The question morphed into "Why was the steam engine invented when it was, and not the entirely different internal combustion engine?" To which a bunch of answers suddenly come together that then expand into a big cloud of explanationness. Answers to questions like "What's with the big wheels?", "Where the gears at?", and "Coal? Srsly?"
How this fits into Civilization is this. Civ is based upon the fact that technologies cannot exist until other technologies they depend upon exist. I figured it was possible to do an analysis to determine why each were invented when they were invented.
So, let's begin, and I'm going to begin with some pictures demonstrating a locomotive type:
Here's a steam engine that uses this layout:
This is what's called a "Pacific", the term doesn't refer to any specific locomotive, but rather a class of steam locomotives that have a particular wheel layout. They're also classified as "4-6-2", which means "four non-driving wheels at the front, six driving wheels in the middle, and two at the end." (If you're thinking "Shouldn't it be 2-3-1?" - think "two rows of wheels, one on each side") The tender isn't included in this. I'm picking on the Pacific because, well, it's a good example of all of the elements that made up a classic stream locomotive just before everyone switched to electric and diesel. In some ways, it's a great big kludge, a collection of hacks designed to make the thing work with contemporary speed and power requirements - but that's not a nice thing to say, and most train enthusiasts would want to smack me for saying that, because the vast majority of Pacifics were beautiful machines.
Anyway, notice the following elements:
- Big driving wheels, attached directly to the body of the locomotive.
- More, smaller, wheels at the front and back
- A transmission that involves big long metal rods pushing and pulling things. There are no gears. None whatsoever.
This is what's known as a Co-Co diesel locomotive. It has nothing to do with Conan. All that means is that it has two trucks (American terminology, and what I'll use from here on) or bogies (British terminology, also slang for dried mucus), each with six wheels (three per side), all of which are driven wheels (wheels connected to an engine.) Engines with four wheels per truck are also common, and are called Bo-Bos.
How a diesel driven vehicle transmits motion to the wheels differs from application to application. For road vehicles (ie cars, trucks), and for a type of train called a Diesel Multiple Unit (DMU), typically a mechanical transmission that uses gears is implemented. Locomotives are another matter, and the vast majority use an electrical transmission - the diesel engine itself drives a generator, that drives electric motors attached directly to each axle. Those that don't use electrical transmissions usually use some other, non-mechanical, system, such as a hydraulic system.
Diesel engines were pretty much like this from the beginning, they were generally never incorporated any of the lessons learned from steam engine design. They pack more power into a smaller space and are much lighter, in large part because of the differences in form factor. And while some efforts have been made to make steam engines that have that form factor, they haven't been very successful, in large part because they suffered virtually all of the disadvantages of a steam system and a diesel engine without retaining many positives of either design.
Internal combustion engines differ from steam engines in a large number of respects:
- With the exception of diesel, virtually all IC engines require a timed spark generation system, which in practice needs to be electrical.
- Each stroke of the engine starts with a high power, high intensity, explosion. To minimize stresses on the engine's cylinders, this needs to be converted to kinetic energy as quickly as possible. Diesel engines typically use a large number of cycles per second, using smaller cylinders than a steam engine, as a result of this.
The first "railways" date back millennia. Initially little more than gooved tracks for wagon wheels, these evolved into plateways and finally modern railways as we know them today. Plateways involved horse drawn lines of carts, with flanged wheels on wooden or cast iron rails.
When the first steam locomotives appeared, largely in the early nineteenth century, that was the infrastructure, and the first locomotives wouldn't run on them. New railways were built, initially with cast iron rails, and then with wrought-iron when it became frighteningly obvious that cast iron was completely unsuitable. (When low cost methods to manufacture steel in bulk came along, rails were switched to steel for obvious reasons.)
Here you already see evidence of why certain decisions were made in the early evolution of steam. Cast iron was replaced with wrought iron. In the 1700s, cast iron was a cheap, easily obtained, metal for which mass production processes had already been developed. Cast iron is a carbon-rich iron alloy famous for its compressive strength and endurance but also infamous for how brittle it is. And wrought iron - well, wrought iron is as close as anyone could make to pure iron prior to the last hundred years or so, but was always extremely labor intensive to produce. Softer than cast iron, it was at least malleable. But it wasn't as strong as steel.
What about steel? Established methods for making steel in the 1700s and early 1800s were both energy and labor intensive. Melted wrought iron would be fused with carbon over a period of days of constant heat. It simply wasn't an option for early railway builders because steel was much, much, more expensive than either cast or wrought iron.
What about power? Portable power sources were plentiful in the early 19th Century, with solid fuels like wood and coal being by far the most common. Gases were rare, and oils existed but there was no serious industry to extract them - be that from the ground, or from plants - in massive quantities. Refining processes for oils extracted from the ground were very much in their infancy. Kerosene, the fuel that started the oil industry, didn't really start to be produced in high quantities until the mid 1800s. Fuels we know and "love" today like Gasoline and Diesel didn't come until much later.
And it's worth mentioning that electricity was more of a theoretical quantity than a practical technology in the early 1800s. The first practical battery, as we'd know it today, wasn't invented until 1836, and it wasn't portable.
Requirements of each engine
Steam engines consist of:
- A strong, high pressure, boiler, to make the steam
- A heat source for the boiler
- A valve/piston system to convert the pressure of the steam into movement
- A timing method to inject steam into the valve/piston system, and to allow it to be released
- A transmission method to convert the movement into rotational action upon the wheels
- A fuel that can easily be inserted into the system as needed right to where it will be combusted
- A system to insert that fuel, with air, into the system when needed.
- A system to combust the fuel and air.
- Strong, high pressure, valves/pistons to contain the explosions, and convert each into movement
- A transmission method to convert the movement into rotational action
- A system of gears, or some other transmission method, to reduce the rotational action to a useful speed.
In practice, these systems require the following technologies:
- Steam boilers require materials strong enough to contain a large amount of steam. In the 1700s/early 1800s, this in practice meant iron and cast iron. The systems weren't perfect, but they worked.
- Any heat source will do for heating the boiler, and so simply building a hot fire as needed would be all that was necessary. Coal quickly became the fuel of choice, as it was cheap and easy for a fireman to manage. In the 20th Century some very late steam locomotive designs used oil, largely because it made the process of managing the fire easier, but it wasn't necessary.
- The valve/piston system again needed to contain high pressures, and convert high amounts of energy into torque. In practice, we were talking about a largely solid box containing a piston, with valves operated by a system of control rods. Again, iron/cast iron were perfectly adequate for the job.
- A system of connected, pivoted, rods called a "valve gear", was attached to the near-edge of a driving wheel. As the wheel turned, it pushed the rods to and from the valves, ensuring they'd be opened and closed at the right times.
- The transmission for an engine of this kind needed to be simple, and extremely strong. In fact, with the design settled upon by locomotive designers, it needed high tensile and compression strength. A heavy, strong, rod was attached to the piston, which in turn was connected to a point on the near edge of a driving wheel. The other driving wheels were connected to one another with a horizontal bar that, likewise, was heavy and strong. As the piston rod moved in and out, it turned the attached driving wheel, which in turn drove the other wheels. Again, the iron technologies of the 1700s were more than up to the task of implementing this.
- Realistically, an automated process needs to inject fuel into the cylinders due to the large number of combustions per minute. This, in turn, makes it extremely difficult to use solid fuels. Liquid and gaseous fuels certainly existed in the 18th and early 19th Century, but they weren't cheap. They wouldn't have been on anyone's list of potential ways to drive something that required the enormous amounts of power a locomotive requires.
- Certainly had gas or oil been more cost effective in the time period, it wouldn't have been hard to create a method to inject fuel into the engine. No high pressures are required, just a pump and some method to time the injections, which could be connected to the output.
- However, actually exploding the fuel when needed might have been a problem. For most fuels, some form of reliable spark generator, one that doesn't exhaust itself after several hundred thousand cycles, would have been needed. The immature state of electrical technology pretty much rules out anything requiring a spark generator. For diesel, no spark is required, just compression, but diesel was not available until long after the oil boom.
- Steam got away with using regular and cast iron in the early days because while both suffer from fatigue, the number of stress cycles is relatively low for a steam engine. For an internal combustion engine, it's hard to imagine that this would be the case. Now, don't get me wrong - cast iron has been a component of engine blocks for many decades, but we're talking about considerably higher power requirements than that used by the average car engine. It seems improbable that the state of iron technology in the period would have produced a reliable, high MTBF, cylinder system.
- The transmission method for an IC engine tends to be similar, in some ways, to a steam engine's, except that there's more than one piston, and so each drives a rod (crankshaft) connected to a bar with kinks on it, that's made to rotate by having each rod connected to the kinks. Again though, this is something that would be rotating much more quickly than a steam engine's driving wheel, and so would be suffering greater stresses, again raising the question of whether the period's iron technologies were up to the task.
- Because of the speed of an IC engine's crankshaft, this can't be used to drive wheels directly. The available options are:
- Gears. But... again, huge stresses are involved. Remember I was saying earlier that almost all diesel locomotives use electrical transmissions, and those that don't use other weird systems like hydraulics? Well, that's because even today's materials don't lend themselves to making gears that require this kind of strength.
- Electrical transmissions - not available in the period in question.
- Hydraulics - it'd be interesting to know if anyone could have thought of this at the time, but you'd have had problems producing flexible materials capable of withstanding the high pressures involved.
A system of hacks
Earlier I described a 4-6-2 as being, in some ways, a kludge. What do I mean?
The 4-6-2 was usually built as an express locomotive, used for fast, non-stop, trains.
Let's start with the "6". The big driving wheels are a product of the fact that a steam engine generally drives the wheels directly. The speed of the engine is finitely linked to the speed you can inject steam, and expel it, from the valves. The power of the engine is going to be linked to the size of the valves, which will have a negative effect on this speed. And finally, just to add insult to injury, if you start adding more than two valves (one per side of the locomotive) as a way around these limitations, you'll start having serious problems with timings. (Lest anyone think this is theoretical, actual four-cylinder locomotives, using two sets of wheels per side, were made in the early 20th Century, and these engines suffered from severe wheel slip issues, amongst other things.)
So, given this, how do you increase the speed of the engine? Well, the obvious solution is simply to increase the size of the wheels, which is exactly what a steam engine does - it has great big wheels, so each piston movement translates into a larger amount of movement.
The problem you get at this point is that big wheels have two problems. The first is that they're more prone to slip, and second their size makes them somewhat more difficult to turn curves, to the point that they're very likely to derail.
For the first, you have six wheels, rather than two. The issue with the locomotive following curves however required a little more work.
So our next hack involves the "4-" - the truck at the front. This has small wheels, that hug the track, pulling the locomotive in the right direction. The driving wheels thus need to make contact with the track, but they can have much more liberal flanges, allowing them to slide left and right without there being any risk of them leaving the track altogether.
Finally, the 4-6-2 needed a fairly large boiler to provide enough power to drive the engine at speed. With vertical space at a premium (and likewise width - think tunnels) the one solution to this is to make the engine longer. However, a long, heavy, boiler needs to be supported, and the two wheels at the back are for that.
So that's it. You probably knew all that anyway, I just enjoyed writing it down and figuring out the thought process.