The piston-driven steam engine has been almost entirely replaced by the internal combustion engine for automotive, marine, rail and stationary power applications. The reason for its demise is acknowledged to be its lower thermal efficiency in comparison with that of the internal combustion engine.
Some specific disadvantages of classical steam engines are now mentioned as they have a bearing on what follows:
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This type of engine uses steam at relatively low pressure (up to around 1500kPa) and this means that the cylinders must be of rather large capacity when compared to those of an internal combustion engine of the same power.
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In the piston steam-engine, admission and exhaust valves open and close once for every single revolution of the engine as opposed to once in every two revolutions for a four-stroke internal combustion engine. This means that the valve mechanism in a steam engine must operate at twice the speed for given engine revolutions as compared to a typical internal combustion engine. Valve operating speeds are an important limiting factor for the maximum running speed of any piston engine. Therefore steam engines are obliged to run at much slower speeds than internal combustion engines and are unable to take advantage of the increased potential for higher power output provided by high-speed operation.
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Also relating to valve mechanisms, steam engine operation requires that valve timing and the duration of opening (valve events) should be variable, which makes the steam engine valve system significantly more complex than the valve system in standard internal combustion engines.
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In order to achieve thermal efficiencies similar to those found in the internal combustion engine, heat must be recovered from the exhaust steam (in a condenser) and furnace exhaust (in a heat exchanger) and returned to the feed water and combustion air. Condensers and heat exchangers are bulky and costly additional items.
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Steam passages and valve openings must be relatively large for a steam engine to function well due to the volume occupied by steam (1600 times the volume of water at atmospheric pressure). This factor increases the size and complicates the construction of these elements as compared to the internal combustion engine whose air passages and valve openings can be simpler and smaller.
For these reasons, classical steam engines are typically much bigger than internal combustion engines of equal power ratings, even without adding external heat-recovery systems. Their power density is significantly lower and their first-cost is higher. If heat-recovery systems are not fitted, their fuel consumption is higher. If bulky condensation systems are not fitted they require constant replenishment of the water consumed.
It is worth noting here that the steam engine in the form of a turbine continues to be widely used for electrical power generating applications. When combined with efficient heat recovery systems, such generating plants may achieve thermal efficiencies of over fifty percent. By contrast, the classical piston steam-engine, such as found on railway locomotives, achieved thermal efficiencies in the range ten to fifteen percent. It is therefore reasonable to conclude that the classical piston steam engine cannot compete in today’s applications against the internal combustion engine which can achieve thermal efficiencies in the range of twenty five to thirty five percent, has a high power density and is cheap to make.
