Brayton cycle engine beginnings

[myrickman]

Underneath the cylinder is the exhaust chest. This was straightforward and uses a conventional poppet valve inside the combustion chamber.

 

[myrickman]

Detail of the stuffing box on the top side of the cylinder. It has a captured o-ring seal and a nut to adjust the seal tension. The top side gets two check valves which pump air from the cylinder to the receiver. Once some pressure is built up in the receiver, a valve is turned on, an ignitor turned on and the fuel pump adds combustible material to the air stream. The screen keeps the burning mixture from the mixing valve like when you place a screen over a flame and it chops it off.

 

[AussieJimG]

Coming along nicely. I am still watching.

Jim

 

[myrickman]

Here are the valve bits, cams and actuators. On the right side, the cam forces the c-shaped actuator down, pushing the fuel pump plunger down and pulling down the air valve on the top side of the cam. The pressure in the receiver is always higher than the cylinder so it functions like a steam engine. On the left is a conventional valve assembly sans spring and retainer.

 

[myrickman]

The air check valves are on the top of the cylinder. The one facing forward is the intake and the one on the left delivers compressed air to the receiver. The links for the connecting rod are made from brass with little oil cups. You can see how the entire linkages go together now. The brass piping delivers receiver air to the inlet assembly on the bottom of the cylinder. I found a nice little 10 mm spark plug to tuck into the head to ignite the mixture.

 

[myrickman]

Here I am mocking up the air lines before I commit to putting holes in the receiver tank. I wanted to use hard plumbing to make it look more attractive. The brass cock is to turn off the air supply. A pressure gage and safety valve go on the left side of the tees. Once I get these in, I can disassemble the whole mess and paint the parts. I need to get a 10x1 mm tap for the spark plug and attach the 1/8 copper tube to the mixer to deliver the fuel.

 

[myrickman]

Today I reversed the positions of the intake and exhaust chests by doing a 180 with the head. The exhaust has to come out at an angle to clear the head bolt and will shoot exhaust into the flywheel...grrrrrr. Now the exhaust comes out in a better location. The porous sintered stainless steel disks which mix the fuel and air are installed. I made spacer gaskets so that all the chests are tight when the ports are at the correct angle. Luckily I had a 5-40 tap and die to thread the fuel line and mixer chamber. A little 10 mm thread spark plug will serve as the igniter for now. All the threaded connections in the receiver tank are welded in. With luck, we'll give it a try before Christmas ..... More pix tomorrow.

 

[myrickman]

Going to try it out later today. The 1/8 copper tubing was really stiff so I annealed it with a propane torch. Bends with no problems now. The oiler is actually for storing a small amount of kerosene which the engine runs on. I made the bracket to hold the oiler and use the threaded hole on the big shaft collar (which is welded to the cylinder) to mount it to the engine.

 

[myrickman]

Had a couple of minor air leaks where the pillow blocks mount to the top of the air receiver. I coated the underside of the cover plate around the bolts with gas tank sealer which fixed those. The pressure goes up to about 10 psi just cranking it over a half dozen times and the leak down rate appears low. My buzz coil needs a new condenser, but I have another spare one for the first trial. I know most of you folks tend towards the smaller engines but this is as small as I felt comfortable making the test mule.

 

[myrickman]

The underside of the cylinder is much busier than one would glean from the patent drawings and model. You can see the fuel mixer now on the left side with the fuel pump. The spring on the fuel pump serves both to return the pump plunger and to hold the air admission valve closed. The air is delivered for about 40% of the power stroke. I hope this is enough to get it running. The early descriptions of the engine say 60%... Those little fuel check valves were sure worth the 12 bucks from Grainger. I could not have made them that precisely and with the positive seals (Viton) for the $$$.

 

[AussieJimG]

That's a great looking engine and much more complicated than I thought it would be.

I am sure you will get it running but in the meantime it is a work of art: beautiful.

Jim

 

[myrickman]

Well I tried exhaustively yesterday to get the engine to run...close, but not quite. Reading the original operating instructions, the engine calls for fuel with a 55 degree rating. This turns out to be similar to 2 parts gasoline to 1 part diesel. Initial trials with this fuel resulted in ignition straight away. First I tried running it with a 3/16 plunger in the fuel pump with a 1/8 stroke. Turns out this is FAR too much fuel and all I ended up doing was fouling the plug. Next I reduced the pump diameter to 1/8 diameter and 1/8 stroke and added a needle valve to regulate the inflow of fuel to the pump. Also made up an adapter to turn over the engine with a big electric drill. As I cranked over, I would slowly turn the fuel and there would be a brief period where it would run for a few revs then get too much fuel and foul the plug. I'm getting a nice fine fuel-air mix which comes out of the exhaust when the plug fouls. Otherwise when it does fire, I'm getting big plumes of smoke and loud pops as the gasses exit the cylinder. I suspect my porous disks in the mixer may be holding fuel and then releasing it all at once ? Going to try it with non porous disks in the mixer. The other item is to add a spring to hold the air valve shut and then make the fuel pump a variable stroke so I can regulate the fuel per stroke. This thing needs a precise amount of fuel delivered per stroke to operate and the continued plug fouling leads me to believe I need to cut the fuel waaay back.

 

[myrickman]

If I put a separate spring to keep the intake valve closed at the end of the lever arm, I can then install a ramp to vary stroke. Once I do that, I'll disconnect the fuel line and watch how the juice is delivered. That clearing the charge is an important point and using porous disks ain't going to cut it. If I make a Venturi and spray fuel into it, it should help
Vol of gas needed for stoiciometric burn at 13:1 is 1.2 cu in. At stp for full stoke but were delivering at two bar for 50% of stroke so same as full volume at 1 bar
Assuming vapor volume is 1300x times liquid this is 0.015 cc of fuel !
My pump delivers 0.025 cc, almost double.
So if I halve the stroke, I should be ok.... Close but no cigar...

 

[AussieJimG]

It won't be long now

Jim

 

[myrickman]

Got disgusted with it and with the cold winter, I let this project sit for a few months. Tried another go at it today. I reduced the stroke of the fuel pump and reduced the pump plunger diameter from 3/16 to 1/8. Put some fuel to it and got some nice pops but no sustained running. Further reduction in the fuel pump stroke seemed to work better as far as getting a burning mix. Just at the time I thought I almost had it I got a strong kick and it locked up. After disassembly, it appears the piston cocked in the bore and the rod is bent. I think I'll make a cast iron or aluminum piston with a longer skirt. This piston is only about 1" high on a 2-3/8" bore. The longer skirt should prevent the cocking; about 2" -2-1/2" should be better. Also made a little starting crank and that was time well spent to feel how things were working. The good news is the reduced fuel did stop the flooding and spark plug fouling. Removing the spark plug is a real pain in the rear.