How Does a Pulsejet Engine Work?

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Bruno Ogorelec
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How Does a Pulsejet Engine Work?

Post by Bruno Ogorelec » Sun Aug 22, 2004 3:07 pm

The picture above shows one of the many possible layouts of a valveless pulsejet engine. It is a bit simpler than the ones with valves, so we'll start with that. It has a chamber with two tubular ports of unequal length and diameter. The port on the right, curved backwards, is the intake pipe. The bigger one on the left is the exhaust, or the tailpipe.

Fuel is most often gaseous or liquid propane, but some engines, especially those with valves, run on gasoline, methanol and other liquid fuels, even the Jet-A.

In some other valveless engines, it is the exhaust pipe that is bent into the U-shape, but the important thing is that the ends of both ports point in the same direction. (In this website, you may see a number of engines that are not bent, but this is often done for simplicity. For experimental purposes, builders often ignore the fact that thrust in exerted in two directions at once. They build a bent engine only when they actually want to power something with it.)

Back to the picture. Imagine fuel squirted into the chamber and mixed with air. Imagine the spark plug producing a spark.

When the fuel-air mixture combusts in the chamber, the process generates a great amount of hot gas. The pressure inside rises very quickly – so fast, in fact, that to an observer it looks and sounds like an explosion.

The rising pressure forces the hot gas to expand out of the chamber and pass through the two ports at high speed. As they leave the engine, the two jets of hot gas exert thrust. They push the engine in the opposite direction. This is jet propulsion.

As the hot gas blows out of the engine, the pressure inside the chamber drops. Due to inertia, this drop continues even after the pressure falls back to atmospheric. At the lowest point of the process, there is partial vacuum in the chamber. The outside (atmospheric) pressure is now higher than the pressure inside the engine.

The process now reverses itself. Fresh air starts rushing into the ends of the two ports. At the intake side, it quickly passes through the short tube, enters the chamber and mixes with fuel. The tailpipe, however, is rather longer, so that the incoming air does not even get as far as the chamber before the pressure peaks and the engine is refilled.

One of the prime reasons for the extra length of the tailpipe is to retain some of the hot exhaust gas within the engine at the moment the suction starts. That gas gets greatly rarified in the expansion, but the incoming fresh air pushes it back, increasing its density and mixing it vigorously with the fuel/air mixture that enters the chamber from the intake. The heat and the dissociated free radicals in the gas will eventually cause ignition and the process will repeat itself.

Ignition is thus automatic. You only have to ignite the mixture once, at the start, either with a spark plug or anything else you may think of. Once it fires the first time, the engine provides its own ignition.

The above cycle is very brief. In a small (flying model-sized) pulsejet, it happens 100 to 250 times a second! The individual cycles cannot be distinguished by ear. Instead, there is a (very loud!) continuous roar.

The cycle is not much different in the conventional flap-valve pulsejet, like the big Argus or the small Dynajet (see the next picture). There, the rising pressure inside the chamber makes the valve flaps at the front of the chamber snap shut and there is only one way for the hot gas to go -- into the exhaust tube.
Some valveless engine designers have developed designs that are not bent backwards, but employ various tricks that work in a similar fashion to valves -- i.e. they allow fresh air to come in but prevent the hot gas from getting out through the intake. Some of those tricks are described elsewhere on this website.
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