Shock Force Piston Movement

[t644]

I designed a cylinder for an opposed force engine, the two pistons are half-hemispheres, that move laterally to the direction of the up-down cylinder (the drawing should show two pipes both perpendicular at the level where the two pistons are to accomodate their lateral movement). My question is, I originally had the top of the cylinder open and short, but I wonder if it should be a very long coil that is "closed at the end", upon explosion, so that the explosion in the cylinder reflects a shock force, as opposed to a compression force, like an explosion in a gas pipeline that reaches supersonic speeds. Then the engine could use less fuel if the local explosion is 7 ATM and the reflected explosion in a gas pipeline is 1000 ATM, I assume using a small amount of fuel would move the two pistons. The picture says top torus, no, those are the two pistons, shaped as half-spheres, with it being more flat at their contact point. Just beneath the pistons is a ring of repeated metallic X's, or ribbon X's, and looking directly at them inside the pipe, they look like X's with no width, but the ribbons have width in the other coordinate, but they're just ribbons, and they're used to control the flame. I'm wondering if that vertical pipe needs to be very long to generate a shock wave that moves the pistons that move left-right in an opposed force cylinder, as opposed to generating compression in a short closed cylinder. The design seems to me that the vertical pipe (which is closed at the bottom) is that way so it's not really short, but perhaps quite long and coiled.

This is a video of an explosion in an open pipe on youtube:

https://www.youtube.com/watch?gl=UG&hl=en-GB&v=2--CWLW_TvE

If you can generate an explosion, or really shock wave like in that youtube video, it seems the engine would burn (imagine if the fuel had to "catch up" with the shock wave front, it would create a long string of flame outside the piston area that (and due to the other things like the shape of the piston and the X's) also had better combustion profile so that it burns more efficiently.

 

[BrianPetersen]

No. Just, no. You're dreaming in technicolour.

Get an understanding of how a normal piston engine works, and why.

Normal engines don't make use of "shock" waves, and with good reason. Combustion occurs quickly but in a controlled and progressive manner - not an explosion. Explosions blow stuff up ... including engines.

 

[t644]

Maybe, but maybe it needs some additional work. This is the paper that mentioned the shock front:

https://www.icheme.org/~/media/Docu...evention/Hazards Archive/III/III-Paper-06.pdf

And yes an explosion can distort metal. You're not walking on your right leg.

 

[BrianPetersen]

What relevance is an explosion in a pipeline to what you are attempting to do?

 

[RodRico]

t664,

I can't tell for sure what you're trying to accomplish from your drawing, but I'm guessing you're trying to effect compression ignition using shock waves. Compression ignition requires the charge be held above autoignition temperature for a prolonged period before the reaction begins (see

http://james-keck-memorial-collecti...aAyalaKeck-CombustionFlame-133-467-2003.pdf),

and it seems it would be very hard to make a shock wave with a pedestal that is sustained for the required amount of time. If you're familiar with Fourier transforms, you know it would require a good number of shock waves combining to create a sustained pressure peak, and this suggests a pretty complex shape producing multiple impulses timed to overlap.

BrianPeterson,

Note HCCI combustion is essentially an explosion that produces a very high pressure spike. Some designers try to spread the pressure spike in time using stratification and/or EGR. I don't; I simply use pistons of small enough bore to limited mechanical loads below the yield pressure of the steel I use for pistons, rings, cams, bearings, etc. The speed of HCCI combustion is nicely illustrated by the video I snagged from Sandia Labs.

Rod

 

[enginesrus]

Piston engines work using expanding gases. If it was an explosive event either it will be too much instant pressure for the components to handle, like bearings etc. or too little to cause the desired effect and desired power, since you would have to tame down the shock to save the parts. As compared to explosions engines run on slow burning conditions.

 

[btrueblood]

Detonation does give higher combustion pressures and temperatures than slow deflagrations normally used in piston engines. But, as others have noted, extraction of power using pistons, turbines, or other devices pretty much precludes using shock waves and rapid loading implied by detonations. A way around the problem is to use detonation waves for jet propulsion - google "pulse detonation engine", but as noted in the wikipedia article, there has been no production implementation of the device. The engine still has to be fairly bulky for its thrust output, in comparison to turbojets, due to higher pressures, and the lack of a simple turbine for power extraction means you can't just directly compress incoming air with a fan or blower. And they are louder than F$%$.

 

[RodRico]

enginesrus and btrueblood,

Detonation (as the forced by HCCI) creates very high gas pressure per square inch of surface area, but reducing the piston surface area (diameter) allows tailoring of the mechanical loads to within the yield strength of the materials used to transmit power through the engine. Given the loads are thus tamed, detonation yields the highest possible pressure and temperature resulting in improved efficiency. From the many published papers on the topic, HCCI is ~5% more efficient as a result of its near instantaneous combustion.

Rod

 

[RodRico]

By the way, ARPA-E funded a shock wave rotor engine some time ago. There's a nice overview of the technology at

https://www.egr.msu.edu/mueller/NMR...eviewOfWaveRotorTechnologyAndApplications.pdf

 

[enginesrus]

HCCI still not explosive it is very similar to diesel, and not as bad as gasoline engine detonation with the associated shock, the ratio of the fuel charge is a controlling factor too.

 

[btrueblood]

Wave rotors work at lower pressures and densities (weak detonations) more typically, and don't directly impinge shock velocities onto moving surfaces (there's a lot of plumbing and shock turning between the bang and the whirr). Yes, there's also been some work on PDE's as part of a wave rotor topping cycle, but I don't think much has come of it (yet?)

 

[BrianPetersen]

The original poster is nowhere to be found.

 

[t644]

In response to the person who mentions the shape of the shock waves. If everything occurs in the area as in the picture and the vertical pipe -is not long-. Well, the explosion would make a diamond shape pattern between the pistons, in terms of the shock wave pattern. However, I don't believe it's like that initially, as I believe the vertical pipe is involved with it ie it may be a long pipe that is narrow and filled with vapor that is lit on explosion in the core, and a string of flame trails the shock wave, which bounces back and the shockwave when returning is magnified in pressure by a large amount to drive the pistons, but after bouncing off the end of the long vertical pipe. If you read the pipeline paper, it does mention -two- discontinuities in relation to the flame that trails the shockwave and the shockwave ie there is a relationship in their discussion that creates a scenario with two discontinuities which may be related to the delay in combustion of the hydrocarbon of the two species.

 

[RodRico]

HCCI still not explosive it is very similar to diesel, and not as bad as gasoline engine detonation with the associated shock

Take another look at the video showing HCCI, spark, and diesel combustion. Diesel combustion occurs at the leading edge of the fuel spray and has the longest duration of the three. HCCI ignition and detonation in a spark ignition engine are similar but differ in a couple of ways: First and foremost, explosive combustion in a spark ignition engine is an unplanned event occurring before TDC in a spark ignition engine and produces stress outside design limits while explosive combustion in an HCCI engine is planned and produces stress inside design limits. Furthermore, detonation in a spark engine is caused by a hot spot, excessive compression, or a combination of the two while in HCCI it is solely the result of compression. When detonation is caused by a hot spot, combustion occurs at a speed similar to that seen when a spark ignites the fuel; a flame front is developed which takes time to spread through the air/fuel charge. When detonation of a premixed air/fuel charge is caused by compression, the chemical reaction is theoretically simultaneous throughout the charge. In an actual engine, the air/fuel charge is not perfectly uniform, however, so ignition starts in one or more regions then spreads rapidly to the other regions due to the pressure waves that spread through the intake charge increasing charge temperature.

 

[enginesrus]

Gasoline engine detonation has other causes as well and can happen at a moderate CR.