Engine Case Selection - page 1 of 3

February 1, 2001

This month we begin with the most basic engine component -- the case. In keeping with a low-cost philosophy we'll use as many off-the-shelf items as we can.

Any engine case can be used to hang Tesla turbine components on, so put some time & thought into the end application: 

• Will I use this engine for high horsepower or low horsepower work?
• Will it be stationary or portable?

Since old, dead snowmobiles are in abundance here in Michigan we have access to thousands of cases designed for lightweight, medium-horsepower use. Most two-cylinder snowmobile engines put out anywhere from 30 hp to 60 hp. The bearings for these engines are produced in large numbers so the cost is low for refitting new bearings into the case -- but we'll leave the in-depth study of bearings for next month.

Photos A and B show a typical two-cylinder Hirth snowmobile from the outside; photos C and D show interior views. 

Notice that the volute air compressor housing is an integral part of the case. This limits the direction of shaft rotation clockwise only (fan end) if a main shaft mounted compressor is planned. If belt-driven or electric compressors are used, the volute housing can either be used to mount components on, or simply cut off to get it out of the way.

The next thing to consider is how the bearings and shaft will work inside the case. 

Photo D shows a close-up of the bearing-crank assembly. Since a new shaft and bearings will replace this whole assembly, we need to figure how the bearings and shaft will be set into the case to prevent shaft creep -- or axial movement. This is important, to prevent end-hung rotor components from contacting each other. 

Photo D shows the right-most bearing secured in place by metal snap rings on both sides. The inner bearings also have snap rings on their inner surfaces, and the left outer bearing is free to move with shaft "growing" due to heat.

Photo A

Photo B

Photo C

Photo D