Making pulsejets less thirsty
Last Updated: 8 November, 2002
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How Much Fuel Do Pulsejets Use? One of the biggest drawbacks of traditional pulsejet designs is the rate at which they consume fuel. Whereas a modern high-bypass turbofan engine of the type used in passenger uses just 0.3-0.4lbs of fuel for every lb of thrust generated, a traditional pulsejet will consume over ten times that amount (3-4lbs) for every lb of thrust it generates. This means that a pulsejet generating 100lbs of thrust will consume over 300lbs of fuel per hour. If that fuel is regular gasoline, we're talking about using 55 (US) gallons per hour! By comparison, even a humble piston engine driving a propeller can generate the same amount of thrust at a cost of just 2 gallons per hour or less. Obviously this appalling inefficiency is one of the reasons that pulsejets are seldom ever considered to be a practical powerplant for anything other than small models.
Why Do Pulsejets Use So Much Fuel? There are a number of reasons for this -- some are obvious, some are not. Firstly, have you noticed how, unlike other types of engines, pulsejets tend to glow red hot when running? All that radiated heat is energy which is not contributing to thrust -- it's simply wasted. By comparison, the turbofan and piston engines waste far less of the energy the get from burning fuel -- converting more of it into thrust and less into waste-heat. Then there's the fact that pulsejets don't really compress their air/fuel mixture before its ignited. It just happens, that the more you compress a combustible mixture of air and fuel vapor before igniting it, the more efficiently the energy stored in the fuel can be released. This is why "high compression" piston engines usually produce more power than "low compression" ones. A typical piston engine compresses the air/fuel mixture it sucks in by a factor of between 8 and 11 before it is ignited. A typical turbofan engine also significantly compresses the incoming air to over 250psi before the fuel is added and this produces far more efficient combustion than we can ever hope to achieve in a traditional pulsejet engine.
How Do We Improve Pulsejet Fuel Efficiency? Unfortunately this an elusive goal that seems very hard to achieve in practice. A number of designs have been proposed, some of which attempt to use two engines joined by way of a common pipe which allows the combustion in one engine to pre-compress the air/fuel in the other. This approach looks good on paper but, as far as I'm aware, hasn't actually yielded any significant improvements in power or fuel economy. Another concept receiving huge amounts of attention is the Pulse Detonation Engine or PDE. The PDE takes advantage of the incredibly high pressures that exist when an air/fuel mixture is detonated rather than just burnt. Unfortunately, convincing conventional air/fuel mixtures to detonate is not a particularly easy thing to do -- they'd much rather just burn (or deflagrate) as they do in a regular pulsejet. However, all is not lost, there are some relatively simple ways to increase the fuel efficiency of a pulsejet.
The Thrust Augmentor The augmentor won't reduce the amount of fuel an engine burns, but it will increase the thrust it produces without increasing fuel consumption. When a well designed augmentor is fitted to a pulsejet the thrust can be increased by as much as 50%-80% while burning no additional fuel. If you needed 100lbs of thrust you could then simply use an engine that produced 65lbs of thrust and get the extra 35lbs from the augmentor. Assuming the basic engine had an efficiency of 3 lbs of fuel per lb of thrust per hour, it would consume just 34 gallons of fuel per hour. With the augmentor fitted, we'd then have an engine that produced 100lbs of thrust but which used some 20 gallons per hour less than a plain, non-augmented engine producing the same amount of power.
Timed Fuel Injection The problem with this setup is that for a fair percentage of the engine's operating cycle (combustion/exhaust), that fuel is being wasted -- often simply passing out the tailpipe and being ejected from the tailpipe and contributes nothing to the engine's power output. Fortunately, the period during which injected fuel would be wasted just happens to coincide with the period during which pressure inside the engine is highest. It therefore becomes a very simple job to create an injection system that automatically shuts off the flow of fuel when it's not going to be of any use.
The picture on the right is a very crude example of this concept and has been used to great effect in a number of my prototype engines. As you can see, the reed valve is created by bending a thin strip of metal into a U shape so that it sits over the hole through which fuel (propane in this case) is injected. It's worth noting that there are some issues to be considered when designing such an timed injector system: Firstly, the pressure inside a pulsejet, even at the peak of the combustion cycle, is not very high. This requires that you have quite a large area against which the combustion gases can press in order to close the valve -- or it won't close at all. It's actually the *flow* of gas that initially closes this type of valve.
I have however, built a "better" injector that relies solely on pressure
and not flow.
As you can see, this injector is a little more complex and requires some machining operations to make. It does however, have the advantage that, because it's pressure rather than flow activated, its placement within the engine is far less critical. It's been my experience that this "timed injection" can produce an additional 10%-15% improvement in fuel consumption when compared to the same engine with untimed injection.
Summary Now, combine the timed injection with a thrust augmentor and, if you're lucky, you've nearly halved the fuel consumption of a conventional pulsejet. Piston engines and turbofans are still a whole lot more efficient so you will still need deep pockets to run a big pulsejet engine for any significant length of time.
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The simplest way to do this is to fit a simple reed-valve to the end of the
fuel injector.
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