A switchless pneumatic engine mounted in a steam machine design.

A wanted for a while to test the switchless pneumatic engine, in order to continue my work about Lego Pneumatic Engine (LPE). The aim here is to not use the classic switch lego which has a poor dead point which blockes completely the air flow in middle position. This causes many stop during the functioning and a high needed force to overcome this dead point (therefore a big wheel of inertia).


The custom switch is based on work of DrDueNL which has made a brilliant work about switchless engine. The custom switch works as the same as the lego official switch. The swinging arm presses alternatively the input (chamber in pressure) and output (exhaust chamber) of each chamber of the pneumatic cylinder by pressing directly on the pneumatic hose. For example, at the right, the swinging arm pinches the hose connecting the lower chamber which causes pressure in it, and do not pinch the hose on the higher chamber which causes the exhaust of the airflow. The movement is repeated for each chamber alternatively. The important thing is that on the deadpoint, the two chamber are on the exhaust which doesn’t block the airflow.


The ignition timing is controlled by the 40 tooth rotating and push the swinging arm. A little timing delay is needed but less than a classic lego switch. In order to prevent the slack of functioning, the mechanism is doubled with 4 40 tooth gears. But there are also a little gap between the two gearing by slightly twisting the axle and move back for on tooth. This configuration causes a little tension in the axle which overcome completely the slack of gearing which improve the efficiency.


Consequently, the engine runs smoother than an engine which lego switch. The custom switch enables to have  free dead point which causes to work with low pressure and less friction (only two hand pump needed or a 3 pump compressor on a RC motor). Of course this solution is more imposant and bulky but the efficiency is way better.

I have created the building instructions has a detailed photo sequence in order that you can rebuild it.The engine needed no ajustement, just respect the timing delay as seen on picture and the trick about the 40 tooth gearing (not needed but better).

See the rebrickable bill of material here : http://rebrickable.com/mocs/nico71/switchless-pneumatic-steam-machine (228 parts)

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