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Watt's Steam Engine


Thank You, and Goodbye!

Thank you to all my followers and supporters, to all the people whoe encouraged me with their comments, to everyone who made this worth the while. I enjoyed presenting this model just as much as building it.

Keep bricking, and farewell!



Watt's Steam Engine LDD File for Download

For those who want to build their own steam engine right now, here's the LDD file of my model. A parts list can easily be exported from LDD (File -> Export BOM).

Watt's Steam Engine LDD file

The lengths of the various pushrods must be adjusted carefully, so the piston and in particular the sliding valve center precisely in between the steam ports.

Have fun!

PS: This project is being featured on the Ideas Showcase!



A video, and some more pictures

Dear readers,

finally, the long-promised video is ready! It can be found at:

Now here's some pictures of real beam engines that I found inspiring. A the German Museum of Technology (DTMB) in Berlin, two beam engines are on display. They are regularly run for demonstration, although only on electricity.

A machine by Thomas Horn, 1860. This one is shown powering a work shop with a host of 19th century tooling machines. Photo by me, 2017.

A machine by Borsig, 1859. This machine has been modified several times during the ca. 80 years it was in operation. It now features two cylinders in compound, that means the steam from the high-pressure cylinder operates the low-pressure cylinder, before finally being exhausted into the condenser. Photo by me, 2017.

This diagram is from the 10th ed. (1895) of Louis Thomas 'Die denkwürdigsten Erfindungen im XIX Jh.', and is now in the public domain. It shows a cross section of a later Watt-type steam engine, around 1800.

I have also run some more renders to check out other possible colour combinations. With existing bricks, the flywheel can only be built in yellow, and that only thanks to the Technic bucket wheel excavator. It would look more realistic in black. However, there have also been original engine prototypes in more colorful designs. Besides the sand green and the dark orange combinations shown on my project title page, blue combinations have also been around.

I am looking forward to your feedback. Please let me know how you like the video!

See you around,




So... How does it actually work?

This update delivers some background on the workings of Watt's Steam Engine. If you shun a long read, just skip it. The next update will be more pictures (and hopefully a video :-)

A steam engine works on the fact that water turned to steam occupies about 1.700 times more space than when condensed. Fire is used to turn the water into steam, but since the boiler is steamtight the steam cannot escape and builds up pressure. The pressure ist applied to the surface of the piston inside the cylinder, and tries to push it into a direction where there is less opposite pressure. Obviously, pressure alone won't do any work without a second pole of lower pressure where the steam can flow to. In the simplest case that means letting the exhaust steam escape into the open air. Better results are obtained when the steam is cooled down again, and thus condensed. The condensed steam now 'shrinks' back to a tiny fraction of its volume, namely the original volume of the water it is composed of. That leaves an imperfect vacuum on the the other end of the cylinder, which gives a larger difference of pressure as our driving force. At the same time, the condensate can be fed back to the boiler as feed water.

These principles are the basis of the steam engine in the times of James Watt. Now let's look inside the cylinder.

The cylinder (blue) contains the piston (dark gey) which can move up and down and is fixed to the piston rod. The steam chest (dark red) with the steam ports (yellow) contains the sliding valve (dark grey), which controls steam intake and outlet. In the position shown, the steam in the steam chest passes the sliding valve on its upper end and enters the cylinder through the upper steam port. While the steam pushes down on the piston, the space underneath the piston is evacuated, as the exhaust steam flows through the lower steam port, passing underneath the sliding valve towards the exhaust port. The exhaust steam is then cooled in the condenser by injection of cold water.

The air pump (the larger upright light grey cylinder in my model) draws the remaining air and the condensed water from the condenser, so it doesn't swamp. By the way, the smaller, dark grey pump on the model, closer to the center of the beam, is the boiler-feed pump.

Now, as the piston reaches the end of its travel, the sliding valve is reversed, and fresh steam enters on the lower side of the piston. And thus, the process keeps going to-and-fro.

The movement of the sliding valve is derived from the crankshaft via the valve gear. There are many different types of valve gear. My model employs a pushrod on a step of the main crank, which actuates the sliding valve by an L-shaped lever.

The motion of the piston is transferred to the large horizontal beam which is the most recognizable feature of this type of engine. In order to keep the piston rod bushing constantly steam-tight, it is of utmost importance for the piston rod to move in an absolutely straight line. But the end point of the beam, being suspended in its center, is travelling on a circular path. What is there to do about it?

The piston rod (yellow) is coupled to the beam (dark red/black) by a link (dark green). Furthermore, the end point of the piston rod is guided by two levers (dark red and blue). The end point of the blue lever which is in line with the piston rod is fixed on the frame.

It's easy to see now that when the piston rod end is travelling in a straight line, the dark red and blue levers will always be in same but opposite angles to the horizontal.

The trick is to couple both ends of the dark red lever to the beam in such a way that the two form a parallelogram. If the length of the dark red lever is equal to half the radius of the beam, and its inclination is always the same as the beam thanks to the two dark green links, then the blue lever moves like the mirrored inner half of the beam. That means, its inclination is forced to be same but opposite as the dark red lever. Going back to the beginning, that means the piston rod's end is forced to go exactly straight up and down! And there is hardly any extra friction added, as sliding shoes would. So there is less wear, too... A most elegant solution in deed.

The engine does not use a simple crank, but a planetary motion drive. A crank is such a simple mechanism, there's no point in patenting it - so thought James Watt. Someone else thought differently, and so Watt had to come up with a different solution to turn longitudinal motion into rotary motion. This is his idea. It didn't violate the crank patent (a crank is a lever that's fixed to the crankshaft, here's a lever that rotates about the crankshaft), and even better, it will multiply the speed of rotation. In my model, the crankshaft makes one full revolution on each up- and downstroke.

The red gear is fixed to the dark red push rod. The yellow lever rotates about the main shaft (green), which I shall no more call crankshaft ;-) As the yellow lever keeps the red and the green gear constantly in mesh, the fixed red gear wil roll about the green gear. From the viewpoint of the yellow lever, the red gear will rotate counter-clockwise, while the green gear will rotate clockwise. The clockwise rotation of the yellow lever is actually overlayed on both movements: as the red gear is forced to stay fixed,  the green gear must rotate clockwise at twice the speed.

The reason why this arrangement was used for quite some time is exactly this: at twice the speed you may use a smaller flywheel. That means less cost and less space, while keeping the same effect.

If you have followed me this far, each of you is entitled to a drink at the local coffee shop!

One obvious reason why I am so passonate about Lego is that mechanical relations can easily be experienced by building Lego models. That holds true for simple things a the static stability of a Duplo tower just as much as for Watt's parallelogram. I truly believe that we learn a lot about the laws of nature by playing. This kind of intrinsic knowledge is not missed by those who have never attained it, and canot be learned from books. We still live in a world of mechanical objects, and will for some time.

Allow your kids to play, and never stop!



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