What is it? Potentially, a sort of evolutionary "missing link" between traditional internal combustion engine technology and the new world of alternatives.
How does it work? An electric motor and batteries are added to the traditional petrol/diesel engine. Energy created by the engine that would otherwise be wasted, for example, during braking, is stored in the batteries to be used when needed, say, when accelerating hard. Some hybrids can run for short distances on electric power only. You do not plug them into the mains (although there are companies that will convert them and deliver a claimed equivalent of 130mpg).
For: They do conserve energy and they do have better fuel consumption and lower emissions, but sometimes worse than claimed in official figures. Silent at low speeds. Tend to attract tax breaks.
Against: A certain amount of hype is associated with them. There is the potential for a lot of "leakage" as energy is moved to and from the batteries. Modern small diesel engines often offer superior economy and low CO 2 emissions. Relatively high prices deter consumers. The cars are usually ugly and heavy. Batteries take up a lot of space.
Prospects: Usually regarded as an "intermediate" solution, but the pioneer, Toyota, insists they are there for the long term. Sales are growing quickly but are still to reach the levels that would mean the manufacturers would recoup their investment.
Examples: Honda Civic Hybrid; Toyota Prius; Lexus 450h (as used by David Cameron); Lexus RX300h; Ford Escape Hybrid SUV, Ford Explorer Hybrid SUV; Chevrolet Silverado Hybrid pick up; Mitsubishi Fuso lorry.
The electric car
What is it? Something you plug into the mains to charge. Its greenness is down to the fuel used to generate the electricity in the power station. If it is wind power, the car is pretty eco-friendly.
How does it work? Batteries take electricity from the grid and store the power. The car is driven by an electric motor. Modern batteries and other advances mean they are faster and have a better range than the familiar examples, the golf-cart and milk-float.
For: Very green, because even a power station run on fossil fuels is much more efficient than the internal combustion engine. Silent.
Against: With the exception of the (expensive) 130mph Tesla sports car, they have a short range and low top-speed.
Prospects: The Tesla shows what can be done. There is an electric version of the Smart city car on trial which can do 70mph and has a range of 70 miles, far better than the G-Wiz (40mph and 40 miles). Memories of the Sinclair C5 are fading.
Examples: Tesla, Smart ForTwo Electric; Reva G-Wiz. Electric models from Piaggio, EFFIDI/Sakura and Aixam compete in this tiny sector of the market.
What is it? A conventional car that can run on unconventional fuel from sustainable sources, from rape-seed oil to sugar-cane and woodpulp. Usually the bio-fuel is blended with petrol or diesel: for example "E85" is 85 per cent bioethanol and 15 per cent petrol.
How does it work? The same as a conventional car. It is green because the source of the fuel can be replenished.
For: Greener than the older fuels. Biofuels produce 65 per cent fewer greenhouse gas emissions because carbon emissions during production and consumption are almost equal to the amount removed when the crops from which they are made are grown.
Against: In the UK, there is little tax advantage to buying the fuel to offset the normally high cost of purchase and poorer fuel economy. Few filling-stations for bio-fuel (a handful in East Anglia and the South-west). Chip-fat is a crude "bio" fuel.
Examples: Saab 9-5 biofuel; Ford Focus Flexi-Fuel.
Compressed natural gas
What is it? Environmentally clean alternative to those fuels. It is made by compressing methane (CH4) extracted from natural gas. It is stored and distributed in hard containers, usually cylinders. Natural gas could be pumped directly into your car from your domestic supply.
How does it work? Just as an alternative to petrol or diesel, so no substantial new engine technology is needed and existing vehicles could be converted.
For: Lower emissions, convenience if it can be linked to a domestic gas supply so refuelling can be done at home.
Against: Needs plentiful cheap supplies of natural gas, just at the time when that is being depleted.
Prospects: Popular in parts of South America and the Indian sub-continent, but less attractive in Europe as reserves start to run down.
Examples: CNG Honda Civic GX (USA); Volvo V70 dual-fuel; some refuse lorries; anything from buses to auto rickshaws in countries such as India, Pakistan and Chile. Liquefied natural gas (LNG) is similar but more expensive to produce because it needs to be cooled. Neither should be confused with liquefied petroleum gas, or LPG, which is not much greener than existing fuels but is certainly much cheaper and has about 1,000 UK filling-stops, as opposed to a mere 25 nationally for CNG.
The hydrogen fuel cell
What is it? Revolutionary.
How does it work? Hydrogen is "liberated" from, say, water via electrolysis, then cooled to turn it into liquid hydrogen, which is distributed in pressurised cylinders. Hydrogen is turned into electricity through fuel cells which then power electric motors. Although it produces only water from its exhaust pipe, the energy required in producing and transporting the hydrogen gas on which it runs takes the edge off its green credentials. Alternatively, hydrogen can be burnt in existing engines as an alternative to petrol or diesel.
For: Potentially much cleaner and greener, depending on how the energy to create and transport the hydrogen to fuel stations is generated. Pure water as emissions would make city air cleaner.
Against: Feasibility depends on making the hydrogen generation efficient. Large tanks of hydrogen gas may have additional safety concerns. Need more space to carry the fuel.
Prospects: Most agree this is the most likely contender to replace the old technology on a 10-year time scale. Honda has been piloting its hydrogen fuel cell technology for some time in Japan and the US. Mercedes-Benz also has concept A-Class hydrogen fuel cell cars on the road, and have trialled a London bus with the new technology. BMW has also experimented with hydrogen, but as a simple replacement fuel in conventional engines.
Examples: Honda FCX; Mercedes-Benz A-Class (experimental); Mercedes F600 Hygenius; Mercedes-Benz bus; GM Opel/Vauxhall Zafira; Peugeot, Nissan and Hyundai test vehicles; various BMW concepts.
What is it? The ultimate green car, powered by sunshine.
How does it work? Energy from the sun is converted directly via photo-Voltaic cells and stored in batteries.
For: Zero emissions, the fuel is freely available on an inexhaustible basis and needs no energy to create or transport.
Against: Poor performance. Obvious difficulties of daylight and season are compounded by low power outputs.
Prospects: A very long shot. Vehicles need to be extremely light (and thus use expensive materials) and aerodynamic. They normally carry only one person. Still, in 1993 the Honda Dream won the world record for a solar-powered journey, crossing Australia from Darwin to Adelaide, so they work, after a fashion.
Examples: Schools, universities and eccentrics usually the source of strange-looking but working machinery that may be mainstream in a century.Reuse content