Compressed Natural Gas (CNG) School Bus Operations
Compressed natural gas (CNG) school buses use natural gas stored in high-pressure cylinders at 3,600 pounds per square inch (psi) to power an internal combustion engine modified for gaseous fuel. CNG buses have been in school district fleets since the 1990s and offer reduced particulate emissions and lower fuel costs than diesel, though the infrastructure investment for fueling stations is significant. CNG is primarily methane, which is lighter than air — unlike propane or gasoline vapors, methane disperses upward and dissipates quickly in open air, reducing the risk of ground-level vapor accumulation and ignition.
The high-pressure CNG cylinders on school buses are typically mounted on the roof or rear of the bus. These cylinders are designed to withstand significant impact and are equipped with pressure relief devices that vent gas safely if cylinder pressure exceeds design limits. Drivers should be aware of the cylinder locations on their specific bus and avoid situations that could cause physical damage to the cylinders (low overhead clearances, scraping against overhead structures, rollover scenarios). Cylinder inspections are required at intervals specified by the cylinder manufacturer and DOT, and any cylinder showing signs of dents, gouges, corrosion, or heat exposure from fire must be removed from service immediately.
CNG bus pre-trip inspection includes checking the fuel pressure gauge (minimum starting pressure varies by manufacturer but is typically 300–500 psi minimum for normal operation), inspecting visible cylinder mounting hardware and fuel lines for signs of damage or corrosion, and checking for gas odor (while natural gas itself is odorless, an odorant — mercaptan — is added to aid leak detection). A smell of natural gas at the cylinder area or in the engine compartment requires that the driver not start the vehicle and immediately notify maintenance. CNG buses should not be fueled or serviced in enclosed spaces with inadequate ventilation, though the upward dispersal characteristic makes CNG safer than propane in enclosed fueling areas.
In the event of a fire on a CNG bus, the driver should evacuate all students immediately and keep them at least 300 feet away from the vehicle. CNG cylinders exposed to fire may vent gas through pressure relief devices, producing large flames. Emergency responders need to know the vehicle is CNG-equipped before approaching a fire scene.