Well-to-wheels
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Description
Well-to-Wheel is a specific type of Life Cycle Analysis for motor vehicles. It is based on the efficiency of the mechanical energy of a car with respect to the primary energy available at the source, e.g., the chemical energy in the crude oil extracted at the wellhead.
Detailed description
Depending on the vehicle, the figure varies due to differences in energy extraction and transfer. For instance a vehicle with a SI (Spark Ingition) engine that used gasoline, would, according to the DOE[1], use a factor of .83 for petroleum refining and distribution efficiency, in addition to it's EPA fuel mileage rating.
On the other hand an electric vehicle, powered by a mix of electricity generation methods, would use a weighted sum of the energy needed to acquire the energy received from the various sources, as well as it's charging/discharging efficiency, drive-train efficiency, and energy required to move it per unit distance.
Effect of Well-to-Wheel Analyses
The following table gives an approximation of the efficiencies and indices for carbon emissions and other pollution for various types of cars. It must be considered that these may vary a lot, according to local conditions, so the figures are purely indicative. The table is followed by some explicative notes.
| Type of car | W>W efficiency estimate, per cent |
W>W carbon index |
Pollution index |
|
|---|---|---|---|---|
| Petrol-electric hybrid | 20 | 1 | 1 | |
| Small efficient diesel | 20 | 1.0 | 1.2 | |
| Conventional petrol or diesel | 16 | 2.0 | 2.0 | |
| Plug-in hybrid | Nuclear/renewable | 18 | 0.9 | 1.2 |
| Fossil fuel | 2.5 | 2.5 | ||
| Electric | Nuclear/renewable | 14 | 0.8 | 1.0 |
| Fossil fuel | 3.0 | 3.0 | ||
| Fuel cell | Reformed CH4 | <10 | 2.5 | 2.0 |
| Nuc/ren electrolysis | 0.8 | 1.0 | ||
| Fossil electrolysis | 3.0 | 4.0 | ||
Notes:
1. The reference car is a mid-sized hybrid car, such as a Toyota Prius or a Honda Civic Hybrid. Cars of different sizes have to be pro-rataed to equal these in performance, size and weight.
2. The figures are approximations, based on what are considered average conditions, with modern commercial cars, in developed countries, most of which have to import or transport their energy over considerable distances. With cars that are not yet commercially available, the figures are assumed from available data and engineering practice. Special "concept" and small series cars are not considered.
3. The indices refer to the reference car.
4. The "small efficient diesel" refers to European marques, such as Volkswagen or Peugeot, fitted with particle filters. They definitely pollute more than the reference vehicles a) because the particle filters are inefficient with ultrafine particles and b) because of other pollutants due to less refined fuels and different combustion conditions, notably nitrogen oxides, sulfur dioxide and hydrocarbons.
5. Nuclear/renewable electricity is never totally carbon-free or pollution-free.
6. Electric cars and plug-in hybrid cars when running on electricity charged from power grids have very poor overall efficiencies because the average power station has an efficiency even lower than the modern internal combustion engine.
7. Fuel cell cars are assumed to use hydrogen.
8. Most hydrogen is made from reforming methane from natural gas, a process that emits carbon dioxide and many other air pollutants.
9. No attempt has been made to indicate relative costs because of different conditions, such as fuel taxation, distribution and electricity costs etc., from country-to-country and even within countries.
