September 27, 1995


Subject:  Assigned Deterioration Factors for Gaseous-Fueled
          Vehicles and Engines

                                   CD-95-14(LDV\LDT\HD\GF)

Dear Manufacturer:                 

A proposed draft of this document was provided to the industry
for comment on July 21, 1995.  Comments were received from
several manufacturers and manufacturer organizations.  To the
maximum degree possible, changes were made to the factors
themselves and clarification was added to several points raised
by the commenters.

One commenter wanted this document to address the status of any
changes to the fees required by 40 CFR 86.907-93.  There has been
no change in the regulations and, therefore, these remain in
effect.  Any changes to this regulatory requirement will be
addressed in a separate document.

A major automobile manufacturer suggested that the alternative
durability protocol, depicted in Enclosure 9, be made less
burdensome.  EPA has to weigh the emissions risk for all
potential users of this procedure against the minimum testing
needs.  EPA believes that it would be inappropriate to reduce the
requirements of Enclosure 9. 

On September 30, 1994, EPA published emission standards for
Clean-Fuel Vehicles and Engines in the Federal Register.  This
rule also established the certification requirements for original
equipment manufacturers and for vehicle conversions wishing to
certify as Clean Fuel Vehicles (CFVs).  In addition, a September
21, 1994, rule established emission standards and certification
requirements for all on-highway gaseous-fueled vehicles, and also
a provision to allow aftermarket conversions to be certified.

In both final rules cited, manufacturers seeking certification
are required to meet durability requirements for each engine
family. Under EPA's certification regulations, small-volume
manufacturers may use assigned deterioration factors (df's) for
use in certifying new vehicles instead of operating the vehicle
for its full useful life.  The standard protocol EPA uses to
assign df's is described in EPA Advisory Circular No. 51-C.


Certain small-volume manufacturers may choose one of the two
methods described in this letter for deciding the appropriate
df's for their product line.
  
In the first method, applicable manufacturers may calculate and
use df's based on either the average or the 70th percentile of
the manufacturer's durability data whichever is less. 
Alternatively, EPA may assign df's for small-volume manufacturers
based on the Agency's estimate of the 70th percentile of the
industry-wide durability data base.  EPA is assigning df's (see
Enclosures 1 through 8) based on the Agency's estimate of what
the 70th percentile df's would be for such vehicles based on
industry-wide data, if such data were available.  The method used
to assign the df's is designed to ensure that the df's used are
as accurate as possible given the lack of data from durability-
data vehicles from previously certified engine families.

Manufacturers seeking to certify a particular gaseous-fueled 
engine family pursuant to 40 CFR 86.094-24(e) may also use these
assigned df's.  This provision allows any manufacturer to request
to certify engine families with combined total sales of fewer
than 10,000 (light-duty vehicles, light-duty trucks, and heavy-
duty engines) using the small-volume manufacturer procedures for
emission-data vehicle selection and the calculation of
deterioration factors.

In the second method for determining the applicable df's for
their product line, manufacturers can follow the abbreviated
durability protocol depicted in Enclosure 9.  The df's calculated
using the actual emissions data obtained via the test protocol in
Enclosure 9 will become the assigned df's for that manufacturer. 
The criteria to use the data generated by a specific engine
family to satisfy the df data in another engine family or in
another model year will be the same as described in Advisory
Circular 17F.

For vehicles converted to dual-fuel operation (that is, vehicles
that can operate on two separate fuels but only one fuel at a
time and not on a mixture of fuels), the df's depicted in this
document can only be applied to emission values obtained when
certifying the vehicle/engine on a gaseous fuel.  The df's that
apply to such vehicles when certified on the fuel for which the
vehicle/engine was originally certified must be based on the
original fuel.  EPA publishes on the Technology Transfer Network
bulletin board (modem telephone number 919-541-5742, information
number 919 541-5384) information for most model years.  If the
df's needed are not on the bulletin board you may contact Mr.
Clifford Tyree for additional information.


The df's assigned by EPA for gaseous-fueled vehicles may be used
by eligible manufacturers through model year 2000, at the latest. 
EPA expects that by MY 2000 the industry can generate data that
EPA can use to reassess the appropriateness of the values
assigned in Enclosures 1 through 8 and the test protocol
described in Enclosure 9.  EPA is assigning these df's with the
understanding that in-use field experience will demonstrate that
vehicles converted to operate on gaseous fuels will continue to
comply with full-life emission standards.

Commenters also had several suggestions for determining df's
besides those described in this document that would be applicable
in MY 2001.  It is premature to describe in this document
procedures that could be applicable beginning in MY 2001.  For
example, EPA is considering revising the durability requirements
for light-duty vehicles.  These revisions could have an impact on
the durability procedures that would be in place for MY 2001. In
addition, the experience gained by the Agency and the industry
following the issuance of this document may cause a complete
reassessment about how conversions should be handled.  It is,
therefore, appropriate to address the MY 2001 and later
durability requirements for manufacturers eligible to certify
under 86.094-14 at a later date.

EPA may rescind these assigned df's at any time if the Agency
receives information (including in-use emissions data) suggesting
that vehicles converted to operate on gaseous fuels pursuant to
the requirements of 40 CFR 88.306-94 (Clean-Fuel Vehicle
provisions) and 85.503 (aftermarket conversion provisions) are
not mechanically durable or that such vehicles exhibit increased
emissions levels.  EPA may rescind the df's for a particular
subset of the industry only, or in total, depending on the
available data and information.

If EPA rescinds the applicability of the df's contained in this
document, in total or for a specific segment of the industry, the
validity of Certificate(s) issued will be addressed then.  EPA
expects that the assigned df's will provide benefit to applicable
small-volume manufacturers who certify gaseous-fueled vehicles
and engines.  EPA retains the right to rescind the assigned df's,
however, if data and information demonstrate that vehicles
converted to gaseous fuel exhibit mechanical durability concerns
or/and increased vehicle emissions.  EPA also will pursue
appropriate action, including recall or revoking or voiding
certificates, based on the facts of each situation on a case-by-
case basis. 


As described above, Enclosures 1 through 8 are a set of
mathematically derived df's for the emission standards for the
two Final Rules referenced.  Enclosure 9 is a description of an
alternate means for generating engine family-specific assigned
df's through MY 2000.  If you have any questions, please contact
Mr. Clifford Tyree, 313-668-4310.

Sincerely,



Robert E. Maxwell, Director
Certification Division
Office of Mobile Sources

Enclosures