Next on the list of critical elements of a pulsejet must surely be the fuel system.
Atomization
Smaller engines such as the Dynajet have traditionally used a very crude form of carburetor that using the incoming air to create a spray of rather coarsely atomized fuel droplets.
This atomizing process occurs right at the front of the engine when the incoming air is forced through a slight venturi.
An Italian by the name of Bernoulli discovered that the faster air flows, the lower its pressure becomes. This observation was promptly labeled (wait for it…) the Bernoulli Effect.
The atomizer on these small pulsejets uses a venturi to squeeze the incoming air through a narrowing in the intake. As it squeezes through, it has to speed up. As it speeds up – the pressure drops.
Now, if we stick a pipe carrying some fuel into the middle of this low-pressure area, the fuel is literally sucked out and turned into a fine spray of droplets.
What could be simpler?
Unfortunately, although this system does work, the magnitude of the low-pressure area created in the pulsejet’s venturi is quite small and this means that there’s not much energy available to suck that fuel through.
A Note About Atomization and Vaporization
Another problem with the simple atomizer is that the fuel droplets created tend to be very large and therefore do not vaporize particularly well. It should be remembered that liquid fuels themselves don’t actually burn – only the vapors that they emit will ignite. In order to obtain good vaporization, the goal should be to create the smallest possible droplets because this results in the largest surface area (from which vapor is emitted) for a given volume of liquid.
Fortunately however, the inside of a pulsejet engine is a very hot place so, despite the fact that the simple atomizer does a poor job of converting liquid fuel into a nice fine spray, the high internal temperatures of the engine greatly assist the conversion of those large droplets of fuel into vapor.
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The end result is that most of these small pulsejets are extremely sensitive to just where the fuel tank is placed relative to the atomizer assembly.
If you place the tank too low then the engine won’t have enough “suck” to pull the fuel up to the atomizer nozzle.
Place the tank too high and gravity will draw the fuel through – effectively flooding the engine.
What’s worse, even if you do get the engine running nicely, moving the fuel tank up or down by even an inch or two can cause it to stop because the fuel flow is affected.
There are ways to reduce this sensitivity to fuel-head however and perhaps the simplest is to use a pressurized fuel tank.
By delivering the fuel under pressure, the effect of a changing fuel-level is dramatically reduced. The big problem is how do we generate this pressure?
One option is to simply pump some compressed gas into the fuel tank then seal it up. In order for this to work, the tank should only be filled with fuel to only about 25 percent of its capacity otherwise the pressure inside will drop significantly as the fuel is drawn off.
Alternatively, the compressed gas can be stored in a separate container and fed into the fuel tank through a regulator. This is how the fuel system for the Argus engine that powered the V1 flying bomb was configured and is illustrated in the diagram above.
Rather than rely on a large reservoir of compressed air inside the tank, it is possible to tap into the pressure produced by the combustion of the pulsejet itself.
This diagram shows how some of that pressure can be directed into the tank to keep it pressurized. Note the small reed valve that stops the pressure from leaking back into the engine during the intake phase.
In practice, the reed valve should be placed in the pipe that leads from the engine to the tank rather than in the tank itself. Surprisingly, there’s little risk that the hot gases from inside the engine will ignite the fuel in the tank. This system can be used with both atomized and injected fuel systems.
Another simple way to achieve fuel pressurization is to use something like a small balloon for a fuel tank. This configuration is called a “bladder tank”. The elasticity of the balloon will automatically pressurize its contents – but be aware that some fuels will quickly break down the rubber from which normal balloons are made and if it goes “pop”, you’ll have a very real fire danger.
Some of those using pulsejets in model airplanes often use these bladder tanks to ensure good pressurization and reliable fuel feed under varying G-forces. It’s worth noting however, that the rubber tank is usually contained inside another leak-proof container such as a plastic soda bottle. This way, if the bladder bursts, the fuel remains contained
Most flyers of pulse-jet powered model airplanes also use a device called a Cline regulator to ensure not only that the fuel pressure remains constant but also to automatically shut off the fuel flow if the engine stops unexpectedly.
You should also be aware that any leak in a fully pressurized fuel system can result in large amounts of flammable liquid being dumped onto the
ground or in the general area of the engine. This is an obvious fire risk. What’s even worse is that if the engine stops for any reason, the flow of fuel will continue to flood into what is now a red-hot steel tube. That can result in a very impressive fireball that could also be very dangerous.
Injection
Virtually all engines over 20lbs of thrust use direct fuel injection rather than atomization.
In such a system, the fuel is squirted directly into the engine’s combustion chamber under some form of pressure.
This makes the engine’s operation far more reliable and adds the additional benefit that by varying the amount of fuel being injected, the engine’s power can be varied. Yes, a throttleable pulsejet!
The Argus V1 engine used direct injection but, to the best of my knowledge, no attempt was made to provide any form of throttle control – not that it would have been of any use on a flying-bomb anyway.
The downside of fuel injection is that you need some method of pressurizing the fuel to force it into the engine in a fine spray.
There are really only two options – use a fuel pump or pressurize the entire fuel tank.
The V1-flying bombs used the latter option and the fuel tank was pressurized using the same compressed-air source which drove the missile’s gyroscopes and other onboard systems.
Most of my injected engines use propane as a fuel because this has the advantage of being self-pressurizing. Your common BBQ tank has around 100psi of pressure in it so you can use this for direct injection without the need for a supply of compressed air or a fuel pump.
Using such a system, the pulsejet remains a stand-alone engine that requires no extra bits and pieces to keep it running.
The simplest injection system for a petal-valved engine simply involves locating a cross-drilled injection nozzle directly behind the valve-retainer plate.
This nozzle is drilled so that the incoming fuel is sprayed out directly towards the side of the combustion
chamber. This ensures optimum mixing with the air and (in the case of liquid fuels) means that any droplets of fuel that aren’t vaporized by the incoming air will be instantly flashed into vapor when they hit the hot combustion chamber walls.
A more recent innovation I’ve come up with however involves placing an additional disk behind the valve retainer, separated by just a small space.
By injecting the fuel in the same radial pattern as with the previous system but between the two disks, the fuel is not only vaporized more effectively but also serves to cool down the valve retainer disk (and the valves). Building a system like this does however, require access to a lathe in order to turn up the key component which is this radial injector nozzle.
Using this double-disk setup I’ve been able to double the life of the reed valves used in a petal valve engine while also slightly increasing the engine’s performance and throttle range.
Timed Injection
One disadvantage of direct fuel injection is that simple systems such as the one used in the Argus V1 engine tend to spray fuel throughout the engine’s operating cycle.
Fuel will only burn efficiently (or at all) when mixed with exactly the right amount of air. This combustible mixture of air to fuel is referred to as the “stoichiometric ratio” and it varies depending on the type of fuel being used.
It makes little sense therefore, to waste fuel by injecting it when there is no incoming air to mix with it as that fuel will be unable to burn inside the engine thus contributes nothing to the thrust being generated.
Back in 1947, the guys at Princeton University came to this same conclusion and suggested that using timed fuel injection would be a way to improve the fuel-efficiency of pulsejet engines.
Now there are two ways in which timed fuel injection could be done: the simple way and the complex way.
Given that the simplicity of a pulsejet is its single greatest virtue, I’m all in favor of keeping a timed fuel injection system simple too.
I regularly get email from people who think it would be a good idea to use an electrically driven fuel injector like the ones used in modern car engines – but I disagree.
In order to make one of these injectors work you’d need a rather complex system that involved a battery to drive the injector, sensors to measure the pressure inside the combustion chamber for timing, and some electronics to tie the whole thing together.
This setup, although I’m sure it could be made to work, would be costly, complex and offer only minimal benefits over the system I use to obtain timed fuel injection.
Fortunately it is a simple job to synchronize the injection of fuel into the engine with the intake of a fresh air charge. This is because the pressure inside the engine falls to below 1
atmosphere (14.7psi at sea-level) during the intake phase and rises to as much as twice atmospheric (30psi+) during combustion and exhaust phases.
A valve placed over the fuel jet is sufficient to provide a degree of injection timing and the addition of this mechanism can provide a noticeable improvement in the fuel-efficiency of a large pulsejet.
I’ve experimented with a number of different valved injectors ranging from a simple bolt drilled length-wise with a flap of spring-steel over the end like the one illustrated here…
To this carefully machined injector made from stainless steel and nickel-plated steel components I fitted to the 100lbs-thrust engine on my gokart. I noted a very definite improvement in the fuel-efficiency of this engine after fitting the timed injector system.
What Fuel is Best?
One of the great advantages of pulsejet engines is that they can, at least in theory, be made to run on almost any type of combustible liquid or gas.
Pulsejets aren’t limited to liquid or gas fuels however – on at least two occasions, coal dust has been used as a fuel. It is rumored that the Germans attempted to run the Argus V1 engine on coal dust when liquid fuel supplies became almost unobtainable near the end of WW2 and some of Reynst’s pulsed combustors were designed specifically to use this unusual fuel.
Before you start worrying too much about what is the best fuel, it’s worth citing part of a report published by Princeton University in 1947 that summarized a large amount of the research done into pulsejet engines up to that time. It said “the pulsating jet engine of contemporary design ran on almost any common fuel with negligible variations in performance.”
The only caveat the report included was that “principal [sic] differences were in the degree of body heating and the rapidity of valve destruction.”
They found that even the use of exotic fuels such as nitropropane or nitromethane offered only a slight power increase at the expense of doubling an engine’s fuel consumption.
It makes sense therefore to choose your fuel on the basis of whatever’s cheapest or most convenient to use.
For most of us however, the choice of fuels is fairly simple and boils down to one of these:
Gasoline
This has the advantage that it’s relatively cheap, very easy to obtain, and is pretty clean burning.
It’s also quite volatile so atomizes easily to promote easy starting.
Note that, contrary to what you might think, higher-octane gasoline is not going to produce any more power than regular gasoline. In fact (in theory) it may produce slightly less power. If you plan to use gasoline, just use whatever’s cheapest.
Propane (LPG)
Thanks to the popularity of gas-fired BBQs, propane has also become quite easy to obtain and suitable 20lb refillable tanks can be bought for well under $50.
In some countries, propane is even cheaper than gasoline and it burns very cleanly indeed – leaving no smell and very little residue at all. Despite the fact that it’s stored under pressure, it is actually quite a bit safer to use than gasoline because its vapors dissipate very quickly in the open air.
Since the boiling point of propane is well below normal room temperature, it either comes out of the tank as a gas (thus avoiding the need for vaporization) or, when drawn off as a liquid, instantly boils into a vapor. This makes a propane-powered pulsejet one of the easiest to start.
Note that bigger engines will almost certainly demand to be fed with liquid propane because an average BBQ tank simply can’t provide gas at a sufficiently high rate to keep up.
If you’re planning to use a BBQ tank of propane as a fuel, you’ll have get rid of the regulator that is normally used to limit the flow of gas. This regulator reduces the pressure of the gas to just a few psi, far too low for a pulsejet’s needs.
To give you an idea of just how much gas is needed to run a pulsejet, my own 15-lbs-thrust engines (PJ15) will drink all the propane gas you can feed them – with the regulator removed.
The Lockwood valveless engines will drink all the liquid propane you can feed them without any regulator in place.
If you try to use propane without removing the regulator then all you’ll get from a pulsejet is a few bangs and pops – it won’t run.
However, you will still need some form of control over the flow of gas into the engine and for this I recommend buying a cheap propane/air torch – of the type often used for soldering or brazing.
These torches are available from almost any hardware store and cost just $25-$30. Note that depending on the exact make/model of torch you buy, you may need to purchase an additional adapter fitting so that it can be screwed directly onto a 10lb or 20lb propane tank.
To use a torch like this as the gas-control valve for
your pulsejet, simply unscrew the burner fitting on the end and slide your propane-certified plastic fuel pipe over the end, securing it with a small hose-clip.
The gas-flow knob on the torch will now enable you to control the amount of gas that is delivered to your engine. If you invert your BBQ tank of propane, the torch will still serve as a very simple way to control the flow of liquid propane to larger engines. Very simple, very inexpensive, and very effective.
Another method of controlling the flow of propane to your engine is to simply use a device called a needle-valve. These valves are readily available from a number of sources and, just like the gas-torch, offer a very fine degree of control over fuel-flow.
Butane
It should be noted that although it is also often sold for use on small camp stoves, butane is not a good substitute for propane. It contains less energy and doesn’t produce as much pressure as propane at room temperature. In short – don’t waste your time or money trying to use butane as a fuel for pulsejet engines.
White Spirit/Coleman fluid
This is simply a very low octane unleaded form of gasoline which has no fancy anti-knock or combustion “improvemnet” additives included. It’s actually a better fuel than high-octane
gasoline for pulsejet use. Many of the early small pulsejet engines such as the Dynajet run best on this fuel.
Methanol
This is my second-favorite pulsejet fuel. It has the advantage that it will burn over a very wide range of rich/lean mixture settings – making an engine less sensitive to fuel head or starting conditions.
It also burns very cleanly with no smelly or oily residue and creating little more than water vapor and some C02 as combustion byproducts.
On the downside, methanol is more expensive than gasoline, your engine will burn more of it for a given amount of power, and it can be very dangerous if spilled because it burns with an almost invisible flame. Many people have been burnt because they’ve walked straight into a methanol fire without seeing it.
Despite the downsides, I prefer to use methanol for all my aspirated engines because it generates a little more power, allows the valves to run cooler, and doesn’t leave my hands stinking of gasoline.
Note that you shouldn’t use pre-mixed model airplane fuel instead of straight methanol.
Model airplane fuel contains up to 20% oil that will leave significant deposits inside your pulsejet and also affects the vaporization of the mixture. It’s also a lot more expensive than plain old methanol so you’ll be wasting money.
Your local hot-rod or drag-racing club ought to be able to help you find a source of methanol but if all else fails, try one of the major oil companies like Mobil – they sell me 5-gallon drums of the stuff when I want it.
Another thing to watch when using methanol as a fuel is that it is very hydroscopic – which is to say that it tends to absorb moisture out of the air. If you leave a can of methanol uncapped then it may well absorb so much moisture that its combustibility is affected and this can result in hard-starting.
Also be aware that when you use methanol as a fuel, one of the combustion byproducts is water (albeit as water vapor). This means that the spring-steel reed valves used in an engine
Also be aware that when you use methanol as a fuel, one of the combustion byproducts is water (albeit as water vapor). This means that the spring-steel reed valves used in an engine