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Last week we looked at the efficiency of Internal Combustion Engines (ICE) and ways they could be improved. This week I would like to look at External Combustion Engines (ECE) and ways they can be improved. ECE play a huge part in our history and the development of the high pressure steam engine set the stage for the industrial revolution. They also play a huge part in the developed world today and are used to generate the majority of the world's electricity.
An external combustion engine is an engine that derives its power by burning a fuel in a chamber that is open to the atmosphere. Being open to the atmosphere means that ECE are more tolerant to variations in fuel and do not require the highly refined fuels that most ICE do. Examples of ECE are:
- Reciprocating Steam Engines There are two main types of reciprocating steam engines
- Vacuum Engines were the first steam engines and worked by using the drop in pressure that occurred when steam is allowed to cool and condense creating a vacuum in the cylinder.
- Expansion Engines were developed as a replacement to the vacuum engines and worked by harnessing the energy in high pressure steam. They are more powerful than vacuum engines but require the handling of high pressure steam
- Steam Turbines are the next step in the evolution of the steam engine. Rather than using expanding steam to drive a piston it is allowed to expand through a series of radially mounted blades that cause rotation.
- Stirling Engines are closed cycle reciprocating engines with a pair of cylinders arranged in a way that enables the total volume to vary as a function of the position of the crank. They hold the potential of being the most efficient of all the ECE but are limited by the rate energy can be exchanged with the gas in the closed cycle.
ECE require some sort of system like a boiler that is used to collect the energy from the combustion and this makes them relatively inefficient when used in transport. The steam locomotive that was in common use till the middle of the last century, whilst being powerful was only about 5% efficient overall.
ECE do however have advantages and in fixed situations where the mass is inconsequential and complex control of the combustion process can be implemented, they can be relatively efficient. The most common external combustion engines in use today are the steam turbines used to generate electricity. The source of heat can be virtually anything that burns or produces enough heat to cause a change in the state of the medium used within the system. Energy sources include natural gas, nuclear reactors, geothermal and oil but the most common is coal which is used to generate slightly more than half the world's electricity.
Whilst coal fired power stations are far more efficient than steam locomotives they are only around 30% efficient overall and this means that over twice as much energy is wasted than used. Coal is also one of the dirtiest fossil fuels and produces more carbon dioxide and sulphur compounds than most other fossil fuels.
When you look at the amount of electrical energy that is consumed around the world and realize that twice this amount is wasted generating the electricity then it is clear that improvements in efficiency here could have massive benefits. If the efficiency could be increased to 60% the amount of pollution generated by burning coal could be halved ant the total global production of CO2 reduced by over 10%.
The question is what can be done to improve the efficiency of external combustion engines? Can the heat that is currently wasted be utilized? Is there a better fuel for ECE than coal? Can ECE be modified so the pollution is not released into the atmosphere?
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