Under the Microscope: As we all know, conserving the world's limited supply of fossil fuels and finding alternative means of generating power than burning these fuels has become an urgent priority, writes Prof William Reville
Not only are we fast losing a non-renewable resource, but burning fossil fuels releases the greenhouse gas carbon dioxide to the atmosphere to dangerously warm the world. About 30 per cent of our usage of fossil fuel is the gasoline and diesel used for transportation and it is well past time we found ways to greatly reduce this dependence. A very promising move in this direction is the development of the gasoline-electrical hybrid vehicle. This new technology is described by JJ Romm and A A Franklin in Scientific American, April 2006.
But before I describe the hybrid car, I would like to briefly ponder on what seems to me to be an obvious and easily effected measure that would quickly allow us to greatly cut down on our consumption of gasoline.
Most of us live in or near towns and drive relatively short journeys to and from work, shops, etc, unaccompanied in big saloon cars. Why is all such local motoring not done in special little cars that have a fuel economy comparable to the motorbike, reserving our current saloon cars for inter-city driving?
A hybrid car uses a battery-powered electric motor in combination with a petrol-burning engine to reduce gasoline consumption, reduce emissions and to improve range and acceleration. The battery motor and sophisticated power electronics add to the initial cost of the car but this extra cost is recovered over a few years in improved fuel efficiency. The extra "electrical costs" should drop dramatically as technology advances and hybrid cars become more popular.
The commitment to fuel economy in hybrid car design ranges from "full" to "mild" hybrid cars, ie from making every effort compatible with current technology to economise to merely shutting off the engine every time the car stops in traffic. The principal device used by the full-hybrid design to economise on fuel is the simple but elegant mechanism of trapping braking energy and storing it as battery power. In our conventional cars the energy released in braking is dissipated and lost as frictional heat. Current hybrid technology can convert about half the braking energy into battery energy.
The electrical assistance in full-hybrid cars allows the manufacturer to use smaller internal combustion engines and even to run these engines on a different cycle to conventional engines. Engines run most efficiently only at certain speeds and torques but the conventional car engine must operate over a wide range. The engine in a hybrid vehicle can be restricted to run at the most efficient settings, with electrical power kicking in to boost performance as driving demand requires.
Also, conventional petrol engines run on an internal combustion cycle called the Otto cycle which is more powerful but less fuel-efficient than the alternative Atkinson cycle. The hybrid car manufacturer has the option of using the more efficient Atkinson cycle and boosting its performance with the electrical system. Hybrid cars also improve on fuel economy by running the car's air-conditioning, power steering, water and oil circulation, etc off the electrical battery. In conventional cars these devices run off moving parts in the engine with motive power transferred by belts.
The full hybrid car can also switch over entirely to electrical power under certain conditions such as when the engine is idling or the car is moving at low speed, thereby burning less gasoline. A full hybrid car, such as the Toyota Prius, can achieve an improvement in full economy of 60 per cent and more over the conventional car. On the other hand, the minimal design of the micro-hybrid, which switches over to electrical power when the car stops, can achieve an improvement of 10 per cent in fuel efficiency in city driving but produces little advantage on the open road.
Battery technology is at the heart of the hybrid car. The next generation of hybrid vehicles will have much improved batteries and this will allow plug-in hybrids to become commercially viable. These cars will run in either hybrid or full electrical mode and will allow further significant improvements in fuel efficiency and reduced emissions. The "plug-in" in the title of these new hybrids refers to their capacity to plug into the electrical power overnight to charge up their batteries at off-peak power rates. Such vehicles could also feed power back to the grid during the day at peak power demand hours.
The new improved batteries in the plug-in hybrids will allow manufacturers to use ever smaller petrol engines. Romm and Franklin describe a prototype plug-in hybrid Mercedes-Benz Sprinter van currently being tested with a 143 horsepower engine and a 120 horsepower electric motor. It can travel in electric mode for 20 miles, uses 40 per cent less petrol than a conventional van and has better acceleration. Improvements in battery technology will allow much better fuel economy in the future and modified internal combustion engines will allow plug-in hybrids to run on a mixture of 15 per cent gasoline and 85 per cent biofuel. Economies like this will be essential when world oil supplies go into decline, but it would be wise to develop and use these engines as soon as possible anyway.
Hybrid cars are available in Ireland. The Toyota Prius, is the best known. Lexus sells the GS450h hybrid saloon car and Honda will introduce a 5 door hatchback hybrid later this year.
• William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie>