The pre-industrial atmospheric CO2 concentration was 280 ppm. Presently the CO2 concentration is about 393 ppm, and the global average temperature increase to date is about 0.7 ºC (1.3ºF). Both these numbers are growing as mankind uses more and more fossil fuels and emits more and more CO2 (and other greenhouse gases) into the atmosphere. Since transportation accounts for about 25-30% of global CO2 emissions there is a strong motivation to make this sector more fuel efficient, when using fossil fuels, and to decarbonize the movement of people and goods wherever possible (i. e., eliminate the release of atmospheric greenhouse gases).
Internal combustion engines are highly inefficient. Personal passenger transport is powered by internal combustion engines (ICE) that are fueled mostly by gasoline, refined from crude oil. Burning fossil fuels injects the greenhouse gas carbon dioxide into the atmosphere, in a one-way flow from the geological deposits containing the oil to the release of a car’s exhaust to the atmosphere. Yet use of ICEs is highly inefficient in terms of converting the chemical energy contained in the oil into useful mechanical energy, namely, propelling a car along the road. This is shown below.
Energy use and losses in driving an automobile powered by an internal combustion engine, for combined city/highway driving. The useful energy is “Power to Wheels”, lower right. Its percentage is slightly lower for all-city driving, and slightly higher for all-highway driving (see the website below). Source: Energy Efficiency & Renewable Energy, U. S. Dept. of Energy; http://www.fueleconomy.gov/feg/atv.shtml
It is remarkable that only 1/7 to 1/4 (depending on city to highway driving) of the energy contained in the fuel is used in moving the car. It is even more surprising that “Engine Losses” include heat that is deliberately dissipated via the car’s radiator and exhaust, which constitutes about 56-64% of the energy in the fuel (depending on city to highway driving).
The energy that propels the vehicle along the road must overcome the forces opposing forward motion, namely wind resistance, rolling resistance and braking (Power to Wheels, see graphic above). These are susceptible of improvement. Yet even if they were fully eliminated, which of course is not possible, there would still be the very high thermal losses (Engine Losses, see graphic above) that arise as long as the power source is an ICE.
Reducing losses and increasing efficiency are considered in many sources (see References). Among the most significant is weight reduction. The inertia of an object is directly related to its weight. It takes energy to change its inertia, for example when accelerating a car from a stop. A lighter car will need less energy for acceleration than a heavier one. A lighter car, needing less energy, can then incorporate a smaller engine, thereby decreasing weight even more.
Smaller ICEs, in addition to being lighter, are also more efficient in converting the energy in the fuel to the forward motion of the car. The Canadian Automobile Association, citing data from Natural Resources Canada’s 2008 Fuel Consumption Guide, shows that there is a much larger percentage increase in fuel economy in a compact car than in an SUV or a pick-up truck by making the engine smaller. This factor is in addition to the considerable fuel economy achieved just by driving a compact car as opposed to an SUV or a pick-up truck.
Streamlining the body lines of a car reduces its aerodynamic drag resistance to forward motion, a second important factor in optimizing efficiency of automobile transport. In addition to the improvement in body shape that is obvious to the observer, shielding wheel wells and the underbody of the car further would improve its aerodynamic flow properties.
Manufacturers of these electric cars emphasize their environmental advantage in having zero or minimal tailpipe emissions of CO2. Electric motors such as used in electric cars are highly efficient, capable of converting more than 90% of the electrical energy into the mechanical energy of motion. As pointed out in the earlier post, however, these cars actually have low or zero emissions only to the extent that the electricity used to charge the batteries itself is obtained from renewable or low-CO2 emitting generation sources. Coal-fired electric generation is the least efficient, whereas modern natural gas-fired plants using combined cycle generation attain quite high efficiencies and much lower emissions of CO2. By 2035, the U. S. National Academy of Engineering estimates that even for all-electric vehicles, the greenhouse gas emissions will remain at 30-50% as much as currently emitted by ICE-powered cars because electricity will still be generated to a considerable extent from fossil fuels. Optimally, use of renewable sources such as wind power, solar power, hydroelectric power and geothermal power will provide truly zero emission generation of electricity.
Actual growth in number of passenger vehicles (1980-2008) and projected growth (2020, 2035). Other non-OECD (developing) countries includes
Current technology emphasizes powering passenger cars with fossil fuel-driven ICEs, leading to a large increase in greenhouse gas emissions worldwide from this source. But the Edmunds Auto Observer reports that, as of 2009, China’s fleet-average fuel efficiency, including SUVs and minivans, was already 36.8 miles per gallon (mpg), and that the country has mandated an increase to 42.2-mpg by 2015. A tax on vehicles based on their engine size provides a further economic incentive impelling Chinese purchasers toward smaller vehicles. The current tax rates are shown in red bars in the graphic below.
Vehicle excise tax in
© 2012 Henry Auer