A simple and compact Inline 4 Lego pneumatic engine. This engine operates at around 1200 rpm at 60 psi.

It requires modified pneumatic switches and cylinders to perform smoothly. This process isn't too difficult, though the switches can be more challenging to do.
Switch modification video here https://youtu.be/dsJajPITMqE
For modifying the cylinders, you need to disassemble the cylinders, and bore out the ports to 1.5mm (1/8")
You will also need pneumatic tubing, I used 2mm ID*4mmOD silicon tubing. 
Lengths of tubing is as follows
And a length of tubing to connect to an external air source to the engine. 

Connections as follows 

make four of these
Connect  25mm and 30mm sections of tubing and connect to the top parts of the pneumatic tee, and connect a 60mm piece to the bottom of the tee.

And one of these
Connect two 35mm length sections to either sides of a pneumatic tee.

Finally connect all of the parts to the valves like this.

Following the assembly I lubricated the model with WD-40 specialist PTFE, this isn't harmful for plastics, as many lubricants contain acetone, or large quantities of mineral oils, these can cause damage/failure of parts. 
To lubricate it, apply some spray around the crank journals and attach a small piece of tubing to the end of the applicator and connect that to the engine, and spray some into the inlet port while turning the engine over.
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Name I4 LPE
Designer TommyStyrvoky
Designed 2018
Inventory 230 parts
Theme Technic > Model
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Help Guide - BI File Types

This MOC also has a 3D digital file available (eg LDD, MPD, etc): 3D Model