How to use a pulsejet engine to power a model vehicle
Copyright 2002 Larry Cottrill
_____________________________________________________
Note from the author:
A few weeks ago, a question was posted on
Kenneth Moller's website, on the Valveless Pulsejet Forum,
concerning how to use a valved or valveless pulsejet
in a radio-controlled vehicle. I posted a response that
was well-received by many participants of the forum,
since I tried to pretty well 'cover the bases' of design
problems that might be encountered. In this Q & A article,
revised slightly from that original forum post, I'll share my
response. This information is drawn from ongoing discussions
and other research, as well as from my own jet modeling
experience.
While this individual was asking about using a pulsejet
in a model car, all of the issues I addressed in my
answer could apply just as well to aircraft or marine
modeling, to some extent. Since I felt from the question
that this young man had no pulsejet experience, I decided
to talk in some detail about the special design issues that
need to be considered in using these fascinating devices.
I hope you find it informative! - Larry Cottrill
_____________________________________________________
Q:
> I want to build a small R/C car powered by a pulsejet.
> I have 2 major demands:
>
> 1.Performance
> 2.Reliability
>
> Can those two demands be met in a valveless
> pulsejet engine design?
>
A:
I won't try to describe how to design and build an engine --
there are plenty of discussions on Kenneth Moller's Pulsejet
and Valveless Pulsejet forums (and several other places) where
these issues are discussed in detail. Instead, I'll try to
concentrate on how to actually use such an engine to power
a land-based vehicle.
There are many, many potential problems here --
so many, in fact, that you might find the situation quite
daunting. On the other hand, maybe you are the kind of
individual who will let nothing stop you -- in that case, go
for it! At a MINIMUM, here are some of the design problems
you will have to face to get the job done:
- A lot depends on what you mean by "small":
Lately (since Fall 2001) I've been working on what are
considered "tiny" valveless engines, and have yet to be really
successful at getting either reliable operation or good
performance. My old DynaJet (a valved engine that I bought
back in the 60s) is about 20 inches (over 500mm) long and
exactly 2.5 inches (approximately 64 mm) maximum diameter and
weighs ONE POUND (half a KG)!. All of the few very small
pulsejets I've ever seen photos of are about that same length,
even if they are less than an inch (25mm) in diameter and very
light in weight! (And, incidentally, none of these supposedly
successful engines were valveless designs.) So, at the present
state of the art, the "smallness" of achievable pulsejet engines
is limited -- there are a lot of technical reasons for this, but
I won't burden you with the details here.
[ This photo shows me with the original Synchrodyne(TM) valveless
engine prototype -- only 3/4 inch (19 mm) in diameter, but
around 20 inches (approx. 500 mm) long. I have only been able to
shorten this design a couple of inches so far (of course, more
experimental work needs to be done, so it could end up a little
more compact, eventually): ]
- Uninterrupted fuel flow (if liquid fueled):
A pulsejet of any type MUST have absolutely smooth fuel delivery
-- in the case of liquid fuel, it will stop immediately if a
bubble comes up the fuel line. In a pulsejet, there is simply no
"flywheel" action to carry the operation of the engine through
even a split-second fuel interruption. For a vehicle that runs
on ground or water, this makes it VERY difficult to design
and build fuel systems. Many things have been tried -- for
model planes, I like to 'fill' the fuel tank with nylon scrubber
material (by this, I mean the 'kitchen scrubber' kind) to baffle
the slosh of the fuel. Probably the best option, though, would
be a 'bladder' or 'balloon' tank, carefully purged of air when
refilled. But, most balloon materials don't last long when
exposed to the kinds of fuel needed (white gasoline, ether,
etc.) so they need frequent replacement. [See the article by
Bruce Tharpe, which follows -Ed.]
[ Here is a proposed fuel tank design based on capillary
action (i.e. fuel ' wicking') -- NOTE: keep in mind that at
the time of this writing (summer 2002) this is only a
proposed design that has not yet been built and tested: ]
- High heat output:
By their very nature, pulsejets run literally red hot from the
primary combustion zone near the front all the way back to
the exit nozzle (I need to modify this statement a little --
once they're moving and getting plenty of airflow around them,
the red hot zone is more limited, but is still a potential
problem). You MUST provide (a) some sort of 'heat shield' (sheet
aluminum is ideal) between the engine body and the vehicle
structure, and (b) really good airflow to and around the engine
itself once the vehicle is in motion. Also, you need to "get
moving" right away after startup, and avoid "static running"
(full power without forward motion) as much as possible.
THEY DO GET HOT!
Closeup of Pablo Becker's homebuilt "Chinese valveless" engine running - this clearly shows the
kind of operating temperatures we typically get with pulsejets. Note the dark 'cold spot' formed by
the cooling action of air pulled in at high speed through the intake [the "Chinese" design is unusual,
in that the air intake faces rearward - the intake and its Propane gas fuel connection can be seen
at top center].
Photo Copyright 2002 Pablo Becker
- Engine mounting:
Because you have to insulate the vehicle body from the
engine heat, mounting can be difficult. Your mounting, besides
supporting the weight and inertial "maneuvering forces" of the
engine, needs to be a good heat radiator, without conducting
much heat to the mount points on the vehicle. Remember also
that since these engines are long, you need to provide support
not just for weight and thrust, but for the rotational
"thrashing" forces as you corner your car. This could just be
some lightweight metal bracing out near the tail end of the
engine.
- Poor throttleability:
Generally in something like a model car, you would like to be
able to slow down to a crawl when you feel like it. Pulsejets
are VERY limited in their ability to be throttled, although
pulsejet expert Bruce Simpson of New Zealand makes a good case
for throttleability for gas-injected (as opposed to liquid-fuel
carbureted) engines -- apparently, his successful propane-fueled
jobs are throttleable over a fair range (I can guarantee you,
the bottom end is nothing approaching the low output of a piston
mill at 'idle', however). What this means is that your "slowing
down" is going to be mostly a matter of good braking and ground
traction, rather than the "engine braking" you're probably used
to (of course, you'll want the best brakes you can put on your
car, anyway).
- Poor thrust/weight ratio of valveless designs:
So far, it looks to me like none of us have really achieved a
lightweight valveless engine that really delivers the goods,
EXCEPT for the Lockwood-Hiller design in its best configuration.
The L-H would be a fine engine for a car, although it's rather
bulky, especially in terms of overall cross-section (however,
that's probably not as critical as it is in an airplane). Note
that this is an injected, not carbureted, design. Also, it is
still considered experimental (in other words, not commercially
available).
- Starting:
I have read many people saying you can light a pulsejet from
the rear of the tailpipe, and in some cases I'm sure you can get
away with this, but in general, spark plug ignition somewhere
in the combustion zone probably provides more reliable results.
I have experimented extensively with several commercial glow
plugs with pulsejets and can tell you one thing they have in
common: they don't work. Even using ethyl ether, I have never
gotten a single bang out of one of my jets using glow plugs at
their rated voltage. Spark plugs the size and thread of standard
glow plugs (1/4-32) are available (Champion V-2 and V-3), though
pretty expensive (around $12.00 US each), and they work
beautifully, after opening the gap way up. You have to provide
high voltage spark for starting (not needed after the engine is
running) -- I use a Model T Ford 'buzz box' type spark coil, but
much more modern solutions are available, of course. At any
rate, it's quite a rig to set up. For carbureted engines, you
need a supply of compressed air with steady (i.e. regulated) and
easily adjustable delivery pressure, a 'push button' valve,
etc., all of which is another complication and expense. For all
my experimental work, I use the same pressure tank and spark
coil rig I put together back in 1963 and it still does the job
for all small engines.
- Fuel injection:
As you may have gathered, at the moment, fuel injected (and
especially gas- rather than liquid-fueled) designs have some
advantages. However, there are some drawbacks. Injection of
liquid fuel requires a regulated pressure source such as a pump
(which needs a battery in the vehicle, naturally) or a pressure
tank and regulator (and if you're doing all that, you might as
well use gaseous fuel like propane and just eliminate the liquid
fuel). Some experts (for example, Bruce Simpson) could make a
compelling argument that a fuel gas such as propane is the only
reasonable way to go -- but, I don't know how small (and light
weight) you can get cylinders, regulators, etc. to fit the
vehicle design you may have in mind. I have always used liquid
fuels in model aircraft I've designed and built for jets.
"CHINESE DESIGN" IN OPERATION
Pablo's "Chinese" valveless - this is a good shot of the engine as a whole, showing the
Propane tank, fuel line and connection at the intake. A properly running "Chinese" design
doesn't show any flame coming out of the tailpipe - the combustion zone is entirely contained
in the engine. The big 'funnel' on the ground beside the tank was used to pipe in air from a
'shop vac' for starting the engine [most pulsejet designs require forced air for starting].
Photo Copyright 2002 Pablo Becker
- Fuel economy:
In pulsejets, you can forget about fuel economy at the speeds at
which a car can be expected to operate. They are "gas hogs"
until you get up to a few hundred MPH. This is not usually a
great concern to hobbyists, except that if you want to run for
more than a minute or two, you're going to have to allow enough
physical space in your vehicle design to house a BIG tank,
whether your fuel is liquid or flammable gas.
- Vibration:
Pulsejets are vibration generators by their very nature,
because of their multiple explosions-per-second operating mode.
Make sure your radio gear is well cushioned against vibration
(you'd probably be careful to do this, anyway). Back in the days
of 'reed' radios (unless you're about 50 years old, you probably
don't even know what I'm talking about) there would have been a
concern about the jet actuating a servo by acoustic resonance,
but I can't imagine the modern stuff (especially FM, 'digital',
etc.) having a problem like this. The point is, just make sure
nothing critical in your model vehicle is vibration-sensitive at
frequencies around a few hundred hertz (cycles/second).
- Safety:
Because of the very high operating temperature, the flame
ejection (this would be only two or three inches behind a small
engine, but extremely hot and totally invisible in daylight) and
the extremely volatile fuels used, safety is a big concern.
Always keep a good fire extinguisher (Type A, B AND C) close at
hand during starting, and always take it with you if you need to
"chase down" a model that has stopped or crashed somewhere "down
the road". You'll need to stay away from areas where the model
might accidentally run off into dry grass, etc.
- Noise:
You may or may not know that pulsejets are the all-time loudest
engines for their size and power ever devised by man. The
reason, of course, is the multiple explosions (pulses) per
second that deliver the thrust. This means that many areas you
would normally use for electric or even piston-powered cars will
be unacceptable for a jet-powered model. It would be nice to be
able to enjoy your hobby without arrest and prosecution, paying
EPA fines (US), etc.
If you can handle all that (and I'm sure other jet modelers can
think of a few more!) then I say, "Go for it!"
Good luck -- if you go ahead with this project, be sure to write
back to me with plenty of photos, and permission to publish
them!
- Larry Cottrill
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Photo Credits:
Many thanks to Pablo Becker for permission to use the fine photos
of his "Chinese Valveless" pulsejet in action, as part of this article.
Pablo is a student in his homeland of Argentina. You can discover more
about his "Chinese" engine at his Website: pulsejetweb
Pablo had this to say about his Chinese Valveless engine:
Before building this engine, all my attempts to make a pjet run
were unsuccessful, so I was prepared for another failure, but ...
I started my vacuum cleaner and placed the nozzle about 50 cm
away from the engine intake, and gradually I opened the
propane valve, till it started to make some booms and then a nice
roar. Once the engine was hot I started to move the nozzle towards
the intake while increasing the amount of propane delivered. And
suddenly, when the hose was 1 cm away from the intake, the
frequency changed to about 250 hz and the roar got much louder.
I couldn't believe it !! I finally had a pulsejet running.
I stopped it and started jumping around the garden shouting,
"It works! It works!"
I wonder if Pablo had any neighbors close by who were anything near as
excited as he was ;-)
_____________________________________________________
Larry Cottrill is Director of Product Development and acting
CEO of Cottrill Cyclodyne Corporation of Mingo, Iowa, USA -
striving to create the world's smallest, safest and most
practical hobby jet engines.
You may contact me concerning
this article at:
email: jetZILLA@cottrillcyclodyne.com
I reserve the right to publish any submitted question along
with my response, so please indicate whether you want to be
anonymous or allow me to use your name (or just first name,
if you prefer) and your city/state or province, etc./country.
Thanks - I'll be waiting to hear from you!
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