Combustion Operated Impulse Drive Unit

Basics

Cut away view of a typical Combustion Operated Impulse Drive Unit

     Shown above is a representation a Combustion Operated Impulse Drive Unit. The Valve Unit has a piston like feature which defines one end of the combustion chamber. The Exhaust valve seals the other end. The Inlet valves operate as one way check valves. High pressure gas from the intake channels push open the inlet valves and fill the combustion chamber. The inlet valves remain open until the pressure force inside the combustion chamber nearly equals that of the intake channel. When the pressure between the combustion chamber and the intake channel essentially become equal, the spring assist will close the valve. Other valve arrangements are possible.

 

     Fuel injectors inject fuel into the intake channel stream as the combustion chamber fills with the compressed gas. Once the predetermined pressure is reached, the spark plug can initiate burn.

 

     The above drawing is just one version of the combustion operated impulse drive unit. Though it does show key features, variation in design execution are possible. To further clarify the concept of the Combustion Operated Impulse Drive Unit a graphic representation of the operation is shown below. Four phases of operation are shown, but it should be noted, these four phases are one firing of the unit.

Load

     Phase 1: Loading Phase — Compressed gas, typically air, is force through the intake channel and into the combustion chamber. Fuel, timed for the event by some controller, is injected into the stream and carried into the combustion chamber. The amount of fuel injected is matched to the predetermined pressure and volume of the combustion chamber. This representation has the fuel injector in line with the intake channel. Other versions may have injection straight into the combustion chamber.


 

Burn Phase

     Phase 2: Ignition Phase — Once the predetermined pressure is reached in the combustion chamber, the inlet valve closes. An ignition source is then presented initiating burn within the combustion chamber. A spark plug works for this task.

Expansion Phase

     Phase 3: Expansion Phase — The burn created by the ignition leads to hot expanding gases which push on the piston face of the valve unit. This force causes the valve unit to move, pulling the exhaust valve from the exhaust port, allowing the expanding gases to escape as a thrust burst. Force of expansion also pushes on the inlet valve, further seating it. The motion of the valve unit has a force associated with it. Its distance of motion will determine work available there. The arrows at the back of the valve unit show the force of the valve unit. The expanding gases escaping from the exhaust port can simply be used as thrust. Or the mass flow can be applied to an object. The total expansion of the combustion chamber will go into determining how much usable work is available from thrust. The small arrows inside the combustion chamber show the action of the expanding gases.

Recovery Phase

     Phase 4: Recovery Phase — Once the bulk of the expanding gases have left the combustion chamber and the pressure inside drops some point near the predetermined pressure needed for combustion, the recovery force can exert itself and return the valve unit to the initial sealed position. Once this position is established the process can be repeated. The arrows at the back of the valve unit show the action of the recovery force. Arrows are used instead of a spring to underscore that no particular force device is needed. The top drawing shows a spring, but the forward return mechanism for a Combustion Operated Impulse Drive Unit could be pneumatic, hydraulic, even electrically based, although springs certainly work well enough.

Example — Below is one iteration of a combustion operated impulse drive unit. This is a proof of concept build used only to determine the effectiveness of certain design aspects. Currently an official Demo Model is being built. It is hope that that model will be available soon, but this proof of concept test should give a reasonable overview of what goes into running one of these designs.

     Picture 1 — This is an overview of the test platform and the iteration of the drive unit that was being tested at the time. Each version of the combustion operated impulse drive unit needed to be tested and this same platform, with some modifications, was used to test most of them. Under the Hall Effect Sensor is a cylinder. This cylinder can be rotated by means of a handle. Embedded into this cylinder is a magnet, which is the trigger for the ignition system. To the right of the sensor is a typical compressed air trigger found on many consumer style air compressors. The handle also turns two cams. One cam operates the compressed air trigger, the other typically runs the fuel pump. In this version, however, that cam was not used to run the fuel pump. The fuel pump was operated manually, simply as a means to speed up the testing schedule. Some parts are labeled, and others, their functions should be reasonably easy to infer.

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     Picture 2 — This is the same platform, and same iteration as above, at a different angle. The spark plugs are removed and the fuel injectors pulled from their mounts. Also the compressed air trigger, the compressed air trigger mounting fixture, and the compressed air trigger actuator are removed from the platform. These two pictures should give a reasonable approximation of the support mechanisms needed to run a combustion operated impulse drive unit.

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The Video

     The Video — Unfortunately, the video is of low production value. This was taken as an after thought. The Drive Unit had been run earlier, and since it was all ready set up, it was thought it may be a good idea to get some video documentation of one of these units operating. There is obvious reluctance to use proof of concept units for demonstration purposes. That’s what Demo models are for. The Demo model is still being built, so this will have to do for now.

     Yes, those are mis-fires. It is preferred to call them failures to fire. The root cause for the failures to fire are most likely air bubbles in the fuel lines. As stated, the unit was operated earlier, and operated until it ran out of fuel. Fuel had to be added to make this video. This unit has a rather primitive fuel injection system with no pre-firing purge mechanism. The system should have been manually purged for the video, but again, this was simply for documentation, not demonstration. The fuel used in this version is gasoline, bought from a gas station. The only prep to the fuel was to let it settle in a clean container in case debris was in the transport container. Other fuels used in other iterations include alcohol, both ethyl and isopropyl, alcohol/ether blends, alcohol/gasoline blends, and alcohol, gasoline and ether blends. The above iteration was run a few more times after this video, but it only used gasoline in every operation. At the time of this build maintaining correct blends was determined problematic, and gasoline has since been the only fuel used, and has proven to be very consistent.

     The irritating noise at the end of the video is the air compressor turning on to refill its air reservoir. Audio editing was used to diminish the sound. Other than the audio editing and converting the raw video from .avi to more web friendly formats, no other editing was performed on this video. This is essentially the same as the raw file. This iteration was run at approximately 5:1 compression (or about 70 psig). Approximately because the gauge on the air compressor was the means to determine the actual pressure. The fuel pump was designed to meter out a portion of fuel to match the combustion chamber volume of air at a 5:1 ratio. There was very little effort to assure the fuel pump metered out exactly the designed amount. In the earlier run, time was spent adjusting the air compression to better match the fuel being injected into the intake channels. No video was made of that process nor the subsequent operation at the determined compression value. Also, this video was taken no more than an hour after initial operation, and the air compressor was not tampered with in any way at the time. It was not even turn off. This is why it is felt the compression or the compressor itself is not a root cause for the failures to fire. It may also be noted the that timing of the firing event is very loose. These drive units can operate on a very generous timing schedule. When injecting into the intake channels, it is important to make sure the fuel is completely introduced before reaching combustion pressure. Direct injection into the combustion chamber adds some criteria, but can operate with a similarly generous timing schedule. But more importantly, these type of units can be fired with a much tighter, shorter timing schedule.

 

     That summarizes the video. If for any reason you could not access the video on this website, the link below will take you to the YouTube video. Also, if there are any questions or would like more information use the email link on the Home page.