Lego Steam Engine , Reversible Pneumatic Motor

Oktober 2005. When I presented my 6x6 LegoTruck at the LowLug meeting in Deurne, someone told me that my truck, with these motocycle wheels reminded him of pictures of steam-trucks. So, I did a search on the web with the keywords "steam truck" and I found lots of pictures that simply ask for Lego modeling.
This is just one of the links from:

Most of these steam trucks have a two cylinders in a horizontal plane and belts to the rear wheels. Lego models of such trucks could have a Lego steam Engine like this.

This creation was designed to allow for experimentation. In this page I will try to explain how it works and what other solutions for Lego steam engines or pneumatic motors can be found on the web.

An explanation of steam engines with animations can be found on "How stuff works". The probably most famous Lego 2-cylinder engine with lots of explanations and web-links can be found on CSSoh's Lego pneumatic pages. Another one with examples of cars is from Joe Nagata:

Other web-pages that should be mentioned:
The oldest one might be this one of No Quarters VW.
Dan's workshop
with engins's that work with cylinders made of bricks!
Anders Isaksson built a 1 cylinder steam engine with a brick pressure switch
Rob Stehlik's 4 cylinder motor
Tom Eastaway built a 2-cylinder, then a 3-cylinder and .. finally? a 4 cylinder engine.
This a compact 4 cylinder motor from an unknown person. Another 4 -cylinder,
A 2-cylinder motor with a very simple reverse!
An 8-cyliner in Alex Technic Lego workshop His site features also an excellent LEGO Techic Forum with lots of information about engines!!!
Some of these are also descibed on CSSoh's pages "Pneumatocs of yesteryeas" including a radial pneumatic engin!
Another radial engine was found on BrickShelf
A reversible pneumatic motor with use of a differential was found on Brickshelf built by Kevin

A number of these pages mention difficulties with timing of the valves. Some metion the need of flywheels and then there is a debate if 4 cylinder engines run better or smootherthat 2 cylinder engines.

I have builded a number of them and I couldn't make them work. One of the reasons might have been the fact that tuning of these engines was difficult and that I had built them in one stage, while their creators had probably dome some tuning between the stages of building. Therefore, I decided to built one in small steps. The result is shown in the pictures at the end of this page.

I started with just two cylinders and two valves. Each cylinders controls mechanically a valve that controlls the air pressure for the other cylinder. A considreable force is needd to push and pull the valves. This is not a problem because this force comes directly from the pneumatic cylinders. I have applied some Vaseline on the valves and cylindertubes to make them slide smoother. It appeared that the one cylinder made larger sweeps than the other. I expect that this will happen be very often and I didn't try to make the sweps equal.

Then, the next step was to connect the crankshafts. By applying pressure, it will become clear that full rotations canl not happen without helping the gear manually between half cycles.

The last step is the connection of the second pair of gears. The phase shift can easily be changed and is best for 90 degrees. Now the engine runs smoothly. I played with different lenghtes for beams and crankshafts. The engin also runs when they are not the same for each cylinder. More symmetry in construction and behaviour of cylinders and valves reduces friction. 4 cylinders will not improve performance if all cylinders and valves do not behave equally. A flywheel is not needed, but I expect that it will help to make the engin run smoother. I hope to get more practical results in the near future.

One idea is to make the engine reversible by inserting a differential between the two halves with a wormwheel to control the phase shift.

The following pictures show an extension to make the engine reversible.


The left and right side of the engine can be decoupled and coupled by shifting the black handle. By slowly pumping a phase shift can be introduced of 180 degrees. When the coupling is activated again, the engine rotates in the other direction. Due to lash in the number of gears (4 transitions) the timing could be not optimal for both directions. However, this engine still runs smoothly in oth directions. The coupler/decoupler is constructed in this way to be able to withstand large forces. Especially, when this engine is used to drive a truck this forces might be to big for reversible constructions with a differential.

December 2005. This time, I wanted to make a reversible and tunable steam engin, or, in other words, a reversible LPE. Firstly, I started to study on How stuff works the Johnson bar , but I could not come up with a Lego version. Then, I studied again the reversible pneumatic motor with use of a differential on Brickshelf built by Kevin. His approach seemed to me the best and I tried to do a "minor" modification of my engin. The next pictures show the result.


This engin is easy to built and it can be reversed by rotating the differential 180 degrees. I am happy with the result and I plan to use it in a LEGO model of a steam truck.