ASE F1 Compressed Natural Gas Vehicles key terms

Natural gas, mostly methane, that has been compressed to a high pressure (typically 3,000 or 3,600 psi) so a large amount of energy can be stored in a small onboard tank volume for vehicle use.

A rating that expresses a fuel's resistance to knock on a scale where pure methane is 100 and hydrogen is 0; CNG's high number allows higher compression ratios than gasoline in a dedicated engine.

A unit that compares the energy in a quantity of alternative fuel to the energy in one gallon of gasoline, about 126.7 cubic feet of natural gas at standard conditions, used for pricing and fuel economy comparisons.

An engine designed and built to run only on natural gas, with no gasoline fuel system, usually featuring higher compression, hardened valves and seats, and gas-specific fuel and ignition controls.

A vehicle carrying two separate fuel systems, typically CNG and gasoline, that can run on either fuel but not mixed in the same combustion event; it usually starts and warms on gasoline.

The rated pressure at which a CNG cylinder or component is designed to operate when fully filled at a reference temperature, commonly 3,000 psi or 3,600 psi in vehicle applications.

A pressure higher than service pressure, commonly 1.5 times service pressure, that a cylinder or component must withstand during hydrostatic or proof testing without rupture or permanent deformation.

A pressure test in which a cylinder is filled with water or another liquid and pressurized to verify structural integrity; the incompressible liquid limits stored energy and makes failure less violent than a gas test.

The periodic inspection and testing a CNG cylinder must undergo at intervals set by code, such as every three or five years depending on type, to confirm it remains safe for continued service.

Construction classifications for CNG cylinders: all-metal, metal liner with hoop-wrapped composite, metal liner with fully wrapped composite, and plastic liner with fully wrapped composite, differing in weight and inspection needs.

Harm to the fiber/resin overwrap of a wrapped CNG cylinder, such as cuts, fraying, impact whitening, or heat damage, which can reduce strength and may require the cylinder to be condemned.

The high-pressure shutoff valve threaded into each CNG cylinder that seals gas in the cylinder and supplies it to the system; it may contain an excess-flow device and a pressure relief device.

A safety device on a CNG cylinder or valve that vents the cylinder contents when temperature or pressure reaches a set limit, protecting the cylinder from overpressure or fire exposure.

A pressure relief device that opens when a fusible element or shape-memory element senses a high temperature, such as in a fire, allowing gas to vent before the cylinder is weakened by heat.

A device in the cylinder valve or fuel line that restricts or stops gas flow when flow exceeds a set rate, limiting the release if a line downstream is severed or badly leaking.

Rigid tubing or hose, typically stainless steel or reinforced material rated for full cylinder pressure, that carries gas from the cylinders to the regulator or fill point.

A rigid metal tube, usually stainless steel, used for high-pressure CNG plumbing because it resists pressure, abrasion, and vibration better than flexible hose in fixed runs.

A compression-style connection in which a cone-shaped flare on the tube end is clamped between a mating seat and nut to seal high-pressure gas; correct angle and torque are critical to prevent leaks.

A hand-operated valve, usually at the cylinder or a service point, that lets a technician or driver stop gas flow from the cylinders for service or in an emergency.

A device that reduces high cylinder pressure to a lower, stable working pressure supplied to the injectors or mixer, using a diaphragm and spring to hold outlet pressure nearly constant as demand changes.

A pressure-reduction scheme in which gas passes through two regulators in series, the first dropping cylinder pressure to an intermediate level and the second trimming it to injector or mixer pressure for stable control.

Icing or frost on a regulator caused by the cooling that occurs as high-pressure gas expands; it can stick the diaphragm or block flow, and is managed by engine coolant heating of the regulator.

A dash-mounted control on a bi-fuel vehicle that lets the driver choose gasoline or CNG operation and often shows fuel level or pressure status for each fuel.

An electrically operated valve that meters natural gas into the intake port or cylinder, opened by the engine control module for a calculated pulse width to deliver the needed fuel mass.

The length of time, in milliseconds, that the ECM holds a gas injector open each cycle; it is the main command used to set fuel quantity delivered to the engine.

A fuel delivery layout in which gas is injected into the intake port just upstream of the intake valve, mixing with air before entering the cylinder.

An air/fuel metering device that uses a venturi in the intake airstream to create a low-pressure signal that draws and proportions gas into the incoming air, common on older or simpler conversions.

The air-to-fuel mass ratio at which all fuel and oxygen are consumed in combustion; for natural gas it is roughly 17:1 by mass, higher than gasoline's approximately 14.7:1.

Short- and long-term corrections the ECM applies to fuel delivery based on oxygen sensor feedback, adjusting injector pulse width to keep the mixture near the target ratio.

An oxygen sensor that reports a continuous air/fuel ratio over a broad range rather than only rich/lean switching, giving the ECM precise mixture feedback for CNG fuel control.

A sensor that detects engine vibration from detonation and signals the ECM to retard spark timing, protecting the engine especially when fuel quality or pressure is off.

The number of crankshaft degrees before top dead center that the spark fires; CNG's high knock resistance often allows more advance than gasoline, improving efficiency if calibrated correctly.

The electrode spacing and thermal rating of a spark plug; CNG's higher ignition voltage demand and lean burn often call for a specific gap and colder or gas-recommended plug to avoid misfire and pre-ignition.

The transformer that steps battery voltage up to the high voltage needed to jump the spark plug gap; CNG mixtures can demand more voltage, so coil condition strongly affects misfire.

The fill connection on the vehicle, commonly a 3/8-inch or 1/4-inch quick-connect style, where the dispenser nozzle attaches to deliver high-pressure gas into the cylinders.

A one-way valve in the fill path that allows gas into the cylinders but blocks backflow out through the receptacle, keeping the fill line depressurized after fueling.

The controlled removal of gas from a vehicle's cylinders, done with approved equipment and procedures before service, cylinder removal, or disposal, never by simply venting to atmosphere indoors.

Finding gas releases using methods such as soap solution, electronic combustible-gas detectors, or pressure-decay observation, performed in ventilated areas with the system pressurized.

The visual and physical examination of CNG cylinders for damage, corrosion, label legibility, and valve condition, performed at scheduled intervals and after incidents like impact or fire exposure.

A diagnostic method that pressurizes a closed section of the CNG system, isolates it, and watches for pressure drop over time to reveal leaks that are too small to hear or see.

Using a diagnostic scan tool to read live ECM parameters such as fuel trim, injector pulse width, and sensor voltages, then comparing them to specifications to isolate CNG driveability faults.