greenhouse gas emissions and fuel efficiency standards for medium- and heavy-duty engines and vehicles - phase 2 629 pages in english preepa and nhtsa on behalf of the department of transportation are each proposing rules to establish a comprehensive phase 2 heavy- duty hd national program that will reduce greenhouse gas ghg emissions and fuel consumption for new on-road heavy-duty vehicles. this technology-advancing program would phase in over the long-term beginning in the 2018 model year and culminating in standards for model year 2027 responding to the presidents directive on 18 february 2014 to develo new standards that will take us well into the next decade. nhtsas proposed fuel consumption standards and epas proposed carbon dioxide co2 emission standards are tailored to each of four regulatory categories of heavy-duty vehicles combination tractors trailers used in combination with those tractors heavy-duty picku trucks and vans and vocational vehicles. the proposal also includes separate standards for the engines that power combination tractors and vocational vehicles. certain proposed requirements for control of ghg emissions are exclusive to epa programs. these include epas proposed hydrofluorocarbon standards to control leakage from air conditioning systems in vocational vehicles and epas proposed nitrous oxide n2o and methane ch4 standards for heavy-duty engines. additionally nhtsa is addressing misalignment in the phase 1 standards between epa and nhtsa to ensure there are no differences in compliance standards between the agencies. in an effort to promote efficiency the agencies are also proposing to amend their rules to modify reporting requirements such as the method by which manufacturers submit pre-model mid-model and supplemental reports. epas proposed hd phase 2 ghg emission standards are authorized under the clean air act and nhtsas proposed hd phase 2 fuel consumption standards authorized under the energy independence and security act of 2007. these standards would begin with model year 2018 for trailers under epa standards and 2021 for all of the other heavy-duty vehicle and engine categories. the agencies estimate that the combined standards would reduce co2 emissions by approximately 1 billion metric tons and save 1.8 billion barrels of oil over the life of vehicles and engines sold during the phase 2 program providing over 200 billion in net societal benefits. as noted the proposal also includes certain epa-specific provisions relating to control of emissions of pollutants other than ghgs. epa is seeking comment on non- ghg emission standards relating to the use of auxiliary power units installed in tractors. in addition epa is proposing to clarify the classification of natural gas engines and other gaseous-fueled heavy- duty engines and is proposing closed crankcase standards for emissions of all pollutants from natural gas heavy-duty engines. epa is also proposing technical amendments to epa rules that apply to emissions of non-ghg pollutants from light-duty motor vehicles marine diesel engines and other nonroad engines and equipment. finally epa is proposing to require that rebuilt engines installed in new incomplete vehicles meet the emission standards applicable in the year of assembly including all applicable standards for criteria pollutants. pre