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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Lopez Mike
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Re: digital steam engine indicator

Post by Lopez Mike » Thu Oct 06, 2016 8:15 pm

Yes. That's what I said. "For the better part of 200 years we have been recording the relationship between piston travel and pressure . . ."

But suppose we have two instruments, one of which uses piston travel and one that uses crank angle. Will they give the same numbers when you measure the 'card' with a planimeter (or a spreadsheet)? And if not, which number should be subtracted from the BHP to give us the efficiency of the engine?
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Re: digital steam engine indicator

Post by troye_welch » Thu Oct 06, 2016 9:06 pm

I've never used a prony brake, but the way I understand them to work is that the flywheel is gripped with a belt with friction blocks and this is tightened to a degree and the torque is transferred to a lever which exerts a force on a scale. The torque can be figured simply by multiplying the weight (in pounds) by the length of the lever (e.g., if 200 pounds are exerted on the scale with a 2 foot lever, then 400 foot pounds of torque are present at that moment)

Since we are talking about getting a reading on the scale and since the flywheel is still spinning at a pretty good clip, it's safe to say that the flywheel (and the rest of the test system) is averaging out any fluctuations or variations in torque (and for that matter speed) into a single number. This average torque multiplied by the speed gives HP.

The indicator has finer granularity since it shows the pressure (and thus force) fluctuations WITHIN a single stroke, whereas a brake test averages everything out (though the bonus is that it does account for friction and mechanical losses).

Since work is defined as a force acting through a distance, the indicator diagram is textbook for meeting the definition of work, since you have a linear graph of pressure versus distance. Power is just work divided by time (1/RPM).
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Re: digital steam engine indicator

Post by barts » Thu Oct 06, 2016 9:17 pm

Lopez Mike wrote:Yes. That's what I said. "For the better part of 200 years we have been recording the relationship between piston travel and pressure . . ."

But suppose we have two instruments, one of which uses piston travel and one that uses crank angle. Will they give the same numbers when you measure the 'card' with a planimeter (or a spreadsheet)? And if not, which number should be subtracted from the BHP to give us the efficiency of the engine?
The integral of the piston travel vs pressure is work (PV). For the crank angle vs pressure case, we'd need to convert pressure to torque so that the units work; this would add an effective crankarm term which would be a function of crank angle.

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Re: digital steam engine indicator

Post by Lopez Mike » Thu Oct 06, 2016 10:28 pm

Again, I know all of that stuff. What is bugging me is this:

Let's say that you have a fixed 100 pounds of force acting against the piston all along the stroke. Between TDC and 10 degrees after that point, you get very little motion. Around 90 degrees of crank rotation you get a lot of motion. Work is force v.s distance. It seems as though a given area on the indicator diagram means a great deal of difference whether it is at one end of the diagram v.s near the middle.

Now given that we agree that the flywheel averages most of this out, this all may be a moot point.

But it seems obvious that two indicators, one based on piston travel and one based on crank angle are going to produce rather different diagrams. Which area should we comparing with the results of a brake measurement to determine efficiency?

All this sounds very much like Bart and I arguing about angels and the heads of pins while lounging in a chair under a fir tree. You had to be there.
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Re: digital steam engine indicator

Post by barts » Thu Oct 06, 2016 10:39 pm

Lopez Mike wrote:Again, I know all of that stuff. What is bugging me is this:

Let's say that you have a fixed 100 pounds of force acting against the piston all along the stroke. Between TDC and 10 degrees after that point, you get very little motion. Around 90 degrees of crank rotation you get a lot of motion. Work is force v.s distance. It seems as though a given area on the indicator diagram means a great deal of difference whether it is at one end of the diagram v.s near the middle.

Now given that we agree that the flywheel averages most of this out, this all may be a moot point.

But it seems obvious that two indicators, one based on piston travel and one based on crank angle are going to produce rather different diagrams. Which area should we comparing with the results of a brake measurement to determine efficiency?

All this sounds very much like Bart and I arguing about angels and the heads of pins while lounging in a chair under a fir tree. You had to be there.
We're not arguing, we're agreeing.

The crank angle and piston location methods are not comparable, and the difference is not trivial to compute. A plot of crank angle verses cylinder volume will demonstrate this nicely.

This is why Buckeye locomobiles could simply vary eccentric position/phase angle with respect to the crank and get such a large change in cut-off as a result (I'm using the same technique on our big boat engine).

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Re: digital steam engine indicator

Post by Lopez Mike » Thu Oct 06, 2016 10:45 pm

But which one should we be using to compare? The obvious answer is the old standard piston travel type if for no other reason that to be backward compatible with a ton of prior art.

But for one reason or another I have this thing rattling around in my brain that since we use crank angle (however much its rate of change is pretty much constant) to measure speed on a brake, that we should be using crank angle with an indicator so that we are not comparing apples and oranges. Both pretty much round but they taste different.
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Re: digital steam engine indicator

Post by barts » Thu Oct 06, 2016 11:08 pm

Lopez Mike wrote:But which one should we be using to compare? The obvious answer is the old standard piston travel type if for no other reason that to be backward compatible with a ton of prior art.

But for one reason or another I have this thing rattling around in my brain that since we use crank angle (however much its rate of change is pretty much constant) to measure speed on a brake, that we should be using crank angle with an indicator so that we are not comparing apples and oranges. Both pretty much round but they taste different.
These have nothing to do with each other.

Suppose you were trying to come up with a way to divide a planet's land area fairly (equally) amongst two species. Would you use a Mercator projection? No, because it distorts the land areas at the poles. In the same way, a crank angle chart over-emphasizes the effect of pressure in the cylinder at the beginning and end of the stroke.

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Re: digital steam engine indicator

Post by Lopez Mike » Fri Oct 07, 2016 12:40 am

Ah, then the counter argument is that the piston travel chart under-estimates the effect of pressure at the beginning and end of the stroke.

So if we had an engine with zero friction and losses, which type of chart would give the same h.p reading as the brake?
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Re: digital steam engine indicator

Post by barts » Fri Oct 07, 2016 1:04 am

Lopez Mike wrote:Ah, then the counter argument is that the piston travel chart under-estimates the effect of pressure at the beginning and end of the stroke.

So if we had an engine with zero friction and losses, which type of chart would give the same h.p reading as the brake?
The area inside the cylinder pressure vs piston travel plot directly gives us work done on the piston, ignoring mechanical losses.

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Re: digital steam engine indicator

Post by Lopez Mike » Fri Oct 07, 2016 1:21 am

So you're pretty sure that the geometrical transform between the piston motion and the crank rotation doesn't make our traditional indicator cards wrong?

As I said, I have this niggling suspicion that there is a problem. That we have been getting wrong data all these years for IHP. Hard to imagine, I know, given how much this was beaten to death over a hundred years ago.

So in our design of a digital indicator we had best stay with sensing piston travel to avoid all the horrors of needing to know the connecting rod length.

Looking back a few posts, I don't think much of a home made linear potentiometer. The environment is too grubby for any sort of exposed ohmic contact. Either a relatively pricy commercially available linear or rotational pot or perhaps a capacitive or inductive sensor. But either of the last two come with additional complications.

Best to stick with a quality pot with a ball bearing shaft. I've not seen any high quality linear (straight line motion) potentiometers. Worth a look. But since there will be a fair range of strokes to be adapted to, a rotational pot with either a variety of pulleys or the traditional levers to transform the piston stroke to the sensor travel.

There is little reason to go away from an ordinary indicator for most of our launch engines since used indicators are floating about on EBay for $200-300. But for small engines the advantage of small displacement sensors are overwhelming.
Last edited by Lopez Mike on Fri Oct 07, 2016 4:15 am, edited 1 time in total.
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