Torque versus speed for steam engine

A special section just for steam engines and boilers, as without these you may as well fit a sail.
addiator
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Re: Torque versus speed for steam engine

Post by addiator » Fri May 12, 2017 9:02 am

The mentioned PLAN formula is hardly relevant here. There are some other factors to be considered.

In an ideal engine, a torque-rpm characteristic would be flat, and different for any given cutoff. That is to say, the power would proportionally increase with RPM, while the torque should remain constant. This is not actually the case, because of port throttling losses. Those increase with rpm in a non-linearly proportional fashion. They also depend on other factors, and can be evaluated from an equation given by Strahl many decades ago, which is fairly complicated to say the least. If anyone is interested, I can share some theoretical papers on the subject of engine thermodynamic analysis I have published some time ago by PM.

To illustrate what I am talking about, let me share a torque, power and RPM characteristic evaluated from theoretical analysis with all losses considered. This is is evaluated on a 1.2 kW uniflow engine, but the overall trend is valid for any steam engine. Nind is indicated power, M is torque. The different colored lines stand for cutoffs from 10 to 50%.
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Re: Torque versus speed for steam engine

Post by cyberbadger » Wed May 17, 2017 2:22 am

So, I'm confused about the PLAN formula and addiator's
theoretical analysis with all losses considered.
The original question in this thread:
wsmcycle wrote:Since the pressure of the steam is 100% at zero rpm, how does the Hp unfold as the rpm increases? I have a Navy K engine that "seems" to run too fast (rpm)
It seems like the graph from addiator shows hp going up with rpm.

I can tell you right now Nyitra is a my open experiment figuring this out for myself experimentally - Because I don't want a really fast running engine either . Right now my Toledo is geared heavy, I have two ratios between the two sprockets and the two pulley's. Both are trading torque for more rpm at the prop. I have tried this arrangement - and it is workable -but not optimum. I need more testing without seaweed... My current final ratio is 1:3.43. So if the Toledo was doing ~60rpm in my recent video the propeller is doing 205rpm.

The Toledo didn't want to go much faster than this, but I don't have enough data to say why because the seaweed was heavily influencing the results. :x

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Re: Torque versus speed for steam engine

Post by addiator » Wed May 24, 2017 10:45 am

cyberbadger wrote:It seems like the graph from addiator shows hp going up with rpm.
Look at how power is defined as a physical quantity. It is the amount of work done in an amount of time. The unit itself (kW being a kJ/s) shows it clearly. At zero rpm power will always be zero, because you are doing zero rotations in a minute. In terms of rotating shafts, power can be described as N=M*omega or torque x angular velocity, angular velocity being just another way to note down the speed of the shaft (omega = PI*n/30).
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Re: Torque versus speed for steam engine

Post by cyberbadger » Thu May 25, 2017 12:26 am

addiator wrote:
cyberbadger wrote:It seems like the graph from addiator shows hp going up with rpm.
Look at how power is defined as a physical quantity. It is the amount of work done in an amount of time. The unit itself (kW being a kJ/s) shows it clearly. At zero rpm power will always be zero, because you are doing zero rotations in a minute. In terms of rotating shafts, power can be described as N=M*omega or torque x angular velocity, angular velocity being just another way to note down the speed of the shaft (omega = PI*n/30).
This makes sense but...

When is PLAN formula relevant vs your theoretical analysis and vice versa...

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Re: Torque versus speed for steam engine

Post by addiator » Fri May 26, 2017 11:54 am

cyberbadger wrote:This makes sense but...

When is PLAN formula relevant vs your theoretical analysis and vice versa...

-CB
PLAN has little to do with torque, and all to do with power. It is always true, but it does not say much by itself, as mean indicted pressure which is one of the inputs is itself a product of many factors, and as I have written it actually depends on rpm due to the port throttling. If you look at the characteristics I have shown, you could actually very well replace the torque with MEP, the values would be different, but the trend identical. So, if torque is what you need and you have the MEP, then from aformentioned definitions, denoting torque by M and power by N=PLAn, M=N/omega=PLAn/(PI*n/30)=PLA*30/PI.
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Re: Torque versus speed for steam engine

Post by cyberbadger » Fri May 26, 2017 2:08 pm

addiator wrote: PLAN has little to do with torque, and all to do with power. It is always true, but it does not say much by itself, as mean indicted pressure which is one of the inputs is itself a product of many factors, and as I have written it actually depends on rpm due to the port throttling. If you look at the characteristics I have shown, you could actually very well replace the torque with MEP, the values would be different, but the trend identical. So, if torque is what you need and you have the MEP, then from aformentioned definitions, denoting torque by M and power by N=PLAn, M=N/omega=PLAn/(PI*n/30)=PLA*30/PI.
Ok, that makes sense. Is/how much is port throttling affected by valve design. D valve vs Piston valve. I'm assuming we are talking about the ports inside the engine?

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Re: Torque versus speed for steam engine

Post by addiator » Fri May 26, 2017 2:55 pm

The port area, opening, and design of the ports and valves, in the sense that the coefficient of discharge is always different. There is a semi-empirical formula for the pressure drop given by Strahl. Unfortunately not in SI units but the old kGf based system - thought conversion is trivial. I have pasted it as an image.
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Re: Torque versus speed for steam engine

Post by Lopez Mike » Fri May 26, 2017 2:59 pm

So there!!
If you think you are too small to make a difference, try sleeping with a mosquito.
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Re: Torque versus speed for steam engine

Post by cyberbadger » Fri May 26, 2017 3:12 pm

I always love generic coefficients like mu in that equation.

You might as well throw out the whole equation because without experiment that equation is as useful as the spring equation until you experimentally find your spring constant.

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Re: Torque versus speed for steam engine

Post by addiator » Fri May 26, 2017 3:22 pm

cyberbadger wrote:I always love generic coefficients like mu in that equation.

You might as well throw out the whole equation because without experiment that equation is as useful as the spring equation until you experimentally find your spring constant.

-CB
It is actually quite easy to find mu for generic ports. This has been experimentally done thousands of times. I can refer you for example to Stumpfs 1922 book. This is only a simple approximation, and as all engineering formulas for flow losses it has a certain margin of error. This very equation was one of the basics in German locomotive design and was in excellent agreement with experimental results.
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