valveless pulsejet features

[isprins]

I suggest that a valveless engine should have these features:
easy start( no need for compressed air or blower)
good fuel economy
good compression
possible to start without a spark plug ( show a burning rag soaked in fuel into the combustion chamber, and turn on the fuel flow ? )

feel free to post any suggestions

[larry cottrill]

I suggest that a valveless engine should have these features:
easy start( no need for compressed air or blower)
good fuel economy
good compression
possible to start without a spark plug ( show a burning rag soaked in fuel into the combustion chamber, and turn on the fuel flow ? )

isprins -

These would be good "features" (they are really more like operational characteristics); they have all been considered "design goals" at some time or other. However, I think a couple of them (good fuel economy, good compression) are good ideas in the sense that "ending poverty" is a good idea.

Many engines have been started without the use of forced air. Even some small pulsejets can be started without "starting air" if they are equipped with a "fast" propane stinger that causes the intake to briefly act as an eductor (ejector); that is, the fuel flow entrains air efficiently. It takes a well-developed intake flare and a well-placed stinger with very fast flow to do this. The "Rosscojector" has such a high velocity that it will work with even very badly tuned engines, if it is moved in slowly until the "right spot" in the intake is discovered. So, conceptually, what you're after here is not that difficult (though I've never tried to achieve it).

We've seen remarkable improvements in pulsejet "fuel economy" in terms of Thrust Specific Fuel Consumption, which is basically how much fuel you burn per hour to get a lb or kg of thrust. If, however, you mean the more common idea of fuel economy (miles or km traveled per unit fuel), the only way a jet can achieve that is with speed. The only way a jet propelled system can get better distance per unit of fuel than a piston-driven system is by getting close to the ideal of "forward velocity = 1/2 the exhaust gas exit velocity". That is pretty fast, even for a pulsejet (better than half a Mach number in normal air)! Of course, air-entraining augmentors behind the exhaust port change this picture somewhat.

Compression is highly elusive in valveless pulsejets, where each explosion finds two large "leaks" out of the chamber instead of just one. The real way to maximize compression is to design the pipe so that maximum outside air is taken in by Kadenacy action in the available time. But, the available time is not much -- for a small pulsejet, the "breathing" part of the cycle is a couple thousandths of a second, maximum! As far as optimizing the ingested air goes, there is a point of diminishing returns where improvements are more and more difficult to come by, so there is some theoretical maximum that you just can't get to (though perhaps some future "exotic" design could push higher). Better breathing in pulsejets is a fine goal, but is not a simple problem.

Many schemes have been devised to start without a spark plug. The biggest problem with flammable objects is that they are usually ejected from the jet at high speed once the first good bang is achieved. This is generally undesirable from an environmental standpoint, especially under dry conditions! Again, the venerable Rosscojector could be useful here -- a small one could be pre-lighted and introduced as a "starting wand", something like the electric wands that some valved engines use from the tailpipe end. At least these options wouldn't produce "flame thrower" starts that could get you sued or arrested ;-)

Anyway, just some thoughts. Not meant to be discouraging in any way, but just a reminder that "if it was easy, everyone would be doing it." Ha.

L Cottrill