What is working principle of steam engine?

Answer 1

Basic structure

Like the more familiar car engine, a steam engine has a piston that moves when pressure is applied, and valves to control the intake and exhaust of the contents of the cylinder. On an internal combustion engine, air and fuel are drawn in; they are exploded, and like the elephant in the cannon barrel, push on the piston trying to escape. In a steam engine, the inlet valve opens, and steam under pressure pushes on the piston, until you open the exhaust valve to let it out.

While they both have a piston moving in a cylinder, valves, and a crankshaft, there are a lot of detail differences. While steam engines can be quite simple, most have more parts than a comparable internal combustion (IC for short) engine.

Double Acting: A piston engine that uses both sides of its pistons for the production of power.

One other difference with normal internal combustion engines is the use of a "double acting" engine. By sealing both ends of the cylinder, we can push it both directions, and double our power. All it needs is an extra seal, and space for a somewhat taller (or longer for horizontal cylinders) engine. The engine we found was double acting. While it is possible to make a double acting internal combustion engine, (the 19th century Merry engine is such a machine) the higher speeds, temperatures, and pressures that get you efficiency and power in a modern IC engine mean that there are no recent examples.

In the picture above, we see a single cylinder, with the style of valve gear present on our engine. The steam is piped into the steam chest (shown cut open), and will flow thru any uncovered steam inlet ports, in that picture, the bottom one. It will flow thru the passageway, and into the cylinder. At the same time, the top inlet is connected to the exhaust port. With the valve in the position drawn, the piston is being pushed to the top of the cylinder.

Multiple cylinder engine: Any engine with more than one cylinder. Multiple cylinders can eliminate dead spots when starting, and reduce vibration when operating

Simple engine: An engine where all cylinders are fed steam directly from the boiler.Compound engine: A multiple cylinder engine that uses the cylinders in series to expand the working fluid in stages. The exhaust of the first cylinder is fed to a second cylinder, for further energy recovery.

Steam engines can have more than one cylinder, just like an IC engine. They come in two basic styles, multiple identical cylinders all fed directly from the boiler, and the series connected "double expansion", or more generally a "compound" engine. As mentioned above, the steam exhausted from the cylinder is still at possibly significant pressure. If you dump this to the atmosphere, you throw away a lot of energy, and with it, fuel efficiency. To get this energy back, you need to give it some confined place to expand in, and something to push against. So you add another piston, and feed it this previously waste steam. The second stage lets you capture the energy as it expands further. Again, this is not the same as a multiple cylinder engine, they feed the same full pressure steam to all the cylinders. Without compounding, multi-cylinder engines are no more efficient than a single cylinder engine of comparable capacity. With the cylinders connected out of phase, the engine can be counted on to start no matter what the crankshaft position. Most railroad engines are of this style. (multiple cylinder, single stage).

Since the second cylinder of a compound is operating at lower pressures, they are normally much larger than the cylinder in the high-pressure stage. If they are correctly scaled, power output per cylinder is comparable, which helps it run smoothly. The engine used by The Beach Boys was of this type. The higher the steam pressure, the more stages you can make good use of. I have seen some triple expansion engines, and assume that someone has tried a 4-stage engine (if not five or even more stages).

Our engine choice: A compound engine would recover more of our steam energy, but as a compound it could have the same starting problems that a single cylinder engine has. (a "dead spot" when the piston is at the top of its stroke). However, with an appropriate bypass valve in the right place, a compound engine can act like a multi-cylinder "simple" engine, (able to start in any crankshaft position).

While the extra efficency and especally the "start everywhere" feature of multiple cylinders was appealing we considered the significant added complexity a liability. Such a liability, that we passed up the more complex engines that were available. We chose a very simple single cylinder, flat valved design. Because our engine had just one cylinder, with a very tolerant valve system, it was fussy about crank position when starting, and not as efficient as it could be. This poor starting was more than balanced in our opinion by its great tolerance of poor lubrication and the effects of having sat around in the rain for some time.

Engine controlsJohnson bar: A lever that adjusts steam engine valve timing. Sometimes fitted with a stepped locking mechanisim, the source of the term "Notching up". Called a "Reverser" in the UK.Regulator:Called the Throttle in the US, it is a valve between the steam supply and the engine. By adjusting the throttle, you can control the speed of the engine. For maximum efficency, you will want to leave the throttle open as wide as possible (minimizing pressure drop across the valve), and adjust the engine output with the Johnson bar.

The internal combustion engine uses fixed valve timing, as you always want to let as much air and fuel mixture into the cylinder as you can, likewise, you want all the combustion byproducts removed before you start the cycle again. Some that run at very high speeds, may use a system that allows a little adjustment of the valves opening and closing "overlap", to better allow for the time it takes the air to start moving. They do this to give a bit more time for the cylinder to fill, and to help clear combustion products out. The amount of adjustment is quite small compared to steam practice. On most piston steam engines, the valve timing can be varied by quite a bit. By changing the valve timing, you can control both the power and steam consumption of your engine.

One other thing you can do if you can vary your engines valve timing far enough - you can get your engine to run in both directions. You need the valve stuff anyway to save steam; for essentially no extra cost, you can give the mechanism enough range to let you change the engine's direction of rotation. For vehicles, this is a great thing. It means your vehicle can be driven backwards, without the extra parts that adding a transmission with reverse gearing would involve. It's this ability that earned the control one of its names. British railway engineers call it a "reverser", while Americans call it a "Johnson bar" presumably after its inventor, or an early proponent. When we ran our car, we used it as the primary control of our speed, not the throttle (called the "regulator" by UK steam engine operators).

For wheeled vehicles, there is one thing to be careful of: Get reverse by getting the engine to turn the other way, and your car will run just as fast in reverse as it does forward. Useful on a train, but this feature comes as a real surprise to modern car drivers.

For maximum economy, you let a small amount of full pressure steam into the engine, and let it expand as it moves the piston. The inlet valve is held open for a very short time, only when the piston is near the top of its stroke. When you open the exhaust valve at the end of the stroke, the steam has expanded as much as the engine can manage. Since the inside of the cylinder is now at a low pressure, very little energy is wasted as it leaves the cylinder its' pressure dropping to atmospheric pressure. (Note: any pressure drop here wastes some of your energy. You want the pressure difference between inlet and exhaust to be as large as is possible, and the drop between exhaust and atmospheric pressure to be as small as possible).

Unfortunately for many uses, a steam engine that only has a single valve setting won't do. While they can make good use of the expansion of steam once things are moving, it could take a lot more power to get everything moving. Adjustable valve timing gives you an option - you can set the mechanism to admit steam for almost the whole piston stroke, stopping just before you open the exhaust valve. This setting is called "full gear".

Operating in full gear, you get lots of power, at a significant cost in efficiency. When the exhaust valve opens, you vent steam at essentially full boiler pressure, wasting a lot of the energy it contained. This increases fuel consumption, and if you intend to do it for more than just the occasional start, you will need a very large boiler to keep up with the rate the steam is drained. The legacy of the 19th century (especially rail) steam practice, are a bunch of different systems of mechanical linkages, added onto the basic valve operating "cam", to allow you to vary the valve timing. They are usually named after their inventor.

Plumbing - Steam engine accessories

Left needing descriptions are two steam-specific fittings. One gets oil into the engine, and goes by the name of a "displacement lubricator". The other is a steam operated water pump, called an injector. They have one feature in common; they do their jobs, with the only thing that could conceivably be called, a moving "part" being the steam itself.

Displacement Lubricator: A very simple device for adding lubricating oil to the steam supply.

Attached to the steam Plumbing near the valve gear on our engine (the part of the engine called the valve chest) was a displacement lubricator. Like a two-cycle engine, there isn't an oil-filled crankcase to lubricate pistons and valves; you need a way to add small amounts of lubrication to the inside of the cylinder as it runs. On a two-cycle engine (like the one on a string trimmer), you mix some into the fuel.

It's not that simple with steam. You can't just add some oil to the water in the boiler, it won't mix in; it will just float on the top. When this layer of oil atop the water, hits the very hot metal just above the water line, (where the metal surface is at red heat), the oil would just burn to a disgusting crust inside the boiler.

In fact, keeping the lubricating oil out of water inside and that fed into the boiler, is the big problem you will need to solve if you want to condense the exhausted steam back into water and reuse it. This is sufficiently difficult, so that most locomotives, and some steam cars (including Frobette) didn't even consider re-using water. They just released the steam into the air, and carried water to replace what was used in this way.

Since you can't add oil before you turn the water into steam, you need to add it to the steam itself. You don't need to add much, but since the steam line is under pressure, you can't just use an open-to-the-air gravity drip or some sort of wicking, like the engine crankshaft bearings can. Whatever system you use, it will have to work despite the pressure.

You could (and some steam engines do) use a mechanical oil pump to inject a little oil into the engine. But these have a lot of parts, including a need for sliding seals, and some kind of metering nozzle that you have to worry about clogging. The displacement lubricator does the same job in a simple, elegant and very clever way. As mentioned above, it does its job dripping a little oil directly into the engine as steam flows past it, without any complicated moving parts.

They are not much to look at; just a vertical metal cylinder (ours was semi-shiny brass) closed top and bottom, and connected to the steam piping by a narrow tube near the top of the cylinder. Fancy ones include a valve in this tube. A valve here allows you to service the thing (add more oil) while the engine is still running.

The top of the cylinder unscrews, and you fill it with steam oil to just below the passageway in the small tube connected to the engine. With the cap replaced, you start the engine and open any valve in the small feed pipe. You then leave it alone to do its job, checking it from time to time until you learn how often it needs to be re-filled.

Steam travels down the small pipe, and since the cylinder is sitting in the open, cooler air, the steam will condense back into a couple of droplets of water. Since oil is lighter than water, the water sinks to the bottom of the oil filled cylinder. The added water causes the oil level to rise above the bottom of the steam inlet tube, and a like number of drops of oil flow back thru the tube, into the engine. The water has displaced the oil, giving the gadget its name.

After the engine has been running a while, you will have to service the lubricator. You need to drain out the water that has collected, and top up the oil supply. If the lubricator has one, you close the valve in the inlet tube, without a valve, you will have to shut off the steam to your engine. Next you take the top off the lubricator. Before you can add more oil, you have to first open the drain valve at the bottom of the lubricator and let the water that has collected out. With the water gone, all that's left is just to add a fresh supply of oil, and put the lubricator back into service.

Injector: A steam powered water pump. Uses Bernouli's principles to pump water into a pressurized boiler. The only moving parts are water in various forms (liquid and vapor).

The other steam specific part is called an "injector". This is a no-moving-parts way to pump water with steam. Steam enters the device from one side, where it passes over a water inlet. Like a perfume atomizer, it follows the principle discovered by Bernoulli, and it siphons some water out of the water inlet tube, and carries it along. Unlike the atomizer, instead of next spraying everywhere, it's caught by the housing, and the water it picked up is actually forced into the boiler by the force of the flowing steam, and by expansion of the water as it takes heat from the steam.

They can be finicky devices. The ones that I had seen before (in scale model engines) had very tiny passages that were prone to clogging. Some of the parts need to be very well polished for the things to work well. They were also somewhat sensitive to the temperature of the water they were asked to pump.

The one I found was actually a bit too big for our boiler; it used a lot of steam, and pumped more water than we needed. We installed it, so we had our required second water feed, but planned to use the manual feed pump as our primary way to get water into the boiler. (You usually want at least one mechanical pump, you need some way to get water into an empty boiler.)

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