Opening Pressure

Fnord's rambling on hybrid opening pressure. Currently under construction/no images.

Opening pressure is a big concern you should have when constructing a valve. In a hybrid, you want the pressure to build until most of the fuel has burned, and then released at a near-peak pressure for the most power. In a burst disk design this is easily achieved by using thicker disks. But how do you control what pressure a piston valve opens at?

The answer is simple, though it involves some math. We'll start by looking at a barrel-sealer.

Barrel Sealing Valves

The easiest way to understand this is to think about what is keeping the valve shut, and what is trying to open it when ignition occurs. The diagram on the right shows how air pressure affects the valve.

As you can see, a valve with a large seat-diameter-to-piston-diameter ratio has more force holding it shut than a valve with a small barrel seat. The less 'red' area, the less force combustion can exert on the piston. This is critical for a power-optimized hybrid, as ignition will cause the force pushing the piston back to rise fast, REALLY fast. A bad valve design will open at a pathetically low pressure and let large amounts of unburned fuel and air out the barrel. A good rule for barrel-sealing valves is to use a seat diameter that is 80-90% of the piston diameter.

The equation for figuring out at what pressure your valve opens looks like this:

m * a / (p - a) + m = x

Where m = mix pressure a = valve seat OUTSIDE area p = piston area

For example, here we can see what a valve using 1" and 1.5" sch40 steel will do at ~5x mix:

60 * 1.38 / (2 - 1.38) + 60 = ~193 psi absolute opening pressure.

When you build a valve, you should try to shoot for an opening pressure within 30-60% of the gun's peak pressure. Since 5x produces ~500 psi peak, this valve is good for hybrid use, and uses commonly available pipe dimensions.

You will most likely need a near-perfect seal around the piston when building a valve (a single floating o-ring is easy and ideal). This is because any pressure that leaks around the piston will raise its opening pressure, while decreasing the force available to open the valve. Too much leakage will cause the valve to remain shut, containing the explosion completely.

Also, keep in mind using a large seat will reduce flow if you extend it too far back into the valve (what the previous diagram shows is bad in this sense). In a good valve you'll be able to see the side of the piston when you look through the chamber port.

Chamber Sealing Valves

Chamber-sealers can also be used, though the rules governing opening pressure are reversed. This means you want a smaller porting diameter, and a bigger piston diameter. The following equation will give you the opening pressure for a chamber sealer:

(p - a) * m / a + m = x

Where m = mix pressure a = valve seat INSIDE area (porting area) p = piston area

Plugging in the numbers for 1" and 1.5" pipe gives a low opening pressure, while 1" and 2" pipe give good results.

(2 - .785) * 60 / .785 + 60 = ~152 psi (3.14 - .785) * 60 / .785 + 60 = ~240 psi

As of writing this, no one has used a chamber sealing piston on a hybrid. They are more difficult because they require o-ring seals and seamless pipe to build. Carbon grit from repeated combustions may interfere with o-rings, so they should be cleaned occasionally.

Snap valves

These valves are ideal for small-bore hybrids, as they can be adjusted on the fly for any opening pressure. The only limiting factor is the spring compression.

Because they use a spring instead of air pressure, bore sizes over 1" diameter can be difficult to seal. A 1" porting valve disigned to open at 500 psi would require 392 pounds of spring closing force.

Toolies-Style Piston Valves

Toolie valves should follow the same rules as a barrel sealer, meaning the seat is 80-90% of the piston diameter. However, in a toolie valve, the piston is the same diameter as the chamber. This means the sealing face will cut flow through the chamber down to almost 20%, when the port could normally handle about 65%.

The easiest way to remedy this is to cut the piston housing diameter with a bell reducer or reducing bushing, then scale the sealing face+porting down appropriately.

Toolie valves may be difficult to implement due to the fact that they occupy the chamber itself, reducing available space for fans and spark gaps.

Diaphragm Valves

Diaphragm valves have been used before and work roughly the same as piston valves, but off-the-shelf valves may run the risk of being damaged by high pressures. When designing a valve, it is important to construct a pilot area that does not allow the diaphragm to stretch excessively when under ignition pressure. Backing the diaphragm with a metal plate for strength is advisable.

Equations for piston valves may not give accurate results for diaphragm valves, as flexing rubber behaves differently than a solid piston when under pressure.