Vehicle Efficiency Improvement
Design Fuel Consumption vs.
Design Fuel Economy
Transportation Work Group
Phase III - Meeting 1
October 30, 2003
Meszler Engineering Services
906 Hamburg Drive
Abingdon, Maryland 21009
410-569-0599
www.meszler.com
Sound Design is Essential to Success
• Theory -- promote GHG reduction through
penalty/reward.
• Previous fee/reward structures based on EPA fuel
economy (FE).
• Although GHG goes down as FE goes up (and
vice versa), changes are not proportional.
• GHG changes are proportional to fuel
consumption (FC) changes.
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Design (EPA) FE vs. Design (EPA) FC
• Basic Relationships:
– FE = miles per gallon (mpg) as published by EPA.
– FC = gallons per mile (as derived from EPA FE).
– FC = 1 / FE ----- and ----- FE = 1 / FC
• If one is just a restatement of the other, does it
really make any difference?
– It absolutely does, as shown in the following slides.
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Discussion Clarification (a)
• Throughout this discussion fuel consumption
means the design fuel consumption rate equivalent
to the design fuel economy values published by
EPA.
• FC is not intended to capture differences in annual
gasoline use due to different mileage accumulation
rates.
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Discussion Clarification (b)
• Vehicles may accumulate mileage at different
rates, but lifetime mileage will be similar.
– A 50,000 miles/year vehicle will be gone in 2-3 years.
– A 10,000 miles/year vehicle will be around for 12-15
years.
– The total emissions load will be similar for both
(ignoring other differences like maintenance habits).
– High VMT drivers will be subject to fee/rebate more
often as their vehicles “consume” their lives quicker.
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Fuel Economy or Fuel Consumption
mpg or gallons/mile -- What Difference Does it Make?
Fuel Consumption (gallons per mile)
0.10
0.09
Dashed vertical lines denote boundaries
of successive halvings of fuel consumption
(beginning from 2.5 mpg).
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
0
20
40
60
80
100
120
140
160
180
200
Fuel Economy (miles per gallon)
10 mpg to 20 mpg: FE increases by 10 mpg - FC decreases by 5 gals/100 miles.
20 mpg to 40 mpg: FE increases by 20 mpg - FC decreases by 2.5 gals/100 miles.
40 mpg to 80 mpg: FE increases by 40 mpg - FC decreases by 1.25 gals/100 miles.
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Fuel Economy or Fuel Consumption
Increased GHG mpg
Practical Difference
22
21
20
19
18
17
16
15
14
13
12
11
10
Equivalent GHG change above and below 22 mpg average FE.
Dashed line is correct relationship (e.g., a 14 mpg vehicle
produces a GHG emissions increase equal to the GHG
emissions decrease produced by a 51 mpg vehicle).
Solid line is equivalent fee/rebate from Phase II linear-no
deadband program structure.
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Equivalent Reduced GHG mpg
Under FE-based design 13 mpg vehicle pays $3000 for consuming 3.1 gal/100 mi more fuel than a 22 mpg
vehicle. 37 mpg vehicle gets $3000 for consuming 1.8 gal/100 mi less. Reduction is “worth more” than
increase. From an “equal value” perspective, a 72 mpg vehicle would consume 3.1 gal/100 mi less.
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Additional Detail and Expanded
Explanations of Technical Issues
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Fuel Economy or Fuel Consumption
• Question: What difference does it make, aren’t
they the same thing?
• Answer: No. They are related, but not in a linear
fashion. Thus, a change in one does not induce an
equal change in the other. Direct GHG insights
can only be gained through fuel consumption.
• Why: Because CO2 emissions are directly and
linearly related to fuel consumption.
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FE vs. FC - Example Illustration (a)
• Take 2 vehicles with fuel economy (FE) values of
15 and 45 mpg. This is often interpreted as
implying an “expected” average mpg of 30.
• Balance is presumed because the 15 mpg vehicle
gets 50% worse than expected FE, while the 45
mpg vehicle gets 50% better than expected FE.
• However, actual average FE is not 30 mpg, and
average CO2 is not that of a 30 mpg vehicle.
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FE vs. FC - Example Illustration (b)
• 30 mpg is the average FE only in a restricted case
where fuel is rationed equally across vehicles.
• E.g., each vehicle is allotted 5K gallons of fuel
over its lifetime. At 15 mpg, vehicle 1 can travel
75K miles, while at 45 mpg, vehicle 2 goes 225K
miles. Together, 300K miles are traveled on 10K
gallons of fuel for a net FE of 30 mpg.
• However, in reality each vehicle has a similar
lifetime mileage and uses fuel accordingly.
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FE vs. FC - Example Illustration (c)
• E.g., each vehicle travels 150K miles over its
lifetime. At 15 mpg, vehicle 1 uses 10K gallons,
while at 45 mpg, vehicle 2 uses 3.3K gallons.
Like before, 300K miles are traveled, but on
13.3K gallons of fuel (instead of 10K), for a net
FE of 22.5 mpg.
• Since fuel consumption (FC) increases with the
inverse of FE (1/FE), average FC is biased toward
lower FE.
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FE vs. FC - Example Illustration (d)
• At 22.5 mpg, average fuel consumption is 6666.7
gallons per lifetime (or 0.0444 gal/mi).
• At 15 mpg, vehicle 1 uses 50% more fuel (0.0666
gal/mi) and produces 50% more CO2.
• At 45 mpg, vehicle 2 uses 50% less fuel (0.0222
gal/mi) and produces 50% less CO2.
• Even though vehicle 1’s FE is “only” 33% worse
than average while vehicle 2’s FE is 100% better!
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FE vs. FC - Example Illustration (e)
• Conversely, if we run the calculations using an
average FE of 30 mpg, vehicles 1 and 2 are 50%
worse and 50% better respectively in terms of FE,
but their CO2 emissions do not vary equally.
• At 30 mpg, average fuel consumption is 0.0333
gal/mi), so the 15 mpg vehicle produces 100%
more CO2 while the 45 mpg vehicle produces
“only” 33 percent less.
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FE vs. FC - Example Illustration (f)
• Conclusion: If the penalty/reward structure of an
efficiency program is intended to accurately
reflect GHG performance, it must be based on FC.
• Of course, fees can be assigned to specific FE
values, but all underlying calculations must be
performed in FC “space.”
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2003 FE by Class (National Data)
60
Minimum Gas/Diesel/HEV
Maximum Gas/Diesel/HEV
Maximum Diesel
Maximum LPG
Fuel Economy (mpg)
50
Median Gas/Diesel/HEV
Maximum Gasoline
Maximum CNG
Maximum E85
40
30
20
10
4WD Pickup
2WD Pickup
4WD SUV
2WD SUV
M inivan
Cargo Van
Passenger Van
Large Car
M idsize Car
Compact Car
Subcompact Car
0
Data include automatic transmission vehicles only, manual transmission versions with 3-5
percent lower fuel consumption are available for many models.
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October 30, 2003
2003 FC by Class (National Data)
10.0
Fuel Consumption (gal/100 mi)
9.0
8.0
7.0
6.0
5.0
4.0
3.0
Maximum Gas/Diesel/HEV
Minimum Gas/Diesel/HEV
Minimum Diesel
Minimum LPG
2.0
1.0
Median Gas/Diesel/HEV
Minimum Gasoline
Minimum CNG
Minimum E85
4WD Pickup
2WD Pickup
4WD SUV
2WD SUV
M inivan
Cargo Van
Passenger Van
Large Car
M idsize Car
Compact Car
Subcompact Car
0.0
Data include automatic transmission vehicles only, manual transmission versions with 3-5
percent lower fuel consumption are available for many models.
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October 30, 2003
2003 CO2 by Class (National Data)
CO2 Emissions (grams per mile)
700
600
500
400
300
200
Maximum Gas/Diesel/HEV
Minimum Gas/Diesel/HEV
Minimum Diesel
Minimum LPG
100
Median Gas/Diesel/HEV
Minimum Gasoline
Minimum CNG
Minimum E85
4WD Pickup
2WD Pickup
4WD SUV
2WD SUV
M inivan
Cargo Van
Passenger Van
Large Car
M idsize Car
Compact Car
Subcompact Car
0
Data include automatic transmission vehicles only, manual transmission versions with 3-5
percent lower fuel consumption are available for many models.
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October 30, 2003