My version of LPE3i based on scotch yoke system (original ideas from LPEpower) but with new switches ! It works smoothly at 200RPM with 4 hand-pumps and without friction (thanks to the scotch yoke).
The major problem when designing a Lego Pneumatic Engine (LPE) is the dead points of the switches. In deed, the switches are the most important parts with the cylinders, they enable the engine to distribute correctly the energy (air-pressurized) in the cylinders. The problem is when a switch is on its dead point, no energy enter or no get away from. The result is consequently a blocking during the rotation of the engine.
There are different ways to reduce the dead point (evaluate at 30° of a complete revolution of the crankshaft). The first method is to use a inertia wheel also called flywheel in order to give enough inertia to the engine to break the dead point(s). The problem will be then the vibration at high speed (amplify by the mass and inertia). The second method is to modify the mechanics : for instance with the scotch yoke instead of the classic connecting rod which reduces the dead points.
The third factor to consider is the number of cylinder. For instance, a 1-cylinder engine can’t start itself without flywheel. Two or more cylinders is necessary to have a automatic start but you must set up the crankshaft at 90°. If you set up the crankshaft at 180° (in classic real explosion engines), the engine will not start by itself, again because the Lego parts have not enough inertia. If you want more than 2 cylinders, be aware that a lot of cylinders will generates lot of friction. That is why, a configuration angled at 60° with 3 cylinders is a good compromise to start with.
I have made two instruction, one for free as a photo sequence, and one for sale with rendering instructions. As usual, you can buy it for 5€ and then download the instruction immediately.
You can also checked it on rebrickable (or make any export you want)
By using the next up paypal button, you will get the pro-building instruction in PDF for 5€.
After your payment, click on “return to MocShop” and wait for the redirection, you will be redirected directly on the PDF file. If you browser can read the PDF, please wait until the end of the download (it can take some minutes) and after save it on your computer. Please Note that this page will be opened only one time so please be patient to download and saves you copy of the PDF. If your browser can not read the PDF, you will be ask to save it directly on your computer. If you have trouble to download your copy of the PDF after payment, please contact me using the “About Page”. Note that you can print it, but not copy or distribute. Many thank you for supporting my work 🙂
Sample of the Instructions :
Photo sequence instructions
Designed in 2009