The Moriya Story - #3 A new piston for the Major

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Mar 1, 2021
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Spring Valley WI
The 15" Stirling Engine Fan Moriya Major has been put to use quite a bit around here.


Of late its leather piston seal was starting to fail. Not completely, but after running at full speed for an hour or so, the piston would lose some of its seal and the engine would slow down. Restarting from cold, the engine would be fine, until the same thing would happen. The elevated temperatures and wear were taking their toll. A leather seal for a simple Stirling engine such as this is a delicate compromise between minimizing friction and establishing full contact against the cylinder wall to effect a good seal.

Rather than experiment further with different leather seals, it was decided to follow my own advice as given on page 20 of the Moriya Major book and make a solid close-clearance piston. This required a more accurate bore than that which comes in standard DOM tube, good as that typically is.

Diamond compound has become very reasonable in cost these days, so I decided to try it for lapping the cylinder. Since it was my first time using diamond, I followed to the letter the excellent advice provided by Ken Croft in his article on "Piston and Cylinder Finishing" for model aircraft engines. Here is a photo showing the lap which I made following Croft's pattern:

MM Cyl Lap.png

The cylinder bore turned out nice and accurate. Meehanite was selected for the piston, although solid graphite was an option at this point. A plain piston was turned to a nice close easy sliding fit. For lubrication I use a mixture of powdered graphite suspended in lamp oil. A drop or two every half hour or so provides adequate boundary lubrication . If I had to do it over again, I would provide a rather deep groove midway along the piston to fill with loose yarn or felt to hold a supply of lubricant for longer runs

Moriya Major is running just fine again. It is a draw as far as performance goes - not any faster, not any slower. But the cast iron piston and steel cylinder combination should be good for a long time. There is a little more mechanical clicking (mostly from the piston wrist pin) because the air pressure across the piston now swings above and below atmospheric, but overall the fan is still a nice quiet runner.

The Major now starts up from cold in much less time than before. This is because, as a general rule, mechanical efficiency is better in a Stirling engine having its average cycle pressure equal to the external (buffer) pressure than it is when buffer pressure is always lower as it was with the leather seal. In these simple engines, the external pressure is atmospheric, and a flexible leather cup seal acts as a one way valve. It lets air past it when atmospheric pressure exceeds the pressure inside the engine. So after a few turns, the minimum pressure inside the engine equalizes to atmospheric. Now this actually slightly 'supercharges' the engine giving it a little more air internally with which to work. This in turn increases the thermodynamic work that each cycle produces, all else being the same. But the price always paid for this is decreased mechanical efficiency due to the mechanism having to do more (forced) compression work. The end result, good or bad, depends upon the effectiveness of the mechanism, the compression ratio, and the temperatures between which the engine is working (cf. "Mechanical Efficiency of Heat Engines", Chapter 6). The output at the shaft is not always greater, but sometimes is the same - as in the Major - and sometimes is actually less - as in an experiment to which the original Moriya once 'volunteered'.


At the time of the experiment, Moriya the First was running with a plain graphite piston, and a 'snifter' valve was fitted. The snifter was a very sensitive check valve into the workspace. This caused the internal pressure of the engine to always be above or at least atmospheric. Moriya had become pretty noisy in her old age, but with the snifter valve working, she became wonderfully quiet. This was due to the unidirectional loading on all the bearings. However, it was found that the fan speed was around 10% lower with the snifter working. And startup from cold took longer. So the neat little snifter valve is now in parts unknown.

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