Gasoline

Gasoline cars can be green (or greener) if they are highly fuel-efficient – in other words, if they have high fuel economy, 'miles-per-gallon' or 'mpg'. Cars with good fuel economy need not necessarily be small or under-powered – within all vehicle classes, 'mpg' varies between 25% and 40%.

Higher 'mpg' implies less fuel use, which in turn means fewer emissions of carbon. However, for emissions other than carbon dioxide, the correlation between fuel economy and vehicle emissions is far weaker – in other words, some small cars with good fuel economy may emit as many particulates or NOx as larger cars.

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Hybrid

Hybrid cars are part battery-electric and part conventional cars. The underlying principle of all hybrid vehicles is the use of a temporary energy storage device (usually a battery), which enables the main engine to be operated at close to its maximum efficiency.

On North American roads, most gasoline-hybrids reduce greenhouse gas emissions (per mile) by around 25% compared to an equivalent gasoline car. Non-carbon emissions, such as carbon monoxide (CO) and nitrogen oxides (NOx) are also significantly reduced by over 70%.

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Ethanol E85

Ethanol E85 is a mixture of 85% ethanol and 15% gasoline which can used in 'flex-fuel vehicles' (FFVs); these can run on any percentage ethanol blend up to 85%. Ethanol made from plants (biomass) is called bioethanol.

Bioethanol has the potential to be 'carbon-neutral'; all the carbon emissions generated during use of the fuel being balanced by the absorption from the atmosphere during the fuel crop's growth. However, in practice the extent of life cycle greenhouse gas emissions is strongly dependent on the crop grown and the fuel processing employed.

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Diesel

Diesel engines (more popular in Europe) are generally more fuel-efficient than gasoline (spark-ignition) units due to their higher combustion temperature and lower rate of heat loss. Diesel also has a slightly higher energy content than gasoline per unit volume.

As a result, diesel vehicles produce 15%-20% fewer CO2 emissions, whereas gasoline vehicles emit fewer NOx and particulates (per mile). To some extent, there is a trade-off between reductions in local and global emissions – diesel's relatively high NOx and low CO2 emissions are both the result of the engine's high combustion temperature.

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Natural gas

Natural gas is a naturally occurring mixture of gaseous hydrocarbons consisting of methane (CH4) with lesser amounts of propane, ethane and butane. In the US, the main uses of natural gas are heating and electricity generation.

In principle, carbon emissions from natural gas vehicles (NGVs) are reduced due to the fuel's low carbon content. However, methane (the main constituent of natural gas) is also an important greenhouse gas. Taking carbon dioxide and methane emissions into account, natural gas cars show an improvement in greenhouse gas emissions of around 10%-15% as compared to those using gasoline.

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Electric

Electric vehicles (EVs) use an on-board rechargeable battery to store electrical energy. When required, energy is drawn from the battery and converted to motive power by the use of an electric motor.

EVs charged from the US grid (using an average fuel mix) show a significant reduction in greenhouse gas emissions (per mile). Compared to gasoline, the figures suggest a reduction of around 40%. The benefits are mainly due to the fact that EVs are significantly more energy efficient than conventional vehicles; electric power-trains are particularly well suited to stop-start, low-speed operation and use almost no energy when stationary.

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Fuel cell

Fuel cell vehicles (FCVs) dispense with the internal combustion engine altogether. Fuel cells are electrochemical devices that convert the energy stored in chemical form directly into electrical energy, water and heat.

If non-renewable primary energy is used, the impact on emissions is difficult to quantify, depending as it does on the method of on-board fuel storage and fuel production. However, considering the main options, fuel cell cars are predicted to show a significant reduction in life cycle greenhouse gas emissions (per mile) of up to 55% as compared to petrol.

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