Driving cars uses energy – most of it non-renewable oil - and produce emissions – Carbon Dioxide (CO2) , Nitrogen Oxides (NOX) and 'particulates' (some unpleasant substances you might end up breathing in) -but you can do your bit by making the correct choices.

Energy is consumed by making the car in the first place, when it is driven and when it is disposed of at the end of its life. Recent research has calculated the following split:

Manufacture For materials used and energy consumed making the parts and assembling them all together	Use Fuel and oil used, repairs and replacement parts	Disposal Removal of recyclable parts and materials, disposal of the rest

There is therefore a balance between driving a newer car with better fuel consumption and lower exhaust emissions or a second-hand car that has already been produced.

Emissions
Carbon dioxide (CO2) has been taken as the simplest way of comparing different cars – the lower the output of CO2 (measured in grams per kilometre) the better for the environment it will be. It is directly related to how much fuel is used by the car. In the UK this now affects the cost of taxing your car if it was first registered after March 1st 2001.

But CO2 isn't the only thing that comes out of the exhaust pipes of cars. Since the early 1990's exhaust catalytic converters ( 'catalysts' ) have been mandatory fit on petrol engined-cars. These convert some of the other nasty chemical compounds produced by the engine into nicer things. Diesel engined cars have a higher level of particulate output – some you can see (black smoke) and some you can’t. These particulate emissions have been reduced recently but the better fuel consumption of diesels has improved their popularity in most of Europe. Some of the newest diesels on sale have filters aimed at trapping and recycling the particulates - you see them referred to as 'DPF' - Diesel Particular Filter'.

If you're looking to buy a car with the lowest CO2 emissions have a look at the ActOnCO2 site. Several manufacturers have recently launched models, under names such as BlueMotion (VW) and Econetic (Ford), which have improved aerodynamics, engine and gearing tweaks to get under the 100g/km tax band A limit.

It is likely that taxation based on CO2 emissions will increase in the near future, particularly in the higher bands. The EEC has recently passed a bill that will tax cars that produce most heavily but it is not certain how it will be implemented in each of the member countries. Their aim is the get the CO2 emissions down from the current average of 180 to 130g/km by 2013, which is a huge task. So buying the right car now might help you sell it when its time to move on to urnextcar.

Alternative fuels
In the UK both petrol and diesel now have to contain 5% 'bio fuel' - that is fuel derived from a renewable source. This may be reviewed in the future as people have realised that turning food-producing land over to fuel-producing crops is forcing food prices up around the world.

There are already greener alternative fuels on sale but unfortunately they currently have limited availability, though this is likely to improve sooner rather than later.

LPG ( Liquified Petroleum Gas) has been 'round for a while and although it is only about 2/3rds of the cost of petrol your car needs a second fuel system installing at the cost of more than a grand and it takes up space, so it's not great for smaller cars. LPG availability is limited.

E85 bioethanol is 15% regular petrol and 85% ethanol and is sometimes referred to as ''FlexiFuel' as the car will run happily on E85, 'normal' petrol or a mixture of the two. As ethanol is highly corrosive you can't just put it in a regular car, you need to buy one that is specially designed to suit it - like certain Ford, Saab and Volvo models. Morrisons currently has the widest E85 availability.

Biodiesel is produced from vegetable oils and can used on most diesel cars without modifications but many manufacturers recommend a mix of biodiesel and 'normal' diesel to maintain the reliability of the engine. CO2 emissions is claimed to be reduced by 60% over 'normal' diesel. Biodiesel availability is limited.

Whereas LPG is a petroleum-based product, both E85 and Biodiesel are extracted from crops like sugar beat. The reduction in CO2 emissions results from the fact that plants absorb CO2 while they grow and it is released again when the fuel is burned - hence they are 'renewable' forms of energy.

The future
The change that is just starting is that of 'downsizing' of engines - a 1.0 litre engine instead of a 1.3, for example. Many companies are now offering derivatives with smaller engines than before, that are more economical and have lower emissions and are often turbocharged not for more power but for more torque (pulling power). Although the engine has a smaller capacity the extra technology will keep the price up but that's OK providing the driver sees the benefits and the drivability doesn't suffer - finally proving that there is a substitute for cubic capacity.

There is now a hugh amount of research going on into developing bio fuels from other things apart from crops, such using left-over food production. This would be used to produce ethanol or methanol.

Some say that the future is already here as you can already buy high-tech hybrid engined cars. The first, 'parallel hybrids', use a complex powertrain that uses a smaller conventional engine and an additional electrical charging, storage and drive system, allowing to use either or both systems to drive the car. The Toyota Prius is an example of this technology. They can offer improved fuel economy in certain conditions as energy usually wasted when braking can be stored and re-used but there are still more economical, conventional vehicles available and they are cheaper to buy in the first place.

A second type of hybrid - a 'series hybrid' - has just been announced in the US, in the form of the Chevrolet Volt. Only the electric motors actually drive the car and the small petrol engine is only there to keep the battery topped up. The Volt will be on sale in the US in 2009 and Vauxhall/Opel versions will probably on sale from 2010.

'Plug-in' electric vehicles are also likely to appear, also in the US at first, and these will be very economical for people who make relatively short journeys. Their range and re-charge time is being driven by the development of new battery technologies.

There is also a hugh amount of research and development into improving current petrol and diesel engine efficiency, improved efficiency transmissions, regenerative systems and alternatives such as hydrogen to power fuel cells.The latter might appear in the next decade, so don't hold your breath as the overall 'energy equation' ( what energy you have to put in to produce compress and deliver the hydrogen doesn't add up.Oh, and hydrogen also goes 'bang' unless you're very careful.

Lotus Engineering are involved in most of these new technologies and have shown a 'tri-fuel' Exige. This might be an easier interim technology before the wholesale change to hybrids and plug-ins.

Just don't expect any of the new technologies to be cheap to buy…