Steam: Take some water, heat it up until it boils. Yet this everyday substance fills factories, will power trains until the end of time, powers the SS something or other, and has an entire genre named after it. Oh, and in real life, is one of civilization's most important inventions, powered the industrial revolution, and led to the development of many modern technologies.
Most of this useful notes is about steam engines, but as those steam filled factories in movies suggest, it has other industrial uses. So, to get things started:
Non-Engine uses of Steam
If you've lived or worked in a building with a radiator, you've experienced a common use of steam: Heat transfer. Steam is used in heating systems, chemical factories, and other places to move heat from hot places to cool ones. Steam has a couple advantages: first, water is nontoxic, easily available, well understood. Second, steam can easily transfer heat through boiling and condensing. Heat transfer through boiling and condensation is much faster than other methods, requiring less space and a lower temperature difference to move the heat. A boiling or condensing material also releases and absorbs heat without changing temperature: If you cook, you take advantage of this when boiling, steaming, or frying foods, the boiling liquid keeps the same temperature however much heat is added (compare pan cooking where things can overheat/burn easily). this temperature control makes it particularly useful in chemical plants: operations like distillation and some chemical reactions require specific temperatures for best results. Any liquid boils at a different temperature depending on what pressure it is at, so by simply changing the pressure, the temperature of heat transfer is chosen.
Sometimes, high pressure steam can be used similar to pneumatic systems, to directly operate or power devices. The stereotypical train whistle is an example: steam is easily available in these locomotives, so some is drawn to power other systems.
Steam is useful in material treatments/cleaning. You've probably heard of steam cleaning carpets, but it can also be used to harden concrete, sterilize objects in autoclaves, among other treatments. In these cases, steam does a few things: it avoids drying if that is a concern (important with concrete), provides controlled, often lower temperature heat than a furnace might, and avoids any fires if that is a concern.
In most steam and flames factories, the steam is being used for..well, it's there to look cool, so don't think too much further. However, if you need a real reason, it is likely a leak from some kind of heating system or steam controlled machinery. or cold pipes or a cold leak creating condensation.
And now for our main feature: Steam engines
Steam Engines: How They Work
All steam engines are based on a couple properties of materials: gases take up more space than liquids, and gases expand more than liquids over the same temperature and pressure changes.
Most steam engines today and historically follow a similar cycle. Start with low pressure water. First, pressurize it using a pump. Then, feed it into a boiler. Here, the water is boiled and heated. Next, feed the steam into a power device, either a piston or turbine. Today, turbines are the device of choice, but they were invented later. In the power device, steam pushes against the piston or turbine, doing work. The energy for this work comes from the heat in the steam, as a result, the steam cools. As the steam pushes, it also expands, and the lower temperature and higher volume mean its pressure drops. Once the team is expanded as much as it can be, send it to a condenser, where it loses heat to change back into low pressure water, and the cycle repeats.
The work it takes to pump a liquid is the volume multiplied by the change in pressure, the work done by an expanding gas is roughly the change in volume multiplied by the pressure (That;s not a typo, the reason for this difference is a bit much to explain. Expanding gases change pressure as they expand, so more exact formulas exist to measure work). Liquid water takes up much less space than steam, and gases expand by quite a lot more the liquids do, so expanding steam does far more work than it takes to pump water, this leftover work powers whatever the engine is powering.The fraction of heat added that is captured as useful work is called the thermal efficiency of the engine, the best steam engines can reach around 40-45% of heat energy to work, the rest goes out the condenser, friction in the engine, or heat losses elsewhere in the system. Because the boiling point of any liquid rises with temperature, the boiler temperature must be higher than the condenser temperature. A higher pressure difference between boiler and condenser is useful if possible, it allows the steam to do more work in the power device, increasing thermal efficiency.
Actual steam systems can be more complex than this: steam may be used elsewhere, preheating and precooling of certain flows is common, reheating of steam and passing it through a second turbine before condensing it is sometimes done. These work around or take advantage of the properties of the water, or are other ways to get that little extra efficiency increase from a steam system. Extra systems, inlets and outlets are also used to dump impurities, replace water, or control power of engines, among other safety and maintenance functions. Some steam engines do not have a condenser, instead, outside water was fed to the boiler, then steam was vented after being used for power: steam railroads ran this way, as did many early engines in all uses. A reversal of a steam engine is how most refrigeration and air conditioners work. A liquid is boiled at a low temperature and pressure, compressed instead of expanded, condensed at a high temperature and pressure, and the pump is usually replaced by a simpler to use valve, that lowers pressure by restricting flow. The low pressure partially evaporates some liquid, cooling it to the low temperature needed, and the cycle repeats.
The first steam engines used a different method: using steam to create a vacuum. Steam boiling at atmospheric pressure would fill a piston, then be condensed. The condensed water took up a lot less space, sucking the piston (more properly, allowing outside atmosphere to push the piston) back into place, where a valve would open to let in more steam, and the cycle would repeat. It's a much less efficient system, and takes up for more space for the same amount of power than high pressure steam systems, but was easier to build using the tools of the time.
In physics/thermodynamics, a steam engine is a type of heat engine: a system that converts a temperature difference into work by absorbing heat at a high temperature, converting some of it to useful work, and releasing the rest at a low temperature. In a steam engine, the boiler acts as the high temperature source, and the condenser as allow temperature source. Thermodynamics is well tested, some results show that heat loss is necessary even if the engine works perfectly, that a temperature difference is required, and makes clear a number of results earlier in this section. Today, thermodynamic results are used to help optimize steam systems.
The History of Steam Engines, Explaining the Steam in Steampunk
Devices and ideas using steam to do cool things have existed for a long time, but the first practical engines were created around 1700 in Western Europe, Britain in particular. These first devices were used as pumps for coal mines to remove groundwater that often seeped into deeper mines. Coal at the time was being used more often for heating, since a high population was using up lots of forests for wood. The engines were vacuum engines as described above: boiled water filled a piston, letting it rise, then cool water was added, condensing the steam and sucking the piston back down, to start the cycle again. The piston was attached to a beam, on the opposite end of the beam was the pumping mechanism. These engines were very inefficient, but coal near a coal mine was cheap, and they were easier to take care of than work animals who otherwise would have powered the pumps.
At the time, the word "engine" was used to mean any sort of complicated or clever device (Think the word ingenious). Siege towers, catapults, and such were Siege Engines, an early mechanical computer was a "calculation engine". (The word kind of sticks around today in Game Engine). Along these lines, a clever device using steam was a "Steam Engine". As power generating engines got common, the word got its modern meaning.
Steam engines were improved very slowly over the 1700's, and used in pumps or other niche uses. However, in the late 1700's, a man named James Watt added a couple important improvements. The first, a so called double acting piston, where a steam cycle took place on both sides of the piston. Steam entering a piston would push against a vacuum on the other side, then roles would reverse, meaning each motion of the piston produced power instead of one power stroke alternating with an unpowered stroke to reset the device. This allowed engines to take up less space and weight, a single piston could do the former work of 2 pistons. Second, he added a separate condenser, sending the steam to a separate location to be condensed instead of inside the cylinder itself. This kept the power cylinder and the steam inside hot, taking up more space and needing less from a boiler to fill the cylinder. This outside condenser improved the engine's efficiency, enough that factories began using steam engines to power their machinery.
Watt's engines were an improvement, but he also fought the next big improvement in steam engine design: using higher pressure steam. Higher pressure steam allows for greater thermal efficiencies. It also means a smaller piston can generate the same force and the same power., reducing the needed size and weight of engines. These engines had been proposed in the 1700's, but started being built in the 1800's.
It is also during this time that the first steamboats and railroads were created. Early steamboats were not as fast as sailing ships, and unlike sailing ships they needed to carry fuel, too much for long ocean travel. However, they were reliable, running no matter what the wind was doing, and were used in rivers where they could stop to pick up fuel. Several experiments in land transport were tried, but freely moving vehicles turned out to be hard to build with steam. Travel on rails was a different story: horses had been used to pull carts on rails, travel on rails takes less energy than over ground. Stationary steam pullers and later steam powered locomotives could successfully fill this role, and the first railroads started service around 1800. Britain was a leader in these technologies, but they also expanded in the early United States as well.
Railroads would have an unusual economic effect in history: For most of history, water transport was far, far cheaper than land transport. Ships are still cheaper today for moving goods, but no longer have as big an advantage. As a result, land based regions became much more economically connected in the past hundred years or so. One example of the effects of this may be the U.S. Civil War, most waterways other than the Great lakes went at least somewhat north/south, but railroads helped tie the northern states together, which all stayed as a unit to fight the war. Faster land travel improved military supply, ab army could follow or build railroads instead of stocking to water or finding food locally. Industries could be sited differently, more flexibly located and still access raw materials or reach buyers.
In 1824, in an attempt to better understand steam engines and how to improve them, a French engineer named Carnot wrote a book
proposing some general theories on how to get useful work from heat . His original goal was to help France better compete in building these engines. Instead his ideas were ignored for some time, later discovered and used by other scientists and engineers, joined several other discoveries to have some exciting scientific adventures as the field of thermodynamics, and later reunited with its parents as the field of thermodynamics. While Carnot was working with an incorrect understanding of how heat works, he still managed some useful discoveries: a general model of heat engines, and how much work can actually be produced from temperature differences, among other things.
Steam engine builders may not have known about Carnot, but they would have known his work's most important implication: higher temperature and pressure differences were more efficient. However, limits to materials and manufacturing techniques restricted how much pressure steam engines could contain. Boiler explosions were a common issue, one shows up in Adventures of Huckleberry Finn for example. But as part of industrialization, these things improved over time (powered partly by said steam engines), and steam power did as a result. Steamboats, Railroads, and steam powered factories continued to expand. In 1837, a steamship taking advantage of the Square-Cube Law (The drag on a ship that the engine most overcome goes as the square of the length, so required fuel does as well, while the room inside the ship increases as the cube. Build a large enough ship, and it has room for fuel to cross an ocean, build larger and it has room for useful things as well) crossed the Atlantic for the first time, followed by more ocean going steamships.
In the 1850's and 60's, some engineers designed and created new types of engines. Instead of steam, combustion products themselves pushed the pistons. This will become important later.
And now we get to the era of Steampunk. In the late 1800's and 1900's, if you lived in the right place, you would buy products from steam powered factories, and use steam powered ships and railroads to travel. Britain's advantages in these technologies helped it control the largest empire in the world, and British culture became highly influential as a result. (Certainly in the Steampunkgenre itself.)
Becoming more common during this time were so called compound steam engines: Instead of passing steam through one piston and than a condenser, steam was routed through several pistons in a row. This was to deal with an efficiency limitation: Cylinder walls in any engine will tend towards the average temperature in their cycle. As temperature differences in steam engines became higher, this mean that hot steam entering a cylinder would loose a lot of heat to medium temperature walls, cooling the steam, causing a lower pressure, causing it to do less work. Later in the cycle, this heat would go back into the steam, which would take the heat to the condenser. More heat loss and less work means lower efficiency. Compound engines reduced this problem: Steam would first be sent to one piston where it expanded, then the some what cooler, lower pressure steam sent to another piston, possibly another, etc., doing work,cooling, and lowering pressure in each step. Each piston experienced a lower temperature range, and therefor less heat was lost. However, more pistons had to be balanced with energy losses from flowing through more valves and pipes, plus the cost of a more complex engine. Most common were three stages in so called triple expansion engines.
The final major advance in steam power came in the early 1900's, when steam turbines were first used. Turbines were a lot smaller for the same amount of power, and also generally more thermally efficient. Their inventor dramatically demonstrated turbine's advantages by crashing a naval review at the British Queen's jubilee, running the ship around and outracing several naval ships being displayed. Turbines use spinning blades to capture energy. Fixed blades act like nozzles, using the higher pressure of input steam (or another fluid) to accelerate it, the accelerated steam than interacts with moving blades and transfers energy to them, either using lift as airplane wings do, or by directly bouncing off the blades. Turbines are smaller than piston system of the same size, sometimes more thermally efficient as well.
Turbines were first used on warships, later spreading to passenger ships as well. Using a combination of piston engines and turbine was an obscure White Star Line ship ending in -ic that few have heard of, as well as its sister ship, The Titanic. (And a third sister ship). At the time, turbines were still new, somewhat noisy and caused vibrations, the ship's designers decided to use a smaller turbine to get some of its advantages combined with well tested piston engines to keep noise down. The turbine used steam that had already been through the piston engines, improving the efficiency of the system. Another important change was increasing use of oil for fuel: Almost all steam power to this point had burned coal, but oil was easier to pump (so no gangs of stokers needed to shovel, no so called trimmers needed to keep coal evenly spread in a ship), and often produced less smoke. Oil or coal powered turbine ships fought most naval battles in World Wars 1 and 2.
Later in the 1900's, descendants of that 1850's engine a few paragraphs ago became much more common. That was the first internal combustion engine, and these engines have replaced steam in many uses. Steam power has itself gained new uses also: most famously, the development of nuclear power. Steam today continues to improve, using new materials, running at higher temperatures and pressures, being further optimized, though the cycle is still largely the same boiler/turbine/condenser/pump that it has been for about a century. The general design internal combustion engines comes from steam devices: Pistons are used in diesel and spark plug engines and variants, turbines are used in gas turbines including most jets.
Where is it used?
The main competitor to steam engines are various types of internal combustion engines: Gas turbines and jets, diesel and spark plug engines and variants, chemical rockets can also be included in this group. It's not just a fancy name: In a steam engine, the heat source and the "working fluid" are separate, steam and water push the piston, turbine, etc., while an outside source provides the heat. In internal combustion engines, gaseous combustion products do the work, the same material acts both as a heat source and working fluid. Other types of "external combustion engines" exist besides steam, but aren't commonly used for various reasons.
The big advantage of steam engines is an ability to use almost any source of heat for energy. Internal combustion engines need a heat source that also produces lots of gas, and most actual engines need fuel with even stricter properties: the fuel most flow easily, piston engines require fuel with particular ignition properties, burning at a certain speed and igniting under specific conditions. Steam, however, takes up more weight and space than similar power internal combustion engines, and does not change power, start, or stop as easily. Heat transfer in boilers and condensers requires a lot of surface area and space, and boilers full of water take a long time to heat up, plus will stay hot even when a heat source is cut. Combustion gases, on the other hand, are already hot and will be replaced with fuel and air anyway, eliminating the need for boilers and condensers. They also start, stop, and adjust power more quickly: adjust the amount of fuel added, spin up the engine and/or ignite the fuel if needed, and it is good to go.
Thermal efficiencies of steam engines vary. If a steam engine can be built large, with enough room for everything, properly built ones are among the more thermally efficient engines in existence. If saving space or weight is need, and the engine must be squeezed down, the thermal efficiency drops by a lot.
Other power sources, like batteries, photovoltaic cells, windmills, and such, are harder to generalize.
As a result, steam power is most useful when fast response is not needed, space and weight are available, and/or unusual heat sources must be used.
Electric Power Production, Powering Buildings
Electrical power production is a great fit for steam power, and where it is mostly used today. Power plants do not need to travel anywhere, so weight and space are not a problem. Electricity is used constantly, so power plants run more or less constantly (see a few paragraphs down for more detail). Electricity also makes use of sources of power that vehicles do not use for various reasons: stationary (geothermal sources), unsafe (nuclear) among other reasons.
Among those sources is coal. The original fuel for steam engines, better way to use its energy have never been found. Early internal combustion users tried using coal dust, but injecting it into engines and mixing it with air never went well, and the engineering problems to doing this have never been solved. Coal can be converted to liquid fuels, but this process loses a lot of energy.
Nuclear and Geothermal power are sources of heat that do not release combustion products, and steam is the go to way for using this heat. Those giant towers you associate with nuclear plants? Those are cooling towers. The actual nuclear reactor in such a plant takes up a very small amount of space, fission releases a far larger amount of energy per fuel used than chemical reactions. But all that energy has to go somewhere, and large steam systems with large cooling towers are needed to capture this energy.note Geothermal power can sometimes exploit natural steam within rocks, but if it isn't there, outside water can be added to fill this role. Both these plants are expensive to build, especially nuclear: Geothermal power requires skilled drilling to reach hot rock sometimes far underground, nuclear requires containment to prevent radiation release, but fuel costs are low once built. Sunlight can also be used as a renewable source of heat, it can be concentrated with mirrors to get high temperatures, though photovoltaics have proven cheaper.
A newer use of steam power is in so called combined cycle plants. In these, an industrial gas turbine is used to burn fuel while extracting power. The exhaust from gas turbines is very hot, and is the right temperature to power a steam engine boiler. Combining the two cycles in this way extracts more energy than ether system alone: A steam system itself operating at a higher temperature would need far too expensive materials and design, gas turbines cannot also be built that efficiently. Such systems have the highest thermal efficiency of anything every built.
Steam based power plants usually act as "base load" plants. They cannot be adjusted easily, and their fuel costs are usually lower than other sourcesnote , so it makes sense to produce as much power from an existing base load plant as possible while using faster adjusting, more expensive power sources, or energy storage that stores power when not needed and releases power when it is, as to handle high power demand that the base load plants cannot.note
Steam power was historically used for factories, to power machinery, but most modern ones use outside electricity instead. However, chemical plants do use steam power systems sometimes. Chemical reactions being what they are, they can release a lot of heat, and it often makes sense to use this heat to power a steam system, producing power to sell to the electrical grid or to help power the plant.
Ships and Trains
Think of a non navy ship name, an "SS Something" probably comes to mind. For their size, ships require little energy to move, and as a result can have space for relatively large engines. They also run constantly for days or weeks at a time, usually running at a mostly constant speed, and at port take a long time to load and unload (allowing the engines to be started and stopped), so are also a good fit for steam engines. Fuel storage on those long trips is another issue, even requiring little energy, long trips can require a lot of space to be used for fuel, so the generally high thermal efficiency of large steam engines is useful.
Culturally, steamships were the only way to cross oceans before airplanes were introduced, which may explain why SS remained the abbreviation of choice in pop culture for ships for a long time. (the abbreviation for "generic internal combustion engine powered ship" is MS or MV, for "motorship" or "motor vessel". Future types of ships, such as battery or fuel cell powered ones, would get their own abbreviation.) Originally SS stood for Steam Screw (in competition with paddle wheels), but as propellers became standard, the meaning shifted.
Most ships have shifted to other sources of power, but steam systems are still used on nuclear powered ones: nuclear power, as mentioned above, produces heat, but a steam system is needed to convert that heat into useful energy. Nuclear power is used by navies and some icebreakers. On navy ships, it allows long patrols while using little space for fuel, on submarines, nuclear power requires no oxygen and allows the sub to stay underwater for months if needed (crew supplies are the limit here). Militaries also usually have the money and clout to be allowed to use and spend money on such systems.
Also using steam power are some liquefied natural gas ships. Such gas is carried in insulated containers, but no insulation is perfect, and burning off the so called "boil off gas" produced as heat leaks in by burning it in a steam boiler has proven common. However, newer modified diesel engines that burn off this gas have been produced, so LNG carriers will likely switch to these if natural gas continues to be used for long enough.
Railroads were made possible by steam engines. Travel on rails is more energy efficient than on roads or the ground, so like on ships, locomotives have some extra space for a bigger engine. The basic design of steam locomotives never changed much outside some early experiments, and you can recognize the design in pop culture. Fuel and water are fed from a tender (that cart behind the engine) or from tanks on the locomotive. Both are fed to the boiler, in your standard western or Thomas & Friends, this is the large horizontal cylinder that takes up most of the locomotive. Fuel is burned in a firebox for heat, water is pumped separately through the boiler. Exhaust from burning fuel goes up the chimney, steam goes to pistons at the bottom of the locomotive (those small horizontal cylinders on either side.). Pistons power the wheels, they push those bars you see attached to the wheels to turn them. The familiar "chug chug" sound you hear comes from steam travelling through the piston, as the train moved faster, the piston did as well, as did the sound. Locomotives vented steam out the chimney: condensers would take up a lot of space, especially air cooled ones, so bringing extra water proved simpler. Steam turbines were never commonly used on locomotives despite the size and efficiency advantages they theoretically would bring, they only run well at one speed, but locomotives need to change speeds often.
On both ships and trains, diesel engines were the main replacement for steam. Diesel engines are very thermally efficient engines that do not produce much power for their weight, so are suited to similar roles as steam, though they can remain efficient at much smaller sizes and take up less space for the same power. Fuel savings were not a big concern: coal cannot be used in diesels, so on trains the greater thermal efficiency was traded for needing to use more expensive diesel fuel. On ships, the sorts of large diesel engines used can burn fuel oil, already on use on many steamships, so little gain was made. The main reason to switch was labor costs: Diesel engines take far less time to start up, require no cleaning of boilers and condensers, saving the need to pay people to do all of these things.
Other (Non-)Uses of Steam
Road vehicles and aircraft rarely used steam power. Steam is not a good fit for these roles: saving weight is important, and cars and trucks need to adjust speeds easily, and convenience of starting and going right away is very, very valuable for cars. Steam cars were produced early on as cars were developed, but they were quickly outcompeted by spark plug engines, plus battery power in some specialized uses. An early vehicle resembling a truck was tested around 1800, but railroads proved a better way to go. Later, steam trucks were also outcompeted by spark plug engine trucks, the need to keep the engine small badly hurt steam's efficiency.
Steam never been seriously tried to power airplanes, much less helicopters. These vehicles need fuel to stay in the air, and a lot of it (see airliners where most weight can be fuel), any small increase in weight needs more fuel and engine power, carrying that needs more fuel and engine power, etc., quite possibly too much to carry, so saving weight is vitally important. Steam engines were used in early zeppelins: Lifting gas does not need extra fuel in this way so is friendlier to heavy stuff. However, diesel engines quickly became to go to zeppelin engine, for similar reasons as in ships and trains.
Weight restrictions are even more important in space travel, so steam power has never been used here either. Even proposed nuclear reactors to power spacecraft usually use less efficient but lighter ways to convert heat into useful power. An analogy can be made between steam engines and nuclear thermal rockets: in these rockets, a pressurized liquid is fed past a nuclear reactor, heated and evaporated, and shot out a nozzle. This resembles the boiler and power device of a steam system that vents steam instead of condensing it. However, this technology owes more to rockets than steam systems.
Space colonies, if effective heat sources are found, could conceivably use steam systems to get power from them. Depending on the planet's environment, they may not use water for such systems, which leads to:
The Future, Plus Non-Steam Steam Engines
How often steam is used in the future depends on which energy sources get commonly used. As of writing this, steam use will likely decrease in the near future. Global warming means that fossil fuel plants, especially coal plants that release lots of other nasty pollution outside of warming, will likely get phased out. They are also getting more expensive than alternatives, windmills and solar cells have been rapidly getting cheaper and better at producing power. However, is research into new nuclear power techniques payoff, or controlled fusion is actually developed ("It's 20 years away. It always has been and it always will be."), than steam would be the go to choice for extracting power from the heat released.
There is also a possibility that shipping would switch to nuclear power. It is the only plausible way to power ships without greenhouse gases that has actually been tested and used. However, with cost cutting being important in shipping, and accidents being somewhat common, widespread nuclear powered ships have some regulation issues to work out. Heat recovery in chemical plants, geothermal power, and existing nuclear ships will continue to be used.
And heat recovery in chemical plants brings this useful notes to non-steam steam engines. "But its called a steam engine! how can it not use steam?!" you maybe thinking. Actually none of how these engines work requires steam, any liquid that changes to a gas when heated and back to a liquid when cooled can in theory do the job. (It doesn't even need to boil and condense: a liquid that chemically decomposed into gases when heated, then reformed from those gases when cooled, would also work, though no such liquid has been proposed.) Water is by far the most common: it is easily available, non-toxic, boils/condenses at convenient temperatures and pressures, is well understood, among other reasons. However, there are a couple non steam based systems worth mentioning here.
First is the organic Rankine cycle[[note]]Rankine cycle is the name for an ideal steam engine cycle, as it might be analyzed using thermodynamics. As an analogy, "Otto cycle" is the idea spark plug engine cycle, "Diesel cycle" for ideal diesel engines, among other such cycles, used for low temperature waste heat (maybe around 100-200 Celsius/200-400 Fahrenheit) or so. At such low temperatures, the pressure difference between water in a boiler and condenser would be very low, making for difficult steam engine design. However, many organic chemical have low boiling points, and can be used effectively in steam cycles at these temperatures. low temperature differences mean low thermal efficiencies, and the fraction of heat recovered compared to whatever process generated it is very low, but in a big industrial operation, even this small fraction of heat is worth building a system to capture.
Second is the kalina cycle, which uses a mixture of chemicals (the original proposal suggested an ammonia/water mix). Why use a mixture? It takes some explanation, but using a mixture can more effectively exploit heat from certain sources.
One result from thermodynamics is that heat transferred through smaller temperature differences is more efficient than heat transferred through larger temperature differencesnote This useful notes has been written as if boilers just produce heat and it goes into the engine, but that is not how most heat sources work. Instead, exhaust gases from combustion, gases or liquids with waste heat, or sometimes other heat transfer materials (Nuclear reactors heat one coolant, this coolant than boils water in a second cycle that generates powernote ). These material, as they flow through the boiler, continuously cool.
Ideally as much heat as possible will be extracted from whatever its source is, and that heat will be transferred to a boiling material that closely matches it in temperature. The boiling working fluid and source of heat would flow in opposite directions, the working fluid smoothly heating to closely match the heat source material near it in temperature. However, a single chemical does not do this when boiling. Instead, it heats up smoothly, then stays at the same temperature while absorbing heat and turning into a gas, than the gas goes back to smoothly increasing in temperature. However, multiple chemical mixed together behave differently when boiling. when the liquid mixture first starts boiling, the more easily boiled chemical turns to gas more easily, becoming concentrated in the gas while leaving the liquid slightly more concentrated in the higher boiling liquid. This leftover liquid requires a slightly higher temperature to boil, so heats up slightly. the lower boiling chemical continues to evaporate more easily, leaving a liquid more concentrated in low boiling chemical, and the process continues. The result is that the mixture increases in temperature while boiling, creating a temperature change closer to the ideal smooth increase than a single chemical could.
This efficiency increase isn't as high as, say, increasing the pressure difference in the engine, but does help a bit, and Kalina cycles are used a little bit in waste heat recovery. They could conceivably become more common if conditions are right.
(One of mankind's most important inventions to extracting a little bit more energy from waste heat. Ironic way to end isn't it? That's useful notes for you.)
