SpudFiles

I was doing some research on free piston engines, when I got this idea for a piston hybrid. It functions like a two-stroke engine, except instead of a crankshaft, you have a spring that resets the piston.

Orange = O-rings
Blue = Venturi-fed Fuel Intake Check/Reed Valve
Green = Check/reed valve
Red = Exhaust Port

When you pull back the piston with, say, some sort of charging handle, the fuel mix that is behind the piston is pushed through the green check valve, and into the chamber. When you release the handle and let the piston fly forward, the fuel mix is compressed, hence the "hybrid" functionality, and a new fuel charge is pulled through a venturi/carburetor into the space behind the piston. Upon firing, the small piston in the tee opens first, firing the cannon. The combustion causes the big piston in the chamber to fly back, opening the exhaust port and pushing the fuel charge into the chamber all at the same time. The spring pushes the piston forward, and the process begins again.

My advice...? draw a better diagram (not that mine are any better)
... and add this video

[youtube][/youtube]

kjjohn wrote:The combustion causes the big piston in the chamber to fly back, pushing the fuel charge into the chamber.

Wrong, I'm afraid.

Water does not flow uphill. If the piston has been pushed back, the pressure behind it must be lower than on the chamber side.
And thus, the fuel charge will not flow through the check valve, as its pressure is lower than that of the chamber side.

And if the piston is NOT pushed back, there's no pressure there to encourage the fuel mix to move through the check valve.

If the exhaust valve opened before the fuel mix enters, the pressure would not be higher on the chamber side. Fuel behind the piston would be 1x mix, so you wouldn't have to worry about losing very much fuel through the exhaust port, although, like in a two-stroke engine, there would always be some lost fuel.

The problem is, with two strokes, there is no check valve (which takes a certain pressure to "crack"), and there's no spring that increases the pressure differential between sides.
Think about it. If it takes 100 psi to force the piston back, 50 of that coming from the spring, and 50 from the compressed fuel/air mix, then that's a 50 psi uphill gradient before you even consider the check valve.

It may work, but it has got several problems a two stroke hasn't got:
- the above
- Energy being taken to drive the projectile, rather than the piston
- Lack of the inertic "flywheel" (which may not be an actual flywheel) and sine wave like piston motion that two strokes enjoy.

... so it's going to be very dependent on the mass of the piston and it's never going to fuel as well as a conventional two stroke.

The problem with using internal combustion engines as inspiration is we are borrowing so heavily from the power stroke and that's not paid back into the compression stroke - which means you can't get the same precompression.

EDIT: One other thing. The small size of two stroke cylinders means they don't have problems with flame front speed. The principles may not work as well in a larger chamber, where the flame front may not have yet burnt out, thus igniting the new fuel/air.

I see the possibility of flatulence, I would make it like so:

@JSR
That's not a problem. Rag expressed what the real problems are

Anyway... this design is all about directing flow of gases. Basically speaking you go to great lengths to build a thing that does what DCVs do
(Who would have thought that I'd write it ?? )

though I am not saying that spring loaded pistons are not a bad idea

It looks interesting and I think you should persure it and develope the idea. It looks tricky as hell to make it work. Good luck.

oooh oooh how about this?

How about I put a barrel on a 2 stroke whippersnipper, cut the grass and pwn stuff?

I might try that, dakka dakka with marbles perhaps.

Zeus wrote:How about I put a barrel on a 2 stroke whippersnipper, cut the grass and pwn stuff?

This is an idea I've thrown around for years, small 2 stroke geared down to drive gatling gun barrels propelling the projectiles with the exhaust

oooh oooh how about this?

Not a bad idea... the MOAR pumps the MOAR powah