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Old 05-23-2017, 11:47 AM
Goody Goody is offline
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Default Small Steam Engine System Design



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Disclaimer: This post is for education purposes only.

The prospect for making use of steam engines is often discussed on these forums. In all cases I have seen, the discussion centers on old technology that was abandoned for good reason. Generally, these engines are bulky, inefficient, labor intensive, and dangerous. They should NOT be considered. However, it is possible to devise a small steam engine system that does not have these undesirably qualities. Small steam engine systems of the kind I will describe were never commercially manufactured. So, people generally are not aware of the possibilities. The history is interesting. In short, there was a push during the 1960's and 1970's to develop automotive systems with low emissions. A small number of very sharp people were able to advance steam technology in the automotive arena to make it a lot more compact and more efficient than traditional steam cars of the early 1900's - and low emissions were achieved along with a very wide fuel capacity. However, the problem was solved with computer controlled fuel injection and catalytic converters applied to the internal combustion engine. The steam engine systems were never fully developed. It turns out that applying the basic design criteria to a small wood-fueled steam engine system eliminates most of the problems with the old technology.

This will be a long post, but I will keep it as brief as practical. If anyone wants details, then I can provide references and/or address questions. That said, I recommend against anyone attempting to design and build a steam engine system without doing serious research. It would be a big job to properly build a useful system. So, it's not something that should be casually considered. Also, this is not a cookbook presentation meaning I don't have plans you can follow. I am presenting DESIGN PRINCIPLES from which a good system can be built. Although, I do provide some specific suggestions most of which have been done with good results.

A steam engine system requires a furnace, a water feed pump, a boiler or steam generator, a piston expander and crank, a condenser, water storage, controls, lubrication. Achieving a compact system with good power density and good efficiency requires the use of steam at pressures and temperatures higher than the old technology. Since this cannot be contained safely in traditional boilers, then boilers will not be used. Rather, the system will use a very compact steam generator. This one deviation from traditional design (i.e. no boiler) is perhaps the single most important change that can make a practical system possible.

Steam generator: This is a single length of tubing coiled and shaped to pick up heat efficiently from a furnace. There are few constraints on design. However, good efficiency requires sufficient tubing and a counterflow heat exchange where the water/steam pumped through the tube moves in a direction opposite the flow of furnace gases. It is impressive how small a tubing coil is required. For example, there were automotive steam generators built with only 50 pounds of tubing that could achieve a continuous 100 hp from the engine. It's possible for a small 1 KW engine to use only 50 feet of 1/4" tubing. The reasons for this include the use of higher pressure steam that allows the steam to spend more time in the tube exposed to the heat in the furnace. Also, there is high turbulence as the water/steam moves through the tubing that increases heat transfer, all else equal. This also means the actual temperature of the tube is very close to the temperature of the steam/water mixture inside. Good efficiency requires a high temperature furnace. With wood as the desired fuel, then it's important to use a gasification furnace and provide good thermal insulation. The gasification furnace is hotter, cleaner, and more efficient. Good steam engine design starts with good furnace design. The entire system rests on this foundation.

Expander design: By far the simplest design that can show good results is often called a single-acting uniflow piston engine. The valve gear is critical to steam engine design. Traditional steam engines often use slide valves, but these are not compatible with steam at high pressures and temperatures. Some use piston valves, and these can do better. However, the best alternative is the bump/bash valve or a small poppet valve. The bump/bash valve uniflow is the simplest design. The bump valve admits the steam to the piston. The uniflow describes how the steam is exhausted. Here, there is no exhaust valve. It's set up like a two stroke gas engine. You can find examples all over YouTube that depict these kinds of expanders, and all but a very small number of examples I have seen should not be referenced for fear of getting very bad ideas. Basically, they are all hack jobs. The kind of design that can achieve good results and with a relatively simple build is to use a small ball valve with a stainless steel seat. A very hard pin on the piston face taps open the ball valve with a short nominal lift only on the order of 1 mm for a small engine (with perhaps a 2" stroke length). Steam fills the clearance volume above the piston, the ball reseats, and the steam expands perhaps 6-8 times to force the piston before the exhaust ports or channels are exposed near the end of piston stroke. Then the cycle repeats. The simplest build would convert a single cylinder gas engine with a cast iron sleeve. There are two general approaches: (1) Use the gas engine piston as the steam piston, or (2) Connect a separate steam cylinder/piston on top of the gas engine cylinder and connect the steam piston rod to the gas engine piston using it as a crosshead. There are advantages and disadvantages to each. I prefer (2) because it avoids mixing steam/water with the lube oil in the crank case. However, this can be overcome. Here is one video depicting a (2) conversion: https://www.youtube.com/watch?v=-85qglTdsE4 . Now, this provides a useful glimpse into the approach. However, the specifics of this design has important limitations. Mainly, the valve he uses would not likely allow high steam pressure and temperatures for long if at all. Also, it should go without saying, but if (1) is used, then the cylinder must be very well insulated. So, grind off the cooling fins and insulate the cylinder.

The basic idea of what I'm talking about is depicted here: http://www.rossen.ch/solar/wcengine.html That design used steel ball bearings as the steam valves and used tapered pins installed on the pistons to lift the valves. For a small engine, I suggest harvesting one of the gas engine valves to use as the lift pin. Grind off most of the stem and mount the valve on the piston face. Use the stem nub to bump open the valve. I recommend a ceramic ball bearing for its hardness. You don't want to deform the ball as sealing will be compromised. I speculate, but I would experiment with using a stainless steel compression tubing ferrule as a seat for the ball valve. Of course, you would have to machine a new head for the cylinder to accommodate the ball valve, and connect the steam line to the valve. Building the expander is the greatest challenge to the build. The peak pressure in a small gas engine is roughly 500 psi. So, keep steam pressure no higher than this value to prevent stressing the bearings. A good engine could use saturated steam. However, superheated steam will increase efficiency.

The last thing I'll mention is controlling the steam generator. A small steam generator requires precise control. However, this is not so challenging with an engine designed for a more or less constant low output. The basic idea here is to match the water feed rate into the tube with the furnace output. Too much water results in low steam temps and/or water slugs sent to the engine (and low efficiency and possible damage). Too little water results in high steam temperature and possible engine damage. So, a feedback control system is necessary. Basically, some kind of thermostat sensing steam temperature is used to control the water feed rate to keep steam temperature in a narrow range. Alternatively, it is possible to control the furnace output to achieve the same results - and this is perhaps simpler. Another approach is to sidestep the feedback control system by operating the feed pump to always send a little excess water at all times. So, there would always be a little water in the tube. The steam /water mixture goes to a compact separator that is just a short vertical length of steel piping. Pressurized steam leaves the top and pressurized hot water leaves the base through a small orifice and into a tube. The steam can then be directed through a length of steel tubing in the hottest part of the furnace to add superheat if desired. This heat in the water leaving the bottom of the separator can be regenerated back into the cycle by sending it through a small heat exchanger to preheat furnace air, or the heat can be sent through a small copper heat exchanger to heat the lubricating oil and drive off any water condensation. This would be done with the engine design (1) briefly described previously because the steam exhausts into the crankcase. I speculate, but one might pump the oil continually out of the crankcase at a low rate and to an external heater, then return the oil - this would eliminate splash lubrication which is actually primitive and provide an opportunity to reintroduce the oil directly onto the cylinder wall and other strategic locations. Perhaps merely insulating the crankcase and heating the oil sufficiently will solve any emulsion problem, but perhaps not - so one may need to be imaginative. Lubrication in steam engines is always a challenging endeavor. With the type (2) conversion the lubrication should be simpler - although, fundamentally the same. Here, the oil can be pumped directly onto the piston rings when the piston is in the bottom position. The crankcase oil would be completely separate and left to lubricate the lower crosshead (remove compression rings to lessen friction there). So, the steam piston rings will be flooded with oil pumped onto them. The steam and oil are exhausted together, and they move to an insulated separator. A mechanical separator would probably be too complicated for a small system. So, I believe heat could be used to basically boil off the water. The hot oil would be taken from the heated vessel by the oil pump and the steam driven off would move to the condenser.

NOTE: A properly designed small wood gasification furnace used to efficiently heat a steam generator could be used to power a traditional piston steam engine, and this would eliminate the need to design the expander. Of course, the efficiency would be much lower. However, it would be safe - and it can be designed for unattended operation for long periods. This approach can make sense in a cold climate where wood is plentiful as the engine could provide heat and while providing electricity at a low rate. Again, the main problem with the old designs is the use of a boiler and need to babysit the system. Finally, I will mention one possibility -albeit reluctantly. The most common problem that caused boiler explosions was failure to control the water level. Also, metals were not nearly so strong as today. The process went something like this: water level went low, so metal temperature rose due to loss of cooling, the metal weakened and the boiler exploded. So, a strong pressure vessel that is not directly fired would make a reasonably safe boiler. This could be done with a small water heater or small propane tank by connecting a tubing coil to the vessel. The tubing coil is heated separately and it works by natural circulation where the water/steam mixture in the tube is less dense than the water in the vessel, so the water flows into the tube forcing the hot water/steam up and into the top of the vessel. Steam accumulates in the vessel and this is used to drive the expander. Yeah, there remains an element of danger, but a properly designed system would be safe assuming proper maintenance, operation, and safety features (things like keeping a tight system to prevent air introduction, regular hydrostatic testing, pressure relief valves, control system using pressure and temperature feedback, etc.). However, this is just an idea. Don't monkey around with this if you're not qualified. Log it back in the corners of the mind as a primitive means for heat and power during a TEOTWAWKI event. NOTE: Control for this kind of system might be done by using the pressurized water in the system to position a small hydraulic actuator to position a furnace damper - or use your imagination. Of course, the pressure must be relatively low - less than 150 psi for the water heater vessel - a propane tank could go higher. Also, while it should go without saying, this control system is no substitute for a steam relief valve - or two.

WARNING: Working with steam power is like drinking alcohol. If you can handle your liquor, then you know who you are. Those who are not sure are often those who cannot handle their liquor. If you are not certain you can do this safely, then you cannot.

Last edited by Goody; 05-23-2017 at 05:08 PM.. Reason: Minor editing.
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Old 05-23-2017, 05:31 PM
jdemaris jdemaris is offline
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What is the purpose? I.e.. what would this theoretical steam engine power? My wife helped run a steam-powered saw and grist mill for 20 years. This was at a water-powered mill that used steam in the summer when water was too low to power the water-wheel. Seems to me a steam-engine fire is a large part of what some call inefficiency. But at least the steam-engine works with renewable energy. We drive new cars that are maybe 30% efficient with petro fuel that is not renewable. Or worse yet - electric cars that are really indirect coal-burners. By some standards - a Stanley Steamer car is more efficient.
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Old 05-23-2017, 06:30 PM
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What is the purpose? I.e.. what would this theoretical steam engine power? My wife helped run a steam-powered saw and grist mill for 20 years. This was at a water-powered mill that used steam in the summer when water was too low to power the water-wheel. Seems to me a steam-engine fire is a large part of what some call inefficiency. But at least the steam-engine works with renewable energy. We drive new cars that are maybe 30% efficient with petro fuel that is not renewable. Or worse yet - electric cars that are really indirect coal-burners. By some standards - a Stanley Steamer car is more efficient.
That's up to the end user. The topic pops up regularly on these forums, and most of the ideas I've seen on the topic are bad - even dangerous. The only application I have considered that makes sense is micro scale combined heat and power with wood fuel in a cold climate. Assuming a good design and build, a system using the same fuel as a good furnace can supply the same heat while also providing electricity at a useful rate. Shaft power could be useful for some applications as well. Steam also is a very useful medium for transferring heat - and there are some interesting possibilities that include water heating, water pasteurization, and water distillation.

It's very difficult to get high thermal efficiency in a small steam engine system. This is why I have considered only the combined heat and power setting as viable. The system could run for long periods unattended and be very quiet. The design discussed here can show fully 10% conversion of wood fuel energy (lower heating value) to shaft work at 1-2 hp - which is the output I consider most useful at this scale. However, if heat is the primary use, then "efficiency" doesn't matter so much. Still, a 1 hp engine will provide DC electricity at a rate of 500 watts and heat at a rate of 5 KW.

Last edited by Goody; 05-23-2017 at 06:37 PM.. Reason: Minor editing.
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Old 05-30-2017, 08:36 PM
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Default The Cobber - Wood Fueled CHP Steam System

https://www.youtube.com/watch?v=PGuQx7mu-zI

Here is a video depicting a system that has been under development for some time. It is based on a system designed by the late Ted Pritchard. The system here is a lot more sophisticated than what I suggested previously. The main difference is the speed control necessary to generate AC power directly. Also, it's a lot more powerful than the 1 KW system I suggested (this system will deliver 5 KW electrical continuously). The system is also designed to deliver space heating, water heating, AND distilled water during operation. There also are plans to incorporate various belt driven systems (water pumps, compressors, etc.).
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Old 05-31-2017, 11:44 AM
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Default Steam motor

Could a hydraulic cylinder be used as a motor for steam generator
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Old 05-31-2017, 02:09 PM
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Goody, are you building a prototype now?

I'm your target audience but much of your explanation I'm unable to internalize as my mind has not lived inside it like yours has. Explain a bit conceptually why a wood gasifier is potentially an improvement over a directly fired boiler. You mentioned this would allow you to eliminate the need for an "expander" and potentially be safer for prolonged unattended operation. I do understand that as a desired design element but can't picture how that would work. My instinct is that wood-gas is not the best way to transfer the heat of the hearth to the steam generator over a directly fired "boiler". The wood gas itself will create pressures and require induced circulation- are you talking about a secondary pressurized system? What then, are you taking about using that wood-gas as a conventional fuel or just venting it? I'm not challenging it conceptually, just seeking to be better informed.

Whether a direct fired or a wood-gas system, what are the benefits of heating steam to drive your engine over a conventional wood-gas internal combustion? Again, I'm not poking holes, You've obviously given this considerable thought, I'm just looking to hear it.

I share your intrigue with this, that Pritchard-"Cobber" system in the youtube is pretty compelling, and appears to be the brass-ring for off-grid self-sufficiency, (Wood in, walk away, power out) I am curious to see what they've done with it in the last 8 months.

Prolonged unattended (safe) operation is the golden egg when it comes to a system like this. I'm working on a wood-gasifier that runs on wood pellets which might provide this, This would power my pellet milling tools allowing self-sufficiency. I've got an independent design for a furnace (not a wood gas system, more like a rocket stove forced air system) that would incorporate a water heating coil and storage tank, a thermostatically controlled exhaust vent that would selectively heat a closed ammonia loop refrigeration unit integrated into a small walk-in freezer-refrigerator combo, some TEGs mounted on external surfaces (to make use of waste heat) and a secondary forced-air heat exchanger that would retrieve heat from the exhaust pipe and send it into the home, or vent it outside contingent on the season. I've confident I can pull a very large percentage of thermal energy out of the system, just not sure there will be enough of it in there to do all of the jobs I want it to at once. My target is 4 days of continuous operation without maintenance (potentially a week or more)- which is about the same as the conventional pellet stove we use to heat our home now (which are an unappreciated genius system on my opinion)

Do you think a steam system could potentially help me accomplish these things?
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Old 05-31-2017, 02:15 PM
PalmettoTree PalmettoTree is online now
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Not qualified to input but I'll be following.
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Old 05-31-2017, 03:23 PM
Goody Goody is offline
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Originally Posted by whatzup69 View Post
Could a hydraulic cylinder be used as a motor for steam generator
A hydraulic cylinder will certainly work to seal a steam piston, and this has been done. However, I do not have details. Also, commercial hydraulic spool valves have been used as steam valves. However, these valves will not work well for high temperature steam. Saturated steam with generous oil lubrication is required. Of course, the stock hydraulic piston seals will not work with steam. So, you would either have to change these seals or replace the piston altogether.

Last edited by Goody; 06-01-2017 at 02:33 PM.. Reason: Condensed the post.
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Old 05-31-2017, 04:22 PM
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Originally Posted by KeyserSoSay View Post
Goody, are you building a prototype now?

I'm your target audience but much of your explanation I'm unable to internalize as my mind has not lived inside it like yours has. Explain a bit conceptually why a wood gasifier is potentially an improvement over a directly fired boiler. You mentioned this would allow you to eliminate the need for an "expander" and potentially be safer for prolonged unattended operation. I do understand that as a desired design element but can't picture how that would work. My instinct is that wood-gas is not the best way to transfer the heat of the hearth to the steam generator over a directly fired "boiler". The wood gas itself will create pressures and require induced circulation- are you talking about a secondary pressurized system? What then, are you taking about using that wood-gas as a conventional fuel or just venting it? I'm not challenging it conceptually, just seeking to be better informed.

Whether a direct fired or a wood-gas system, what are the benefits of heating steam to drive your engine over a conventional wood-gas internal combustion? Again, I'm not poking holes, You've obviously given this considerable thought, I'm just looking to hear it.

I share your intrigue with this, that Pritchard-"Cobber" system in the youtube is pretty compelling, and appears to be the brass-ring for off-grid self-sufficiency, (Wood in, walk away, power out) I am curious to see what they've done with it in the last 8 months.

Prolonged unattended (safe) operation is the golden egg when it comes to a system like this. I'm working on a wood-gasifier that runs on wood pellets which might provide this, This would power my pellet milling tools allowing self-sufficiency. I've got an independent design for a furnace (not a wood gas system, more like a rocket stove forced air system) that would incorporate a water heating coil and storage tank, a thermostatically controlled exhaust vent that would selectively heat a closed ammonia loop refrigeration unit integrated into a small walk-in freezer-refrigerator combo, some TEGs mounted on external surfaces (to make use of waste heat) and a secondary forced-air heat exchanger that would retrieve heat from the exhaust pipe and send it into the home, or vent it outside contingent on the season. I've confident I can pull a very large percentage of thermal energy out of the system, just not sure there will be enough of it in there to do all of the jobs I want it to at once. My target is 4 days of continuous operation without maintenance (potentially a week or more)- which is about the same as the conventional pellet stove we use to heat our home now (which are an unappreciated genius system on my opinion)

Do you think a steam system could potentially help me accomplish these things?
I am not building a prototype. I did serious research for several years and even sourced a lot of parts, and purchased some parts for testing. In the end, I simply could not justify it financially. This was about 7 years ago. Also, I was going to relocate to an area where this kind of system would have been particularly useful. However, things didn't work out that way. Oh, then there is the rapid cost decline for photovoltaics. PV with battery storage makes a lot of sense for micro scale power generation.

I totally understand about internalizing. There are so many nuances to language that dialog is imperative. I have to differentiate between a "wood gasifier" and a "wood gasification furnace". A wood gasifier is something that generates an engine grade gas for use in internal combustion engines. It is fundamentally different than a wood gasification furnace. The latter is a lot simpler. If you desire details, then I will made a post dedicated to it. Until then, I will emphasize only that a gasification furnace generates pyrolysis gases at a controlled rate and mixes this fuel gas with excess preheated air for complete combustion at high temperatures. A wood gasifier generates CO and H2 fuel gas using an endothermic process, and mixes these fuel gases with air. Both achieve clean combustion, but the former is simpler and can be done at a low burn rate with little fuel processing.

The potential benefits of using a wood fueled steam engine system over a wood gas engine system include the following. Note that I restrict consideration to micro scale combined heat and power:
(1) The steam engine system would require less processing of the fuel. So, it could use small wood splits as opposed to screened wood chips or wood pellets. The only way around this fuel processing difficulty for wood gas is to use a very large system that can use larger wood chunks. So,...
(2) The steam engine system could be a lot smaller. What I'm considering as a useful system would operate for long periods, unattended, at a low output literally on the order of 1-2 hp. Compared to a large wood gas engine system operated intermittently to charge a large battery and large thermal mass, the system here would require a much smaller battery and smaller thermal mass.
(3) The system could be very quiet.
(4) It is much easier and more efficient to harvest heat from such a system.
(5) Steam is a very useful medium for transferring heat. For example, water distillation would be easy.
(6) A steam engine system with a proper design can be more efficient than commonly understood (see NOTE).
(7) No concerns about tar fouling engine. Although, there are other concerns with steam including proper lubrication that can be tricky.

I did a lot of design work on various systems and I considered wood gas engine systems as well. In the end, my research showed clearly if wood is the energy source, and combined heat and power at a micro scale is the goal, then a properly designed piston steam engine system will beat the pants off any wood gas engine system. Again: (1) less fuel processing, (2) operation at a low output for long periods unattended, (3) superior heat recovery, (4) quiet. Imagine how one might configure a Cobber system to heat and power a large off grid home. I might operate at a low output to provide AC power directly and while charging a modest battery system to support low power systems at night. I would capture all condenser heat with hot water storage and provide space heating hydronically. The low speed flywheel could power loads directly to avoid many generator and motor losses (for example, air compressors or even refrigerant compressor for air conditioning might be done). Also, the steam exhaust could be used to recycle water in the home through distillation, and while retaining the heat for use in heating applications. The heat can even be used to rapidly dry wood fuel when space heating is not required. This all becomes a practical possibility.

NOTE: The Power Pallet 20 KWe wood gas engine system from All Power Labs shows a peak operating electrical efficiency right at 15%. So, they can convert 15% of the wood energy to AC electricity. With their generator operating at about 85% efficiency, then this implies a thermal efficiency for the engine of 17-18%. Their system is sophisticated. A good micro scale wood gas engine system could show 15% overall thermal efficiency (fuel to shaft work). A micro scale piston steam engine system can achieve 15% with proper design and attention to minimizing the losses - and without exceeding 600F steam temperature - but only IF the thermal losses from the system are absolutely minimized. I believe a fairly compact wood gasification furnace would be necessary where pyrolysis gases are shunted to a compact combustion chamber and mixed with preheated air. This would get complete combustion and high temperatures while helping to contain the heat before it is transferred to the steam generator. Also, the clean combustion would make it possible to place a small finned heat exchanger on the exhaust to preheat feed water without fouling this heat exchanger. While this seems complicated, there are no moving parts. So it just has to be configured properly. I'm considering a system operating at a more or less constant output, and things get so much simpler under these conditions. Anything better than 15% at this scale should consider compounding and reheat - and that starts to get complicated.

Last edited by Goody; 06-02-2017 at 03:02 PM.. Reason: Minor editing. Added NOTE.
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Old 06-04-2017, 06:01 PM
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Years ago I did some design work on a DIY steam engine. Takes a lot wood to make power.

Aluminum block 4 cylinder engine run inverted. No rust or danger from hydraulic lock. Grease zirks on all bearings, lubed with high temperature water resistant grease like Mystic JT-6 Cam made to convert it to 2 cycle. Steam injected with electric solenoid valves at spark plug holes. Steam and rpm control with new distributor. Pistons re-cut for multiple hi temp o-rings. Use a manual transmission with gears reversed to step up the engine rpm's to drive the generator head. Add a large mass flywheel to the system.

Flash boiler. Liquid lead at about 900 degrees, atmospheric pressure is heated in a pot in the fire, then circulated to flash boiler by a EMF liquid metal pump. Feed pump to the boiler needs to be about 1500 psi. Solenoid valves and piping need to be able to handle that temperature and pressure. Water side of the boiler has bolted on cover to allow easy scale removal.
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Old 06-04-2017, 08:45 PM
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Back in the 1970's I remember a person in Australia developing a steam engine for vehicular use, the article was ran in something like Mechanics Illustration. According to the article it was about the size of a typical four cyl. motor, worked off anything that would provide a pilot light for heat (as in gas, diesel, propane etc...multi fuel capable), had a full head of steam in approximately one minute and used a eight gallon water tank with water recirculation that would give it roughly a 800 mile range before refill. He built a couple of them for testing, one was ran at 100% output for 100,000 hours then torn down and analyzed for wear, only minimal wear was detected. After reassembling it he put it back in operation.

The second one he put in a '68 Corvette. By the end of the article he only test drove the vette once, he commented that at i/3 throttle it was running over 100 mph and scared him so bad he parked it until he could get different gears for the rear end.

The article ended with a promise of a follow up on his progress and that he was in negotiations with FMC. I watched for the promised followup but never saw it.

I can think of lots of possibilities with such a motor.
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Old 06-05-2017, 12:49 AM
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Originally Posted by charliemeyer007 View Post
Years ago I did some design work on a DIY steam engine. Takes a lot wood to make power.
True. This is why combined heat and power with wood is the logical configuration. That said, a proper design can be 2-3 times more efficient at producing work than traditional designs. For example, the system depicted in the recently linked video should be a able to produce 1 KWh of electricity from five pounds of seasoned firewood while being no more complicated than the traditional system - just different. Things get really interesting when one looks for ways to make the most of the heat like space heating, water heating, water pasteurization, water distillation, and wood fuel drying. The only thing I don't like about the Cobber system I linked is its size and output. Most off grid homes are modest in their energy needs. A quality system rated at only 1 KW and much smaller than this system would be more useful to a lot more people.
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Old 06-06-2017, 01:45 PM
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True. This is why combined heat and power with wood is the logical configuration. That said, a proper design can be 2-3 times more efficient at producing work than traditional designs. For example, the system depicted in the recently linked video should be a able to produce 1 KWh of electricity from five pounds of seasoned firewood while being no more complicated than the traditional system - just different. Things get really interesting when one looks for ways to make the most of the heat like space heating, water heating, water pasteurization, water distillation, and wood fuel drying. The only thing I don't like about the Cobber system I linked is its size and output. Most off grid homes are modest in their energy needs. A quality system rated at only 1 KW and much smaller than this system would be more useful to a lot more people.
I actually think that 5KW to 10KW output should be the minimal target output for a steam system, completely independent of whether or not a smaller system is achievable and efficient. I think that a 1KW demand is MUCH better addressed with a PV system, there is just no off-grid scenario where using steam power as a complete alternative to PV is practical if even feasible. A PV system can provide 1KW for days, weeks, months, and years without maintenance and minimal degradation. Yes, current battery technology used in a PV system does have a finite life span, but I guarantee the practical lifespan of a daily or constant use steam system would be considerably smaller.

Where I do see a steam system as excitingly practical is as a redundant charging and backup system, but more importantly as a self sufficient means to provide power in the 5KW to 10KW range for larger loads; Irrigation Pumps! Welders! Walk-in Freezers. Lathes, Mills, and other industrial tools (the sort of tools you'd need to build, repair, and replace your steam system for example). I view the PV system as perfect for powering the efficient off-grid house you describe, and a larger spot-use system as needed to power the shop/barn it would take to keep the whole endeavor running. In that context an efficient, simple to operate, and durable wood-powered generator would be a game changer in an off-grid or grid-down scenario.
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Old 06-06-2017, 02:19 PM
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http://www.thesteamboatingforum.net/...ile.php?id=186
http://www.thesteamboatingforum.net/...ile.php?id=187

These pics show a micro scale steam engine system built by a young power plant engineer during the early 1980's. The system operated 24/7 to heat and power a modern home in Pennsylvania during four consecutive heating seasons. It was fueled by anthracite coal acquired from the region.

Here are some specs on the system I was able to acquire. The engine used saturated steam from a small firetube boiler that operated at about 150 psi. The exhaust pressure was atmospheric. The heat from the condenser provided space heating and water heating. I don't know how the electrical system was configured. I do know the efficiency of the expander was measured at a very high value - at least for the system parameters. The peak efficiency of the engine was measured at nearly 11% - that is, nearly 11% of the energy in the steam delivered to the engine was converted to shaft work. While that doesn't seem high, it is about 85% of theoretical maximum. You see, piston steam engines CAN be highly efficient - they just don't use low pressure steam very efficiently. However, the boiler was not efficient at only 70% which is typical for a firetube boiler. Of course, this guy was probably getting coal really cheap and didn't care so much about "efficiency".

A little on the engine conversion. The engine was based on a small cast iron industrial gas engine. The gas engine piston was not used as the steam piston but used as the crosshead [see type (2) conversion in the first post of the thread]. The wide cylinder seen is an exhaust manifold surrounding the steam piston that collects the steam exhausted from the uniflow exhaust ports. A relevant design feature of this engine is a cam operated poppet valve to admit steam. This makes a difference to increase the efficiency. It's more efficient than the bump valve (although, I like the bump valve for its simplicity). Using a poppet valve makes it possible to fully recompress the residual steam in the cylinder back up to boiler pressure before opening the valve. This works to preheat the cylinder head before admitting steam which reduces steam condensation. With the bump valve the high pressure steam mixes with residual steam that is wet. Using superheated steam helps to compensate, but all else equal the poppet valve will be more efficient.
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Old 06-07-2017, 05:04 PM
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Thank you. What type of piston could I use.
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Old 06-07-2017, 07:28 PM
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Thank you. What type of piston could I use.

Conventional small engine pistons will work. After all, the hydraulic cylinder is not fundamentally different than an engine cylinder. However, this would require a single-acting expander (steam forcing only one side of the piston). Of course oil lubrication is required. You would need to replace the piston rod. Hydraulic cylinder rods are very thick and the seals would almost certainly not do well with the steam exhaust. So, I suggest removing the thick rod, then pressing a linear bearing into the hole to support a smaller chromed rod that connects to the piston. You would have to place a packing gland to seal the rod. This bearing would not likely do well to support the loads from the crankshaft, so I recommend a crosshead - and this may be necessary for good rod sealing. If you do actually try this, then I encourage you to first try keeping the hydraulic piston and replacing the piston seal with high temperature compression packing products. I speculate, but I suspect this can work well with a low speed engine to eliminate oil lubrication requirements - assuming a very tight system that eliminates all free oxygen - so the system would have to be hermetic or nearly so (no air entry and little or no water/steam escaping). Of course, you could use oil with such a piston seal as well if desired, and this will both reduce friction and help sealing. However, doing away with oil would solve a lot of problems.

I can see certain benefits to using a hydraulic cylinder (also consider a small engine cylinder liner), but only under a very limited set of conditions. In fact, I considered it years ago as a possible way to configure a highly efficient and slow moving compounded steam engine. However, this was more sophisticated than a typical design. It would be much simpler and less expensive to convert a small gas engine such as the 6.5 hp rated Predator engines available at Harbor Freight for $99. I warn that the "engine" (which should be called the "expander" including the crank) is the simplest part of a steam engine. The most difficult part is the rest (furnace, feed pump, steam generator, condenser, and controls). Hence, a serious project should work on the "engine" last.
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Old 06-08-2017, 07:05 PM
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I actually think that 5KW to 10KW output should be the minimal target output for a steam system, completely independent of whether or not a smaller system is achievable and efficient. I think that a 1KW demand is MUCH better addressed with a PV system, there is just no off-grid scenario where using steam power as a complete alternative to PV is practical if even feasible. A PV system can provide 1KW for days, weeks, months, and years without maintenance and minimal degradation. Yes, current battery technology used in a PV system does have a finite life span, but I guarantee the practical lifespan of a daily or constant use steam system would be considerably smaller.

Where I do see a steam system as excitingly practical is as a redundant charging and backup system, but more importantly as a self sufficient means to provide power in the 5KW to 10KW range for larger loads; Irrigation Pumps! Welders! Walk-in Freezers. Lathes, Mills, and other industrial tools (the sort of tools you'd need to build, repair, and replace your steam system for example). I view the PV system as perfect for powering the efficient off-grid house you describe, and a larger spot-use system as needed to power the shop/barn it would take to keep the whole endeavor running. In that context an efficient, simple to operate, and durable wood-powered generator would be a game changer in an off-grid or grid-down scenario.
You make a good argument. Yes, I see the demand for the larger system. Still, I see the Cobber system as overkill for most small off grid homes I have seen. Of course PV with battery storage should be used in almost any off grid setting I can imagine. The main value of a much smaller wood fueled steam engine system in my mind is the ability to provide an efficient controlled source of heating. Basically, it could double as a backup means to bulk charge the battery system whenever PV is insufficient, and it could be a primary source of heat and electricity during extended periods of inclement weather (such as during the winter months). Also, the system should be set up to operate the furnace and steam generator without operating the steam engine expander (I consider that a very important design feature). Any home that uses wood as a primary source of heat during the winter months could benefit from a well engineered small steam engine system. From what I've seen of small off grid homes, the heat provided from a 1 KW steam engine system is sufficient for all space heating and water heating needs during the winter months. I am also interested in a system that is much smaller, simpler, and less expensive than the Cobber.
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Old 06-21-2017, 01:06 PM
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I'm working on a wood-gasifier that runs on wood pellets which might provide this. This would power my pellet milling tools allowing self-sufficiency....

Do you think a steam system could potentially help me accomplish these things?
Have you looked into pelletizing grass? Grass pellets actually burn quite well. One benefit of an external combustion engine would be the ability to operate with a much wider source of biomass fuels such as grass. I've looked into various perennial biomass crops grown specifically for energy, and grasses have the highest yields. Just cut it periodically, and it grows back. Of course, some grasses are better than others.
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Old 06-21-2017, 04:51 PM
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Have you looked into pelletizing grass? Grass pellets actually burn quite well. One benefit of an external combustion engine would be the ability to operate with a much wider source of biomass fuels such as grass. I've looked into various perennial biomass crops grown specifically for energy, and grasses have the highest yields. Just cut it periodically, and it grows back. Of course, some grasses are better than others.
Yes, that's part of my thinking. The idea is to be able to create a consistent fuel. Sticks, wood chunks, logs- is easy enough to burn and has been done for eons, but to have the capacity to convert that fuel into pellets so you can automate the system that feeds the fuel for sustained durations could take the technology to the next level. But to also be able to utilize waste, like phone books, or unlimited supply of pine needles, oak leaves, or grass would also be a huge bonus to pellet fuel.

Full disclosure, I've not yet purchased a pellet mill, so far I've only experimented on a small scale device that can convert my wood chips from my chipper into sawdust, which is generally what you need to have to create pellets. There is much discussion on using newspaper, grass, dead leaves, pine needles, corn kernals, etc. to make pellets from, but from what I've read, this is more difficult to actually do in reality than is often advertised.

Good clean, dried sawdust will make consistent sturdy pellets without too much hoopla because the wood has a lot of lignins in it that allows it to cake together into a hardened pellet. Using anything else generally requires the addition of a binder agent, that effectively replaces the lignin's from the wood to bind the pellets. That is not a deal-breaker for me, IF I'm able to find renewable and producible sources for a binder that works. But, having read the feedback from those who have tried and done this sort of stuff, it still seems problematic.

the best solution I've seen is to mix clean sawdust with some of this other material, with maybe some additional binding agent, the way ethanol is added to gasoline in a smaller percentage (what is it 8-15%?). So that is where I'll focus my efforts, but like I said, I have yet to make a pellet, so this is all theoretical for the time being.

I have an overall plan for a system that will produce clean dried sawdust, and also potentially mix in some of these other organic materials. right now my Achilles heal is converting wood chips to sawdust, which is my primary focus. I also have at least 20 other projects going on at any given time, so this may likely not happen at all until I retire off grid.

Also worth contemplating, is the design used in larger coal steam plants- whereas the fuel (powdered coal, or potentially powdered grass biomass) is injected into a combustion chamber along with air/o2 and combustion is instantaneous and very efficient with these tiny particles. One could skip all the issues and logistics of pelletizing the fuel (and replace them with other issues and logistics). I recognize this is the opposite direction of going with gasification, but worth a think.
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Old 06-22-2017, 09:09 AM
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While I do like the qualities of pellets, I don't like the additional hardware and labor required to make them. I know from first hand experience that any particulate dry biomass will burn wonderfully in a small gasifier furnace. This includes dry cut grass. The test unit I used had a simple auger at the bottom of a bucket shaped to funnel everything to the auger tube. I could drop damned near anything that would fit and burn, and the auger would force everything into the pyrolysis chamber. I tried wood shavings, crass clipping, dry leaves, paper shred. I did not have pine needles available, but I know shredded pine needles would be brilliant - the ash content is fairly low and the energy content is actually higher than most woods by dry mass. Perhaps a system that shreds biomass, dries it using heat from the condenser, then forces the dry biomass into a small pyrolysis chamber is the way to go.
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