Wobbler Improvements (Or General Improvements)

[Nerdz]

Ive nearly completed my first wobbler, and its got me thinking as to how to improve it. Ive come up with a list and want some thoughts on it.

1) Slim Down Cylinder (Trim a bit from the sides not being used)
2) Slim Down the Piston (ie, make it hallow but fitting into the cylinder.)
3) Slim Down the Flywheel. Less weight in the center, more on the outside.
4) Polish All Contacting Surfaces
5) Bearings on axle load surfaces.
7) Exhaust Cooling and Refeeding

#7 Sounds Odd but what if we feed the exhaust into a cooler of some kind? I know this probably wont increase power but it will increase the efficiency.

Im no mechanical engineer, but Im thinking the first 3 would enable a simple wobbler to reach higher RPM's and possibly more power.

Also, what happens as we increase the Bore and Stroke of a Steam engine? Im almost tempted to scale up the wobbler to have a 1'' cylinder (actually how do you scale up engines?)

 

[Tin Falcon]

1) Slim Down Cylinder (Trim a bit from the sides not being used)
2) Slim Down the Piston (ie, make it hallow but fitting into the cylinder.)
3) Slim Down the Flywheel. Less weight in the center, more on the outside.
4) Polish All Contacting Surfaces
5) Bearings on axle load surfaces.
7) Exhaust Cooling and Refeeding

IMHO on your first wobbler focus on getting a runner.

Then if you want to make a nicer looking engine build another one.


while the improvements you propose will make your model more real compared to a larger engine I do not see a lot of performance improvement. While the uniformed think High rpm the true test of a well built model is smooth running at a few rpm. on a few psi of air.
Tin

 

[Nerdz]

How do you increase performance on a steam engine then? Higher Compression?

 

[fcheslop]

Reduce the reciprocating mass around the pivots
Alter the porting
Use a trunk guide along the lines of Cracker
Have a look at Dr Senfits engines A wobbler can be made to run well but never efficiently

 

[abby]

Make a proper steam engine with slide or piston valves.

 

[lennardhme]

abby,
John Penn made PROPER engines in the mid 1850's. A world leader. Look up the DIESBAR, a paddle steamer in Germany, still running with a twin John Penn oscillating engine built in 1857. 622mm cyl. 686mm stroke.
Steam engines came from an era, & all are pertinant. Not all oscillating engines are basic wobblers. I copy John Penns designs with steam delivery & valving through the trunion. A very interesting design. Timing is achieved by rotating the trunion.
Sorry fellow, you belittled a particular interest of mine. Maybe your next post could be more helpful [ and knowledgable ]
Leonard

 

[Tin Falcon]

Well Nerdz tell us what you are tying to accomplish.
First of all Steam engines in general are not particularly thermally efficient to begin with. Then we scale them down to small to tiny models . scaling does some interesting things. lets say for conservative figures we make a model at 1/4 scale. the length of the model will be 1/4 of the original the area of the piston (and all other surfaces) will be 1/16 of the original the power output will be 1/64th and the moment of inertia of the flywheel will be 1/256th of the original.

so my thoughts

1) Slim Down Cylinder (Trim a bit from the sides not being used)

Will make the model look better but purely cosmetic.

3) Slim Down the Flywheel. Less weight in the center, more on the outside.

In a small model cutting out the center actually reduces the moment of inertia.
The reason fly wheels have heavy rims is to keep a high moment of inertia but keep mass and material need small.

4) Polish All Contacting Surfaces

You want contacting ing surfaces smooth to reduce friction but slight surface variations will hold oil better than a mirror finish.

5) Bearings on axle load surfaces

Stirling engines NEED ball or roller bearings plain bearings are fine for steam engines. Will the reduces friction improve efficiency yes will it be measured improvement . Do not think so.

recycling steam not odd at all and a condenser should increase efficiency some but again may not be noticeable.

(actually how do you scale up engines?) the simple anser is multiply given dimensions by some constant. In reality you want the new numbers to give standard stock sizes so you are not having to machine every surface. and you need to be able to use standard fasteners.
Tin

 

[Philjoe5]

Scaling a model up or down isn't all that difficult, it just takes more time. I made a mill engine, then scaled it up by 25%, then made another scaled by 50%. The fasteners are usually where compromise sets in, but not always.

I started by making a chart of screw sizes in the original engine (I tried to copy it here but it came out improperly formatted). In my case the original engine used #4 screws, major diameter = 0.112", for parts attached to the cylinder. At 1.25x scale the nearest screw size equivalent is 1.25 x 0.112" = 0.140", a bit larger than a #6. Sometimes you may go up a little in size, sometimes you go down.

Generally, model engines made from barstock plans have oversized fasteners with respect to scale so there isn't usually a problem stepping down to the nearest size. When stepping up, watch for clearance issues.

Personally, I think re-scaling a model is a good exercise in model building because every change you make forces you to think a few steps ahead.

Good luck

Phil

 

[abby]

Leonard , take your frog pill old chap ! it's just a hobby , boy's toys. The question was "wobbler or general improvements" , and nerdz asked if raising the compression would give more power.
I conclude that Nerdz 1. doesn't know a lot about steam engines. or 2. meant raising the steam pressure.
As I am sure you know simple wobblers cannot handle very high steam pressure and making the trunnions for a proper oscillating engines as fitted to ss Great Britain in 1843 or HMS Warrior in 1860 would possibly be beyond his current skill.
I suggested slide valve or piston valve as the easiest next step for a higher powered , more efficient engine , I don't think he is ready for turbines and superheated steam yet.
So Leonard who was being helpful , me or you ? Before you can help anyone you have to know what they are really asking and why and make a suggestion if you believe it is an answer.
OK You have told us about John Penn and the "DIESBAR" very interesting , don't suppose they could fit a Volkswagen engine , but that wasn't the question.

 

[barnesrickw]

I agree just getting one to run is a good place to start. I also agree that steam engines and thermodynamic efficiency is almost a physics joke. Taking a deeper look at "efficiency", and I know this is not what you really mean, but; On a full scale, the simplicity of a wobbler requiring fewer resources to make, maintain, and less engine weight to push around may be more cost efficient than a complex engine. Big variable there is cost of fuel. So for some applications, the wobbler may be the proper engine.

 

[Nerdz]

Lots of questions!

Reduce the reciprocating mass around the pivots
Alter the porting
Use a trunk guide along the lines of Cracker
Have a look at Dr Senfits engines A wobbler can be made to run well but never efficiently

What is a trunk guide?

Well Nerdz tell us what you are tying to accomplish.
First of all Steam engines in general are not particularly thermally efficient to begin with.
Tin

I'll be brutally honest here. The whole reason being why I would love to build a stirling (as Ive said in my welcome thread) is for power generation. Nothing Large, Just small scale. I just think they are really interesting (For lack of a better word and to sound childish; cool). Ive made solar panels and I have a interest in wind power (but havent done anything yet), so I have a major interest in "green" tech. Never mind the politics and all. Its more of a hobby than anything serious. So Ideally I would LOVE to make something mechanical and have it generate electricity, even if It is a little inefficient. Hell, even solar is 15% efficient at most in all practicality. But the downfall is, your power source isnt always there.

I do know a wobbler isnt going to generate much power, but making one does lead into other research and other engines, Hence the point of this thread. After all, where would we be without curiosity? If I had all the information I could on the subject, I would love to sit down and design something. It makes me wonder why there isnt more information on stirling engines.

I conclude that Nerdz 1. doesn't know a lot about steam engines. or 2. meant raising the steam pressure.
As I am sure you know simple wobblers cannot handle very high steam pressure and making the trunnions for a proper oscillating engines as fitted to ss Great Britain in 1843 or HMS Warrior in 1860 would possibly be beyond his current skill.
I suggested slide valve or piston valve as the easiest next step for a higher powered , more efficient engine , I don't think he is ready for turbines and superheated steam yet.
.

You are Correct Sir. The Best Kind of correct. I dont know a lot about steam other than I know the process is only 10% efficient. Stirlings are up to 50% Efficient (according the carnot cycle). Heat is required to boil water. Thats about all I know about steam (probably a few more tidbits) Before I move on to turbines, I would defiantly invest in a mill first!


I also received my copy of "Making Simple Model Steam Engines" today and Im very excited to read it!

Now as far as efficiency goes, For steam, energy is required to heat up a liquid to the point of where it boils. That Liquid is now a gas, which then starts to collect in a container to build up pressure. Pressure is then used to move a mechanical object, which has friction to overcome. Overall it seems very inefficient. So Im looking at the bigger picture while seeing how each individual part has an effect on the engine itself.

 

[Tin Falcon]

First of all I will say power generation is a little outside the box here. As we focus on models. Models can produce small amounts of power and model dynamos /generators can and have been built.

first of all you need the work backwards when it comes to power generation . How much power output you want or need. then take into account the efficiency of your generator.,your engine and then your boiler.
also you need to take into account local boiler codes . Many jurisdictions have a specification of boiler size designated for personal or hobby use that are not required to be inspected.
Off the cuff guess and others may correct me but if you want kilowatt of output you probably need a 2HP steam engine and a 4HP boiler. dump the steam into a house or shop radiator and you will collect some of the waste heat.
The easiest way to increase power output of a steam engine is to scale up the engine.
a given design with a 1/2 bore scaled up 3x will produce 27 x the power of the original.

tin

 

[Nerdz]

First of all I will say power generation is a little outside the box here. As we focus on models. Models can produce small amounts of power and model dynamos /generators can and have been built.

Off the cuff guess and others may correct me but if you want kilowatt of output you probably need a 2HP steam engine and a 4HP boiler. dump the steam into a house or shop radiator and you will collect some of the waste heat.
The easiest way to increase power output of a steam engine is to scale up the engine.
a given design with a 1/2 bore scaled up 3x will produce 27 x the power of the original.

tin

Thinking Big? I like it, but its too big . "Small" meaning a Few Watts of Output. Thats all.

I realize that Power Generation would be outside of this forums reach, but thats OK. I can handle the electrical part. I just need help with the mechanics Im already planning another wobbler. But Im still working out a small boiler. The book previously mentioned has plans for a simple one.

 

[FSG]

Hi Nerdz:
I built my first wobbler as an exercise in basic machining, to demonstrate that I can actually machine parts that go together and do what they were designed to do (all of the other things I've built lately have been delivered to customers). As I made improvements (larger flywheel, improved crank, bearings for shaft, prettied-up piston rod) I made incremental improvements in piston to cylinder fit, and use a shoulder bolt for the crank pin. It will run on 45PSI air, and will spin over 1500RPM at 85PSI, and sounds pretty impressive. I use it as an attractor piece to advertise machine shop classes at our local hackerspace, pointing out that the engine was built on the very machines installed and running right behind me, and that anybody can learn to build this engine or anything else they want, right here in this shop. But we haven't hitched her to any load yet - she may spin fast, but I don't believe she makes enough torque do do anything meaningful.

 

[Swifty]

Wow, 45PSI and 85PSI is a lot of air pressure, I would have thought that 10 to 15 PSI would have been sufficient. It may be an idea to check to see if there is too much friction somewhere. Having said that, I don't have any wobblers, only conventional piston / slide valve engines, you can just about run them by blowing in the airline yourself.

Paul.

 

[vederstein]

There are other technologies more efficient than steam, but steam has one great advantage: they are external combustion - the heat source doesn't matter. Coal plants, hydrothermal plants, and nuclear plants are all giant steam engines. The conversion from heat to electrical power is with a steam turbine.

The graph below is from a whitepaper published by MAN, the German diesel engine company. As you can see from the graph, diesels and some gas turbines are significantly more efficient than steam. I don't have a graph for Otto cycle engines, but I think they're around 20%.

Steam is reasonably efficient on cheap fuels, therefore steam will be with us for a long long time.

 

[Tin Falcon]

Wow, 45PSI and 85PSI is a lot of air pressure, I would have thought that 10 to 15 PSI would have been sufficient.

I have to agree !!!

One of main considerations in relationship to air pressure in a wobbler or any engine for that matter is piston size.

PSI as most of you know is pounds per square inch. so what does a square inch on a round piston look like ? or a better question what diameter?
pi * r^2 = 1 : r^2=1/pi : r= sqrt(1/pi) : And of course d= 2r

so 1/pi = 0.3183099 r= 0.5642 d=1.128 . so we would be exerting 45 to 85 pounds of force on our hypothetical piston.
This is not a lot for a working engine under load. But is is lot for an oscillator and a lot for a demo engine running at idle speeds.


A well built engine of average model size say 1/2 bore should run on breath power.
this is a bench mark.

1 psi =27.67 inches of water. So breath power is approximately 1/2 psi.

(This is based on some of my own testing and an approximation if someone has better data on breath power please share it)


It should take only 0.1 lbs of force on a piston to run a small engine. anything more and there is likely undo friction.

Like I said benchmark. Do not feel bad if you have an engine that takes more psi and tiny engines often need more psi due to tiny surface areas of the piston.

for instance a piston diameter of 3/16 inch will need approx 4 psi to achieve this.
this being said even after years of running in my senf Minikin still needs about 15 -20 psi to run.
Last but not least I remind you of the laws of scaling areas are proportional to the square or square root depending on direction

so if we take our hypothetical piston and make it half size the area of the piston is .25 square inches .
This is why I do not recommend starting this hobby building anything with a bore of less than 1/4" . 3/8 or 1/2 is much better to start with.
Most of my engines fun nicely on s few psi with no load.

I really tried to be careful with the math but feel free to check me.
And I hope this explanation was helpful. Math and algebra is important in this hobby.
Tin

 

[Tin Falcon]

Interesting graph vederstein but I am not seeing how this applies to model steam engines. Other than showing low efficiency.
I do not see any models in the megawatt output class.
Tin

 

[vederstein]

Tin:

Your point is true - I just wanted to show the relative efficiencies of various engine technologies.

 

[Tin Falcon]

Ok that is valid and valuable.
just checking perspective.
Thanks
Tin