Interpretation ID: nht80-1.28

DATE: 03/10/80

FROM: AUTHOR UNAVAILABLE; F. Berndt; NHTSA

TO: General Motors Corporation

TITLE: FMVSS INTERPRETATION

TEXT:

March 10, 1980

Copies to:

RAR,RS,RLL,JGB,DPD,MRB,CTT,RAW, WLW,MLT,DAL,RFH,WCC,CRS,GPT,RGS

Mr. David Martin, Director Director, Enviromenental Activities Staff General Motors Corporation General Motors Technical Center Warren, Michigan 48090

Dear Mr. Martin:

This is in response to your recent request for an interpretation of the term, "capacity", as used in Safety Standard No. 301-75, Fuel System Integrity. Paragraph S7.1.1 of that standard provides that "the fuel tank is filled to any level from 90 to 95 percent of capacity with Stoddard solvent...." "You ask whether "capacity" should include the vapor volume in the air dome plus the volume of the fuel filler pipe when filling a fuel tank for compliance purposes. (Total tank volume = usable capacity + unusable capacity + vapor volume + fluid in filler pipe.)

The vapor volume can be filled with solvent if the solvent is added very slowly to force the air vapors out of the dome. This has been done in past compliance testing by the agency. Upon reconsideration, however, it is our opinion that the term, "capacity", should not be interpreted to include the vapor volume in the air dome, since fuel tanks are never filled to this level by vehicle users. Fuel tanks are designed to include an area for fuel vapor and pressure build-up. Vehicle users never fill their tanks so slowly that this area is displaced with fuel. Therefore, it would be an unrealistic test to require manufacturers to fill tanks in this fashion. Apparently, fuel is actually squeezed out of the filler pipe during compliance testing if the tank is filled to this absolute level. This would not seem to be an accurate test of fuel tank integrity, since it is leaks or punctures in the tank itself that generally cause fuel loss in real-world crashes.

In consideration of these facts, the agency interprets "capacity" to mean "usuable capacity", as used in the vehicle manufacturer's Part I submission to the EPA, plus "unusable capacity" (i.e.. the volume of fuel left in the tank when the engine fuel pump sucks air).

It should be emphasized that the "usable capacity" should be determined only after the tank has been filled to its "unusable capacity". In other words, when testing a tank that has never been filled, the unusable, residual fuel level should be reached before the "usable capacity" is added to the tank. If this is not done, the actual volume of fuel in the tank will be somewhat below the "usable fuel capacity.

Sincerely,

Frank Berndt Chief Counsel