2. Comparison of Light-Duty Vehicle Greenhouse Gas Emissions

This section provides a comparison of diesel vehicle GHG emissions and GHG emissions from conventional gasoline vehicles, HEVs, FFVs, and PHEVs, using the Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) well-to-wheels model. According to the GREET model, GHG emissions from diesel vehicles are lower than from conventional gasoline vehicles, an advantage that is amplified by the use of biodiesel. However, HEVs, FFVs, and PHEVs, all potential competitors with diesel vehicles as alternative vehicle technologies, provide equivalent or increased reductions in direct GHG emissions compared to diesel vehicles.

The transportation sector is the second-largest emitter of GHGs by end use in the United States, accounting for 28 percent of all GHGs emissions in 2007.¹¹ GHG emissions are unregulated in the United States but continue to garner significant attention both domestically and internationally because of concerns about climate change. Diesel-powered vehicles have entered the climate change debate because proponents claim that they have lower GHG emissions than conventional gasoline vehicles.¹² This report uses the GREET model, developed by the Argonne National Laboratory, to examine and compare projections of GHG emissions from diesel vehicles with those from other vehicle types, including FFVs, HEVs, and PHEVs, in 2010.¹³

According to the GREET model, total well-to-wheel GHG emissions from diesel vehicles using diesel fuel are 15 percent lower than those from comparable gasoline vehicles using E10 (Table 2.1). For diesel vehicles using B20, GHG emissions are 18 percent lower than those from diesel vehicles using diesel fuel, widening a diesel vehicle’s emission reduction over gasoline counterparts to 30 percent. Diesel vehicles using diesel fuel emit between 4 and 137 percent more direct GHGs than FFVs using E85, depending on whether the ethanol is produced from corn or cellulosic material. Diesel vehicles using B20 emit 95 percent more direct GHGs than FFVs using cellulosic ethanol but 14 percent less than FFVs using corn-based ethanol. HEVs emit between 20 and 71 percent less GHGs than diesel vehicles using diesel fuel and between 4 and 65 percent less than diesel vehicles using B20, depending on the HEV engine or fuel type. PHEVs, which use power from the electricity grid for a portion of their propulsion, emit between 16 and 63 percent less GHGs than diesel vehicles using diesel fuel but may emit more GHGs than diesel vehicles using B20, depending on the energy source used to generate the electricity.

GREET’s GHG emissions model provides an accounting of GHGs emitted through the entire fuel cycle and reports comparable well-to-wheels emissions rates for a variety of light-duty vehicle fuel and technology combinations. In the well-to-wheels calculation, emission rates are estimated for three stages of the fuel cycle: feedstock, fuel production, and vehicle operations. The feedstock stage includes those emissions created from the collection of an energy-bearing resource, such as the production of crude oil from an oil field or the growing of corn for ethanol, as well as transportation to the refinery gate. The fuel stage includes those GHGs emitted during the process of turning a feedstock into the fuel that can be used by a vehicle, such as processing crude oil into gasoline or diesel fuel or turning soybean oil or grease into biodiesel, as well as transporting these fuels to retail facilities. Together, the feedstock and fuel stages comprise the “well-to-pump” portion of the well-to-wheels fuel cycle. Finally, the GHG emissions from the actual operation of the vehicle, also referred to as the “pump-to-wheels” cycle, are estimated.

It is important to note that the GREET model projects GHG emissions from direct land-use changes but does not project emissions from indirect land-use changes.¹⁴ Direct land-use changes involve direct displacement of land for farming of the feedstocks needed for biofuel production; indirect land-use changes are those made to accommodate farming of food commodities in other places in order to maintain the global balance of food supply and demand.¹⁵Measuring indirect emissions is a complex and often contentious, though potentially important, issue.¹⁶ Because indirect emissions resulting from land-use change are not included in GREET, they are not included in this report. A complete table of GREET’s well-to-wheels direct GHG emissions projections for 2010 is provided in Appendix B.

Based on projections of GHG emissions associated with the well-to-pump portion of the full fuel cycle, diesel vehicles using diesel fuel emit 18 percent less GHGs than conventional gasoline vehicles using E10 (Table 2.2). At the feedstock stage, there is essentially no difference between GHG emission rates for gasoline and diesel fuel, because both follow similar production paths; however, at a refinery the process for making diesel fuel is less energy-intensive than the process for making gasoline on a Btu basis. Consequently, making diesel fuel from petroleum emits less GHGs in the fuel stage than making gasoline. Blending 10 percent ethanol made from corn into gasoline slightly reduces the GHG emissions compared to conventional gasoline in the well-to-pump cycle, because corn at the feedstock level is a GHG sink, meaning that it removes carbon dioxide from the atmosphere during photosynthesis. However, at the fuel stage, making corn ethanol is a highly energy-intensive process, offsetting much of the GHG reductions at the feedstock stage.

Diesel vehicle GHG emissions associated with the feedstock and fuel stages are reduced by 96 percent when using biodiesel blends, because biodiesel is made from soybeans or other plants that serve as a carbon sink. If diesel fuel is mixed with biodiesel in a blend of 80 percent diesel fuel and 20 percent biodiesel (i.e., B20), GHG emissions in the well-to-pump stage are almost zero. This is because GHG emissions in the feedstock stage of B20 are negative, meaning more GHG is removed from the atmosphere than emitted, and emissions at the fuel stage are low due to the relative ease with which biodiesel can be made from soybean oil.

FFVs using E85 achieve direct GHG emission reductions from the well-to-pump cycle relative to diesel vehicles using diesel fuel. Ethanol in the United States currently is made mostly from corn, but there are large-scale scientific and industrial efforts underway to produce cellulosic ethanol from other herbaceous plants.¹⁷ At the feedstock stage, both corn and cellulosic E85 are GHG sinks. At the fuel stage, however, there is a considerable difference in GHG emissions between corn and cellulosic ethanol because of the higher energy input needed to make corn ethanol and the reductions gained from the utilization of cogeneration at cellulosic ethanol plants. A conventional diesel vehicle using B20 emits more GHGs than an FFV using cellulosic E85 but emits less GHGs than an FFV using corn-based E85.

GHG emissions attributed to the well-to-pump cycle for HEVs are less than those from a conventional diesel vehicle using diesel fuel and either higher or lower than those from a conventional diesel vehicle using B20, depending on the HEV engine or fuel type. If the HEV uses cellulosic E85 or B20, then GHG emission reductions exceed those of a diesel vehicle using B20. GREET measures GHG emissions at each stage on a grams per mile basis that is based on the fuel economy of that vehicle. Thus, for HEVs, emission reductions in the well-to-pump cycle result from their fuel economy gains in the vehicle operations stage. The fuel economy improvement of HEVs over diesel vehicles is described in more detail below.

Conventional diesel vehicle GHG emissions associated with the well-to-pump cycle are generally lower than those for PHEVs that draw energy from the electricity grid. PHEVs are unique in that they utilize battery power at the start of a trip to drive the vehicle for a distance until a minimum level of battery power is reached. The vehicle then operates on a hybrid mixture of battery and internal combustion traction, the same as an HEV. The PHEV battery is then recharged by plugging the vehicle into an electrical outlet using a power cord, much like recharging a cellular phone.

In the GREET model, PHEVs are assumed to travel 33 percent of vehicle miles traveled in all-electric mode and the remaining 67 percent using the hybrid electric-gasoline engine. Because PHEVs use electricity for a portion of their propulsion, electricity generation is an important factor in their GHG emissions. Electricity generation in the United States uses large amounts of fossil fuels, especially coal, meaning that producing the fuel (electricity) needed by a PHEV emits high levels of GHGs relative to a refinery producing diesel fuel at the fuel stage. Even increasing the share of non-GHG-emitting renewable fuels, such as the electricity grids of the Northeastern United States or California, PHEV fuel-stage emissions are still higher than those for a conventional diesel vehicle if the PHEV uses E10 gasoline.¹⁸

Conventional diesel vehicles using diesel fuel emit less GHG in the well-to-pump cycle than PHEVs equipped with diesel engines using diesel fuel or B20, or equipped with flex-fuel engines using corn-based E85. Only by using a flex-fuel engine with E85 made from cellulosic material does a PHEV emit less GHG than a diesel vehicle using diesel fuel in the well-to-pump stage. A conventional diesel vehicle using B20 emits 90 percent less GHGs than a PHEV using cellulosic E85 when the vehicle is recharged using the average U.S. electricity grid; it emits the same amount of GHGs when the PHEV is recharged in the U.S. Northeast region, where slightly more non-GHG-emitting renewable fuels are used for electricity generation than the national average; and it emits more GHGs than a PHEV recharged in California, where the percentage of renewable fuels used for electricity generation is much higher than the national average.

In the pump-to-wheels cycle, conventional diesel vehicles using diesel fuel and B20 emit 14 percent less GHGs than conventional gasoline vehicles using E10 (Table 2.3). This is directly related to diesel vehicles’ fuel economy advantage over conventional gasoline vehicles. Greater fuel efficiency means that less fuel is used to travel each mile, translating directly into lower GHG emissions because such emissions are a byproduct of the burning of carbon-based fossil fuels.

Diesel vehicles emit 12 percent less GHGs than FFVs in the pump-to-wheel stage. This reduction results from the fuel efficiency advantage of diesel engines over flex-fuel engines. FFVs using E85 have lower fuel economy than diesel vehicles, because the heat content of ethanol is 84,600 Btu per gallon, lower than either diesel fuel or gasoline.¹⁹ This means that relatively more E85 must be burned to achieve the same energy output as diesel fuel. For an FFV using E85, fuel economy is reduced by about 15 percent from a similar gasoline vehicle and by 40 to 65 percent from a similar diesel vehicle.²⁰ However, if a flex-fuel engine is optimized to take advantage of the fuel properties of E85, its fuel economy can be increased by up to 18 percent on a Btu equivalent basis, which, depending on relative fuel prices, would make it competitive with diesel vehicles.²¹

HEVs in the pump-to-wheel cycle emit from 21 to 25 percent less GHGs than conventional diesel vehicles using diesel fuel or B20. HEVs achieve relatively high fuel economy because they use an onboard self-recharging battery that shuts off the internal combustion engine at idle and restarts it when needed in acceleration. Because of the battery, HEVs use less carbon-containing fossil fuel, cutting GHG emissions in the process. An HEV equipped with a compression ignition engine using either diesel fuel or B20 can achieve even further GHG reductions by utilizing the fuel efficiency advantages of both the onboard battery and the diesel engine.

PHEVs achieve large GHG emissions reductions—between 47 and 50 percent—compared to diesel vehicles using diesel fuel or B20 in the pump-to-wheels stage. A PHEV using electric power is highly efficient at converting electricity to propel the vehicle, achieving about 105 miles per gallon in electricity mode. Also, when a PHEV switches to its HEV system, it maintains a fuel economy advantage over a diesel vehicle. PHEVs can slightly increase their fuel economy advantage even further by adding a diesel engine.

Notes