Recent developments in alternatively powered road vehicles – electric cars, hybrids, and in particular DaimlerChrysler’s NeCar4 fuel-cell-powered car – would seem to suggest that the internal combustion engine is about to be superseded.
But I believe that such engines, in particular the diesel, have another 30 years of life ahead because at present, all the potential competitors fall down on either cost or performance.
The internal combustion engine has created a particular combination of cost, performance, power density, range and usage patterns against which any competitor must be judged. Without major changes in transportation behaviour, this combination is unlikely to change.
The most fully developed competitor, the battery-electric vehicle, needs to be judged against this background. In spite of having a longer history than the internal combustion engine, it still falls short of the characteristics that customers take for granted. The potential for this challenger rests on the emergence of a breakthrough in battery technology.
Let us compare the weight and volume of different fuel systems equivalent to 50 litres of gasoline or diesel. This amount of the two conventional fuels weighs about 45kg, and will propel the average car about 500km.
A lead-acid battery to do the equivalent job would weigh more than 5 tonnes. Even a sodium-sulphur battery, typical of most advanced batteries today, would weigh over 1.5 tonnes and occupy more than 1,500 litres. Improvements of the order of 20:1 would be required to match the diesel or gasoline system.
In use on the road, the battery-electric vehicle is virtually non-polluting. However, in the UK at least, most of its electricity is generated from fossil fuels at moderate efficiency and then distributed with various losses, so the impact on air quality or global warming of these vehicles would be, at best, modest.
A typical car drive cycle, such as the US Federal Light Duty Cycle, shows why the battery-electric vehicle is superficially attractive. Although the maximum instantaneous power needed is 100kW, with full regenerative braking the mean power is only 4kW. An electric motor has a high overload capacity so the instantaneous power demand can be met.
The need for a range of 500km, however, is a real hurdle for the battery electric vehicle. But a hybrid configuration, with the average power provided by an internal combustion engine and an electric motor to cope with transient demand and for emission-free operation in towns, can achieve high efficiency and lower emissions while still giving a good range.
However, features common to all hybrid vehicles – two sources of power (internal combustion engine and electric motor), two storage systems (fuel tank and battery) and their complex control system – mean they are more costly and weigh more than single power source vehicles.
The least developed but most promising competitor to the internal combustion engine is the fuel cell. It operates on the fundamental principle that the combination of hydrogen and oxygen to form water creates electrical energy.
The cell’s operation is not only theoretically free of pollution, it is also free of the Carnot cycle restrictions – which set physical limits to the efficiency of heat engines – so greater efficiencies should be possible.
However, unless an infrastructure is developed to provide hydrogen as a fuel, the fuel cell will need to rely on a liquid hydrocarbon fuel. This would require a heavy and costly reforming plant, which would degrade the efficiency of the power system because all the carbon in the fuel would be `thrown away’ as carbon dioxide or carbon monoxide.
Although a practical fuel cell vehicle using hydrogen could be available by 2004, it is unlikely that it would come remotely close to the cost of a conventional vehicle, and equally unlikely that the infrastructure to provide the hydrogen would be in place in time.
Oil as a basic fuel source will be available for many years, so it looks likely that the diesel engine will continue to co-exist with gasoline and gas engines. The most important feature of the diesel is its efficiency compared with any other current prime mover. This efficiency is matched by its low CO2 output, which will be particularly important as governments worldwide attempt to meet their obligations under the Kyoto agreement.
Michael Monaghan is technical director at powertrain specialist Ricardo Consulting Engineers