SpudFiles

Has anyone ever thought of a homemade 2 cylinder propane-powered engine? My dad's recently reached his 'concerned' stage of his spudding tolerance cycle, so I'm looking to channel my creativity in a less-parentally-disturbing manner, and this is what I've been thinking of lately.


Let me begin by saying that I have absolutely no experience making anything even remotely related to this type of project, but I'm sure as hell willing to try - so long as I don't lose a finger.

I've been doing some Googling, and found out that it is possible to modify the carburetor of an existing gas engine to accept propane, so I know that this is possible. I figure the good ol' four-stroke cycle would ought to do the trick nicely, with functionality almost identical to that of a car engine. so long as I come up with my own "carburetor" to spit out the ideal propane-air mix.

Which brings me to the interesting part: the carburetor. What creates the airflow to mix the propane with? I can imagine a stopgap solution of simply using pre-compressed air, but that leaves me tied to the air source - which is what I've been trying to avoid with this project. Other than that and bottled oxygen I'm stumped on this part. The rest of the beast - while still freakishly complicated - as far as I'm concerned will simply take an equivalently freakishly long time to piece together given the straightforwardness of the design.

Chances are this project will take a very long time to complete if it ever finishes, but once it is, I'll have a new toy to brag about for years to come - and maybe even mount to my bicycle somehow

A place for you to start.

http://www.impco.ws/products/carburetion.htm

Zippster wrote:Which brings me to the interesting part: the carburetor. What creates the airflow to mix the propane with? I can imagine a stopgap solution of simply using pre-compressed air, but that leaves me tied to the air source - which is what I've been trying to avoid with this project. Other than that and bottled oxygen I'm stumped on this part. The rest of the beast - while still freakishly complicated - as far as I'm concerned will simply take an equivalently freakishly long time to piece together given the straightforwardness of the design.

I'm a little stumped why you're worried about this aspect. The suction of the piston pulls air in through the carburator...it's probably not something you'll have to worry about...it happens naturally. If you can figure out how to feed your propane, you should be ready to go.

There are additives in gasoline that help protect your engine, propane will be missing those. If you are doing this for grins and giggles, you should be able to get running pretty easily. If you are looking at a serious application, you'll want to dig a little deeper.

The are plenty of LP engines out there, for golf carts and fork trucks primarily. These might be a place to start for you as well.

I suggest that, before you start trying propane, you spend a while learning how a normal gas motor works. Pressurized gas motors are a lot trickier than liquid gasoline burning motors.

starman wrote:
The suction of the piston pulls air in through the carburator...it's probably not something you'll have to worry about...it happens naturally.

Yeah I figured that part out now, makes me feel like an idiot for not getting it in the first place.

Before I dare to attempt this beast I figure I'll try to make a simpler one to run off of compressed air to get some experience working with this kind of thing. I've already been doing a lot of research, and at this point I have the entire thing assembled in my head save for the valve system. I can think of the mechanism to control said system, I just need the valves themselves. I need to find a 3 way valve with a push button for control, or better yet some form of reciprocating shaft. I'm aware of the existence of so called 'air cylinder control valves' or some permutation of the phrase, but most sites skip the explanation and jump right to the selling.

One last question for tonight: what sort of pressures can I expect from combusting propane, particularly after a compression stroke - and can a typical air cylinder stand up to the heat?

Should be roughly comparible to gasoline. Dodge actually made a natural gas model of my truck and it uses the exact same engine block, just a different fueling system.

What air cylinder are you referring to?

My two cylinders just got in - happy day!

They're two used 1-1/8" diameter 4" stroke double acting air cylinders
I just tested 'em out an they work and seal properly, and are extremely solid. Both are made entirely of stainless steal, and from what I can tell are in top notch condition save for a little dirt. I'm relatively sure that the steel would hold up to the pressure, but I'm not so sure that the seals would survive the heat. On the subject of pressure, how much of it can I expect to get at the 3-4x hybrid mix compression should achieve?

Well I'm off to our robotics shop for a while
cya


EDIT:

I just got some measurements on the cylinder body - its .12 inches thick stainless steel for a 1-1/8" diameter cylinder, if that helps.

Air cylinders (rams) will work. for a while. As you've already realized, the problem is not pressure, it's heat. . . . combustion engines are more properly called "heat" engines. . . which includes internal and external combustion, piston, turbine and other types.

Some of that heat will 'soak' into the cylinder walls, and eventually overheat the o-rings on the pistons and around the rods. you'll need to think long and hard about how to remove that heat before you'll get an engine that will run well.

Too, carburation is very much an art; and using a gaseous fuel will compound that. I'd start with a more forgiving mix like atmospheric pressure air and a fuel like 91% isopropanol (rubbing alcohol) or (if you can get it) methanol. be very careful with methanol. . . it absorbs through skin, WILL make you blind, and burns with a colorless, smokeless flame. use nitrile gloves and forced ventilation when working on the fuel system. . . . an RC car/plane carb would be a good first cut to get the engine running at a low power.


For an air pressure engine, have a leisurely searchtool on 'steam engine'. The valving is much the same. For a first pass, a passable piston valve can be made from 3/8" and 3/4" brass plumbing parts, a long grade 8 bolt, some compression fittings, and o-rings, using nothing more complex than a drill-press, a 1/8"NPT tap, some drill bits and a suitable set of files.

(yup, been there, done that, wore the tee-shirt out.)

I used mine to automatically operate a pnuematic press I made from PVC and angle iron. I used it to crush soda bottles prior to recycling them. (I don't recommend duplicating that. It almost cost me a thumb. . . .)

A dial caliper (harbor freight/ 'General' brand tools in many big box stores) will be a good investment if you're getting into engines or anything with an o-ring. later you'll want a micrometer and all sorts of arcane stuff. . . all of which will be useful in making intricate spud guns, etc.

The ultimate in easy: an 'oscillating engine.' My first was a drillpress, sandpaper and tap-plastics material special. (it ran about 10 minutes before I broke it trying to hotrod it. )

there is a lot of good info on this site.
http://www.propanecarbs.com/methods.html