by Andrew Porter
England 6 March 2009. Most people think of the internal combustion engine as being the thing that powers cars. But just as many are used for railway trains, ships, planes and more. Although carmakers are looking at alternatives like fuel cells, battery power or even biofuels to run hybrids, there are still plenty of vehicles with plain old internal combustion engines – so is there anything that can be done to improve it that will help lessen environmental impacts?
Infernal Internal Problems
In most cars, there is an optimum engine revolution rate that provides the best fuel economy. This is when the fuel/air ratio is correct, the combustion chamber has peak efficiency, burning occurs at just the right time, and exhaust emissions are the minimum. At about 2500 rpm, engines are usually most efficient. Few cars (apart from the DAF/Volvo Variomatics) optimise this, however – the Variomatics uses optimum torque and engine efficiency despite the changing speed of the car through a constantly variable mechanical transmission system.
Waste heat, wasted sound energy, and wasted kinetic energy (braking energy) are also serious concerns. A car weighing 1 tonne braking from 50 kph wastes 112,500 Joules of energy heat. The higher the speed, the hotter the brakes and the more energy waste. It should also be noted that power (Horsepower or Watts), indicates the rate of energy transfer. The best that is generally accepted for the use of internal combustion engine is about 30% for a petrol-run car and 40% for a diesel-run car – neither of which is very good even before the energy wastage to braking, heat and so on. So any ways to grab some energy back are to be welcomed.
Electronic control of engine timing and injection can also make a difference. Instead of using mechanical timing to decide fuel delivery point and ignition, electronics optimise when and how much fuel and air are injected into the engine and ignited. This means less fuel is used, and the waste products are reduced.
Some Alternative Solutions
- Some railway locomotives have used hot engine cooling water to ensure what would otherwise be wasted energy was used to heat the carriages – this can be done with cars too.
- Braking energy could be turned into electrical energy and stored in a battery (some cars already do this) but this means carrying a heavy battery and generator, which demands more energy. An alternative is to use compression braking. This is when fuel is cut completely from the engine, but it is still turning, obviously, as the car slows. This allows air to be forced out under pressure by the engine pistons, into a special compression chamber. As the air pressure in this chamber increases, it increases the load on the engine, and slows the car. The energy stored in the pressurised air can also be used to help speed the engine up again later. This system works very well, but is noisy.
- The next method, is using an alternative fuel is hydrogen extracted from water (H2O). Hydrogen has a high energy density; the distance travelled on a given amount of fuel is better than petrol or diesel. (Energy Density of Hydrogen by weight: 143MJ/kg3; Energy Density of Petrol/Gasoline by weight: 46.4MJ/kg3). Oxygen helps to burn hydrogen, but extracting them isn’t easy. One solution (http://pinoybusiness.org/2008/08/06/water-powered-car/) uses a battery-powered reaction unit that breaks the water down into hydrogen and oxygen before supplying these to the engine. The waste products include pure water, and heat. However, some of the waste water can be returned to the reaction unit for breaking down once again. As the process is not 100% efficient, more water is always needed.
- BMW has devised a method of recovering waste heat energy to help power the car. As 66% of energy contained in petrol is lost via exhaust emissions and cooling water, it has designed a system to use waste heat to produce high-pressure steam. Heat is extracted from the exhaust pipe, and then the steam produced is used to help turn the engine shaft (an additional system using ethanol extracts even more waste heat). BMW estimates that this system improves overall efficiency by about 15%.
If some of these methods are adopted then we could see notable benefit without changing the basic infrastructure of fuels, fuel filling stations and so on, as referred to in my previous articles. Combined, they can offer improved total energy efficiency without loss of performance, but with a definite reduction in pollution and reduced carbon emissions.
Picture taken from: http://www.theengineer.co.uk/liChannelID/2/Articles/310051/Boiling+point.htm
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