History of The Steam Engine - High-pressure Engines

High-pressure Engines

As the 18th century advanced, the call was for higher pressures; this was strongly resisted by Watt who used the monopoly his patent gave him to prevent others from building high-pressure engines and using them in vehicles. He mistrusted the materials' resistance and the boiler technology of the day.

The important advantages of high pressure engines were: 1) They could be made much smaller than previously for a given power output. There was thus the potential for steam engines to be developed that were small and powerful enough to propel themselves and other objects. As a result, steam power for transportation now became a practicality in the form of ships and land vehicles, which revolutionised cargo businesses, travel, military strategy, and essentially every aspect of society. 2) Because of their smaller size, they were much less expensive. 3) They did not require the significant quantities of condenser cooling water needed by atmospheric engines. 4) They could be designed to run at higher speeds, making them more suitable for powering machinery.

The disadvantages were: 1) In the low pressure range they were less efficient than condensing engines, especially if steam was not used expansively. 2) They were more susceptible to boiler explosions.

The main difference between how high-pressure and low-pressure steam engines work is the source of the force that moves the piston. In Newcomen's and Watt's engines, it is the condensation of the steam that creates most of the pressure difference, causing atmospheric pressure (Newcomen) or low-pressure steam (Watt) to push the piston; the internal pressures never greatly exceed atmospheric pressure. In a high-pressure engine, most of the pressure difference is provided by the high pressure steam from the boiler; the low pressure side of the piston may be at atmospheric pressure or, if it is connected to a condenser, this only provides a small proportion of the pressure difference.

The first known advocate of "strong steam" was Jacob Leupold in his scheme for an engine that appeared in encyclopaedic works from around 1725. Various projects for steam propelled boats and vehicles also appeared throughout the century one of the most promising being Nicolas-Joseph Cugnot's who demonstrated his "fardier" (steam wagon), in 1769. Whilst the working pressure used for this vehicle is unknown, the small size of the boiler gave insufficient steam production rate to allow the fardier to advance more than a few hundred metres at a time before having to stop to raise steam. Other projects and models were proposed, but as with William Murdoch's model of 1784, many were blocked by Boulton and Watt.

This did not apply in the USA, and in 1788 a steamboat built by John Fitch operated in regular commercial service along the Delaware river between Philadelphia PA and Burlington NJ, carrying as many as 30 passengers. This boat could typically make 7 to 8 miles per hour, and traveled more than 2,000 miles (3,200 km) during its short length of service. The Fitch steamboat was not a commercial success, as this route was adequately covered by relatively good wagon roads. In 1802 William Symington built a practical steamboat, and in 1807 Robert Fulton used a Watt steam engine to power the first commercially successful steamboat.

Oliver Evans in his turn was in favour of "strong steam" which he applied to boat engines and to stationary uses. He was a pioneer of cylindrical boilers; however Evans' boilers did suffer several serious boiler explosions, which tended to lend weight to Watt's qualms. He founded the Pittsburgh Steam Engine Company in 1811 in Pittsburgh, Pennsylvania. The company introduced high-pressure steam engines to the riverboat trade in the Mississippi watershed.

The importance of raising steam under pressure (from a thermodynamic standpoint) is that it attains a higher temperature. Thus, any engine using high pressure steam operates at a higher temperature and pressure differential than is possible with a low pressure vacuum engine. The high pressure engine thus became the basis for most further development of reciprocating steam technology. Even so, around the year 1800, "high pressure" amounted to what today would be considered very low pressure, i.e. 40-50 psi (276-345 kPa), the point being that the high pressure engine in question was non-condensing, driven solely by the expansive power of the steam, and once that steam had performed work it was usually exhausted at higher-than-atmospheric pressure. The blast of the exhausting steam into the chimney could be exploited to create induced draught through the fire grate and thus increase the rate of burning, hence creating more heat in a smaller furnace, at the expense of creating back pressure on the exhaust side of the piston.

On February 21, 1804 at the Penydarren ironworks at Merthyr Tydfil in South Wales, the first self-propelled railway steam engine or steam locomotive, built by Richard Trevithick, was demonstrated.