                       UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
                      NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY
                                   2000 TRAVERWOOD ROAD
                                   ANN ARBOR, MI  48105




                                                                                                                 	      			                     
May 4, 2012
                                                         OFFICE OF 
                                                      AIR AND RADIATION

MEMORANDUM

SUBJECT:	Fuel Use with Dosing for DPF Regeneration

FROM:	Lauren Steele, Office of Transportation and Air Quality

--------------------------------------------------------------------------------
TO:		Docket EPA-HQ-OAR-2011-1032

Regeneration Frequency
      During both active automatic and parked manual diesel particulate filter (DPF) regenerations, emission rates may increase for some pollutants, especially nitrogen oxides (NOX) when NOx-reducing aftertreatment devices are not present downstream of the DPF. Because of the potential effect on emissions, engine manufacturers are allowed to provide frequency factors that adjust for the average excess emissions during DPF regeneration, as part of the certification process. During a series of certification tests, the DPF regeneration frequency represents the fraction of tests during which an automatic active regeneration occurs. Passive regenerations would not be counted in this frequency.  Table 1 summarizes some information from EPA certification data regarding DPF regeneration frequency factors (FreqDPF), for two model years and two types of test cycles.
Table 1:  Frequency Factors for Certified Engines
Test Cycle
Model Year
Low FreqDPF
Average FreqDPF
High FreqDPF
Urban
2008
Near zero
4.8 %
Near 10 %
Highway
2008
Near zero
7.1 %
Near 17 %
Urban
2011
Near zero
3.0 %
Near 11 %
Highway
2011
Near zero
4.9 %
Near 18 % [a]
      [a] Excluding one outlier in this set with FreqDPF over 60%
      It is difficult to translate this information into an over-the-road DPF regeneration frequency. We do not have information about the duration in minutes of regenerations that occur during test cycles, nor can we translate a typical 40-minute test cycle into an equivalent over-the-road driving distance.  However, EPA has information from fire chiefs that indicates many fire trucks are experiencing regeneration intervals of approximately 20-30 engine hours.  Further, EPA has learned that individual regeneration events on fire trucks are often estimated to have a duration of between 20 to 40 minutes: closer to 20 minutes for automatic active regenerations, closer to 40 minutes for manual active regenerations.  From the available duty cycle data for emergency vehicles, we observe that typical annual engine hours range from about 300 hours per year for some rural emergency vehicles, to over 1,600 engine hours for some urban emergency vehicles, with a middle range vehicle running about 1,200 engine hours over a year. From this information, some regeneration frequencies can be derived, though these do not have any correlation to any frequency factor that may be associated with the engine certification for these vehicles. 
      Table 2 presents some estimates of the number of regeneration events an emergency vehicle may experience if it had a DPF regeneration interval of 25 engine hours with varying annual operating hours. Assuming that an average regeneration event is estimated to last for 30 minutes, the last row of Table 2 shows this would mean that about two percent of the engine's total annual operating time would be spent regenerating.
Table 2:  Estimated In-Use Emergency Vehicle Regeneration Events

Low Engine Hours
Middle Engine Hours
High Engine Hours
Annual Engine Hours
300
1,200
1,600
Annual Regeneration Events
12
47
63
Percent Operating Time Regenerating
2 %
2 %
2 %
    
Estimated Effects on Regeneration Frequency from This Action
      As a result of this rulemaking, it is possible that some manufacturers will submit applications for AECD's or field modifications with liberalized parameters under which automatic active and/or parked manual regenerations may occur, for engines on emergency vehicles.  The frequency of DPF regeneration in use may thus increase for these vehicles.  
      If a requested modification does not include any recalibration of engine parameters, this simple increase in regeneration frequency would be associated with a decrease in the duration of each regeneration event. This would be expected because the shorter time interval would lead to a reduced soot loading at the time the regeneration is initiated, which would take less time to oxidize.
      It is also possible that manufacturers may submit requests for engine recalibrations that enable more frequent passive regeneration and rely less on active regeneration, thereby reducing the frequency of automatic active and/or manual active regenerations.
      For purposes of this analysis, EPA is providing an illustrative example of a fire truck that typically undergoes an automatic active regeneration every 25 engine hours, and it receives an approved field modification so that it may regenerate more frequently. Using this example, we illustrate the effect that more frequent manual active regenerations and/or automatic active regenerations may have on the number of regeneration events occurring on a truck with 1,200 annual operating hours.  In Table 3, we show the increased number of events if the interval between regenerations decreases to 12 engine hours in one case and 8 engine hours in another case.  In this example, we also consider that the average duration of a single regeneration event may decrease from 30 to 25 or 20 minutes.
Table 3:  Estimated Change in Emergency Vehicle Regeneration Events

                               12-Hour Interval
                                8-hour Interval
Annual Regeneration Events
                                      97
                                      97
                                      143
                                      144
Event Duration
                                    25 min
                                    20 min
                                    25 min
                                    20 min
Percent Operating Time Regenerating
                                     3.4 %
                                     2.7 %
                                     5.0 %
                                     4.0 %
      From this example, we have shown that some increases in regeneration frequency may result from this rule. In a vehicle whose active regenerations are enabled with fuel dosing, this type of modification could lead to increased fuel use from dosing. However, we do not have information about the likelihood of this type of modification, and it is quite possible that some engine manufacturers may prefer solutions that increase passive regeneration, or solutions that generate the extra heat needed for regeneration by means other than fuel dosing.
Fuel Use from Dosing
      Where DPF systems employ fuel dosing to enable active automatic regenerations, the active regeneration event can affect the vehicle's fuel consumption. Some operators have reported that vehicle fuel economy is poorer on vehicles with frequent active regenerations than on previous vehicle models, though this would vary among makes and models of vehicles. 
      As described in the preamble, only some control systems employ fuel dosing as a strategy to initiate active regeneration. There is very limited information about the amount of fuel used during this process. Based on anecdotal information and phone conversations, EPA estimates that fuel use for an individual active regeneration event with dosing is typically between 0.5 and 1.0 gallon. In extreme cases such as sub-zero ambient temperatures, the supplemental fuel from DPF dosing on some vehicles could easily be in excess of a gallon.
Estimated Effects on Fuel Use from This Action
      If a manufacturer requests an AECD or field modification that would increase the number of active regenerations during a given operating period, and if the demand for regenerations is not reduced through recalibration, then it is uncertain whether or how much fuel consumption would increase on a vehicle that uses a fuel dosing strategy for regenerating its DPF.  Potential increases in fuel use from dosing could be mitigated by the expected decrease in the duration of regeneration events, and by enabling regenerations with other adjustments that could reduce the need for supplemental fuel to generate the heat needed for DPF regeneration.
      To illustrate a case where an emergency vehicle has increased fuel use from dosing related to regenerations, we have estimated the additional fuel use for a truck with a dosing strategy where its regeneration interval is decreased from 25 to eight engine hours, due to the increased availability of operator-commanded regenerations.  In this example, we assume a single regeneration consumes approximately half a gallon of supplemental fuel. As presented in Table 4, if this truck has average engine operating hours of 1,200 per year, and if it undergoes a manual regeneration every 8 engine hours instead of every 25 hours, it would experience an increase of 94 regeneration events, under the above assumptions.  If the amount of supplemental fuel use remained unchanged under the new regime (a conservative assumption) then potentially this truck could consume an additional 47 gallons of fuel per year from the increased frequency of regenerations alone.
Table 4: Fuel Use from Dosing with Increased Regeneration Frequency
1,200 Annual Engine Hours
                               25-Hour Interval
                               12-Hour Interval
                                8-Hour Interval
Annual Regeneration Events
                                      47
                                      96
                                      140
Fuel Dose per Regeneration (Gallons)
                                      0.5
                                      1.0
                                      0.5
                                      0.5
Annual Fuel Use from Dosing (Gallons)
                                      24
                                      47
                                      48
                                      71

Cost Estimates
      Table 5 below presents a summary of cost estimates for this exercise. Estimates are provided for a vehicle with annual engine operating hours of 300, 1,200, and 1,600.  
Table 5: Fuel Costs from Dosing with Increased Regeneration Frequency

                               25-Hour Interval
                               12-Hour Interval
                                8-Hour Interval
Fuel Dose per Regeneration Event (Gallons)
                                      0.5
                                      1.0
                                      0.5
                                      0.5
Annual Dosing Fuel with 300 Engine Hours (Gallons)
                                      6.0
                                      12
                                      12
                                      18
Annual Cost of Dosing (at $4/gal)
                                     $ 24
                                     $ 47
                                     $ 48
                                     $ 71
Annual Dosing Fuel with 1,200 Engine Hours (Gallons)
                                      24
                                      47
                                      48
                                      71
Annual Cost of Dosing (at $4/gal)
                                     $ 94
                                     $190
                                     $ 190
                                     $ 280
Annual Dosing Fuel with 1,600 Engine Hours (Gallons)
                                      31
                                      63
                                      64
                                      94
Annual Cost of Dosing (at $4/gal)
                                     $ 130
                                     $250
                                     $ 260
                                     $ 380
      As shown in Table 5, estimating that ultra low sulfur diesel fuel costs about four dollars a gallon, a vehicle operating 1,200 hours a year with an average regeneration interval of 12 hours could incur supplemental fuel costs related to dosing of about $190 per year, or an increase of almost $100 annually over the base case with the 25 engine hour regeneration interval.
      As noted above, EPA believes these estimates are conservative, because the amount of fuel used per regeneration event would likely decrease with increasing frequency.
