What Is A Pulse Jet Engine?
A Pulse Jet Engine (or Pulsejet), is a fascinatingly simple type of jet engine that can be made with few or no moving parts. They can run on almost anything flammable, under a wide range of conditions, and have a very large throttle range. They can run while standing completely still, or flying through the air at great speed.
The reason they are called pulse jets, is due to how they produce intermittent pulses of thrust. The engine takes in and mixes fuel with air, this mixture then burns creating a rapid increase in pressure. This pressure causes the combusting exhaust gases to shoot out of the engine at high speed, producing a momentary pulse of thrust.
Due to the design and shape of the engine, and the momentum of the exhaust gas leaving the engine at great speed, the internal pressure of the engine falls below atmospheric pressure after the thrust pulse. This low pressure causes more fuel and air to flow into the engine where it is then lit by remaining hot gas, causing the cycle to once again repeat.
This cycle of taking in fuel and air, burning it, expelling exhaust, and taking in a new fuel air charge repeats up to hundreds of times a second for small engines! The pulses of thrust average out into a fairly steady usable force, which can propel model airplanes, boats, go-karts, and a wide range of industrial applications.
The two basic types of Pulsejets are “valved” and “valveless”.
Valved engines have a mechanical valve that opens and closes in order to control fuel and air entering the engine. This often requires parts to be precision machined, and valves to be replaced regularly.
Valveless engines have no such valves or moving parts, and the flow of gases is entirely controlled by the shape of its design. Because of this often times the intake and exhaust face the same direction. Despite common intuition, doing this has no negative effect on performance.
Pulse Jet Engine History
Pulsejet were originally invented in the early 1900’s, but did not see any major use until WWII, when they were used to power tens of thousands of German cruise missiles. Several countries tried to copy the German design, most notably the USA, who had the Ford Motor Company mass produce a clone of the infamous weapon based on unexploded missiles that were captured.
Soon after the war, hobbyists started trying to make their own pulse jet engines for propelling control line hobby aircraft. Several businesses were spawned and half a dozen or so designs came into the market. The most popular design was the Dynajet Red Head engine. Because of its popularity it became the standard engine for control line sport speed competitions, where competitors must use the same engine.
The Dynajet however is not optimized at all. There are several very small modifications that can be done to significantly increase performance, and a few other more involved changes that nearly double thrust output. Due to unmodified Dynajet’s being the requirement for the most popular single application of the pulse jet engine, not much was done to advance the technology in the hobby field, and pulse jets had already been left behind by the propulsion industry.
Over the years a blend of propaganda and misinformation slowly became adopted as fact and absorbed into text books. Almost anything you can find in engineering text books about pulse jet engines is completely inaccurate or down right false.
There are a number of extremely interesting aspects about pulse jet engines, which can be applied to novel applications, making for eye opening science and technology demonstrations. They also have some strange industrial applications, such as drying equipment, dust collectors, and thermal insecticide foggers. There are many many other niche markets that pulsejets are being used in, and there are many inventors that use them as part of some novel device to fill some very specific and unusual need.
Pulse jets are just now gaining popularity again in the aerospace industry due to some of these incredible properties, which range from simulating Pulse Detonation Engines for research and development, to increasing the efficiency of standard turbojet based engines by using clusters of pulse jets as pressure gain combustors within the engines combustion chamber.
In the hobby field, pulse jets remain the simplest and most affordable way to get into jet propulsion, and with the rapid advance of hobby RC technology, we will be seeing a increase in the number and variety of pulse jet powered vehicles.
In more recent years pulsejets have made quite a revival, thanks mainly to the internet and the availability of information. Many hobbyists and inventors all around the world became captivated with these amazing engines, and between experimenting and collaboration, many advances are being made.
Modern Day Pulsejet Revival
Pulsejets have become a popular alternative to small turbines for remote control vehicles, since they are a small fraction of the cost and complexity they make a perfect way to enter the RC jet hobby.
Slowly the misinformation about pulsejets is washing away, as there are numerous well documented and credible sources of new information, and many people around the world are building their own engines.
Very recently the interest in pulse jet engines grew even more, due to their similarity to Pulse Detonation Engines, as they made a very simple, cheap, and effective way to study many of the properties needed to advance the designs and support systems for PDE’s.
Making your own pulsejet can be quite addicting, as there are few things as rewarding as turning raw materials into your very own jet engine capable of shooting fire, shaking the ground, and propelling vehicles at slightly absurd speeds.