EPA's Proposal for MOBILE6      
                     Gasoline Fuel Effects

                          March 7, 1997 


Abstract

     For gasoline, MOBILE5 contains estimates for the effect of fuel
volatility (based on Reid Vapor Pressure), fuel oxygen content and the
effects of Reformulated Gasoline (RFG), Phase 1 and Phase 2.  However,
there is growing interest in controlling gasoline sulfur content as a
way to reduce in-use emissions.  For MOBILE6, EPA proposes an
adjustment to exhaust emissions be added to account for fuel sulfur
content as a tool to estimate the effects of fuel sulfur control
strategies on mobile source emission inventories.

Summary of the Proposal

    Time and resources permitting, we would like to address the
     following fuel issues for inclusion in M6:

          Effects of sulfur on HC, CO and NOx exhaust
          emissions

          Re-evaluation of the effects of oxygenates on
          exhaust HC and NOx emissions

          Re-visit and re-evaluate effects of fuel changes
          (mainly oxygen) on exhaust CO emissions

          Currently unsure whether we will be able to
          revisit & revise, if necessary, RVP's effects for
          exhaust
     
Sulfur Effects

    Sulfur Database
          
         Contains data from the EPA-sponsored ATL-1 study,
         Auto/Oil phase I, Auto/Oil Phase II study,
         Auto/Oil Phase II very-low sulfur study , an older
         vehicle study by EPA (SAE Paper 950778), and a
         study of sulfur effects on Tier 1 vehicles
         performed by Ford Motor Company (SAE Paper
         952510).
         
         Vehicle model years range from 1983 to 1994

         Some high emitters included; however, high
         emitters not tested over entire range of relevant
         fuel sulfur space
    
    Plan of Attack

     Prepare simple analysis of % change effects of sulfur on 
     emissions of HC, CO and NOx using all data sources available

     Analyze effects on Bag 1, Bag 2, Bag 3 and composite emissions

     To the extent possible, analyze trends in emitter classes, 
     technology groupings, and catalyst types

     Look at start emission effects by analyzing Bag 1 - Bag 3 
     emissions

     Compare effects on older vehicles, current technology 
     vehicles, and newer vehicles

    Emerging Issues

     Most of data is based on 1986-1992 Model Year vehicles

     Little data on Tier I/LEVs or for vehicles meeting
     better than LEV emission standards

     Development of appropriate methodology for
     determining base emission rates, the effects of
     sulfur on those rates and emission factors for
     possible input into MOBILE6

     Allow sulfur changes to be dialed in only for non-RFG areas?

     Effects of sulfur on exhaust CO emissions

     Resources allocated to fuel modeling in MOBILE6

     Can we say anything about HDVs?

     There are very few LDTs in the current database;
     do we assume that LDV effects can be used for
     LDTs?

     Range of testing on sulfur content is different
     for each of the studies and across the emitter
     classifications; how do we reconcile?

     More data on high emitters is needed

Oxygen Effects

     Have not begun analysis or data collection and will do
     only what we have time for

     Amongst the highest priority items under this
     subheading would be to revisit and possibly revise
     effect of oxygen on exhaust CO emissions

     Must somehow try to account for dilution effects
     (associated with adding oxygen) on emissions

     Revisit effect of oxygenates on HC, NOx and CO
     emissions using any new data generated on these effects
     since MOBILE5 was completed.

     At minimum, we have to substitute Complex Model oxygen
     effects for current vehicles in MOBILE6 and leave the
     old vehicles as was in MOBILE5 if time does not permit
     for a thorough analysis

     Similar issues on resource allocation as listed above