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Copyright © 2001 - 2009 to Bruce Simpson

An Explanation Pulsejets are very simple engines but their operation is not always easily understood -- after all, how can an almost empty pipe run as a jet engine? This page is an attempt to explain the four basic phases in the pulsejet's operational cycle. 1. Ignition This is the instant that the fuel and air in the pulsejet are ignited. The effect is that a fireball is produced inside the engine which creates a great deal of heat and pressure. The reed valves are held closed by this pressure, effectively leaving the flame and hot gasses only one place to go... 2. Combustion After ignition, the air and fuel continues to burn and expand in a phase called the combustion phase. During this phase the burning gases expand and travel down the tailpipe, exiting at the rear of the engine. The force of the gases leaving the engine in a rearwards direction creates an equal and opposite force that tries to move the engine forwards -- this is thrust. 3. Intake Because gases are elastic (they can be compressed and stretched) and because they have mass, the rapidly exiting exhaust gases have a tendency to keep moving -- even after the pressure inside the engine drops below the pressure outside. This causes a partial vacuum to be created inside the engine. The effect of this vacuum is to draw air and fuel in through the valves at the front of the engine -- which are pushed open by the higher pressure outside the engine. 4. Compression As mentioned above, gases are elastic -- so now, having been stretched out to create a partial vacuum, some of the hot exhaust gases are now drawn back towards the front of the engine by the vacuum that was created. Once again, because they have momentum, the gases in the tailpipe continue to move even after the pressure inside and outside the engine is equalized. This means that the gases continue heading towards the front of the engine -- towards the fresh charge of air and fuel that has just been drawn in. Of course, as soon as the pressure inside the engine becomes higher than the air pressure outside, the reed valves slam shut -- stopping the air/fuel mixture from escaping. This continued movement of the exhaust gases causes the air-fuel mixture to be compressed -- until the hot gases finally travel so far up the pipe that they touch the explosive air/fuel mixture and -- back to step one! This cycle repeats hundreds of times a second -- producing the characteristic buzzing sound of the pulsejet engine. What Can Go Wrong? Given that these are such simple engines it is sometimes difficult to understand why they can be so hard to design and build so that they work. Here are the effects of a few design mistakes that can affect the engine's ability to run: PIPE TOO SHORT If the pipe is too short then the engine won't run because all of the hot gases wil leave the tailpipe. This means there's nothing left to ignite the new air-fuel mixture drawn in during the intake phase. PIPE TOO LONG If the pipe is too long then the exhaust gases will "burn out" and cool down too much -- making it impossible for them to ignite the fresh air/fuel mixture. It is worth noting that most engines are far more tollerant of a too-long pipe than a too-short one. If you're designing an engine it always pay to err on the side of making the pipe a little longer than you might think necessary. VALVED INTAKE AREA TOO BIG If the valves in the front of the engine are too big then the vacuum necessary to suck some of the hot exhaust gases back to compress and ignite the fresh air/fuel charge will disipate too quickly and the engine will not run. VALVED INTAKE AREA TOO SMALL If the valves are too small then not enough air/fuel mixture will be drawn into the engine to provide adequate combustion and the engine will either not run or will run with less than optimal levels of power. It can be seen from this that there are two critical elements to designing a successful pulsejet engine: the valve area the pipe length Designing A PulseJet Armed with the understanding of the basic principles and critical factors it now becomes practical to start designing a pulsejet. I hope to add a "design your own pulsejet" section to this website shortly. Stay tuned!

 

Home | Project Diary | My Tools | Contact Me | Links | My Gas Turbine Project | The Afterburner Turboshaft Engine | Jet-kart | Pulsejet-powered Kart | Kitsets | Troubleshooting pulsejets Valveless Pulsejets | Ramjets Explained | 100lbs-thrust pulsejet | Turbo-turbine FAQ Chrysler's Turbine-cars | How Pulsejets Work | Flying Platform | Metal Spinning | My Lockwood engine Starting a pulsejet | Making Reed-valves Last | Pulsejet-powered speedboat | The PDE Thrust Augmentors List of Sponsors | Master Site Index | The Pulsejet FAQ | DIY Cruise Missile

Copyright © 2001 - 2009 to Bruce Simpson