Westport Power Inc.
1691 West 75th Avenue
Dear Mr. Houle:
This responds to your email requesting our interpretation of whether an accumulator vessel is regulated under Federal Motor Vehicle Safety Standard No. 304, Compressed Natural Gas Fuel Container Integrity (FMVSS No. 304). Specifically, you ask if S3 (Application) of Standard No. 304 covered your implementation of [a] small accumulator vessel as a pressure damping device and not a storage device. Based on the information you have provided, we conclude that this tank is subject to FMVSS No. 304.
By way of background, the National Highway Traffic Safety Administration (NHTSA) is authorized to issue FMVSSs that set performance requirements for new motor vehicles and items of motor vehicle equipment (see 49 U.S.C. Chapter 301). NHTSA does not provide approvals of motor vehicles or motor vehicle equipment. Instead, manufacturers are required to self-certify that their products conform to all applicable safety standards that are in effect on the date of manufacture. NHTSA selects a sampling of new vehicles and equipment each year to determine their compliance with applicable FMVSSs. If our testing or examination reveals an apparent noncompliance, we may require the manufacturer to remedy the noncompliance, and may initiate an enforcement proceeding if necessary to ensure that the manufacturer takes appropriate action.
In your email, you described a fuel system consisting of one or more liquefied natural gas (LNG) storage vessels with internally mounted pumps and vaporizers. You said that the LNG system compresses and vaporizes the LNG and delivers compressed natural gas (CNG) to another system that delivers CNG to the engine-mounted fuel system. Your question was whether a 9-liter Type 3 vessel would be subject to the requirements of Standard No. 304 if its purpose was as an accumulator to provide additional system volume to compensate for pump pressure fluctuations.
Standard No. 304 specifies requirements for the integrity of CNG motor vehicle fuel containers. As defined in S4 of Standard No. 304, CNG fuel container means a container designed to store CNG as motor fuel on-board a motor vehicle. You state that The
accumulator is not intended to provide storage capacity but is purely there to dampen pressure pulsations. Despite your assertion, it appears from your description and schematics of the accumulator vessel that the container in question could and most likely does store CNG.
Based on your schematics, NHTSA believes that the tank falls within the category of vessels that FMVSS No. 304 is meant to regulate. Your email explained that the accumulator tank maintains pressure to the engine for combustion: that suggests to NHTSA that the tank is never empty of CNG, and may contain a considerable amount of CNG at any given time depending on how the fuel system is running. Simply because you assert that the additional system volume provided by the tank is only for dampening pressure fluctuations does not remove the tank from being subject to FMVSS No. 304s requirements. The tank still would contain CNG as motor fuel, and the test requirements of FMVSS No. 304 are designed to ensure a basic level of safety for such tanks.
For your additional information, we also note that the vehicle in which your vessel is installed could be subject to Standard No. 303, Fuel system integrity of compressed natural gas vehicles, if it is a school bus or has a gross vehicle weight rating (GVWR) of 10,000 pounds or less. FMVSS No. 303 applies to new passenger cars, multipurpose passenger vehicles, trucks and buses with a GVWR of 10,000 pounds or less and to all school buses that use CNG as a motor fuel.
If you have any further questions, please do not hesitate to contact Rebecca Schade of my staff at (202) 366-2992.
Anthony M. Cooke