The Results Of 18 Months R&D
In theory, pulse jets are probably one of the simplest forms of internal combustion
engines ever designed -- having only one moving part and able to be fabricated
using basic metalworking techniques.
However, as I've discovered , the gap between theory and
practice is paved with unexpected pitfalls and complexity.
Unfortunately, with the arrival of reliable gas-turbine engines, the
development of the pulse jet stopped dead in the early 1950's -- but
this has the upside of allowing plenty of scope for improvement on
these "dated" designs.
I've spent quite a bit of time experimenting with modern materials and
variations on the basic design in an attempt to produce a "better"
engine -- and I think I'm succeeding.
The latest versions of my pulse jet engines are significantly different to the
first prototype I documented on this site some 18 months ago. On this page
I will explain some of the things I've learned about these engines, how they
work, and why so many people seem to have trouble getting their own designs
(or others) working.
Why Some Designs Won't Run
Since I put this site up I've had a lot of email from other pulse jet engine
builders who generally say something like "I designed and built an engine
but it won't run, can you help?"
Well there are two things that are absolutely critical if you're going to build
an engine that will run:
- Not Enough Air?
One of the main reasons that so many engines refuse to run is that they're starved
for air -- they can't get enough fresh air/fuel into the combustion area to create
the powerful explosion needed to set up the pressure waves that are critical to
keeping the thing going.
Perhaps you've wondered why almost all pulse jets have a larger diameter at the front
than they do at the back. People often mistake this larger diameter section to
be a combustion chamber -- but it's not. In a correctly operating pulse jet
you find combustion occuring right down the whole length of the pipe -- with a
lick of flame even protruding out the end.
The sole reason for that wider front section is to provide enough room to
place the necessary air-intake holes and reed valves.
If you check out the design of most smaller pulse jets (including my original
prototype) you'll see that they usually have a ring of holes around the edge
of a circular disk and that the reed-valve is petal-shaped. Just look at
how little of this disk is allowing air through and how much is solid --
not very efficient is it?
Also note how the incoming air must negotiate its way past the reed valves --
requiring some sharp turns which further reduce the efficiency of this
intake system.
Due to these inefficiencies, the diameter of the front section of a small
pulse jet with petal-valves must be significantly larger than that of the tailpipe.
The mistake that many designers make is underestimating just how much bigger
this needs to be.
I've found that to really make your pulse jet work well you need to have an
effective air-intake area nearly half the cross-sectional area of the tailpipe. If your
engine just pops or only runs while you're forcing air in the front then
you probably don't have enough air-intake area.
Of course if the intake area is too large then your engine won't run properly
(or at all) either. This is because too large an intake area will mean that
the vacuum formed inside the engine will be filled too quickly by the incoming
air and it won't be able to suck the flame in the tailpipe back so as to ignite
the fresh air/fuel mixture.
Dont' forget that with petal valves what you see isn't what you get. Because
the valves only open a small amount and the air has to force its way around
them at an acute angle, you only get an effective intake area of about 60
percent the actual hole area.
My "second generation" engines have a totally different reed-valve system that
allows me to utilize over 80 percent of the intake area for air-flow. It also
allows me to use a much simpler reed-valve design than the complex and sometimes
difficult to fabricate petal shape.
- Tailpipe Too Short?
Although there's much debate on the matter, I've found that the smaller
the engine, the longer the tailpipe has to be in relation to its diameter in
order to ensure strong enough pressure-waves to keep the engine running.
My larger engines run well with a much smaller ratio of length to diameter --
probably because the total volume of air involved in the pulse is as important
as the length of the air-column being moved.
Also, if the tailpipe is too long then your engine will still run. It will
have a lower frequency and the reed valves will likely fail very quickly -- but
it will still run. If the pipe is too short it won't run at all -- just pop
and bang or run only until you remove the starting air supply.
On some of my test engines I've made a trombone-type setup of telescoping pipe
sections which allowed me to "tune" the length. If you want to play around then
I recommend that you do the same -- but be aware that the pipe will get very
very hot. In order to work properly, the inner and outer pipes will have to
slide into each other pretty snugly -- and this means that they will start to
jam up within 20 seconds or so of the engine starting because the inside pipe
will expand more quickly than the outside one. This means you won't get much
time to make your adjustments on each run.
- Fuel Not Being Atomized Properly?
In order to explode, the fuel and air must be mixed in the right proportions
and the fuel should be atomized into a fine mix. If the air-fuel mixture isn't
just right then the engine may pop and bang but it won't run.
Also, if the venturi system isn't atomizing the fuel into fine-enough droplets
you may end up with an engine which is very hard to start but, once going,
runs okay. This is because as soon as the engine gets hot (after running
for 20 seconds or so, even large fuel droplets will be instantly vaporized
by the heat radiated from the combustion zone of the pipe -- making the
efficiency of the atomization system far less important.
Most published pulse-jet designs have a very simple venturi-type caburetion
assembly mounted at the front -- but few provide much in the way of fuel
mixture adjustment -- and it's very important that the mixture is set to suit
the type of fuel being used. A setting that works for gasoline probably won't
work for methanol for instance because methanol requires far less air to
burn -- hence the fuel mixture needs to be richer.
Unfortunately these simple fuel atomization systems are often not very effective
and are also very sensitive to fuel head -- ie: whether the level in the fuel
tank is higher than the venturi or lower. Even when running well, a "normally aspirated"
pulse jet engine may stop if you raise or lower the fuel tank by just a few inches.
This problem of getting the mixture correct is another reason why I prefer
to use propane/LPG with direct injection. These gasses have a much greater
range of combustible ratios with air and that means that getting the mixture
"just right" is far less critical. Also, the direct injection method means
that the engine will automatically produce the optimum air-fuel ratio for
a given fuel-flow rate.
Other Traps
Most pulse jets use a spark plug to ignite the air-fuel mixture when starting
and this plug usually creates a steady stream of sparts driven by an auto coil.
It took me a while to figure out that if you don't turn off the ignition circuit
once the engine has started then these continuous sparks can adversely affect
the engine's operation.
When running properly, the new air-fuel charge entering through the front of
the engine is ignited by a flame-front which travels back up the tailpipe
and collides with the fresh fuel in the primary combustion zone of the engine
(at a point some 25%-30% from the front of the engine). If you leave the
ignition running, the fuel can be prematurely ignited far closer to the front
of the engine -- causing low power, erratic running and unreliable operation.
The engine might just quit suddenly for no apparent reason.
Turn off the ignition before you remove the starting air is a good rule.
Choice of Fuel
The fuel you use can have a significant bearing on how easy your engine
is to start and how reliably it runs.
Methanol is an excellent fuel because it allows an extremely wide range of
air/fuel settings to be used. This means your engine will be far more tollerant
of being set too rich or too lean -- an important factor when you're
experimenting.
It's also a very clean-burning fuel -- the main byproduct being water vapor.
Methanol won't leave your engine stinking of fumes or discolored by sooty
or oily residues -- it's lovely. Be warned however that methanol, like
gasoline, is toxic and can be absorbed through the skin. Take sensible
precautions when handling it.
Also be very careful with methanol from a flamability perspective. The fact
that it burns so well over a wide range of air/fuel ratios makes it quite
dangerous if you're in a confined space. Methanol fumes can build up and
only the smallest spark is required to cause an explosion. Note also that
methanol burns with a clear flame that is invisible in daylight. If you
have a methanol fire you may not realize it until you feel the heat (or
burns). Always have a fire extinguisher handy -- preferably a CO2 unit
since foam and dry-powder are in most cases far less effective on methanol
fires.
The final thing to watch with Methanol is that it can cause your reed valves
to rust. Being made from steel they are prone to rust anyway -- but
since methanol leaves no protective residue (as gasoline or other liquid
petroleum-based fuels do) and because the combustion of methanol creates water-vapor,
rust will occur if the engine is left unused for any length of time.
Another extremely good fuel (where practical) is propane or LPG. Because it's
delivered in a self-pressurized gaseous form it can make the fuel system far
simpler and produce very consistent engine runs. I've used LPG extensively
with direct injection into the combustion zone and it's great.
Unleaded gasoline is low on my list of fuels -- certainly during the
testing phase. Once you've got your engine tried and tested it becomes a pretty
cheap and effective fuel though.
Diesel and Kerosene have reportedly been used, alone or mixed with
gasoline in various proportions -- but I've not tried them.
Atomization or Injection?
In order to make the engine run, we have to provide some way of mixing the
fuel with the incoming charge of fresh air during each combustion cycle.
There are two basic wasy to do this and they both have benefits and drawbacks.
- Atomization
Most small pulse jets use an atomization system (similar to a carburetor)
to mix the air and fuel ahead of the reed valves.
This has the advantage of allowing the easy use of liquid fuels and, because
the fuel has a cooling effect on the very hot reed valves, it can increase
the life of these highly stressed components.
On the negative side, most engines with an atomized fuel supply are very
sensitive to the fuel-head. That is to say, if you raise or lower the
fuel tank in relation to the atomization point, the engine will likely
stop because it becomes too lean or too rich. This is more noticeable
when using gasoline as a fuel and less noticeable when using methanol.
Engines using an atomized fuel system are also unable to be throttled effectively.
In a regular internal combustion engine, throttling is performed by reducing
the amount of air and fuel reaching the combustion chamber. This is performed
in the carburetor by restricting the flow of air/fuel into the intake manifold.
Now, if you're read what I had to say about the need to let these engines
breathe -- you'll see that any attempt to restrict the flow of air and fuel
into a pulse jet will cause it to stop.
- Direct Injection
We can overcome many of the limitations created by an atomized fuel system
by doing what modern auto-manufacturers do -- inject our fuel rather than
use a carburetor.
With the pulse jet we can inject a liquid or gaseous fuel straight into the
combustion zone of the engine to great effect.
One advantage of direct injection is that, because the fuel feed is (must
be) pressurized, there is little or no sensitivity to changes in fuel head.
This generally results in an engine that gives more consistent reliable
runs than its normally aspirated equivalent.
However, one of the biggest benefits of a directly injected fuel system is
that it allows us to throttle the engine to certain degree. This throttling
is accomplished by simply varying the amount of fuel that is injected.
Just like a gas-turbine or diesel engine, the pulse jet will respond to a
reduction in the fuel being injected by sucking less air -- thus restoring
the optimum air/fuel mixture for reliable combustion.
My experimentation has shown that it's possible to get a throttle range
of over 50 percent of the engine's power without too much difficulty.
On the down-side, direct fuel injection requires that you either use a self-pressurizing
fuel such as propane/LPG, or that you use a fuel-pump.
My personal choice is for propane/LPG -- as much for the sake of simplicity as
anything. Even if you're using your engine in a model airplane, you can get
cheap (and lightweight) cannisters of camping gas or even butane lighter
refill bottles that will do the job nicely -- acting as fuel tank and pressure
source combined. The only downside being a possible
fire-risk if you crash -- but that's always going to be a risk with any kind
of jet-powered model.
If you're going to use one of these engines on a gokart or other vehicle then
the ability to throttle it will be very handy and for fuel, you'll be able to use
one of the bigger 10lb or 20lb tanks. One problem with these liquefied gases
however can be that when you're trying to draw off significant amounts of gas
in colder climates, the tank will develop a coating of ice and lose pressure
pretty quickly -- long before it's empty.
Dispelling The Myths
There are a number of myths that surround the pulse jet engine, I will try
to dispel them here.
- Pulse jets are hard to start
Not true. A well designed pulse jet can be started in seconds by the following
procedure:
- turn on the fuel (gas or liquid)
- turn on the ignition
- use a source of compressed air (I use a leaf-blower) to blow some air
into the front of the engine.
- once the engine starts running (it should do so almost instantly) the ignition
should be turned off and the compressed air turned off.
If an engine doesn't respond to these simple starting steps then there is some
other kind of problem with the setup or design.
Check out this video
of an injected propane-fueled engine being started and
this page for more details on starting and
video of a normally aspirated engine being started.
- Pulse jets must be moving through the air to work
I've seen a lot of sites, particularly those talking about the V1 pulse jet-powered
"flying bomb" which claim that pulse jets won't run unless they're moving
through the air -- this is patently untrue of a well designed engine.
I suspect that in many cases, the writer has gotten the pulse jet confused
with the ramjet. Ramjets won't work unless they're travelling at a fairly
high speed (several hundred miles per hour) but pulse jets work just fine
while static -- although they will get extremely hot -- red hot in fact.
- Pulse jets can't be throttled
The old-style naturally aspirated pulse jets could not be throttled but
those using direct fuel injection can be throttled over some of their
power range. Larger engines are much more throttleable than smaller ones
but it's typical (in my experience) to be able to throttle a direct-injected
engine by about 50-60 percent without too much difficulty.
Pulse jets will melt if not moving through the air
A properly made pulse jet made from suitably heat-resistant materials and
to a good design will not melt if run continuously while not moving
through the air. I have run my own engines for periods of up to 20 minutes
at a time on a test-stand and, although parts of the engine do glow
red hot and valve life may be significantly reduced, they do not melt.
Still Got A Problem?
I hope this page has provided the answer to the questions that most builders
ask but few sources seem to answer.
If you have any other specific questions that aren't addressed here, feel free
to contact me directly.