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Feasability study - dimensioning of engine parts

Posted by Steam Captain 
Feasability study - dimensioning of engine parts
February 22, 2021 03:06AM

Incited by the hydro engines, I am doing a hobby feasibility study for a 50-100 bar engine and I've run simple torque calcs just to see how big the shafts and how strong the valve train would need to be.
Those small hydro engines use 1000psi and more and are not necessarily constructed with longevity in mind. But I wanted to see if there is a way to dimension everything in a way to do so after all.

With the medium pressure being small with a short cutoff, the torque calculated with the 2-stroke-torque formula is manageable. BUT that's the usable torque (medium torque of work pressure minus medium torque of compression pressure. The REAL force acting upon the bearings would, with a 50x50mm engine, create 20kN!! of force, translating to 500Nm!! of torque at 100 bar (again: not the usable torque, but the factual bearing load and torque to dimension the parts with). For me, this is incomprehensibly insane torque. Suffice to say I've found no gear wheels able to even remotely not to explode in my face under these conditions without being 10 times the size of the engine itself. (interesting enough, belt and chain drives ARE capable of transmitting even much more torque even in these small dimensions).

I have different question marks I want to settle:
  • Am I doing anything wrong with the calculations (I've just used the standard force (F=p*A), torque(M=F*r) equations from school)
  • What crank pin and crankshaft diameter would you say can handle it? My rough calculations say around 25-30mm would be the minimum way to go.
  • Lastly: Would chainsaw crankshafts be strong enough for 100 bar? I'd like to scavenge the crankshaft from existing ic engines if possible, but building it isn't out of question.
Re: Feasability study - dimensioning of engine parts
February 22, 2021 08:54AM
I started to build an engine based on the 23 to 26 cc Zenoah engines I am familiar with. Their crankshafts and bearings have been used in engines that develop 10 hp and can turn 25,000 rpm. Peak power is around 18,000 rpm. I was planning around 1,000 psi steam.

Lohring Miller

Re: Feasability study - dimensioning of engine parts
February 22, 2021 09:17AM
It looks like it's similar to what I have in mind - maybe a small scale bigger. Can you tell me what power output you're planning to expect? Maybe 1-2 HP would be my guess. Or even more, since the rpm is so high. I myself use a small weed-eater crankcase, crankshaft & piston assembly. It's a test bed to see how much pressure these small engine parts can hold. If not, I was thinking about machining my own crankshaft from a special, heat-treated high-tensile steel. But I would only if I had to in the experimental stage.

My concept is basically to use a slightly bigger scale than a hydro engine to reduce the rpm and make it more lasting. The end product is supposed to drive a real launch type boat (maybe even this test engine already).
Do you know what exactly is responsible for the short life of these engines? My guess would be the rpm and the high pressure/bearing loads.
And are you using poppet or ball valves?

I found your forum thread about the flash steam power plant. It's a very interesting idea, that you want to step onto a new territory by adding a RC component to liberate tethered hydroplanes from their tethers. I guess you would need a very big water surface to have enough space to maneuver the RC boat at these speed levels. Very fascinating challenge. Time will tell when someone even adapts steam plants into RC planes.

Edited 1 time(s). Last edit at 02/22/2021 11:47AM by Steam Captain.
Re: Feasability study - dimensioning of engine parts
February 25, 2021 10:54AM
I took data from the GE steam engine study. They found an over 300 psi BMEP at 1000 degrees F and 1000 PSI inlet pressure. That would give somewhere between 10 and 12 hp at 10000 rpm. My design information comes from the series of posts below.

Lohring Miller

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