Four-stroke petrol engines: These will continue to dominate, though the proliferation of electronics under the bonnet will make them ever more baffling for the home mechanic. On-board computers will control timing and petrol-air mixture with increasing accuracy, improving fuel consumption and cutting emissions.
They will be helped by cleaner fuel. Responding to legislation, US oil companies are introducing what they call reformulated gasoline, which is cleaner and burns better. In addition, ethanol - alcohol that can be made from natural gas or from rotting maize - and methanol, a chemical product, are being promoted as fuels: their progress will be determined by tax incentives.
Catalytic converters, which remove noxious gases from the exhaust, are now compulsory on new cars in Europe, but need to be improved. Johnson Matthey, the world's biggest catalyst producer, is using computer-aided design to minimise the chronic problems of lower performance and worse fuel economy. It is also looking at ways of preheating catalysts. They do not work when cold, so are useless on short journeys.
In the Eighties a lobby led by Ford and Rover was pressing for the UK to go down the 'lean burn' route, rather than towards catalysts. Lean burn engines work on a weaker mixture of air and petrol than normal motors, and are thus less polluting and more economical. They have the appeal of simplicity, but have proved unable to satisfy increasingly strict emission limits while giving adequate power. Nevertheless the engine of the future will certainly be leaner, at least some of the time. Toyota already has a motor that uses lean burn when possible, but switches to a normal mixture when under stress.
Two-stroke petrol engines, usually associated with lawnmowers and primitive East German cars, are making a comeback thanks to the Orbital Engine Company of Australia. General Motors, Ford, Volkswagen and Fiat have bought licences from Orbital, and Ford is running a demonstrator fleet of two-stroke Fiestas. Two strokes are theoretically more efficient than four, because they generate power on every piston stroke, rather than on every other stroke. They are also simpler and lighter. They have tended to be dirty, though, because oil has been added to the petrol to lubricate the engine, generating an unpleasant exhaust. Orbital uses fuel injection to separate the oil from the fuel and, using a simple catalyst (developed by Johnson Matthey), the engines are now clean.
The diesel engine is by its nature lean-burn, because there is no limit on the air going into it. That means it has good fuel consumption. Even though diesels emit low levels of the main noxious substances, they do pump out smoke, which has potentially unhealthy particles in it. Diesels also tend to be slower and noisier than other cars. However, special catalysts and sophisticated injection systems (Lucas is a leader here) are overcoming these problems, and diesels' market share is likely to grow. Short-term growth will depend on how heavily the fuel is taxed. If diesel cars become too popular, the price of the fuel could start to move up.
Gas derived from coal was used to power vehicles during the Second World War. Now gas, as a clean and plentiful fuel that can be used in barely modified internal combustion engines, is making a comeback. Several cities in the US and Canada have buses that run on natural gas. British Gas also has vehicles powered by natural gas. The main problem is storage, which is why it tends to be used on larger vehicles, although natural gas can be converted into liquid ethanol.
In theory hydrogen is the cleanest fuel of all: its waste product, after combining with oxygen, is water. Mazda exhibited a hydrogen-powered, rotary-engine town car at the Geneva motor show. But hydrogen is highly explosive, and difficult to store on a vehicle (see fuel cells, below).
Electric motors: There have been great developments in powerful lightweight electric motors: Iwon Motronics of California is working on a motor generating 100hp, the same as many two- litre petrol engines. The concept car Fiat showed at the Geneva motor show uses an intriguing idea borrowed from its rail division, Elettromeccanica Parizzi. Tiny motors are incorporated in the wheel hubs, so the wheel assembly includes motor, brake drum and suspension.
The lightweight high-power battery remains the Holy Grail for the electric car designer. Traditional batteries are lead-acid, but as anyone who has tried to pick up a milk float knows, they are both heavy and bulky. Cars modified to use them - Fiat and Peugeot have led the field - are two-seaters, with batteries taking up the rear seat room.
New types of battery are being tried: all are lighter and give a longer range than lead-acid, but will remain more expensive until demand justifies volume production. Nissan favours the nickel-cadmium battery, which it says can be recharged in 15 minutes, while Fiat and Volkswagen are testing an Asea-Brown Boveri sodium-sulphur battery, using technology developed in Britain and Germany. This has four times the range of a traditional lead battery but operates at 300C: it has to be kept cosy in a special airtight container.
A favoured solution to the battery problem is the hybrid car, which is electric but has both batteries and another small engine to generate power. In town the car would be all-electric, but in the country the petrol engine would be switched on and used to charge the batteries. Most big manufacturers have built hybrids, and the City of Los Angeles has financed the development of the LA301 hybrid by the Worthing- based International Automotive Design. It is scheduled to go into production this year.
Many people see the gas turbine as the best secondary engine for a hybrid. It runs at high speeds economically and with low emissions. There is also great excitement about the fuel cell, part hybrid, part battery, with a technology regarded as so important that even the parsimonious British government has agreed to fund research on it.
William Grove, a Welshman, first demonstrated the principle of the fuel cell in 1839, but its commercial application started only after Nasa adopted it for the space shuttle. Grove showed that if you pass electricity through water it will split into hydrogen and oxygen, and that if you recombine the hydrogen and oxygen, it will produce electricity.
There have been attempts to repeat Grove's experiment in a car, producing so-called 'water-power'. There is scepticism over its practicality, however, and all commercial fuel cell systems have used specially produced hydrogen.
This is the tricky bit. In its pure form hydrogen is highly unstable; to keep it in liquid form requires an extremely low temperature. Johnson Matthey's researchers believe the best bet for a car is to make the gas on board: a conventional fuel tank is filled with liquid methanol; this is heated and 're-formed' to create hydrogen, which then passes into the fuel cell, generating electricity to power the motor. The car is thus similar to the turbine hybrid, in that the power comes from a refillable liquid rather than from heavy batteries. Existing fuel cells use phosphoric acid and run at 200C.