Author Topic: measuring the power of a small steam engine  (Read 749 times)

Offline Zephyrin

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measuring the power of a small steam engine
« on: March 30, 2019, 08:29:30 AM »
Hi,
A simple way to measure the power of a small motor is to wind around a coil on the axis of the motor a rope and thus lift a weight attached to it; the product of the weight by the height gives the work done and this value divided by the time spent gives the power.
My small steam traction engine recently shown has a winch to wind a cable, like any real tractor :
http://www.modelenginemaker.com/index.php?topic=8923.msg197577#new

I could easily make this measurement with a simple setup....
the figures are of course highly dependent on the boiler pressure, around 1.5 bar in these tests, (15-25 psi on the pressure gauge).
In practice, I was surprised by the fast rise of the 1 kg weight, which goes over the pulley, even if the measured power was only about 2 W.




Offline MJM460

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Re: measuring the power of a small steam engine
« Reply #1 on: March 30, 2019, 11:38:10 AM »
Hi Zephyrin, that is a really great demonstration and an interesting result.  It seems to be performing really well.

It looks like you need to move the tractor up to a second floor balcony though, to give you a bit more time.  Though the winch might then need modifications so all the coils are the same diameter. 

But as you say, the boiler pressure and burner heat release are variables that are harder to control, so your result is probably about the accuracy that is possible. 

Do you have any idea of the approximate fuel consumption?

I think I will have to make up some reduction gearing and a winch to see what I can get out of my meths fired steam plants.

MJM460

The more I learn, the more I find that I still have to learn!

Offline Ian S C

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Re: measuring the power of a small steam engine
« Reply #2 on: March 30, 2019, 12:11:46 PM »
Here is one of my Flame Lickers set up for a test run, a tachometre is also required.
Ian S C
         

Offline Zephyrin

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Re: measuring the power of a small steam engine
« Reply #3 on: March 31, 2019, 10:45:23 AM »
Hi MJM,
I thought about trying it through the first floor window, but I was afraid that the string would break and the weight (1Kg !) would fall...
as regards gas consumption, but with this bl..dy filling valve, it's easier to measure what's spilled during filling than what goes into the tank!
with a 12 x16 mm bore x stroke, the power of the engine at 1.7 bar and 600 rpm could be 6W; only 1/3 is available on the rope...

Ian; Nice Prony brake you have made, certainly more cool to use...easier also to determine the power vs. rpm relationship.

Offline MJM460

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Re: measuring the power of a small steam engine
« Reply #4 on: March 31, 2019, 11:38:41 AM »
Hi Zephyrin, you would certainly need to be sure all was clear below if using the second floor.

I must admit that I weigh the modelís gas tank before and after filling to determine how much fuel is filled into the tank, and again after the run to determine how much is burned.  But it is a pain, unscrewing the the tank so I can put it on the scale.  Might not be practical if the tank is securely built into the model.  It is necessary to vent some vapour to transfer fuel into the tank, so some spillage is necessary.  I also weigh the bottle I buy the fuel in so I get some idea of how much spillage, but it is not consistent enough to use in determining the amount of fuel that ends up in the tank.

I know there is significant loss in the gearing and the pulley, but where do you estimate the rest of the power goes?  Or are you estimating the friction in the actual engine?

Hi Ian, I was also impressed by that brake, it looks really neat.  Do you find that the lever is steady enough to work out just what weight is being lifted to create the counter torque?

Measuring the engine power output is a quite interesting problem.

MJM460

The more I learn, the more I find that I still have to learn!

Offline Ian S C

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Re: measuring the power of a small steam engine
« Reply #5 on: March 31, 2019, 12:13:01 PM »
The brake arm is reasonably steady in use, I have a piece of plastic strip running on the steel brake drum(the plastic started life as the piston ring in a car shock absorber). I have tried using a small set of digital scales/ 500grm, but they have now died, don't know why, must get a new one. The weight including the hanger is 1 oz, measurement of the beam in inches in 1/10 ths of an inch.
Ian S C
         
« Last Edit: March 31, 2019, 12:16:06 PM by Ian S C »

Offline MJM460

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Re: measuring the power of a small steam engine
« Reply #6 on: March 31, 2019, 10:45:40 PM »
Thanks Ian, that looks really great.

MJM460

The more I learn, the more I find that I still have to learn!

Offline Zephyrin

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Re: measuring the power of a small steam engine
« Reply #7 on: April 01, 2019, 11:02:12 AM »

I tried a few years ago to measure the power by braking the motor by holding a rope against a pulley pulling a spring dynamometer, a kind of rope brake; as shown in the attached diagram.
but if I remember correctly, the results depended on the direction of the rope on the pulley, depending on whether the motor torque pulls on the spring or pulls on the fixed point of the rope (my hand). there must be a flaw in this device.

Offline Bluechip

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Re: measuring the power of a small steam engine
« Reply #8 on: April 01, 2019, 12:20:24 PM »
A long time ago, ( probably 40 years ) I pondered this HP lark on small engines and even made a small brake as per the Sellars Brake in  K.N.Harris' Book

It did sort of work but being an Electrical Bod I found it to be a faff too far.

It did occur to me that there being some 746W per HP all tht was needed was a generator, a load and a couple meters to measure volts & amps and I would be in business.

Well no, not quite. There is the nasty snag of n ( should be the Greek letter eta ) ie. Efficiency.

I might shove 50W of mechanical power in but it won't be anywhere near that coming out.

If I use a Permanent Magnet DC motor the principal sources of loss are the I2R losses and the windage  of any fan.

So, how to have an idea of what the losses are?

I came up with this idea. I have no clue as to whether it is valid or not. I never pursued the idea further than what follows.



I got two identical 24V DC motors and coupled them together. One as a motor and the other as a generator.

The motor was connected to a variable supply.

The generator was connected to a variable load.

I don't have any REAL recorded figures so I'm going to use my imagination.

Input 12V @ 2A  24W

I now adjust the load resistance until I have 2A through it at some voltage.

( I remember having to fiddle about a bit, the objective is to match the motor and generator currents )

Output 8.8V @ 2A 17.6W

So, the efficiency of the whole set-up is 73%   or 0.73

I HAVE NOW ASSUMED THE LOSSES IN EACH ELEMENT ARE THE SAME.

So the loss in each is SQRT (0.73)

Which gives me about 85.6%

Never been able to prove or disprove the principle.

At the end of it you could well have a generator you could couple to a small engine and read the amps / volts thereby watts and know the engine output is 1.17 times as much ???

Maybe ??  Maybe not ??

Comments ??  Utter claptrap ?? Quite right, free beer for life ??  :shrug:

Dave
« Last Edit: April 01, 2019, 12:25:52 PM by Bluechip »

Offline MJM460

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Re: measuring the power of a small steam engine
« Reply #9 on: April 01, 2019, 12:29:40 PM »
Hi Zephyrin, I suspect the first issue is just what part your hand actually plays.

That is whether you move it to try and maintain roughly constant speed, or roughly constant force at your hand, or try and maintain it fixed. 

I suspect it would work quite well if instead of your hand you used either a weight for truly constant tension on that side of the pulley.  Alternatively use an actual fixed point for the string anchor.  That second method means the tension in the string attached to the fixed point is not known, so I suspect the fixed weight is what is required, but I need to think a bit more about that.

You and Ian are doing a good job of motivating me to put more effort into some experiments, but time is somewhat limited at the moment.  Like Jo, I donít have all that spare time I had when working. 

And now Blue chip chimes in with an idea that I have also pondered.  I came to the same conclusion, that I donít know if the efficiency of a machine as a motor is the same as the same machine used as a generator, and how much that efficiency varies with speed and current.  It is certainly not constant.  I got bogged down with trying to define the two efficiency curves.

MJM460



The more I learn, the more I find that I still have to learn!