established by the system manufacturer and are published in the service information for the specific make and model. Owner's manuals may list some maintenance but are not the authoritative service reference. ASME and state inspection documents do not specify CNG component replacement intervals.

Question 28CNG Fuel System Fundamentals, Safety, and Regulations

A technician is preparing to defuel a CNG vehicle before replacing a fuel line. Which action is required to ensure the high-pressure section is isolated and depressurized?

  1. Open the manual shutoff valve and vent the line to atmosphere
  2. Close the manual shutoff valve at the cylinder and bleed residual pressure from the downstream line Correct answer.
  3. Disconnect the battery and wait for the system to self-vent
  4. Loosen the line fitting slowly and allow gas to escape until flow stops

Explanation

The cylinder valve must be closed to isolate the high-pressure source, and the downstream line must then be bled to a safe location to remove residual pressure before any fitting is opened. Venting the full cylinder or loosening a fitting under pressure creates a hazardous release. CNG systems do not self-vent when the battery is disconnected.

Question 29CNG Fuel System Fundamentals, Safety, and Regulations

A CNG cylinder shows a requalification date that has passed. What is the correct disposition of the cylinder?

  1. It may remain in service if it passes a visual inspection
  2. It may be used only at reduced pressure until the next inspection
  3. It may be returned to service after the label is updated
  4. It must be removed from service and requalified or condemned per the applicable standard Correct answer.

Explanation

A cylinder whose requalification interval has expired is no longer approved for service and must be requalified by an authorized facility or condemned. Visual inspection alone does not extend the requalification interval. Operating at reduced pressure or relabeling does not restore approval.

Question 30CNG Fuel System Fundamentals, Safety, and Regulations

A CNG vehicle is parked indoors overnight and a gas detector near the ceiling alarms. Which property of natural gas best explains why the sensor is mounted high in the bay?

  1. Natural gas is lighter than air and rises when released Correct answer.
  2. Natural gas condenses at room temperature and pools
  3. Natural gas is odorless and cannot be detected at low level
  4. Natural gas is heavier than air and accumulates at floor level

Explanation

Methane, the primary constituent of natural gas, has a density lower than air, so a release tends to rise and collect near the ceiling. Detectors are therefore placed high to sense accumulating gas. Natural gas is not heavier than air, and it does not condense at ambient conditions.

Question 31CNG Storage Cylinders and Valves

What is the primary purpose of the pressure relief device (PRD) fitted to a CNG storage cylinder?

  1. To prevent backflow of gas from the cylinder into the fill receptacle
  2. To vent cylinder contents when exposed to abnormally high temperature or pressure Correct answer.
  3. To filter particulate contamination from gas entering the cylinder
  4. To regulate the working pressure delivered to the fuel injectors

Explanation

A PRD is a safety device designed to release cylinder contents under fire or overpressure conditions to prevent catastrophic cylinder failure. It is not part of the normal fuel delivery or fill path. Its activation is a one-time protective event.

Question 32CNG Storage Cylinders and Valves

Which cylinder type is constructed with a metal liner fully wrapped with continuous filament composite and no structural metal in the cylindrical hoop direction?

  1. Type 2
  2. Type 1
  3. Type 3
  4. Type 4 Correct answer.

Explanation

Type 4 cylinders use a non-load-bearing plastic liner fully overwrapped with composite, so the composite carries all structural loads. Type 3 uses a metal liner with full composite wrap, and Types 1 and 2 are all-metal or hoop-wrapped metal. The distinction turns on liner material and extent of wrapping.

Question 33CNG Storage Cylinders and Valves

A technician is inspecting a Type 4 cylinder and finds the composite overwrap has a deep gouge exposing the underlying liner. What is the appropriate action?

  1. Apply a protective coating over the exposed area
  2. Reduce the service pressure and continue use
  3. Sand the area smooth and return the vehicle to service
  4. Remove the cylinder from service for evaluation or replacement Correct answer.

Explanation

Damage that penetrates the composite overwrap compromises the load-bearing structure of a Type 4 cylinder, so the cylinder must be removed from service. Field repair of structural composite damage is not permitted. Only the manufacturer or a qualified facility can determine fitness.

Question 34CNG Storage Cylinders and Valves

The manual shutoff valve at the CNG cylinder is normally kept in which position during vehicle operation?

  1. Fully open Correct answer.
  2. Half open to limit flow
  3. Open only while refueling
  4. Fully closed

Explanation

The cylinder manual valve must be fully open during normal operation so fuel can flow to the regulator and the full cylinder capacity is available. It is closed for service, storage, or emergency isolation. A partially open valve can restrict flow and is not a normal operating position.

Question 35CNG Storage Cylinders and Valves

What is the function of the check valve located in the cylinder valve or fill line?

  1. To allow gas to enter the cylinder while preventing reverse flow out of the fill port Correct answer.
  2. To measure the quantity of gas delivered during refueling
  3. To limit maximum cylinder pressure during fast fills
  4. To release gas if cylinder pressure exceeds the set point

Explanation

The check valve permits one-way flow into the cylinder during refueling and blocks reverse flow back through the fill receptacle. This keeps gas in the cylinder and protects the fill circuit. It does not regulate pressure or measure quantity.

Question 36CNG Storage Cylinders and Valves

An excess flow valve in a CNG cylinder valve is designed to actuate when:

  1. Downstream flow exceeds a predetermined rate due to a line rupture Correct answer.
  2. Cylinder pressure drops below the regulator set point
  3. Cylinder temperature rises above a set threshold
  4. The ignition key is turned to the off position

Explanation

An excess flow valve closes when flow rate exceeds a design threshold, which typically indicates a broken line or fitting downstream. It is a flow-responsive device, not a temperature or pressure-setpoint device. This limits the release of gas after a line failure.

Question 37CNG Storage Cylinders and Valves

When a cylinder valve is opened rapidly after refueling, a technician hears a sharp report and the excess flow valve trips. What is the most likely cause?

  1. The cylinder is overfilled beyond its rated capacity
  2. The valve was opened too quickly, creating a momentary high flow spike Correct answer.
  3. The PRD has ruptured and vented the cylinder
  4. The regulator diaphragm has failed

Explanation

Excess flow valves can trip from the transient flow surge created when a valve is snapped open quickly. The remedy is to close the valve and reopen it slowly to reset the device. This is a procedural cause rather than a component failure.

Question 38CNG Storage Cylinders and Valves

What is the typical service pressure rating of a CNG storage cylinder used on light-duty vehicles?

  1. 1,000 psi
  2. 5,000 psi
  3. 3,600 psi Correct answer.
  4. 2,400 psi

Explanation

Light-duty CNG cylinders are commonly rated at 3,600 psi service pressure at a reference temperature of 70 degrees F. This allows the system to accommodate fill pressure rise with ambient temperature. Lower and higher values correspond to other applications or test pressures.

Question 39CNG Storage Cylinders and Valves

A cylinder is marked with a service pressure of 3,600 psi. When full at 70 degrees F, the cylinder pressure is approximately:

  1. 4,500 psi
  2. 3,600 psi Correct answer.
  3. 2,400 psi
  4. 1,800 psi

Explanation

Service pressure is defined at the 70 degrees F reference, so a cylinder filled to its rated capacity at that temperature reads its service pressure. Higher ambient temperatures raise the reading, and lower temperatures reduce it. The 3,600 psi value is the reference-condition fill pressure.

Question 40CNG Storage Cylinders and Valves

Which cylinder valve feature is intended to prevent the valve from being sheared off and releasing the full cylinder contents in a collision?

  1. A fusible plug
  2. A pressure regulator
  3. A coalescing filter
  4. A flow-limiting or shear-protected valve design Correct answer.

Explanation

Flow-limiting valves and shear-protected valve mounting are designed so that impact damage does not result in an unrestricted release. Some designs incorporate a breakaway section that seals if the valve is struck. A fusible plug responds to heat, not mechanical shear.

Question 41CNG Storage Cylinders and Valves

How is a fusible plug in a CNG cylinder valve intended to function?

  1. It ruptures at a set pressure to relieve overpressure
  2. It indicates cylinder fill level by changing color
  3. It seals a leak when flow reverses
  4. It melts at a set temperature to vent cylinder pressure Correct answer.

Explanation

A fusible plug uses a low-melting alloy that softens and releases at a specified elevated temperature, venting gas to prevent cylinder rupture in a fire. It is temperature-actuated rather than pressure-actuated. This is distinct from a pressure relief device that responds to pressure.

Question 42CNG Storage Cylinders and Valves

During a periodic cylinder inspection, a technician notices the composite surface has a chalky, faded appearance with fine surface crazing but no exposed fibers. What is the best interpretation?

  1. The condition may be cosmetic surface aging, but it should be evaluated against the manufacturer's inspection criteria Correct answer.
  2. The cylinder is contaminated with oil and must be flushed
  3. The cylinder is overpressurized and should be vented
  4. The cylinder has failed structurally and must be scrapped immediately

Explanation

Surface fading and fine crazing can be cosmetic weathering, but acceptance depends on the manufacturer's published damage criteria and depth limits. Exposed or broken fibers indicate structural damage, which is different from superficial aging. The correct action is to evaluate against the approved criteria rather than guess.

Question 43CNG Storage Cylinders and Valves

What does the fuel gauge on a CNG vehicle actually measure, and how does it relate to cylinder pressure?

  1. It measures mass of fuel directly and is unaffected by temperature
  2. It measures flow rate through the regulator
  3. It typically reads cylinder pressure, which varies with temperature for a given fuel quantity Correct answer.
  4. It measures the volume of gas remaining at standard conditions

Explanation

Most CNG fuel gauges are pressure-based, so the indicated level changes with temperature even when the amount of fuel is constant. A hot cylinder reads higher and a cold one lower for the same mass. This is why indicated level can shift after the vehicle sits.

Question 44CNG Storage Cylinders and Valves

Why does a CNG cylinder's pressure reading rise after a fast fill on a hot day?

  1. The gas temperature rises during compression, and pressure tracks temperature at constant volume Correct answer.
  2. The regulator is malfunctioning and adding pressure
  3. The pressure relief device is leaking gas into the cylinder
  4. Additional gas is being generated inside the cylinder

Explanation

Fast filling heats the gas through compression and friction, and at fixed cylinder volume a temperature increase raises pressure. This is why fill stations compensate for temperature. The gas quantity is roughly constant once filling stops.

Question 45CNG Storage Cylinders and Valves

Which component in the CNG system reduces high cylinder pressure to the lower pressure needed by the fuel injectors?

  1. The excess flow valve
  2. The fill receptacle
  3. The pressure regulator Correct answer.
  4. The check valve

Explanation

The pressure regulator reduces stored cylinder pressure to a controlled delivery pressure for the fuel system. It may be a single-stage or multi-stage design. The other components serve fill, flow protection, or connection functions.

Question 46CNG Storage Cylinders and Valves

A technician must replace a damaged high-pressure CNG line between the cylinder valve and the regulator. Which practice is required?

  1. Use standard fuel hose with hose clamps
  2. Use only lines rated for CNG service pressure and approved for the application Correct answer.
  3. Substitute copper tubing for flexibility
  4. Use any hydraulic-rated hose of the same diameter

Explanation

High-pressure CNG lines must be rated for the system service pressure and approved for compressed natural gas service. Substituting non-approved hose or tubing risks rupture and is not permitted. Fittings and routing must also follow the manufacturer's specifications.

Question 47CNG Storage Cylinders and Valves

What is the purpose of shielding or guarding around a CNG cylinder valve?

  1. To protect the valve from impact damage that could cause a release Correct answer.
  2. To keep the valve clean of dust
  3. To muffle the sound of gas flow
  4. To insulate the valve from engine heat

Explanation

Valve guards or shields protect the valve body and fittings from mechanical impact in a collision or during service. Impact damage to an unprotected valve can cause a sudden high-pressure release. The guard is a structural protection measure.

Question 48CNG Storage Cylinders and Valves

A cylinder's requalification interval has expired, but the cylinder appears undamaged and holds pressure. What is the correct course of action?

  1. Paint over the requalification label and continue use
  2. Reduce fill pressure by 25 percent and continue use
  3. Continue using it since it holds pressure
  4. Remove it from service until it is requalified or replaced per the applicable standard Correct answer.

Explanation

Cylinders must be requalified at the interval set by the governing standard, and an expired cylinder cannot legally remain in service regardless of appearance. Holding pressure does not substitute for requalification. The cylinder must be requalified by an approved facility or replaced.

Question 49CNG Storage Cylinders and Valves

Which statement best describes the difference between a Type 2 and a Type 3 CNG cylinder?

  1. Type 2 is all-composite, while Type 3 has no liner
  2. Type 2 and Type 3 are identical except for paint color
  3. Type 2 uses a plastic liner, while Type 3 uses no composite
  4. Type 2 is a metal cylinder hoop-wrapped with composite, while Type 3 has a metal liner fully wrapped with composite Correct answer.

Explanation

Type 2 cylinders are metal with composite wrap only around the hoop direction, while Type 3 cylinders use a metal liner fully overwrapped with composite. The extent of wrapping and the load-sharing liner distinguish them. Type 4 is the all-composite-with-plastic-liner design.

Question 50CNG Storage Cylinders and Valves

A vehicle is involved in a collision that struck the CNG cylinder area. Which action is appropriate before returning the vehicle to service?

  1. Simply reset the excess flow valve and drive
  2. Inspect the cylinder, valve, lines, and mounts per the manufacturer's post-collision procedure, and replace damaged components Correct answer.
  3. Lower the fill pressure and continue use
  4. Return to service if no gas is heard leaking

Explanation

A collision in the cylinder area requires a documented post-collision inspection of the cylinder and its attachments because damage may not be audible or visible. Components that fail the inspection must be replaced. Absence of a leak is not sufficient evidence of integrity.