This LPE (Lego Pneumatic Engine) is a 4-cylinder in-line model. This is currently the largest LPE I can make, since I only have 4 cylinders and two switches. My LPE #1 was only a two-cylinder, and was very slow, so I added two more cylinders to make LPE #2. I had some problems originally with powering it, because even after hooking up a small Lego pump to the buggy motor and pumping two large Lego pumps by hand, it was still underpowered. I finally solved the problem by introducing it to a bicycle pump, which, although providing the amount of air needed, could not keep up a constant flow. Another problem is that the 3rd and 4th cylinders tend to twist themselves, causing (in some cases) the motor to reverse its output. I look forward to building LPE #3 in the future. I hope it will have drilled-out intakes, silicon oil lubricant, and a constant 20-30 PSI feed.