ASE H1 CNG Engines (Transit Bus) cheat sheet

Natural gas is lighter than air, so it disperses upward in an open area but can accumulate under a vehicle hood, in pits, or in enclosed service bays; ventilate at the high point.

CNG cylinder working pressure is typically 3,600 psi (25 MPa) at 70 degrees F; a full cylinder can read well above 3,600 psi on a hot day due to thermal expansion.

Ignition temperature of methane is roughly 1,000 to 1,100 degrees F, and the flammable range in air is about 5 to 15 percent by volume; a combustible gas detector reads percent LEL.

Before any fuel system service, close the manual shutoff valve at the cylinder(s), run the engine until it stalls to bleed pressure, then vent residual pressure through an approved vent line.

A leak check uses a combustible gas detector or approved leak-detection solution at each fitting; never use an open flame.

NFPA 52 requires the fuel system be inspected after any collision, fire, or component replacement, and cylinders must be visually inspected at least every 12 months.

CNG fuel lines and fittings are rated for high pressure; never substitute standard hydraulic or air fittings, and use only tubing rated for CNG service.

Depressurize and lock out before opening any high-pressure joint; a 3,600 psi fitting released suddenly can cause severe injury from the escaping gas jet.

Cylinders on transit buses are typically Type III (aluminum liner with carbon-fiber composite wrap) or Type IV (plastic liner with carbon-fiber wrap), both mounted in roof or rear enclosures.

Cylinder service life is set by the manufacturer and by the composite wrap; a common rule is 15 to 20 years or the manufacturer's stated life, whichever comes first.

Cylinders must be requalified or replaced at the manufacturer's stated interval; visual inspection checks for abrasion, impact damage, cuts, and corrosion.

The manual shutoff valve (quarter-turn) isolates each cylinder or cylinder group from the rest of the fuel system.

A pressure relief device (PRD) protects each cylinder; a thermally activated PRD vents gas if the cylinder reaches a set temperature (often around 212 degrees F), and it is a one-time device.

High-pressure stainless-steel rigid tubing runs from the cylinders to the regulator; flexible lines are used only where movement is required and must be rated for the system pressure.

Cylinder mounting brackets must be secure and the mounting must not allow the cylinder to chafe against the enclosure; loose mounts are a rejection item.

A cylinder that has been exposed to fire, or that shows a bulge, dent, or deep gouge in the composite, must be removed from service and not returned to service.

A two-stage regulator system reduces cylinder pressure (up to 3,600 psi) to a low pressure suitable for injection, commonly around 100 to 150 psi at the injector rail.

The high-pressure regulator (first stage) reduces cylinder pressure to an intermediate pressure, often about 100 to 200 psi; the low-pressure regulator (second stage) trims it to injection pressure.

A pressure regulator uses a diaphragm and spring reference; a reference to manifold pressure (or a differential-pressure sensor) keeps the pressure drop across the injectors constant.

A coolant-heated regulator prevents freeze-up: expanding gas absorbs heat and can chill the regulator and lines, causing icing and pressure instability.

A low-pressure lockout or fuel shutoff solenoid closes the fuel supply if pressure falls below a set threshold or if the ignition is off.

A pressure sensor at the fuel rail reports actual rail pressure to the ECM; a mismatch between commanded and actual pressure sets a fuel-pressure fault.

A restricted high-pressure filter or a failing regulator shows up as low rail pressure under load and a lean mixture or power loss.

The fuel pressure regulator's vacuum or boost reference must be connected; a disconnected reference causes rich or lean errors that vary with load.

Dedicated CNG engines use gas injectors at the intake port or in a common rail; injector pulse width is calculated from mass air flow, target lambda, and gas density.

Because methane has a lower volumetric energy density than gasoline, the injector pulse width for the same energy is longer, and injector sizing and rail pressure are matched to the gas flow required.

Gas injectors are typically solenoid or peak-and-hold; the ECM uses a peak current to open and a lower hold current to keep the injector open.

The ECM calculates fuel mass using the ideal gas law and the measured rail pressure and temperature; a failed rail pressure or temperature sensor corrupts the fuel calculation.

Injector balance testing compares cylinder-to-cylinder fuel trim or a pressure-drop test; a clogged gas injector shows as a lean trim on one cylinder.

A dedicated CNG engine has no gasoline injectors; the ECM's fuel strategy is calibrated for methane's octane rating (about 120 to 130 RON), allowing higher compression and advanced spark.

Bi-fuel engines switch between gasoline and CNG maps; the ECM must see the fuel selector input and the correct tank pressure before it will enable CNG operation.

Fuel trim (short- and long-term) is the primary diagnostic for mixture errors; positive fuel trim means the ECM is adding fuel to correct a lean condition.

Methane requires a higher breakdown voltage than gasoline mixtures at the same compression, so CNG engines use higher-output coils and wider plug gaps or higher-energy ignition.

Spark plug heat range for CNG is typically one step colder than the gasoline equivalent, and plug gap is set to the engine manufacturer's CNG specification.

CNG burns slower and hotter at the exhaust valve; ignition timing is advanced compared with gasoline to maintain combustion phasing.

Waste-spark or coil-on-plug systems are common; a misfire on a waste-spark pair affects two cylinders, so identify the paired cylinder.

A misfire monitor uses crankshaft speed fluctuation; a CNG-specific misfire under load often traces to a weak coil, worn plug, or incorrect plug gap.

Ignition system diagnosis includes checking coil primary resistance, secondary output with a spark tester, and plug condition (fouling, gap erosion, cracked insulator).

Knock sensor response is critical on CNG because high compression and advanced timing can cause detonation; a failed knock sensor causes the ECM to retard timing and reduce power.

Secondary ignition voltage requirements rise as plug gap erodes; a plug gap that is too wide can cause misfire under high cylinder pressure.

Stoichiometric air/fuel ratio for methane is about 17.2:1 by mass, compared with about 14.7:1 for gasoline; the ECM fuel map is calibrated to this ratio.

Closed-loop fuel control uses an oxygen (or wideband lambda) sensor; the ECM trims fuel to hold lambda near 1.0 in steady-state operation.

A wideband sensor reports lambda directly; a narrowband sensor switches around stoichiometry and is used for trim in older systems.

CNG produces lower CO2 and particulate emissions than diesel but can produce higher methane (CH4) slip, which is a potent greenhouse gas; a catalyst must be matched to methane oxidation.

Three-way catalysts on CNG engines reduce NOx, CO, and methane; a degraded catalyst shows as a P0420-type efficiency fault.

Positive fuel trim beyond about plus or minus 10 percent indicates a mixture problem; a vacuum leak causes positive trim, a leaking injector or high fuel pressure causes negative trim.

Exhaust gas recirculation and evaporative systems differ on CNG: there is no evaporative canister for methane, but EGR is still used to control NOx.

An exhaust leak upstream of the oxygen sensor causes a false lean reading and positive fuel trim; check for leaks before replacing sensors.

Dedicated CNG engines often run higher compression (around 11:1 to 12:1) than gasoline; a compression test must be compared with the CNG engine specification, not a gasoline spec.

A cylinder leakage test locates the leak path: air out the intake (intake valve), exhaust (exhaust valve), crankcase (rings), or coolant (head gasket).

CNG burns hotter at the exhaust valve and seat, so burned exhaust valves and valve seat recession are common mechanical failures on high-mileage CNG engines.

Valve lash must be checked and adjusted at the manufacturer's interval; tight lash holds a valve open and causes a misfire and low compression.

A running compression test or a cranking vacuum test can reveal a valve timing or camshaft problem that a static compression test misses.

Intake manifold vacuum at idle is typically 17 to 22 in-Hg at sea level; a low or fluctuating reading indicates a vacuum leak, valve problem, or timing issue.

Cylinder head work on a CNG engine requires valve and seat materials rated for the higher exhaust temperatures; standard gasoline seats may not hold up.

An engine that cranks but will not start on CNG should be checked for fuel pressure at the rail, spark, and injector pulse before mechanical tear-down.

The ECM controls fuel injection, ignition, and pressure regulation; the CNG system also has a dedicated fuel control module or is integrated into the ECM with a gas-specific calibration.

OBD-II monitors on a CNG bus include misfire, fuel system, oxygen sensor, catalyst, and the CNG-specific pressure and sensor circuits.

A scan tool reads freeze-frame data and pending codes; always record freeze-frame before clearing codes.

Sensor reference voltage is typically 5.0 volts; a shorted or open reference circuit causes multiple sensor codes at once.

A pressure sensor signal is a voltage proportional to pressure; check the signal against a known mechanical gauge before condemning the sensor.

Actuator tests (injector balance, EVAP purge, fuel pump/solenoid) are performed with a bidirectional scan tool; verify the actuator actually responds.

Wiring diagnosis uses voltage-drop testing under load rather than ohmmeter checks alone; a high-resistance ground causes sensor errors and no-start conditions.

After any fuel system repair, clear codes, run the monitor readiness cycle, and verify no pending codes return before releasing the bus.