Most efficient steam engine design?

[cfellows]

Thanks for all the comments and info, although maybe more info than I was looking for. My fault for not being a little more specific... :-[

What I was really groping for was, does anyone see any significant difference in performance between these three engine designs, assuming the same bore and stroke for all.

Double acting twin oscillator:

Single acting V4 valve in head:

Double acting twin with D valves:

Chuck

 

[JorgensenSteam]

For engines of the same bore and stroke, it is going to boil down to how well the steam can flow in and out of the engine.
The engine with the least restrictive ports and passages, and one that has a valve with no lap will produce the most power.

I have a steam engine with a 1.5" bore that produces probably twice the power of another with a 2" bore, but again, it is attention to valves, ports, and passages.

For some reason, many modelers drill very tiny holes (or sometimes a single hole) for ports and passages, and use a non-domed valve, so sure, you can barely force anything through that, and while the engine runs fine with no load, it will not produce anywhere near the power it could under load.

 

[Jasonb]

Chuck is this engine the intended power source for your steam vehicle? That would give us an idea of the size and type of use you are after.

This may give you some inspiration, 1/2 scale model of an actual engine

J

 

[cfellows]

Chuck is this engine the intended power source for your steam vehicle? That would give us an idea of the size and type of use you are after.

This may give you some inspiration, 1/2 scale model of an actual engine

J

Yes, this is for my steam powered vehicle. I want to maximize torque, which means lots of gearing since speed isn't important (or even desireable)

For engines of the same bore and stroke, it is going to boil down to how well the steam can flow in and out of the engine.
The engine with the least restrictive ports and passages, and one that has a valve with no lap will produce the most power.

I have a steam engine with a 1.5" bore that produces probably twice the power of another with a 2" bore, but again, it is attention to valves, ports, and passages.

For some reason, many modelers drill very tiny holes (or sometimes a single hole) for ports and passages, and use a non-domed valve, so sure, you can barely force anything through that, and while the engine runs fine with no load, it will not produce anywhere near the power it could under load.

Pat, I'm assuming, within certain constraints, port size, and volume of steam flow, is more important for higher RPM's and power than just torque alone. The ports on my oscillator engine are 3/32. I couldn't go any larger without risking interference between crossing holes. The bore is 7/16" and the stroke is 7/8" and the maximum RPM I measured at 20PSI is about 1,700. My drive wheels are about 4 3/4" diameter I'm thinking the top RPM will be about 40. That means I'll need a gear reduction of about 40 : 1 or there abouts.

Unless there is reason to think my little oscillator won't be powerful enough, I'm not inclined to build another engine just now.

Chuck

 

[steamer]

Chuck,

Of the three forms

Oscillator

Limited by available cut off.

Single acting.

IMHO Too much friction area compared to swept volume....

This leaves the double acting simple with cast iron pistons and block with piston valves...IMHO as your best choice.

Dave

...and PS....Don't forget a little superheat!...it goes a long way.

 

[11thHour]

For a small engine you are going to have rather large thermal losses. To make the engine more efficient you should look at ways of thermally isolating the cylinder liner and the piston from other metal parts. Keep the steam hot until it comes out the exhaust and you will be more efficient. You want the heat to move the piston, not heat the air.

Hi Kf,
What is appropriate in small spaces as an insulator? for instance, will a 1mm aluminium plate between my brass cylinder and mild steel frames be worth the effort?
Tim

 

[JorgensenSteam]

I would not underestimate the power of a small oscillator, especially at that rpm.

 

[metal]

Hi Kf,
What is appropriate in small spaces as an insulator? for instance, will a 1mm aluminium plate between my brass cylinder and mild steel frames be worth the effort?
Tim

I wouldn't use aluminium (which conducts heat pretty good). Better use something which does not conduct heat, such as heat resistant plastics (e.g. the stuff they use to make electric circuit boards).

For a small engine you are going to have rather large thermal losses. To make the engine more efficient you should look at ways of thermally isolating the cylinder liner and the piston from other metal parts. Keep the steam hot until it comes out the exhaust and you will be more efficient. You want the heat to move the piston, not heat the air.

In my design of small scale (gauge 1) steam locomotives I try to "increase " thermal efficiency by:

--> using a silver soldered cylinder block assembly (air inside assembly acts as insulator) instead of a cylinder block machined out of solid

-->insulating every hot part (steam lines, cylinders, valves, nuts and bolts etc.) from the frame

--> using an appropriate size for supply and exhaust steamlines and make them as short and “flowing” as possible


Besides thermal efficiency there’s also mechanical efficiency, which can be increased by using low friction bearings (e.g. roller bearings or Teflon bearings) and replacing the brass / O-ring piston by a Teflon one.

Cheers,

 

[Aydelott]

im making the rudy k model marine engine how are the valves attached to the rod to me by the print i looks like its just in a slot can u send pictures of the valves? thanks

 

[cbwho]

Interesting read. I am surprised no one mentioned a double tandem compound. Self starting with the benefits of compounding. Simple mechanically as well.

 

[Charles Lamont]

Max power for a given steam supply is a matter of efficiency, as has been said. Thermodynamic considerations are at least as important as mechanical layout. Personally in a smallish model I would optimise both by using a well insulated single-cylinder to minimise both surface area and friction losses, poppet-valves to allow short steam admission with sharp cut-off and low leakage, uniflow ports to get most of the cool exhaust steam out quickly, probably single acting, and with as short a cut-off as the steam quality would allow, and running at a fair lick. I would only consider a condenser if talking about more than 1HP or so.

 

[Charles Lamont]

Considering the question Chuck says he meant to ask, the oscillator has no means of using the steam expansively, whereas the others can. If the Saito or the slide valve engine do have cut-off part stroke, this means that for the same bore and stoke, the oscillator could well produce a higher power output if fed with steam at the same pressure. BUT, for a given quantity of steam, ie if fed by the same boiler, it would produce less power because it is less efficient - the boiler would not have the capacity to turn the oscillator as fast as an expansive engine. If we add the condition that we are to run the engines at the same rpm as well, then an expansive engine with a somewhat larger swept volume will produce more power with the same steam consumption.

 

[cbwho]

A follow up question: will a single acting oscillator expansion engine work?

 

[Charles Lamont]

cbwho - I think there is a way to vary the valve events on an oscillating engine, using an eccentric driven disc valve equivalent to a slide valve. There is a drawing of one in the book 'Model Stationary and Marine Steam Engines' by KN Harris, but I have never taken the time to fathom out how it works.

 

[Rocket Man]

I took engineering in college I can do math and draw models on paper but I need to build engines and test them to know for sure how well they really work.

You need to design each engine for the work it needs to do. Sometimes you need low RPM engines, sometimes high RPM engines, some times medium speed engines.

Engines with short crank shaft throws will turn high RPMs. Low RPM engines are best built with long crank shaft throw and long rod.

If you build 2 identical engines with 1" bore so one engine has 1" stroke and the other engine has 2" stroke you will learn when testing it 1" stroke turns about 7000 RPMs while the 2" stroke only turns about 3500 rpms. The short stroke engines has 2 times more RPMs and the long stroke engine has 2 times for Torque. They both produce the same amount of HP. Both engines use almost the exact same amount of steam when producing the same amount of HP. These engine are both single acting.

When I build the same 2 engines again 1" bore with 1" & 2" stroke this time they are double acting I get same results as before, a high rpm engine & low rpm engines, both use the same amount of steak at the same HP but this time double acting produces about 2 times more HP than single acting. Rod diameter reduces piston are on bottom side of the engine.

Read history of stream, lots of people have tried lots of things. I have not built all those designs to see how well they work. 1 of the most efficient steam engine every built has the engine cylinder inside the boiler fire box surrounded in hot boiling water. It was a long stroke engine with rod & crank on one side of fire box and cylinder head on the other side of fire box. Cylinder stayed hot all the time there was no cold starting the engine it started up at full power.

Build a better high efficient boiler. I did a lot of boiler testing experiments I could upload pictures but don't know how? It appears to me I have to put pictures some where like photo bucket then link them here. Photo bucket has turned into jerks it cost $$$$$ for me to look at my own pictures so I don't go there anymore.

 

[cbwho]

Rocket man, in regards to single acting and double acting, same dimensions, the double acting will double the power but also double the steam consumption. That is what I found with a Mamod TE oscillators.

 

[Peter Twissell]

The simple Mamod traction engine has variable cut off, using the eccentric 'throttle' changes the position of the cylinder oscillating axis, which in turn changes the period of the inlet.
Maximum efficiency of a boiler is achieved using maximum superheating of the steam. The energy required to boil water (latent heat of evaporation) is required whatever the boiler pressure. Additional heat used to increase steam temperature and pressure is realised during the expansion phase and is delivered as output power.
The most efficient steam engines are supercritical, where the steam is superheated beyond the point at which the pressurised stem is as dense as water, allowing for efficient heat transfer. This happens at pressures on excess of 3000psi and temperatures in excess of 400 degrees Celsius. These figures are only realistic in flash steam boilers, where the pressure vessel consists of small diameter tubes which have high strength relative to their inside surface area. An interesting topic, which I intend to pursue at some point.

 

[dazz]

Hi
There are two parts to thermal efficiency in the cylinders. There is the net flow of heat out of the cylinders. There is also the alternating heat loss as hot steam enters the cylinder, expands and cools and energy is extracted, then leaves through the same ports. In both cases, efficiency can be improved using a poor thermal conductor, like stainless steel.

Stainless steel has its problems (like galling) but it is worth considering given your design goals.

Dazz

 

[Peter Twissell]

For the engine itself, have you looked at an opposed piston uniflow design?
The uniflow concept gives an advantage with thermal efficiency, keeping the inlet end hot and the exhaust end cooler, reducing the transfer of heat between the working fluid and the mechanical parts. With ports at each end opened and closed by the pistons, cutoff can be controlled by moving the cylinder tube itself and there is no energy absorbed in moving separate valves.

 

[Rocket Man]

Some steam locomotives in the late 1940s & early 1950s were equipped with a steam exhaust cooling system to turn exhaust cylinder steam to water so it can be used again. Has anyone every built a model engine that cooled exhaust steam to reused water.