Is the Pulsejet an Efficient Engine?

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Bruno Ogorelec
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Is the Pulsejet an Efficient Engine?

Post by Bruno Ogorelec » Thu Aug 26, 2004 4:42 pm


One of the first things most people with some knowledge of jet engines will say when hearing about the pulsejets is that they are inefficient. In some ways, this is true. In others, it is completely wrong. A general statement of this kind cannot do justice to what is a rather complex problem.

For one thing, efficiency has many forms. A study of those forms on the parallel cases of the pulsejet and the turbojet will sometimes yield unexpected results.

But, let us first make clear what we are looking for. A jet engine is a machine that increases the momentum of air stream. Basically, this is done by adding energy in the form of heat, which increases the speed of the stream. The increase in the momentum manifests itself as the increase in thrust. According to this definition, we are really looking at how well the two kinds of engines pump air.

Surprisingly, in terms of mechanical efficiency, the pulsejet performs as well as the turbojet! For the same quantity of fresh air that passes through it, one produces the same amount of thrust as the other. That's not bad, I'd say. After all, a pulsejet is such an incredibly simple and cheap machine that one really expects the turbojet to be much better. It just goes to show that you have to be able to look at things without blinkers.

However, the pulsejet is less fuel-efficient. For the same amount of fuel consumed, a turbojet will pump roughly four times as much air and generate correspondingly more thrust. The reason is that the various energy conversions in the pulsejet cycle take place under less than optimal conditions.

To start with, the pulsejet extracts less heat from a given amount of fuel than a turbojet does. The reason for that is that the fuel-air mixture burns at a low pressure. In the turbojet, the higher pressure is produced by a compressor at the front end, which pushes a huge amount of air into the combustor, in the same way a turbocharger pushes a lot of air into a piston engine.

So, what we are comparing here is a combustor with a turbocharger and another without it. It is not a fair comparison. One would never do it with car engines, for instance.

A pulsejet is more closely comparable to the inner combustor of the turbojet -- the core cylinder you will find when you remove the compressor and the turbine (plus a lot of complex and expensive ancillaries).

The problem is that the turbojet combustor cannot work without all that other gear. The great wonder of the pulsejet is that it can. The pulsejet provides its own aspiration and compression without any additional mechanical equipment whatsoever. This feature has delighted propulsion engineers from day one.

Wait a moment. I said "compression". But, the pulsejet is usually said to be working without compression. Supposedly, it can only muster a feeble 1.2:1 compression ratio -- and only if its acoustic properties are exploited properly.

Well, I can only say that this is another common misconception. What the pulsejet lacks is better called 'pre-compression'. That ratio is indeed maybe 1.2:1, which is completely negligible in comparison to the turbojet, which achieves pre-compression ratios of 25:1 or even higher. However, pre-compression is just a part of the pressure game.

In the pulsejet, as it name says, the fuel-air mixture does not burn steadily, at constant pressure, as it does in the other jet engines. It burns intermittently, in a quick succession of explosive pulses. In each pulse, the gaseous products of combustion are generated too fast to escape from the combustor at once. This raises the pressure steeply. The part of the fuel/air mixture that has not combusted yet is compressed, which makes it burn more efficiently.

The pulsejet is the only jet engine combustor that shows such a net pressure gain between the intake and the exhaust. All the others show a net loss. Other jet engines have the highest pressure at the intake. Inside the combustion chamber and in the exhaust it must be somewhat lower; otherwise the hot gas would be coming out of the intake, not just out of the exhaust.

The great intake pressure must be provided by something. In a turbojet, that something is a complex two-part assembly -- a compressor that generates pressure and a turbine that drives the compressor. Not only are both devices bulky and expensive, but they also consume a great amount of power. In the turbojet, much of the energy generated by the engine goes to drive the turbine and the compressor. Only the remainder provides thrust.

The pulsejet is different. It performs its compression without consuming any of the power generated by combustion. This is very important. According to some rough figures, a 5-percent pressure gain achieved by this method gives about the same improvement in overall efficiency as the 85-percent gain produced by a compressor, all other things being equal. Now, that's rather impressive.

In a fair comparison, a pulsejet thus beats a turbojet hands down. Give it the same intake pressure and it will greatly outperform the turbojet. Or, conversely, it will generate the same performance as a turbojet with much less fuel. It will need appreciably smaller and cheaper turbine, compressor and other ancillaries to perform the same job. It is no wonder that in the last years of WW II and for almost two postwar decades, scores of researchers and engineers threw themselves at the pulsejet to see how far it could be developed.

Alas, they did not get very far. One of the problems is that, for a number of reasons, pulsejets are very difficult to supercharge. That topic goes beyond the scope of this article, so let me just tell you that the most common form of the supercharged pulsejet is the wave rotor engine. It is more efficient than a turbojet, but the technology that will make it as reliable as the turbojet is not with us yet.

(NASA claims otherwise -- look at wave rotor projects at for instance -- but the wave rotor developers have been saying this for ages, yet you still do not see wave rotor engines in practical applications anywhere. Perhaps we will see them in the future.)

Next on our checklist is the working temperature. The higher the temperature, the more efficient the heat generation. In this respect, the pulsejet is superior to the turbojet. The exhaust gas produced by a pulsejet is much hotter than in a turbojet.

Combustion takes place at similar temperatures -- between 2000 and 2500 C -- but the exhaust gas in a turbojet is immediately mixed with a lot of cool air, so that the temperature is lowered to between 800 and 1200 C before it enters the turbine. The main reason is to keep the turbine from melting. There is no need for such mixing in the pulsejet, which has no moving parts, and its exhaust gas travels towards the end of the engine at very close to its initial temperature, which is two to three times hotter than in a turbojet.

Why is this good? Because the propulsive work is done at the top of the entropy curve, where the conversion to mechanical energy is more efficient. The energy is used as soon as it is produced. In contrast, the turbojet first cools its combustion products, then lets them expand to lower pressure, and then makes them turn the turbine and the compressor. Thrust is produced only after all this work has been done, once much of the energy has been degraded. Therein lies much of the comparative pumping efficiency of the pulsejet.

However, as was the case with self-compression, the problem is what to do with this great efficiency. If you do not have a good way to convert heat into useful work, it will be useless. Alas, this is another area in which the pulsejet is in trouble. It converts less of the available heat energy into useful work than the turbojet. More heat is wasted.

This is connected to the definition of the jet engine as an air pump. Namely, one of the big problems is that the pulsejet uses comparatively very little air. A small amount is sucked into the combustion chamber and used for combustion and a slightly larger amount is sucked back into the exhaust tube between explosions. That is all. There is no through-flow of the kind one finds in turbojets.

The small mass of gas is propelled to the maximum speed possible under the circumstances (the local speed of sound) and no further. The sonic choking of the duct prevents the gas speed from rising further, despite the fact that there is sufficient energy for further acceleration. A Laval nozzle would push the speed beyond the Mach barrier, but it does not work with a pulsating flow.

So, much of the available energy has nowhere to go. This creates problems. At sonic speed, gas is not capable of absorbing more heat. This generates compression waves that travel up and down the engine, disrupting the cycle. Once the super-heated gas leaves the tailpipe, it expands wildly, with gas molecules shooting in all directions in the so-called thermal shock. Under such circumstances, the energy-mass transfer ratio is very low and the resulting thrust is much lower than it could be.

The super-hot exhaust of the pulsejet simply cries out for additional propulsion mass to heat up and accelerate. This acceleration must be done under controlled circumstances, however, in a duct that gives the working fluid useful entrainment. Pulsejet designers have come up with two basic ways to provide a greater amount of fresh air as propulsion mass and allow it to be accelerated in an orderly manner. Both increase the thrust and are thus called methods of thrust augmentation.

The primary augmentation exploits the reversed flow in the tailpipe. This is the purpose of the trumpet-like tailpipes of many pulsejets. Engines like the Lockwood or the Ecrevisse or the Escopette have tailpipes that are progressively larger towards the end, so that the final section is sometimes turned into a veritable bustle. This increases the volume of the exhaust duct considerably and also gives the duct a shape that promotes the intake of fresh air during the suction part of the cycle.

The result is an exhaust filled with a large amount of cool air. Each blast from the combustion chamber pushes this air 'plug' mechanically, but the engine also transfers a lot of its heat to the air, both from the hot tube walls and from the pushing hot gas. Heating makes air expand. Expansion increases the static pressure, which increases the speed at which the air is expelled backwards. Much additional thrust is exerted.

Perhaps paradoxically, the secondary augmentation is performed only after the hot exhaust gas leaves the engine. A device popularly called the 'thrust augmenter' is added to the engine at the very end, and is separated from the engine proper by a notable gap. This gap is where more fresh air enters the flow.

The thrust augmenter is really what is usually called 'ejector' in fluid mechanics. It is a simple length of cylindrical or conical tube mounted after the end of the pulsejet tailpipe. Hot exhaust gas blows into this tube and (due to the Bernoulli law) sucks fresh air from the sides and mixes it with the flow. The shape of the augmenter duct allows the mixed gas to expand in an orderly manner, giving it the proper rearward direction. The process is very similar to what happens in the 'bustle' of the pulsejet tailpipe, but the flow is in one direction only.

Thrust augmenters give excellent results. Some reports indicate that the thrust gain may go as high as 100 percent, which is truly impressive, but even the gains of some 50 percent reportedly achieved by garage enthusiasts at times are nothing to be sneered at.

There may be other ways to exploit the energy of the hot expanding gas. The idea of making it compress the fresh charge for the next working cycle has been around for a while. It would raise the mean effective pressure in the cycle and boost the efficiency considerably. Experiments in that direction have been few and the results inconclusive. However, logic would say that this is a promising idea.

Finally, we should mention another useful feature of the pulsejet -- its very low weight. In the power-to-weight stakes, a good pulsejet is already superior to most turbojets -- especially if you take the entire propulsion system into account. If you want to fly, believe me, few things are as useful as low weight. As it directly impacts on performance, perhaps this feature should also be counted under 'efficiency'.

In conclusion, one can say that at the moment, overall, the pulsejet is inferior to the turbojet in several respects. Yet, it is better than many people would think. When you know what you are looking at, rather than bandying popular 'wisdom' about, you can see that in some ways the pulsejet is actually superior.

The most exciting, however, is the clear promise of possible improvement. The pulsejet is a relatively unexplored engine, left on the margins of technological development because no one could properly tame its wilder side in the mid-20th century, when the jet engine saw its most energetic flowering. It is up to the current generation to re-evaluate its advantages and see whether they can be used in practice. This, my friends, is where we come in.

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