Factors Affecting Pulsejet Valve Life

Valves -- The Weakest Link?
As anyone who has built or owned a small conventional pulsejet knows, the reed valves are a consumable item.

Depending on the engine and the conditions under which it's operated, the reed valves can last anywhere from just a few seconds to 20 minutes or so -- but you can be sure that they will fail at regular intervals.

It's not hard to understand why these fragile little pieces of metal don't last long. They're slammed back and forth between the intake and retainer plates with great force, several hundred times per second. What's more, they're usually exposed to the extremely hot combustion gasses.

Here are some pictures of typical petal valve damage:

Factors Affecting Valve Life
As outlined above, there are two main factors which affect valve life:

1. Physical forces
2. Temperature

If the magnitude and or influence of these two factors can be minimized, valve life will be extended accordingly.

Minimizing Physical Forces
The magnitude of the forces exerted on the tips of a pulsejet reed valve are huge and cumulative.

Hi-carbon tempered spring steel is an incredibly tough material. If you don't believe me, just try putting some on an anvil and hitting it with a hammer. Such abuse will usually produce little visible damage. Of course to reproduce the forces applied during just one minute of pulsejet engine operation, you'd have to whack that piece of metal about 3,500 times.

Another effect of all this movement is metal fatigue. If the valve flexing exceeds the elastic limits of the material then eventually a crack will be created and this will rapidly turn into total fracture.

So what can be done to reduce the effect of these physical forces on our reed valves?

1. Keep the valve movement to a minimum.
   Instead of using a small valve that requires a large amount of movement to produce the required effective valved area, you're much better off to use a bigger valve that moves just a small amount. Small valve movements generally produce lower valve-tip speeds and this can significantly reduce the impact forces involved in valve opening and closing. Remember -- when you double the speed, you quadruple the forces involved.

   Unfortunately, the traditional petal-valve design makes it very hard to produce an engine that has large valves with small opening movements. In order to achieve such a set-up, the diameter of the front part of the engine needs to be very much larger than that of the tailpipe -- and that can produce other undesired side-effects.

   As a general rule, I've found that about 6mm (1/4") of tip-travel is about the maximum you can use with petal valves before valve life becomes unacceptably short. This becomes a limiting factor in terms of how large an engine you can build using simple petal-valve technology.

2. Provide a smooth landing surface
   Any bumps or depressions in the area of the valve-plate against which the valves impact will produce very high areas of localized stress within the valve itself.

   To draw an analogy -- if you place a sheet of glass on a smooth table then lift one end and drop it from a small height, it will touch down quite softly -- the air beneath it providing a cushioning effect. However, if you place just a small pebble between the glass and the table-top then repeat the exercise it will land with a thud -- striking the pebble with some force, possibly even shattering.

   This is the effect that you get when the valve-plate is poorly made or damaged. Small ridges or bumps on the valve plate can create huge stresses within the reed valve itself.

   If you make your valve plate from aluminum, it's a very good idea to choose a suitable alloy that allows the plate to be anodized. A layer of anodizing will be far more resilient than raw aluminum and thus and produce a surface that is less easily damaged by the impact of the valves.

3. Don't use excessive overlap
   Don't make the mistake of thinking that it's a good idea to provide plenty of valve overlap against the valve plate. While it might seem sensible to provide plenty of area for the valve to rest against when it's pushed onto the valve plate, this simply isn't true.

   If you have too much overlap, the air which becomes trapped between the valve plate and the valve will cause the tips of the valves to bend backwards quite markedly -- actually increasing the stresses.

   Determining the best amount of overlap is actually quite a critical parameter in obtaining good valve life. Too much overlap and the valves will quickly split at the tips. Too little overlap and you'll damage the valve plate because the impact forces will be concentrated on a small area.

   And you thought designing pulsejets was easy eh?

Reducing Heat Exposure
The goal of the pulsejet designer should be to try and keep the reed valves out of the way of the hot combustion gases. Unfortunately this is extremely difficult to do with traditional designs.

In the ideal engine, the flow of the intake charge/air should be as unrestricted as possible. The incoming air should be dumped almost straight into the engine's combustion zone.

Unfortunately, if we provide such a direct path then there's nothing to stop the hot combustion gases from rushing back towards the intake and hitting the valves.

I have experimented at length with the addition of devices such as the Blast Ring(TM) -- a simple metal ring that disrupts the flow of gases back towards the valves. This is actually every effective at keeping the valves cool -- but it can only be used with direct-injection engines and has a penalty.

Because it tends to restrict the flow of gases in both directions, the Blast Ring(TM) does reduce the engine's power. For this reason I have made it an "optional" component of the engines I sell. If you want to run your engine for an extended period for demonstration purposes or to test something such as an augmentor then the blast ring will extend the life of your valves.

When you want maximum power however, you can simply remove the ring and power output will increase significantly -- albeit at the cost of valve life.

Another method of reducing valve heating include the use of a flame-mesh/screen between the combustion zone and the valves. Unfortunately this has an even greater effect on power than the Blast Ring(tm). Due to the drag created by such a mesh, the flow of the fresh air/fuel charge into the combustion zone is significantly impeded -- and the mesh itself (due to its exposure to the hot gases) tends to have a very limited lifespan.

Non-Traditional Design
As you can probably see from the information above, modifying traditional pulsejet valving systems to provide more durable and reliable operation is not trivial.

It was the irritatingly short life-span of conventional reedvalves which lead me to think long and hard about ways to re-design the entire valving system -- something which ultimately lead to the XJet design which I've been developing.

Other Options
Of course the ideal solution to the valve-life problem is to ditch the valves altogether and use a valveless design.

Unfortunately, valveless designs still have some way to go before they produce similar efficiencies and power levels to a good valved engine.

The best valveless design I've encountered so far is the Lockwood engine -- but all the working examples I've encountered fall short not only of the equivalent valved engine, but also fail to meet Lockwood's own claims for power or fuel efficiency.

Never the less, there is still a strong contingent of pulsejet experimenters who are adamant that the only good pulsejet is a valveless one.