digital steam engine indicator

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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wsmcycle
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digital steam engine indicator

Post by wsmcycle » Wed Apr 27, 2016 9:50 pm

In this day of encoders and transducers, why not a digital steam engine indicator. all you need are pressure and an encoder for crank position (aka volume) and you have a PV diagram. What were the old indicators used for anyway? adjusting the valves? Lets see your PV diagram. No brag, just fact!
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Re: digital steam engine indicator

Post by fredrosse » Thu Apr 28, 2016 12:14 am

Just last month we were running indicator traces (digital) on a 17 inch bore x 22 inch stroke Diessel engine. Used to do diagnostics on the machine (no2 oil plus digester gas).

For steam engines used to setup valve gear, about the only adjustment you could make.
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Re: digital steam engine indicator

Post by wsmcycle » Thu Apr 28, 2016 11:27 am

Wow that's a whopper. What rpm did it run? How was the encoder mounted/driven? Most of them I see are mounted on the end of the driven shaft in CNC machinery. It could be a rub wheel against the flywheel. I am thinking of a setup that would adapt to many varieties of steam engines. It would only be an "indicator" of engine performance. Could the old ones with pen, card and string have been used on a compound engine? Or, any multi cylinder for that matter? Why were there so many of them? You see those old ones for sale constantly.
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Re: digital steam engine indicator

Post by RGSP » Thu Apr 28, 2016 1:25 pm

Use of mechaical indicators was still part of my Engineering degree course (1970 -73) and they work on any slow piston engine with a bit of fiddling about, single or multi cylinder, steam or i.c.

The slow speed limitation was perhaps the biggest reason for switching to computer based systems doing essentially the same thing, but faster, and I'm pretty sure the undergraduates use them on engine test teaching experiments today.

I can call in at the "Heat Engines" laboratory and get a definitive update if there is significant interest
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Re: digital steam engine indicator

Post by wsmcycle » Thu Apr 28, 2016 1:56 pm

I too used a mechanical indicator in 1975 at the University of Arkansas. I have not seen a digital indicator. I will google it and see if one is commercially available or at least the plans and program for one.
thanks
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Re: digital steam engine indicator

Post by fredrosse » Thu Apr 28, 2016 2:24 pm

Wow that's a whopper. What rpm did it run? ANS; 400RPM

How was the encoder mounted/driven? ANS: About 6 feet diameter disc between engine and its generator, pickup of degree marks on this disc.

Could the old ones with pen, card and string have been used on a compound engine? ANS: Yes, typical use on all forms of reciprocating steam engines, also IC engines, 1 or many cylinders

Why were there so many of them? You see those old ones for sale constantly. ANS: They are the main method of doing diagnostics of engine performance, cylinder pressure vs volume graph is the main result of engine design objectives, so displaying what is going on inside the cylinder is most relevant.

For high speed engines, the pressure indicating pantograph had problems with inertia, and it could not swing from full admission pressure to exhaust pressure with accuracy during the quick duration of one piston stroke. So to fix this problem early high speed indicators had travel limit stops. If the pressure trace was, say 0 inches to 2 inches high (main steam pressure to exhaust pressure), a limit stop restrained the pressure pencil movement to only 1/10 th of an inch. Running the engine, the limit restraint would be placed to only allow movement from 1.9 to 2.0 inches, and a trace was formed with this constraint. Then the limit stops were moved to 1.8 - 1.9 inches, another trace made, then lower and lower until the last trace at 0.0 to 0.1 inches. If the engine was working with constant load, these 20 traces better represented the real Pressure vs Volume relationship for the engine. Of course, electronics can respond fast enough that the mechanical device inertia problems are now only of historical interest.

Electronic pressure sensors can be purchased with very fast response, well under a time constant of 0.001 seconds, however not all of them respond so quickly, and depending on the application, the purchaser needs to get the right transducer.

Steam engines often vary their RPM significantly within an individual revolution of the crank, a single cylinder engine with no big flywheel (such as a launch engine) might be turning 300RPM at TDC, 400RPM at 90 degrees past TDC, then back down to 300 RPM at BDC. For this reason, the crankshaft position sensor needs several signals around a single revolution, I would guess a minimum of 10 degree increments, if not finer.

Electronically recording this information, then doing the mathematical manipulations to translate crank angle to piston stroke (and cylinder volume) is necessary. While TDC is minimum cylinder swept volume, and BDC is maximum, 90 degree crank rotation is far removed from 50% stroke in virtually all reciprocating engines using connecting rods.
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Re: digital steam engine indicator

Post by wsmcycle » Thu Apr 28, 2016 3:43 pm

I was not thinking of a few points on the flywheel but an encoder with 3600 steps in a revolution and "geared" by virtue of it's being driven by the flywheel. This would give very accurate instant information on the position and speed of the piston.
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Re: digital steam engine indicator

Post by ron parola » Thu Apr 28, 2016 11:05 pm

i did make one using a laptop scope adapter ( for portability). They can be made pretty simply; you need a pressure transducer ; one that will work within your pressure range and a way to see where the crankshaft lies. This I made out of a optical sensor with light source ( one unit with led source), using that it outputs a voltage depending on how much light it sees. So you can make a disc out of sheet metal that is eccentric on a shaft so that the voltage will vary as the disc is turned. This means you don't need anything else to make a usable signal for the scope. You need to make a holder to support the disc and shaft and optical pick up. a piece of aluminum with a slot for the disc worked. At the other end of the shaft I have a disc with a round magnet that I stick to the end of the crankshaft, so when the motor turns it revolves the optical sensor disc. The only thing to be aware of you HAVE to make sure the trigger disc IS timed to top dead centre of the piston. With the optical pickup (and the engine held at TDC) you just need to turn the magnet ( or the pickup assembly) relative to the crank to get the highest output on the Y axis on the scope. Now granted this isn't probably labratory accurate, but this is steam hobby stuff after all. I haven't used it for many years now but if wanted I'll TRY to dig it out. Cheers Ronp
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Re: digital steam engine indicator

Post by steamboatjack » Fri Apr 29, 2016 7:48 am

that's not a whopper:
Image
this is a whopper, 960mm bore
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Re: digital steam engine indicator

Post by Mike Rometer » Fri Apr 29, 2016 2:04 pm

What was wrong with the other 40mm, did they leave that for the re-bore? :lol:
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