What Is A Pulse Jet Engine?
Pulse Jet Engines, or Pulsejets are a fascinatingly simple type of internal combustion engine. They can be designed to have no moving parts, run on virtually anything flammable, and have many unique properties that can be adapted to very wide range of uses.
The basic idea behind a pulse jet engine is somewhat similar to a piston engine, if the piston were replaced by a mass of air in an elongated cylinder, and the crank shaft were replaced by the elastic nature of air. The engine “RPM” is governed by its design, and the engine runs in a resonant fashion.
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. One of the most popular designs was the Dynajet engine, and it became the standard engine for control line aircraft speed competitions.
The Dynajet design is not at all optimized, there are several very minor modifications that can be made in order to dramatically improve performance. More modern designs have significantly greater performance, but the Dynajet still remains popular, and there are probably more Dynajets in existence than any other single design.
Outside the hobby field, very little effort was made to optimize or research the Pulse Jet Engine, and they quickly fell into obscurity after the war as the development of the turbojet engine advanced quickly.
Besides a few research groups, very little rigorous research was being done in the field of pulsejets, and 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.
Pulsejets do have unique properties, which caused them to be adopted by 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.
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.