How do Pulse Jet Engines Work?
Pulse jet engines, or Pulsejets are a very simple family of internal combustion engine that have few or no moving parts. They were invented in the early 1900’s, and were cutting edge science until after WWII. Pulsejets got abandoned, many feel long before they even reached their prime. They went from being the main source of power for tens of thousands of cruise missiles, to nothing more than something that a skilled hobbyist could build in their garage and used to power planes in Jet Speed contests. Pulsejets have underwent a recent surge in popularity due to many unique properties, and their similarity to Pulse Detonation Engines, making them the perfect test platform for many supporting technologies and devices.
There are two basic types of pulse jet engines, valved engines, and valveless engines. Both types operate on the same principles, and can run on a wide range of fuels, can be scaled to very small or very large sizes, and are surprisingly powerful for their simplicity.
The video above is an animation showing a simplified flow visualization in a TP-180 Valveless Pulse jet. Blue represents cold air being drawn into the engine, through the intake and the tail, forming the tail ‘cold air piston’ Green represents the mixing of fuel with air, and Red represents combustion and super heated gas. Notice how some super heated gas always stays in the engine, to ignite each new fuel charge.
Pulsejet FAQ & Myths
These seem to be the most common questions and misconceptions running around the internet.
Pulse jets cant run unless they are flying through the air at high speed.
- This is false, pulse jets can run at a standstill, or even traveling backwards at considerable speed.
Pulse jets are extremely fuel hungry.
- There are pulse jets that are inefficient, just like there are cars that are inefficient. You always hear about these rather than the pulse jets that have comparable or better fuel efficiency than similar sized highly advanced turbine engines. Chances are the engine someone designs on a napkin and builds in their garage is not going to be as efficient as other highly refined designs.
Pulse jets glow orange and waste so much of the fuels energy as heat.
- Heat is a byproduct of all engines, it is what allows us to do work. Without the energy of expansion from the release of heat, there would be no work. Just because an engine glows orange does not mean its wasting excessive energy. A mild steel engine will radiate away approximately 8 times the energy as a stainless steel engine. This is also why things like thermos jugs are made from stainless, of all the metals it is one of the most insulating.
Pulse jets have a limited top speed and cant be throttled, they are either on or off.
- Well first of all, what doesn’t have a limited top speed? There are designs that can operate at or above the speed of sound, and there are others that start to lose power over 200 mph. Pulse jets can be throttled over a wider range than most turbines, or other engines. Some valveless pulsejets can run at less than 1% output! They can also run on almost any fuel, or even waste material.
Pulse jets are dangerous and likely to explode.
- Hot coffee, wet floor signs, AA batteries, and permanent markers all have danger warnings… does this mean they are dangerous to intelligent responsible people who behave properly with respect to the conditions the items were intended to be used? We’ve never found a single credible, or non-credible for that matter, account of a pulse jet exploding. The closest we’ve found is an Idiot who welded his 100 lb thrust engine together with spot welds and then made a big fuss about it when the spot welds broke and the engine stopped running. Turbojet engines on the other hand, can, and have exploded quite violently on many occasions.
Valved Pulse Jet Engines:
A valved pulsejet engine like its name implies, has a mechanical valve inside it with which to control the flow of air in and out of the engine. This means that it does have a moving part that can wear out, and also more mechanical complexity, but it also has a few benefits.
Valved engines are much like a two stroke engine carburetor system, and an exhaust pipe, but no piston! Air flows in through the intake, lowering the pressure inside the carburetor section, drawing in fuel without need of a fuel pump. The fuel vaporizes and passes through the valve system, into the tail pipe, where it burns, causing a pressure rise and slamming the valve system shut.
Exhaust blasts out of the back of the engine at great speed, so fast that it causes a vacuum to form inside the tail pipe. More fuel and air gets drawn in through the intake system, into the tail pipe, where it builds up until it is again ignited by remaining burning gas.
This all occurs faster than you can blink, as high as several hundred times a second for small engines, and as low as 40 or so times a second for very large engines.
Valved Pulse Jet Benefits:
- High power to weight ratio
- Lower complexity for integrating into RC vehicles (no fuel pump system needed)
Valved Pulse Jet Drawbacks:
- temperamental to start
- valves wear out, sometimes in seconds
- more difficult to build
Valveless Pulse Jet Engines:
Valveless pulse jet engines operate on the same principles as a valved engine, except with no valve, and no moving parts! The shape of the engine entirely controls the flow of air in and out, this usually means that the intake of the engine also faces backwards. When the engine fires, exhaust shoots out both the intake and the exhaust pipe.
Valveless pulse jet engines are basically a specially shaped hollow tube! There are large number of designs, and an experienced pulsejet builder can design and build pulsejets in almost any shape and size and get them to run. However, because they will run under such a wide range of conditions no matter how badly out of whack, this has given valveless engines a reputation as poor performers.
There are many out there that claim that all valveless engines have low thrust output, high fuel consumption, and overall poor performance. There are a number of valveless designs though that are much better in most areas than valved engines, or even small turbojet engines. There are a very few engine designs that approach the efficiency of even turbofan engines.
Valveless Pulse Jet Benefits:
- Easy to start
- No moving parts, virtually no maintenance an unlimited run time
- Can run on almost anything that burns including waste material
- Extremely large throttle range
- Easy to design one that runs period
Valveless Pulse Jet Drawbacks:
- Difficult to design one with really good performance
- Harder to integrate into RC vehicles
Overall pulse jet engines have a wide range of applications from RC hobby, to alternative energy, industrial processes, and many other fields. It simply takes someone who is courageous enough to break away from tradition and to pioneer something old, with a new twist!
Chances are if someone has heard of a pulse jet before, 90% of what they know is wrong. The 10% of information they do know is also applied as broad over generalizations, which does not hold true for all designs.
First developed in the early 1900′s, pulse jets did not see any real use until WW2 when they were used to propel German cruise missiles. Other countries tried to adapt the technology, but pulse jets were quickly pushed aside by the turbojet, and fell into obscurity before any significant advances were made.
Since the 1950′s relatively little research has been done with pulse jets compared to their turbine engine counterparts, and a lot of myths and plain misinformation have developed, and has even been perpetuated by college engineering text books. Most have completely false information, and everyone keeps spreading it.
The mechanical simplicity makes them very well suited for a number of applications, with virtually no maintenance costs. Since pulse jets cost many times less than factory made turbojets, and are much easier to make than building a jet out of a turbocharger, they are increasing in popularity as a low cost way to get into hobby jet propulsion. They have been used to propel everything from go-carts to bikes to boats, not to mention RC planes.
They are not only mechanically very simple, but they also have an extremely large throttle range, high thermal energy release, and naturally low harmful emissions. Pulse jets are one of the most efficient ways of converting fuel into heat, with some of the lowest releases of harmful emissions of any type of engine or combustion device.
They can be designed and optimized for propulsion, thermal output, and a wide number of other applications such as acoustic output or thermal insecticide foggers. Pulse jets can also be made to run on fuels and material that most people don’t even consider fuel. They can run on everything from coal dust, saw dust powder, used cooking oil, to more traditional fuels like gasoline or diesel. As we like to say, if it burns, you can probably run a pulse jet on it.
Just because pulse jets glow orange hot does not mean they are extremely inefficient. Pulse jets have no internal moving parts, and their core gas temperature is not limited by the melting point of high RPM turbine blades. Metals like stainless and Inconel are very good at holding in heat, and while they may glow orange hot, only a small part of the heat is radiated away.
Turbines can only burn a fraction of the air they are able to take in and compress, if they tried to burn all the air in the combustion chamber, the turbine blades would melt no matter how much coolant was pumped through them. This is why military fighter jet engines have afterburner systems, the afterburner injects additional fuel into a duct that is past the temperature sensitive turbine blades, using up the oxygen left over in the exhaust. An afterburner system is not as efficient as combustion in the engines combustion chamber section, and results in extremely high fuel consumption.
Pulsejets can be fitted with specially shaped ducts called Thrust Augmenters, which increase thrust output by harnessing aerodynamic forces, exhaust gas heat, and providing a reactive surface for high velocity pulses of exhaust to pull in additional air and transfer momentum to it. This can actually make most pulse jets significantly more fuel efficient and powerful. Also the augmenter can be designed to double as a Ram Jet Afterburner by injecting additional fuel in for even more thrust output.
Although there are a number of designs such as the classical ‘Lockwood’ which burn through fuel like there is no tomorrow, the design is one of the worse examples of pulse jet fuel consumption. Even just tweaking the classical Lockwood design slightly results in much better efficiency. Unfortunately for decades it was the standard for providing unflattering performance statistics.