ASE H1 CNG Engines (Transit Bus) key terms

Natural gas, mostly methane, stored under high pressure in onboard cylinders for use as engine fuel. It is lighter than air and burns within a narrow range of air/fuel mixtures.

A rating of a gas fuel's resistance to detonation, similar in purpose to octane number for gasoline. Higher methane number means greater knock resistance and allows more spark advance.

A high-pressure vessel, typically Type I through Type IV construction, that stores CNG onboard. It is rated by service pressure and must be inspected and requalified per its marking and applicable standards.

The nominal working pressure a CNG cylinder is designed to hold at a stated reference temperature, commonly 3000 or 3600 psi. Pressures above this during filling are temperature-compensated, not overpressure.

Periodic inspection and testing a CNG cylinder must undergo at intervals set by its marking and governing standards to confirm it remains safe for continued service.

An external and, where allowed, internal examination of a CNG cylinder for damage such as gouges, abrasion, impact, heat damage, bulging, or corrosion that could compromise its integrity.

The shutoff valve threaded into each CNG cylinder that isolates its contents. It may contain an excess-flow or check feature and must be protected from damage and impact.

A device that restricts or stops gas flow when it exceeds a set rate, limiting release from a broken line or fitting. It helps contain fuel after a downstream failure.

A safety device on a CNG cylinder that vents its contents under specified conditions, such as excessive temperature or pressure, to prevent cylinder rupture.

A hose or tube that routes gas released by a pressure relief device or other component to a safe discharge location away from the vehicle and occupants.

A hand-operated valve used to stop fuel flow from the cylinders for service or in an emergency. Its location and operation are part of required safety knowledge.

Rigid tubing or hose that carries CNG from the cylinders to the regulator at full storage pressure. It must be rated for the pressure and routed and secured to resist damage.

The section of fuel plumbing downstream of the pressure regulator that carries reduced-pressure gas to the injectors or mixer. It is rated for lower pressure than the supply lines.

A device that reduces high cylinder pressure to a stable lower working pressure delivered to the fuel metering system, often using engine coolant heat to prevent freezing.

Ice formation on or inside a pressure regulator caused by the cooling effect of gas expansion, which can restrict flow or cause poor running. Coolant heating and moisture control prevent it.

A coolant-heated component that warms the gas before or within the regulator to offset expansion cooling. Loss of coolant flow can cause freeze-up and driveability complaints.

An electrically operated valve that opens to admit fuel only when the engine is running or cranking and the control system permits it. It provides a positive fuel shutoff when the key is off.

Delivery of gaseous fuel to the engine through injectors controlled by the engine management system. Gas injectors meter fuel by opening time and are fed at regulated pressure.

An electrically operated valve that delivers a metered quantity of gaseous fuel to an intake port or cylinder. Its flow depends on supply pressure, orifice size, and pulse width.

The commanded open time of a fuel injector, expressed in milliseconds. The control module varies it to change delivered fuel quantity in response to operating conditions.

The proportion of air mass to fuel mass delivered to the engine. CNG's chemically correct ratio differs from gasoline, so the control system and diagnostics account for the fuel being used.

The air/fuel ratio at which fuel and oxygen are present in exactly the proportions needed for complete combustion. For natural gas this is approximately 17:1 by mass.

Fuel control in which the module adjusts injector pulse width based on oxygen sensor feedback to keep mixture near stoichiometric. It corrects for fuel composition and wear over time.

The learned and immediate corrections the control module applies to fuel delivery, expressed as short-term and long-term values. Large trims indicate mixture errors such as vacuum leaks or low fuel pressure.

A sensor in the exhaust that produces a signal indicating oxygen content, used by the control module to judge mixture. Wideband types report a proportional ratio rather than a simple rich/lean switch.

A sensor that detects abnormal combustion vibration and lets the control module retard spark timing to protect the engine. CNG's high knock resistance allows aggressive timing that depends on it.

Abnormal, rapid combustion that produces a sharp metallic sound and can damage the engine. It is influenced by fuel quality, compression ratio, spark timing, and mixture.

The crank angle at which the spark occurs relative to top dead center. CNG engines often use more advance than gasoline because of the fuel's higher knock resistance.

The air gap between the spark plug electrodes. CNG's higher firing voltage demand makes gap specification and electrode wear especially important for reliable ignition.

A transformer that steps up battery voltage to the high voltage needed to jump the spark plug gap. Coils may be individual or part of a coil-on-plug or distributorless arrangement.

Failure of a cylinder to ignite its charge or to burn it completely, causing roughness, power loss, and elevated emissions. It may be detected through crankshaft speed variation or ion sensing.

The ratio of cylinder volume at bottom dead center to volume at top dead center. Dedicated CNG engines often use higher ratios to exploit the fuel's knock resistance.

A measurement of cylinder pressure during cranking used to assess ring, valve, and head gasket sealing. Comparing cylinders helps locate mechanical faults causing misfire or low power.

A test that applies compressed air to a cylinder at top dead center and measures leakage, with escaping air location indicating the faulty path such as valves, rings, or gasket.

The clearance in the valvetrain between components such as rocker and valve stem. Correct adjustment is needed for proper valve timing, seating, and sealing, and is part of routine mechanical service.

The computer that controls fuel delivery, spark timing, and emissions functions by processing sensor inputs and commanding actuators. It also stores diagnostic information and runs monitors.

A code stored by the control module when a monitored circuit or system falls outside expected limits. It guides diagnosis but does not by itself identify the failed component.

A snapshot of operating conditions such as speed, load, and fuel trim recorded when a diagnostic trouble code was set. It helps recreate the fault for testing.

Adjustment of fill pressure or fuel calculations based on gas temperature, because pressure rises with heat. It prevents under-filling in cold conditions and over-pressurizing when hot.

An engine that can run on either CNG or a liquid fuel such as gasoline or diesel, selected manually or automatically. Fuel control and diagnostics must account for both fuels.

An engine designed and calibrated to operate only on natural gas, typically with higher compression and optimized timing and fuel control for that fuel.

A driver-operated control that chooses which fuel a bi-fuel vehicle uses. The control module manages changeover, including purging and enabling the appropriate fuel system.