External combustion engine

Summary: External combustion engines are characterized by an external combustion process followed by heat transfer into the working fluid within the engine, and are widely employed in the form of steam engines for electricity generation, as well as in Stirling engines for less centralized applications.

An external combustion engine (EC engine) is a heat engine where, in contrast to an internal combustion engine (IC engine), the combustion takes place outside of the engine. Thus, the working fluid in the engine is externally heated through a heat exchanger or the wall of the engine. Most of the world’s electricity generation occurs in fossil fuel or nuclear plants that raise steam to drive a turbine, that is, in an EC engine. Other examples of EC engines include the Stirling engine, which uses gas as the working fluid, or the Organic Rankine Cycle, which uses steam as the working fluid and is particularly well suited to electricity generation from low temperature heat.

Depending upon whether the working fluid operates in one or two phases, liquid or gas, or liquid and gas, the system may be referred to as single or dual phase. The EC engine may further be categorized depending upon whether the thermodynamic cycle is open or closed, and hence whether the working fluid is replenished or remains within the system: A steam engine using water is typically closed, as is a Stirling engine using gas. The upper limit for the efficiency of a heat engine is known as the Carnot limit, and depends on the temperature difference between the working fluid and the surroundings; the higher this difference, the higher the theoretical maximum energetic efficiency.

The most widespread type of EC engine is the steam engine, which is used the world over in centralized electricity generation plants, which are fueled by fossil fuels or nuclear power, and in some cases biomass. The history of the steam engine goes back thousands of years, but it wasn’t until the Industrial Revolution in the eighteenth century that the engines found widespread application for providing motive power, after James Watt made significant improvements to a previous design, including the addition of a separate condenser. The thermodynamic cycle employed in a steam engine is the Rankine cycle, named after the Scottish polymath, Professor William Rankine, which involves four main stages: pumping of water, heat addition, work production, and heat removal. The components of the steam engine that carry out these stages are the pump, boiler, turbine, and cooling towers, if present, respectively. In cooling towers, the excess heat in the form of steam is condensed to water vapor, and this heat is transferred to water in the cooling cycle (not part of the closed-loop Rankine cycle) before being released to the environment. The steam can be exhausted directly to the environment, as in the case of a steam locomotive, but this is very inefficient because of the adverse effect on overall efficiency, due to the fact that the temperature difference between working fluid and environment is not maximized. In the case that cooling towers are present, the condensed hot water is pumped back to the boiler to begin the cycle again. In a small boiler for domestic use, the heat is either directly released, or used to preheat the boiler water as in condensing boilers.

Whether or not the removed heat is used, such as in centralized combined heat and power (CHP) applications with district heat networks, has a massive effect on the overall efficiency of the plant. While pure electrical efficiencies of about 40 percent may be achieved in coal-fired electricity-only plants, in CHP plants the overall efficiency is around 80 percent. The thermodynamic price paid for this higher overall efficiency is a reduction in electrical efficiency, however, because some of the steam is extracted before passing through the turbine. Because this excess heat is generally not used for local heating, so that centralized electricity generation has average overall efficiencies lower than 50 percent on a global scale, this is an area in which there are large potential improvements in overall system efficiencies. In some European countries, particularly Scandinavian countries, the proportion of electricity generation from CHP plants is very high—in Denmark over 50 percent, for example—such that the overall efficiency of the electricity system is also high.

One of the best known and probably most widely employed EC engine other than the steam engine, the Stirling engine, was invented in 1816 by Robert Stirling, who at that time demonstrated the first closed-cycle air engine. It was only in the latter half of the twentieth century that the term “Stirling engine” was universally applied to this kind of heat engine. In this device, the cyclical compression and heating, followed by expansion and cooling, of gas results in the conversion of heat energy to work output. The thermal efficiency of a typical Stirling engine for domestic applications ranges between 15 and 30 percent, but care needs to be taken in interpreting this value, given that a large proportion of the remaining energy input is available, and is therefore used in such an application, making the overall efficiency around 85 percent. A diesel or petrol engine may have a higher overall electrical efficiency approaching 40 percent, but will not be much more efficient overall, perhaps reaching 90 percent. Hence, EC engines typically have a lower power to heat ratio, so are suited to CHP applications with substantial heat loads.

Due to the external combustion process, the Stirling engine is also very quiet in operation, which makes it useful for applications where this is an advantage, such as in small combined heat and power (CHP) applications for households, where the unit may sit in the kitchen or a utility room. It is also very versatile, because it can be applied in any application with an external heat source, and is very reliable compared to IC engines. The latter means that the Stirling engine has lower maintenance costs, but these are somewhat offset by the higher investment compared to IC engines for household CHP applications. Another application for Stirling engines is in concentrating solar power plants, whereby solar radiation is focused with a parabolic dish onto a point at the input to the heat engine. Hence, the solar radiation serves as a heat input for the Stirling engine, which can be used to generate electricity. This application is especially useful for decentralized power generation, in locations where access to the electricity network or other renewable energy sources might be limited.

On the other hand, there are several disadvantages of EC engines compared to IC engines. First, the latter are smaller and therefore lighter, due to the fact that the combustion, heat transfer, and work transfer all occur inside the same chamber. This size difference is only minor when comparing, for example, a Stirling engine with a petrol engine; but when comparing a steam engine and an IC engine for the same application, such as providing motive power for a vehicle, the difference becomes much more significant. The large external boilers required for a steam engine mean that the whole engine requires a large amount of space, even for applications requiring only a small amount of power. Another disadvantage of EC engines is the relatively long time it takes for them to start, compared to IC engines that can be started almost instantly. Steam engines are most suited to operation in the steady state, which is most efficient and therefore economical at their design point. Hence, large steam engines, such as those used in power plants, take a long time to run up and run down; operating them away from this optimal load makes them less efficient. In addition, one large disadvantage of steam engines is the very high pressures at which they operate, which poses a risk of explosion. Ways in which a boiler can fail include over-pressurization, overheating due to insufficient water and/or flow rate, or steam leakage from the boiler and/or pipe work. Typically, steam engines have devices to account for these risks, including an emergency valve that allows a maximum pressure to build up inside the boiler. Furthermore, lead plugs may be employed in the boiler crown, so that if the pressure and temperature reaches an excessive level, these plugs melt and thus automatically allow some of the steam to be released and the pressure to dissipate.

Bibliography

Drbal, L., K. Westra, and P. Boston. Power Plant Engineering. New York: Springer, 1996.

Eastop, T. D., and A. McConkey. Applied Thermodynamics for Engineering Technologists. 5th ed. London: Longman, 1993.

Rogers, G., and Y. Mayhew. Thermodynamics: Work and Heat Transfer. 4th ed. London: Longmann Scientific, 1992.

Yue, Zongyu, and Haifens, Liu. "Advanced Research on Internal Combustion Engines and Engine Fuels." Engines, vol. 16, no. 16, 11 Aug. 2023, p. 5940, doi.org/10.3390/en16165940. Accessed 1 Aug. 2024.