ASE H1 CNG Engines (Transit Bus) flashcards
ASE H1 CNG Engines (Transit Bus) · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- CNG cylinder working pressure
- Nominal full-cylinder pressure is about 3,600 psi (25 MPa) at 70 degrees F; pressure rises above this on hot days due to thermal expansion.
- Methane flammable range
- Methane is flammable in air from roughly 5 to 15 percent by volume; a gas detector reads percent LEL, with 100 percent LEL equal to the lower flammable limit.
- Manual shutoff valve
- A quarter-turn valve at each cylinder or cylinder group that isolates the storage from the rest of the fuel system before service.
- Bleed-down procedure
- Close the manual shutoff valve, run the engine until it stalls to consume residual fuel, then vent remaining pressure through an approved vent line.
- Type III cylinder
- A CNG cylinder with a seamless aluminum liner fully wrapped with carbon-fiber composite; common in transit bus roof-mounted storage.
- Pressure relief device
- A one-time thermal safety device on each cylinder that vents gas when a set temperature (often near 212 degrees F) is reached, preventing cylinder rupture in a fire.
- Cylinder visual inspection
- An annual inspection for cuts, gouges, abrasion, impact damage, and corrosion on the composite wrap; damage beyond limits requires removal from service.
- Cylinder service life
- The manufacturer-stated life, commonly 15 to 20 years; the cylinder must be removed from service and not requalified beyond that date.
- Two-stage pressure regulation
- First stage reduces cylinder pressure to an intermediate level (about 100 to 200 psi); second stage trims it to injection rail pressure, often 100 to 150 psi.
- Regulator freeze-up
- Expanding gas absorbs heat and can chill the regulator and lines; coolant heating of the regulator prevents icing and pressure instability.
- Rail pressure sensor
- Reports actual fuel rail pressure to the ECM; a mismatch between commanded and actual pressure sets a fuel-pressure fault code.
- Fuel shutoff solenoid
- A solenoid valve that closes the fuel supply when the ignition is off or when pressure falls below a set threshold, providing a fail-safe shutdown.
- Gas injector pulse width
- The ECM-controlled injector open time computed from mass air flow, target lambda, gas density, and rail pressure; longer than gasoline for equal energy.
- Peak-and-hold injector drive
- The ECM applies a high peak current to open the gas injector, then a lower hold current to keep it open and limit heating.
- Bi-fuel fuel map
- A separate ECM calibration used when the engine runs on CNG; the ECM enables it only when the fuel selector and tank pressure conditions are met.
- Fuel trim interpretation
- Positive trim means the ECM is adding fuel to correct a lean condition; negative trim means it is removing fuel to correct a rich condition.
- CNG spark plug heat range
- CNG engines typically use a plug one heat range colder than the gasoline equivalent, gapped to the CNG engine specification.
- Waste-spark ignition
- One coil fires two paired cylinders, one on compression and one on exhaust; a misfire affects both, so identify the paired cylinder.
- Knock sensor function
- Detects detonation and signals the ECM to retard timing; a failed sensor causes retarded timing and reduced power on high-compression CNG engines.
- Misfire monitor
- An OBD monitor that detects misfire from crankshaft speed fluctuation; on CNG, weak coils, worn plugs, or wrong gap are common causes under load.
- Stoichiometric methane ratio
- The chemically correct air/fuel ratio for methane is about 17.2:1 by mass, compared with about 14.7:1 for gasoline.
- Wideband oxygen sensor
- Reports exhaust lambda directly over a wide range, letting the ECM trim fuel precisely to hold lambda near 1.0 in closed loop.
- Methane slip
- Unburned methane escaping in the exhaust; it is a potent greenhouse gas, so the catalyst must be matched to methane oxidation.
- Upstream exhaust leak effect
- An exhaust leak ahead of the oxygen sensor lets outside air reach the sensor, causing a false lean reading and positive fuel trim.
- Cylinder leakage test
- Introduces air into a cylinder at TDC and locates the leak path: intake, exhaust, crankcase (rings), or coolant (head gasket).
- CNG exhaust valve wear
- Higher exhaust temperatures on methane cause burned exhaust valves and seat recession; valve and seat materials must be rated for CNG service.
- Valve lash effect
- Lash that is too tight holds a valve off its seat, causing a misfire, low compression, and possible valve burning; check at the manufacturer interval.
- Idle manifold vacuum
- Normal idle vacuum is about 17 to 22 in-Hg at sea level; a low or fluctuating reading indicates a vacuum leak, valve problem, or timing issue.
- Freeze-frame data
- A snapshot of operating conditions stored when a diagnostic code sets; record it before clearing codes to guide diagnosis.
- 5-volt reference
- A regulated supply from the ECM to sensors; a shorted or open reference circuit can set multiple sensor codes at once.
- Bidirectional scan tool
- A tool that can command actuators such as injectors and solenoids and read live data, allowing the technician to verify actuator response.
- Voltage-drop testing
- Measures voltage lost across a circuit under load; a high-resistance ground or connection causes sensor errors and no-start conditions that an ohmmeter misses.