The major limitations or disadvantages of a steam engine or external combustion engine are given below :
1. A steam engine is huge and heavy. (That is, An external combustion engine is huge and heavy). Due to its big boiler and furnace a steam engine is huge, heavy and clumsy. Since the boiler of a steam engine is very heavy, therefore, a steam engine cannot be used for running small vehicles like cars and buses.
2. A steam engine does not start at once. (That is, An external combustion engine does not start at once). Before a steam engine can start, we have to build a coal fire to get steam. This process takes a long time and hence a steam engine cannot be started at a moment's notice.
3. A steam engine is unsafe to use. (That is, An external combustion engine is unsafe to use). A steam engine is not very safe to use because its boiler can burst due to excessive steam pressure. In order to get the maximum work from the heat energy, the steam has to be maintained at high temperature and a very high pressure in the boiler. If, by chance, the steam pressure increases too much, then the boiler can burst leading to loss of life and property. Thus, the most dangerous part of a steam engine (or external combustion engine) is its boiler where steam is produced. Actually, in the early days of the steam engine, good materials were not available for the construction of boilers which could withstand high steam pressure. Even the construction techniques were not perfect. So, a lot of accidents were caused in the early days of the steam engine due to the bursting of boilers, because of poor materials used in making boilers and faulty construction of boilers.
4. A steam engine has low efficiency. (That is, An external combustion engine has low efficiency). The thermal efficiency of a steam engine is about 20 per cent which means that a steam engine can convert only about 20 per cent of the heat energy into work. One of the reasons for the low efficiency of a steam engine is that the steam used in it is not at a very high temperature. Another reason for the low efficiency of a steam engine is the loss of heat because of the large exposed surface area of the steam engine, particularly the boiler and the cylinder.
Because of these limitations (or disadvantages) of the external combustion engine, another engine called internal combustion engine was invented. We will come to it after a while. We shall now discuss some of the advantages of the external combustion engine (steam engine) over the wind-mills and water-mills.
Advantages of the Steam Engine over Wind-Mills and Water-Mills :
1. A steam engine (or external combustion engine) could be located anywhere because water is heated in a separate boiler to produce steam. The steam engine, with its boiler, could be taken to any place of work. On the other hand, blowing wind or flowing water is not available everywhere to run the wind mills or water mills.
2. A steam engine (or external combustion engine) can be used at any time because steam can be produced at any time. On the other hand, a wind-mill or water-mill can be used only when wind and flowing water are available.
3. A steam engine (or external combustion engine) can be used to run trains for going from one place to another but wind-mills and water-mills cannot be used for this purpose.
4. A steam engine (or external combustion engine) is much more powerful than a wind-mill or water-mill.