The earliest steam engines, as developed by Thomas Newcomen and later improved by James Watt, utilized the condensation rather than the expansion of steam to perform work. Unpressurized steam was injected into the cylinder at the top of its stroke and allowed to cool and condense. This created a vacuum relative to the ambient atmospheric pressure, which caused the outside air to press down upon the cylinder and move it.
These early steam engines were extremely inefficient and large, however, and were generally used only in stationary applications (such as coal mines) where large quantities of fuel were available. Subsequent developments brought significant improvements in engine efficiency and reductions in engine size.
High-Pressure Steam Engines
Steam became a major player in industry with the creation of the high-pressure steam engine by Richard Trevithick. Because the energy content of steam at pressures greater than atmospheric is higher, these steam engines could be much smaller in size than their predecessors. In this configuration, the high-pressure steam performs the work against the engine's piston instead of creating a vacuum. The steam is admitted to the cylinder using valves and is converted to reciprocating power by using a connecting rod and crankshaft.
Once the basic design was established, improvements to the high-pressure steam engine began in earnest. Its potential as an industrial power source encouraged its development. During the next century, numerous designs were put forth. The grand majority of these engines, regardless of manufacturer or style, can be placed into one of two general categories: counterflow and uniflow.
Counterflow Engines
Counterflow engines were characterized by having a single inlet/exhaust port into the cylinder. When the steam valve was in the inlet position, pressurized steam flowed into the cylinder and performed work. At the end of the stroke, the valve moved to the exhaust position, which allowed the steam to exit the machine.
Because of their robust nature and predictable torque curve, counterflow engines were used widely in the rail industry. They were also largely inefficient compared to uniflow engines, however, because after the steam had expanded in the cylinder, the steam had to exhaust through the same hole it had entered through.
Passing the cooler gases over the hot inlet port decreased the efficiency of the counterflow engine. In addition, steam develops its greatest work when exhausted into the lowest pressure possible. Under normal conditions, the lowest pressure is the ambient atmosphere. With a counterflow engine, however, the steam exhausts back into the engine, under pressure, before being expelled.
Uniflow Engines
The other major type of high-pressure steam engine was known as the uniflow engine. This engine is capable of significantly higher efficiency rates than a counterflow engine because the heated steam flows in one direction downward along the bore and exhausts through side-ports directly into the ambient atmosphere. This uniflow design utilizes the maximum amount of steam expansion in a single cylinder.
Several technical hurdles limited the uniflow engine's success, however. First, these engines relied upon poppet valves instead of slider valves to control the steam input to the cylinder. Because of the primitive metallurgy available at the time, these parts tended to be extremely fragile. Next, because the steam flowed in a single direction through the cylinder, they developed a heat gradient along the length of the bore that caused expansion at different rates. Unless the bores were properly machined, there was a risk of a seizure in mid-stroke.
Finally, because there was only an intake valve located on the cylinder head, any gases left in the cylinder after the bottom of the stroke would be recompressed as the cylinder fell. This was good for the thermodynamics of the uniflow engine (it reheated the top of the cylinder before the next charge of steam was admitted), but contributed to an erratic torque curve that was already hampered by the non-uniform steam expansion during the downstroke. This, combined with the required large cylinder volume and faster rotating speed, made it extremely difficult to apply to locomotive power.
Generally, uniflow engines were seldom used outside of stationary power generation. But with the advantages offered in terms of efficiency, and improvements to the metallurgy and design, could they have revitalized the flagging steam technology of the 20th century?
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