The Valveless Pulse Jet

The Valveless Pulse Jet
Bringing the pulse jet engine into the 21st century
Last Updated: 13 February, 2002

Why Valveless?
Given that a conventional pulse jet only has one moving part (the valve) one might be tempted to ask -- why bother to design and build a valveless pulse jet?
Well the problem with conventional pulse jet engines is that the valves are subjected to intense physical and thermal punishment. As a result, they require regular replacement and represent a weak link in the engine's reliability.
From what I've been able to discover, many of the pulse jet powered V1 "flying bombs" which were launched by Germany against Britain during WW2 didn't actually reach their targets because the engines failed enroute. Even extensive US government testing undertaken by the NACA produced reed valves that were only good for about 20-30 minutes of continuous operation on larger engines.
So, by eliminating the valves, it should be possible to make an engine that is as simple as a ramjet having absolutely no moving parts.
The Lockwood-Hiller Design
Durnig the 1950s and 60s, some development work was done on the valveless pulse jet concept in the USA by Hiller and Lockwood.
In 1967 they filed a patent for such an engine and produced a number of working prototypes for which some very impressive claims were made.
The picture on the right is one of the Hiller-Lockwood prototypes being tested. This particular engine is said to have weighed in at just 30 pounds and produced around 300 pounds of thrust. Note that most of the engine is glowing with the heat produced. Also note that it appears as if the guy in the background thinks his hearing protection is inadequate.

Model HH 5.25-7
Valveless Engine
Military max thrust (lbs) .... 300 Maximum continuous (lbs) ..... 280 Minimum idle (lbs) ........... 30 Idle to mil. max time (secs).. 0.1 Fuel/thrust (lb/lb/hr) ...... 0.85 Dry weight (lbs) ............. 30

Many of the features of this engine seem to have been inherited from an earlier French design by Marconnet. Lockwood claims to have significantly improved the engine's performance and flexibility by redesigning the combustion chamber and introducing the concept of thrust augmentation tubes.
One of the big claims to fame of this engine was that it was imune to the damaging effects of ingested foreign materials. Gas turbine engines by comparison are very easily destroyed by objects that get sucked into their intakes. In fact, every year many commercial airliners require major engine repairs after birds are sucked into the intakes.

The above diagram documents the basic theory of operation behind the valveless pulse jet and also shows the effect of the thrust augmentors that Hiller and Lockwood added to their engine.
To explain in more detail what is happening:

A mixture of air and fuel in the combustion chamber is ignited. The ignition source is a spark plug when starting but once running, the air-fuel mixture is ignited by residual hot gasses returning down the long exhaust tube.

The air-fuel mixture explodes and the expanding hot gasses start rushing out both pipes -- producing thrust (and much noise).

Like a weight tied to a piece of elastic, the inertia of the outgoing hot gasses causes the remaining gas in the pipe to be stretched beyond its normal volume -- creating a very low pressure inside the engine. As a result, the flow of gasses out of the engine stop and start to reverse.

Fresh air continues to be drawn in by the partial vacuum in the engine and fuel is also injected into the combustion chamber at the same time. Most of the air actually enters through the short (intake) pipe because it has less distance to travel. However, the longer pipe also now has a column of air (and residual exhaust gasses) rushing inwards. These two inflowing columns of air actually collide in the combustion chamber to compress the new air-fuel mixture. The cycle now repeats from step 1.

Thrust Augmentors
These are little more than a cone-shaped tube placed a small distance from the exhaust(s) of the engine.
They work by recruiting extra (cool) fresh air by virtue of the venturi effect. This cool fresh air mixes with the hot exhaust gasses and, as a result, expands -- adding to the mass of air being propelled by the engine.
There is some debate as to the effectiveness of these augmentors, with some clamiing up to a theoretical 50 percent thrust increase while others say that practical testing produced only a 10-15 percent gain.
Other Designs
Kenneth Møller has the plans for a Chinese designed valveless pulse jet engine on his website
There are also a number of other designs around, although none have a reputation for ease of construction, power or simplicity of use.
And here is a BIG valveless pulsejet used for warming orchards. The developer says he'll have the noise problem sorted out within a year and will be manufacturing these pulsejets for about $500 each. Mind you, the article is dated August 1999 -- I guess it's still too noisy.
I Built One
As an experiment to see how easily an existing reed-valved pulse jet could be converted to valveless operation, I modified my mid-sized development prototype engine in just an hour or so.
I've documented the results (with video footage) on this page.
I Also Built A Lockwood/Hiller
In order to see whether Mr Lockwood's engine lives up to its claims, I built one of his valveless pulsejets according to his own specifications.
The engine is now mounted on my gokart and I intend to run some comprehensive performance (and road) tests sometime in the near future.
Unfortunately, starting a Lockwood-style engine is a little more risky than starting a regular pulsejet engine. You can read all about my engine and the accident I had when starting it on my Lockwood Engine page.

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