A fully integrated transport policy should also acknowledge the need to reduce pollution in city centres. Dave Parker reports on how vehicle manufacturers are looking back to the early history of internal combustion engines for a solution.
London at the end of the last century stank to high heaven. Not, as in previous centuries, from raw sewage running down the streets - the new breed of Victorian civil engineers had already diverted such noxious flows into a vast underground sewerage network. No, London, like all other major cities of the time, was choking to death on the exhaust emissions of the transport that clogged its road network. In other words, London, and the rest of the civilised world, was facing a horse dung crisis.
Thousands of horses were needed to pull the hansom cabs and omnibuses, thousands more hauled delivery carts in from the suburbs - many of them loaded with horse fodder.
Each horse produced several kilograms of dung every day. And although a veritable army of men and boys struggled to keep the streets clean, it was clear they were losing the battle. Railways, above or below ground, were only a partial solution. What was needed was a cleaner substitute for the horse.
Enter the liquid-fuelled internal combustion engine, just in time. Within a few decades horses had virtually disappeared from city streets, and with them the billions of flies that fed on their dung - and on the corpses of the tens of thousands of horses that died in the cities every year.
Pedestrians no longer breathed in bacteria-laden sun-dried dung dust, so respiratory diseases declined. Traffic congestion was no better and average city centre speeds no higher, but the city centre was a cleaner, healthier place if only for a few years.
Then, emissions from the millions of internal combustion engines began to be recognised as a major health hazard in their own right. City air now contained an unpleasant cocktail of carbon monoxide, oxides of nitrogen and asthma-inducing particulates from diesel engines. Under pressure from environmental and public health lobbyists, governments across the world imposed tighter and tighter exhaust emission regulations. This downward pressure is expected to continue in the light of this week's Integrated Transport White Paper.
Vehicle manufacturers have responded with computerised engine management systems and exhaust catalysts, succeeding so well that a 1998 car is likely to produce less than 15% of the emissions of a late 1980s model.
But this technology is nearing its limits and emission regulations are likely to get tighter, especially in busy urban centres. Battery powered electric vehicles still suffer from limited performance and high cost. Although electric cars continue to be developed, most manufacturers now believe the future lies with one of the oldest fuels - gas.
Most early internal combustion engines ran on town gas - produced locally from coal - and drove electric generators and the like. But, although they were heavy, inefficient and prone to explode, internal combustion engines still offered potential advantages over steam or electric power. Their main problem was fuel storage.
Early horseless carriage inventors soon realised that liquid fuels offered an 'energy density' that no competitor could match. A liquid fuel storage tank was light, compact and cheap to manufacture. But although the early petrol engines borrowed heavily from stationary gas engine technology, gas as a fuel for vehicles almost disappeared - but not entirely.
Town gas powered vehicles ran on the roads as late as World War II, but the huge balloons of gas they had to carry on their roofs made them impractical except in emergencies. It was not until the oil companies began liquefying and bottling the gas produced during the refining of crude oil that gas-powered vehicles became a practical proposition.
Much later, companies like British Gas began to sell compressed natural gas in cylinders as well. Although standard spark ignition internal combustion engines can burn both forms of gas with only minor modification, no major vehicle manufacturer seriously considered producing a pure gas powered vehicle. Instead, standard vehicles were converted to run on petrol and either liquefied petroleum gas or compressed natural gas, switching fuels as required. (see box )
Initially the motive was to save money. Basic price and tax levels were usually significantly lower than petrol, and even though most engines suffered a measurable loss of power on gas, cost savings were enough to compensate.
Gas has become more attractive as emission controls have grown even tighter. Although both forms of gas are fossil hydrocarbons and produce CO2 when burnt, carbon monoxide and oxides of nitrogen emissions are much lower than those from petrol engines. And there are virtually no particulates.
But a lot of problems remain. Current internal combustion engines are optimised to burn petrol, so are less efficient burning gas. Exhaust catalysts designed for petrol work less well, if at all, with gas. Fuel storage is another problem, especially for CNG.
This has to be stored at a pressure of 200bar, compared to LPG's 10bar. For the same performance and range gas will need up to four times the storage volume of petrol or diesel. Storage costs are inevitably much higher than the equivalent cost for fuels which are liquid at atmospheric pressure. It is also heavy.
Bi-fuel vehicles are therefore heavier, more expensive and less roomy than their conventional alternatives. Refuelling is also difficult. There is only a very restricted network of filling stations - although fleet operators can buy their own compressors and refuel CNG vehicles overnight from the standard gas main. Compared to battery-powered electric vehicles, however, gas/liquid hybrids look a much better bet, having longer range, higher performance and lower initial cost.
Low emission is not the same as zero emission, however. The ultimate fuel for all road vehicles is pure hydrogen gas. Producing only steam when it burns, hydrogen could solve all exhaust pollution problems - if the formidable technical problems associated with storage can be overcome.
Around 20,000 litres of hydrogen at atmospheric pressure would be needed to give a typical private car a range of 200km or so. This can be stored in one of three ways - in a high pressure cylinder, in an insulated low pressure container at close to absolute zero or absorbed into special metal alloys.
Toyota has a research vehicle which can store 20,000 litres of hydrogen at 10bar in 100kg of these special alloys. The hydrogen released when the alloy is gently heated is not fed into an internal combustion engine, although this would be technically feasible. Instead it passes into a fuel cell - effectively a battery in reverse - where it combines with oxygen from the air to generate electricity.
This then drives a conventional electric motor. Together the fuel cell and hydrogen store weigh around half the equivalent battery pack.
Vehicles like this would meet all foreseeable emission regulations. In the short term bi-fuel cars and taxis, pure gas buses and delivery vehicles will appear, followed soon afterwards by true internal combustion/electric hybrids. Fuel cell vehicles are further off, but most experts see these as the ultimate goal. With the environmental issue out of the way, tackling congestion will become even more important.