One of the most significant questions with marginal fuel-cycle emissions is how to treat fuel economy. Several options for allowing credit for low fuel-cycle emissions are summarized below.
Base low fuel-cycle allowance on actual vehicle fuel economy
ARB could allow low fuel-cycle PZEV credits based on a vehicle’s actual fuel economy weighted over the CAFÉ mix of city and highway driving. This approach has the advantage of not depending upon an assessment of vehicle fuel economy. Manufacturers would be incentivized to make vehicles more fuel efficient and lighter. ARB could publish a fuel-cycle rating for each fuel on a g/gal basis that the manufacturer could then divide by the vehicle fuel economy. Manufacturers could also improve the fuel-cycle emission score by increasing fuel tank capacity to reduce spillage emissions per gallon of fuel.
The disadvantage of this approach is that it primarily favors small passenger cars while trucks and SUVs are a growing part of the LDV mix. A small SULEV would likely displace the sale of another small car. Another disadvantage of this approach is that smaller vehicles balance out larger vehicles in the CAFÉ calculation.
Base low fuel-cycle allowance on assessment of vehicle fuel econmy
ARB could certify fuels based on the results of this study. This approach recognizes that a variety of vehicles will be sold in the market and this study takes into account the likely effect of vehicle fuel economy. If the projected vehicle fleet fuel-cycle emissions were below or near 0.01 g/mi then the fuel would qualify. This approach would allow large vehicle to qualify for the low fuel-cycle allowance and would not provide an additional incentive to improve fuel economy or reduce CO2 emissions. A large vehicle meeting SULEV exhaust would provide significant fuel-cycle emission reductions, primarily by eliminating refueling vapor emissions.
Base low fuel-cycle allowance on vehicle size category bins
In order to incentivize highly efficient vehicles and not bias the low fuel-cycle allowance towards smaller cars, ARB could provide minimum fuel economy requirements for each fuel and vehicle size category. This approach would allow manufacturers to make a highly efficient large vehicle and reduce fuel-cycle emissions when compared to other vehicle types.