Methodology
This study is the third in Climate Central’s periodic reports comparing lifecycle greenhouse gas (GHG) emissions for cars. This document provides a summary of the methodology and data sources used to compare a selection of new 2017 model year light-duty vehicles (cars and SUVs) available in the U.S. The methodology and data sources largely follow those of the earlier studies, but with a few revisions that reflect improved data sources, especially regarding estimates of the emissions associated with manufacturing of batteries for electric vehicles and hybrid electric vehicles.
In the present study, we estimated the lifecycle GHG emissions for 88 model-year 2017 cars, including 20 electric vehicles (EV), 17 plug-in hybrid electric vehicles (PHEV), 28 gasoline-fueled non-plug-in hybrid electric vehicles (HEV), 15 gasoline-fueled conventional internal combustion engine vehicles (GSLV), and 8 diesel-fueled conventional engine vehicles (DSLV).
EVs are cars with wheels driven by electric motors that are supplied with electricity from an on-board battery. The battery is recharged by plugging into an electrical outlet. PHEVs also operate using electricity from a battery, but battery recharging is done somewhat differently: a PHEV’s battery is initially charged by plugging into an outlet (like an EV), but if the initial charge is used up during driving, the conventional gasoline engine that is part of every PHEV automatically starts and drives an electric generator supplying electricity to the battery as the battery continues to supply electricity to motors driving the wheels.
An HEV operates using only gasoline. It includes a conventional gasoline engine and a battery that is typically smaller than on a PHEV. The gasoline engine powers the car and/or generates electricity to charge the battery, which provides an assist in powering the car. An important energy-efficiency feature of EVs, PHEVs, and HEVs is that they employ “regenerative braking”: energy that would otherwise be dissipated as heat during braking is instead converted into electricity and used to partially recharge the on-board battery. GSLVs and DSLVs use traditional engine/power-train designs, with no regenerative braking capability.
The lifecycle GHG emissions estimated here include those associated with manufacturing the vehicles and driving them for 100,000 miles. Emissions associated with end-of-life disposal and/or recycling of vehicles are not included, but are generally less than 3% of lifecycle emissions in peer-reviewed studies, e.g., Hawkins (2013). In the case of EVs and PHEVs, the emissions associated with generating the electricity used to charge the vehicles are included as part of the lifecycle emissions. This electricity is assumed to come from the grid.
