The main criteria of a car's eco-friendliness is generally seen as the energy it uses in its operation. Cars have primarily run on oil-based fuel in the form of petrol or diesel for over a century. But we are now at the point where demand for oil is rising so much, and reserves are declining, that oil is actually running out. Oil prices have always been volatile and they react to speculation about events ranging from terrorism to hurricanes, but rising demand and diminishing reserves mean that overall, oil is just going to keep on getting more expensive. However despite this, most cars today still rely on oil as their primary source of energy; and oil is obviously a key contributor to carbon emissions, and climate change.
Petrol
Petrol has been the most popular fuel for cars in the UK for years; petrol engines are generally quiet and smooth, they are responsive and their performance is good. Petrol is currently slightly cheaper than diesel. Petrol engines emit around 10% more Commercial gas engineer London carbon dioxide (CO2) than diesel. However petrol cars pump out less toxic emissions than diesel. Unfortunately at the moment there is no single source of fuel which can compare with petroleum in terms of its instant bulk availability, energy density and (relative) cheapness.
Diesel
Diesel engines are more economical than petrol engines, therefore they emit less CO2. New 'common rail' diesels are approximately 10% more efficient than older diesels, and direct-injection diesel engines give the best fuel economy, diesels emit more particulates than petrol - but diesel engines with a particulate trap help prevent emissions of sooty particulates - ie. the clouds of smoke that you'll experience if you follow old buses through towns. So diesel engines will generally provide you with more miles per gallon than their equivalent petrol models - just look at the differences between similar vehicles in our Green Car Guide. Diesel is currently more expensive to buy than petrol, and the forecasts are that diesel prices will continue to rise more steeply than petrol in the near future.
Diesel engines have always been seen as slow and noisy, however technology has seen some remarkable advances in recent years; for instance Honda has developed their own diesel engine that is designed to be quiet, refined, clean and with instant response - fighting against all the old stereotypes.
LPG
Over recent years, LPG (liquefied petroleum gas) has been a viable fuel option in the UK. LPG produces fewer emissions than petrol and diesel but fuel consumption is worse. It's been possible to convert many existing cars to run on LPG by after-market conversions, and some manufacturers such as Vauxhall have had new cars in their range that are dual-fuel, which are designed to run primarily on LPG with petrol back-up. There is a reasonable network of filling stations.
LPG, and natural gas in heavier vehicles, has been an attractive proposition in the past primarily due to its cheaper cost, as it has enjoyed less fuel duty. However there is no guarantee that the Chancellor will maintain this in the future, and although there are some emissions improvements over petrol, LPG is still derived from a fossil fuel and therefore still releases greenhouse gases into the atmosphere.
CNG
Some vehicles, usually heavier vans or trucks that normally run on diesel, but also cars such as the Volvo (V70 Bi-Fuel), can run on CNG (Compressed Natural Gas), which again results in lower CO2 emissions than standard petrol cars, but the fuel is not as efficient as diesel. Finding CNG for refuelling can be a challenge.
Petrol-Electric Hybrids
Petrol-electric hybrid vehicles run on a combination of a conventional petrol engine and an electric motor powered by an energy storage device such as a battery pack. In simple terms they work on the principle that an electric motor provides the power at low speeds such as in urban driving, and they switch to petrol for driving at higher speeds. The batteries are recharged while driving and hybrids use regenerative braking, which means that energy is put back into the battery when braking, which improves energy efficiency.
Hybrid technologies improve fuel efficiency and therefore provide considerable fuel savings compared with a normal petrol vehicle - as well as carbon emissions savings. While models might cost more than conventional cars, running costs can be two-thirds that of equivalent petrol-fuelled vehicles.
Because of their lower CO2 emissions, hybrids also benefit from reduced vehicle excise duty and are treated favourably in Budgets. In addition they are exempted from the London Congestion Charge.
However at the moment there are a limited number of hybrid vehicle choices; there are currently just four hybrids available in the UK; the Toyota Prius, Honda Civic hybrid, Lexus RX400h and Lexus GS450h. As they are still a relatively new technology, there aren't many available second-hand and so they are quite expensive.
Toyota's first Prius (launched in Japan in 1997) didn't sell in great numbers, however a new model has been introduced and this is now proving more successful. Although it looks like a normal car, it is designed around energy efficiency, and has many clever technological features that assist fuel consumption, including air conditioning and brakes powered by electricity rather than by sapping energy from the petrol engine. Lexus, part of Toyota, has introduced a hybrid Commercial gas engineer London version of the RX300, known as the RX400h. This is an SUV and because of its size, it still only returns around 35mpg compared to the Prius's 65.7mpg.
Although the official fuel economy figures for cars such as the Toyota Prius at 65.7mpg sound great, they only really achieve maximum economy benefits in built-up areas where they primarily run on electric rather than petrol although the Prius can only drive for around a mile on battery power before needing to revert to petrol; in real-world motoring it seems difficult to attain the official figures. On a motorway run, a good diesel is likely to be more economical. Nevertheless hybrids are still one of the best options that the consumer has today to achieve better fuel economy, especially if much driving is done in towns, along with the financial benefits such as lower tax and escaping London's Congestion Charge.
Only petrol-electric hybrids are currently available; diesel-electric hybrids will achieve even better fuel consumption - watch out for these appearing in the not-too distant future.
Battery-Electric cars
Electric cars use a battery and electric motor to power the vehicle, meaning they have no emissions at the point of use. Due to the capacity of the battery, their range is normally limited to about 40-60 miles between recharges, which means they are only really suitable for city-based users.
Electric vehicles can be recharged by plugging them into an existing electrical socket, and some city councils are installing electric recharging points in car parks or on-street. However, they are only truly 'green' if they are recharged with electricity from renewable sources such as windfarms.
Electric cars are not subject to road tax and, as an added bonus for London drivers, they enjoy 100 per cent congestion charge discount. Drivers living in areas where they have to pay for residential parking permits might also find that they get a discount on this cost.
Electric cars are often regarded as the ideal non-polluters, but it's not quite that simple. They're obviously good for lack of tailpipe emissions and are near silent, but they need to carry rechargeable batteries. These batteries are not really energy sources, they are energy storers. This creates two issues. Firstly they are heavy and bulky with very low energy densities. Secondly, the original source of energy, for recharging, has to be questioned, because if it's not renewable, those vehicles will indirectly contribute to climate change. There is a further drawback; if their batteries are lead or cadmium-based, there is a serious pollution problem if they are not properly disposed of at the end of their life.
The only electric car available in the UK at the moment, although this is about to change, is the G-Wiz, a small car for about-town use, claimed to be the greenest, most energy-efficient and cheapest car to run in the UK. It has two front seats plus two children size seats, which can be folded down for luggage. It is in insurance group 1, exempt from VED and the London Congestion Charge, and qualifies for free or discounted parking in some London areas. The G-Wiz is claimed to consume just one quarter of the energy of a similar-sized petrol car and costs around just £1.64 a day to run, equivalent to around 600 miles per gallon.