Frequently Asked Questions About Pulsejets
Since I put up this website I've received hundreds of emails from people with an interest in pulsejet engines. Many of those emails posed very similar questions -- so I've compiled an FAQ to help others.
There are a number of websites on the Net that have plans for various pulsejet engines available for download. Check out the links page for some pointers.
Take a look at the plans and instructions for an Australian design by Ron Bernhardt called the Digeridoo Pulsejet (240Kbyte PDF file). It's one of the simpler engines and the plans aren't too hard to follow.
Another option is to use my pulsejet design calculator which will create a set of plans for almost any sized pulsejet engine.
What Materials Can I Build A Pulsejet From?
The ideal material for a pulsejet is one of the specialist high-temperature alloys such as Inconel. Unfortunately these metals are not only expensive but also difficult to find at your local hardware store so most people will use a commonly available grade of stainless steel such as 304 or 316.
Another material worth considering is automobile exhaust tubing. This is a mild steel tube with a very thin coating of pure aluminum that stops it rusting. Being mild steel it is very easy to cut and weld -- but it is quite thick-walled and therefore rather heavy. However, for static use it's probably the cheapest, most readily available and easily worked material for most people.
The valve-plate at the front of the engine should be made from aluminum since it is a good heat conductor, is soft enough to cushion the impact of the valves and is easily shaped using a lathe/mill, or even just with a drill, hacksaw and file. The best aluminum to use is one of the high-strength alloys such as 7075 or 2024 but 6061 is also suitable if it is anodized so as to create a tougher skin. Other alloys can still be used but they may not last as long and will likely be gradually damaged by the impact of the valves.
What Can I Make My Reed Valves From?
Depending on the size of the engine, the valve material should be between 0.006" and 0.008" in thickness. Shaping tempered spring steel into the complex form of a petal valve (as used in most small engines) might appear a daunting task as it splits very easily if you try to cut it with snips or shears. The solution to this problem is to etch the valves. An explanation of how to do this is contained in this PDF file.
Can I Use Outboard/Motorcycle/Jetski Reed Valves?
By comparison, the reed valves in a pulsejet must operate under extremely harsh conditions and intense heat.
Most reed valves designed for use with regular piston engines are made from metals with relatively low melting points or composites such as kevlar and carbon-fiber. These valves will last only a short period of time if you try to use them in a pulsejet. They're also pretty expensive -- you could buy a lot of spring steel for the price of a single engine reed.
Can I Add An Afterburner To A Pulsejet?
In order to operate, a ramjet (and therefore an afterburner) needs a constant high speed flow of oxygen-rich air. This is exactly what comes out the back of a gas-turbine -- but nothing like what comes out the back of a pulsejet.
First of all, a gas-turbine engine uses less than 25% of the total amount of oxygen in the air it breathes -- so there's plenty left in the exhaust stream to support the burning of more fuel in the afterburner. Turbines are a very "lean burn" design.
By comparison, pulsejets usually consume all of the available oxygen from the air which is sucked in the front -- that leaves nothing to support the combustion of extra fuel that would be injected into an afterburner.
However, as if the lack of oxygen wasn't enough, there's another problem that effectively nixes the use of an afterburner -- and that is the way that the exhaust of a pulsejet is far from a constant stream. In fact, for a short time during each engine cycle, the hot gases actually reverse direction and start heading back up the tailpipe. This would obviously cause all sorts of problems if you were trying to throw an afterburner on the back.
However, all is not lost -- pulsejets have something even better than an afterburner -- an augmentor!
What Is An Augmentor?
So why don't we see augmentors used more often?
Well the downside is that they're quite bulky and their effectiveness drops off quiet rapidly as the engine starts moving through the air. Above a certain airspeed (less than 100 mph) the drag created by the augmentor can easily outweigh the extra thrust it generates.
However, this makes the augmentor a great device for use on slow-moving vehicles where pulsets are normally pretty inefficient.
One of the best things about an augmentor is that the increase in static thrust comes at no extra consumption of fuel. In effect, the fuel-efficiency of the engine is almost doubled along with the thrust.
More information on how an augmentor works and how to design one can be found on this page
How Long Do Reed Valves Last?
On a traditional design -- like the Dynajet, reed valves can last anywhere from a few minutes to nearly an hour's running time. They tend to last longer when static running is kept to an absolute minimum and cooling air continues to flow over the engine after it stops.
This is because one of the biggest enemies of reedvalves is heat. When an engine is run without any cooling airflow, the valves tend to get very hot and this shortens their life significantly.
I've published more information on the factors affecting valve-life on this page.
Some of the work I've done over the past two years has been aimed at extending the valve life of simple pulsejet designs and the result has been innovations such as the blast ring (as used in the PJ8 and PJ15 engines) and, more recently, a new valvehead design that uses the flow of fuel to help cool and insulate the valves.
The use of ideas such as these can significantly increase valve-life when compared to simple designs like the Dynajet.
It's worth noting that the Argus pulsejet engine used on the German V1 flying bomb had a valve-life of around 30 minutes -- just long enough (most of the time) to get across the English channel and reach London.
Can A Pulsejet Be Throttled?
In fact, this is one area where the pulsejet out performs a gas-turbine. Since there are no heavy turbine wheels to spool up or down, a pulsejet engine responds to the throttle instantly -- going from idle to full power as quickly as you can change the fuel-flow.
Can I Use A Pulsejet To Drive A Turbine?
Firstly, pulsejets are very sensitive to any obstructions in their exhausts. Unlike a regular car engine, where an increase in "backpressure" will merely reduce the power, any undue restriction to the flow of a pulsejet's exhaust will cause the engine to simply stop running completely.
Secondly, high-speed turbines tend to be very fragile devices and don't take kindly to the type of pressure pulses created by a pulsejet engine. These pulses, combined with the much higher temperature of a pulsejet exhaust compared to that of a gas-turbine, would make it very difficult to produce an engine that didn't fly apart after a short period of operation.
When you stop and think about it -- if you're going to add a turbine to a pulsejet, you might as well just put a compressor on as well and build a proper gas-turbine in the first place.
Can A Pulsejet Be Silenced?
One area that shows some promise however, is having two engines firing into a common exhaust so that the pulses were anti-phased to try and cancel out some of the noise. This however, has proven to be a very tricky task and I know of nobody who has managed to achieve this so far.
There is another problem -- and that is that quite a bit of the noise generated by a pulsejet actually comes from the intake. The action of the valves slamming shut with significant force actually creates quite a powerful sonic wave -- in fact this wave is so strong you can actually feel it if you hold your hand near the intake of a running pulsejet.
Why Does Stainless Steel Turn Black/Brown?
At normal temperatures it remains a shiny silver color because the chromium forms a thin oxide layer that stops the iron from rusting.
However, at higher temperatures, some of the chromium burns away or chemically bonds with traces of carbon -- thus leaving the iron content exposed to air.
This exposed iron reacts with the air to form iron oxides that are predominantly black in color. Note: this is not rust, it is another oxide of iron.
Because pulsejets run red-hot, this loss of chromium is quite normal and is unlikely to affect the strength or operation of your engine (remember -- it's the nickel is what provides the strength at high temperatures). The chromium content of the stainless steel will gradually reduce over a period of time, and the engine will take on a predominantly black/gray or deep bronze color.
Once your engine attains this color it is important to keep it stored in a dry place, and it doesn't hurt to lightly oil it before putting it away for any length of time as its resistance to rusting is reduced. With a little care and attention, a stainless steel pulsejet will last for a very long time.
Where Can I Get Reedvalve Material?
Why Not Use A Poppet Valve Instead of Reedvalves?
Surely these tougher, harder-wearing valves would last a lot longer than the frail pieces of steel foil normally used?
Unfortunately things aren't that easy.
Poppet valves are many times heavier than those springy little reed-valves and would therefore open and close far too slowly.
In a car engine it takes quite a few horsepower to open and close the valves at the same rate they'd need to operate in a pulsejet engine. However, instead of having a rapidly spinning camshaft to drive the valves (as in a car engine) the pulsejet only has the relatively weak pressure changes which occur in side the engine to do that work.
In short -- they're too heavy and simply wouldn't open and close fast enough -- no matter how much you tried to lighten them.
Why Haven't Valveless Engines Replaced Valved Ones?
Valveless engines are often awkwardly shaped (the Lockwood/Hiller and this makes them difficult to use in many applications.
As my page on valveless engines details, the performance figures of valveless engines also seem to be quite a bit lower than those of their valved equivalents.
Finally, valveless engines are even more sensitive to minor alterations to the size of critical components than other types of pulsejets.
So, in light of these problems, most people prefer to stick with valved engines, at least initially.
Can I Put A Pulsejet On My Bike/Kart/Car/Cat/etc?
The reality is that pulsejets are really an incredibly inefficent way to turn fuel into motive power -- but don't let that stop you.
The most important thing to watch is that the body of a pulsejet will glow red hot (you've seen the videos). This will result in a very significant amount of heat being radiated from the engine to anything nearby.
Bearing this in mind, you should always make sure that pulsejets are not operated within close proximity to any combustible material -- and that includes the painted steel panels of a car!
Remember also that if a reed valve fails, the engine may stop pulsing -- but it could still spew out billowing balls of flame. Perhaps now you can see why tbe engines I've attached to my gokart are very high, with the exhaust well away from where I'm going to be sitting.
How Much Thrust Is Need For A Kart/Bike/Cat/whatever?
You'll need less power to propel a bicycle than you will to push a gokart along -- simply because a bike has lower rolling resistance and weight than a kart.
If you want an exciting ride then you'll probably want around 100lbs of thrust on a kart and maybe 30-50lbs on a bike. The bare minimum for a kart would be about 20lbs (assuming you're running on a nice smooth surface such as concrete) and you'd probably trickle along quite nicely with 10-15lbs on a bike.